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CHAPTER 1
INTRODUCTION
1.1 Corrosion
Corrosion has been an important problem for centuries, but with the
rapid multiplying uses of metal, the increasing occurrence of corrosive
environment and the depletion of supplies of ores, it has became much more
serious in recent years. Except noble metals (like Gold and Platinum) all
metals exist in nature in combined forms as their carbonates, hydroxy
carbonates, oxides, sulphides, chlorides and silicates. Because of their higher
energy, pure metals are less stable and they have a natural tendency to form
more stable metal compounds that is corrosion. Corrosion is defined as the
destruction or deterioration of a material by chemical, electrochemical or
metallurgical interaction between the environment and material. The
interaction between the material and their environment sometimes give
beneficial effects such as passivation, pickling of steel, storage of electrical
energy in dry cells, etc., but most of the time it leads to destructive results.
1.1.1 Cost of the corrosion
Corrosion is a naturally occurring phenomenon commonly defined
as the deterioration of a metal or its properties due to the reaction with its
environment. Like other natural hazards such as earthquake or severe weather
disturbances, corrosion can cause dangerous and expensive damage to
everything from automobiles, home appliances and drinking water system,
pipelines, bridges and public buildings. Corrosion is probably the greatest
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consumer of the metal known to the man. The tonnage of metals like steel,
copper, aluminium, lead, zinc and tin lost through corrosion is extremely high.
The cost of corrosion is enormous considering many costly processes involved
in the manufacture of metals. Business India news on 24 April, 2010 states
that in India the annual cost of corrosion is Rs 2.0 lakh crore. World annual
cost of corrosion is $2.2 trillion is over 3 % of world’s GDP. The annual
corrosion cost of key sectors such as infrastructure has been put at Rs 22,600
crore, utility services Rs 47,100 crore, production and manufacturing Rs
17,650 crore and defense and nuclear waste Rs 20,000 crore. Correspondingly,
cost of mitigation has been estimated at one per cent of the savings. The figure
cannot be off the mark as US and Japan mark their losses around four percent
of Gross Domestic Product (GDP) (Miksic et al 2004, Natesan et al 2006 and
Natesan et al 2008).
Apart from its direct costs, corrosion poses serious threat to our
natural resources. As a result of rapid industrialization of many countries, the
competition for and the price of the metal resources are in the increase.
Although corrosion is inevitable, its cost can be considerably reduced. It is
thus quite justifiable that crores of rupees are being spent on research to find
out means of preventing corrosion.
1.1.2 Theories of corrosion
1.1.2.1 Acid theory
The acid theory suggests that the presence of acid such as carbonic
acid is essential for corrosion. According to this theory rusting of iron is due to
the combined action of oxygen, carbon dioxide and moisture, converting metal
into soluble ferrous bicarbonate which is further oxidized to basic ferric
carbonate and finally hydrated ferric oxide. The reactions are given in the
equations (1.1) - (1.3). This theory is supported by the fact that rust analysis
3
generally shows the presence of ferrous and ferric carbonates along with ferric
oxide and retardation of rusting in presence of lime or NaOH to the water
which iron is immersed.
Fe + O + 2CO� +H�O → Fe�HCO� � (1.1)
2Fe�HCO� � +H�O + O → Fe�OH CO� + 2CO� + 2H�O (1.2)
2Fe�OH CO� + 2H�O → 2Fe�OH � + 2CO� (1.3)
1.1.2.2 Electrochemical theory
The electrochemical or local cell theory considers that corrosion is
due to the existence of separate anodic and cathodic parts, between which
current flows through a conducting solution. This type of corrosion occurs
where a conducting liquid is in contact with metal or alloy. Immersion
corrosion, underground corrosion and atmospheric corrosion are fall under this
category.
Electrochemical reaction involves transfer of electrons between
anode and cathode through an electrolyte. The anodic reaction is dissolution of
metal as corresponding metallic ions with the liberation of free electrons. The
cathodic reaction consumes electrons either by evolution of hydrogen or by
absorption of oxygen, depending upon the pH of the environment. The anodic
and cathodic reactions are as follows,
At anodic part, oxidation (or) dissolution of metal occurs
M → M�� + 2e� (1.4)
4
At cathodic part reduction reaction occurs, which depends on nature
of the corrosive environment. If the corrosive environment is acidic, hydrogen
evolution occurs at cathodic part.
2H� +2e� → H� ↑ (1.5)
If the corrosive environment is slightly alkaline or neutral,
absorption of oxygen occurs at cathodic part.
1 2⁄ O� +H�O +2e� → 2OH� (1.6)
The metal ion and hydroxide ions react to give corresponding metal
hydroxide.
M�� + 2OH� → M�OH � (1.7)
1.1.2.3 Homogeneous theory
It is not necessary that the anodic and cathodic reactions should take
place at spatially separated areas explained by Wagner and Traud (1938), but
shift randomly over the corroding metal in respect of time and space.
According to this, the entire surface supports anodic and cathodic reaction and
is considered as homogeneous theory of corrosion, while local cell theory may
be regarded as heterogeneous theory of corrosion.
1.1.3 Classification of corrosion
Corrosion has been classified in many different ways. In one way it
is classified into low-temperature and high-temperature corrosion. In another
way it is classified into wet corrosion and dry corrosion. The convenient way
of classification is in which it manifests itself. In this way, it is classified into
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eight different forms. Each form can be identified by mere visual observation.
In most cases the naked eye is sufficient, but sometimes magnification is
required. The identification of the different forms of corrosion is very much
helpful in understanding the intensity of problem and finding the solutions.
The eight forms are given below,
� Uniform or General attack
� Galvanic or Two metal corrosion
� Crevice corrosion
� Pitting corrosion
� Intergranular corrosion
� Selective leaching or Parting
� Erosion corrosion
� Stress corrosion
The eight forms of corrosion are discussed in terms of their
characteristics, mechanisms and preventive measures as follows,
1.1.3.1 Uniform or General attack
Uniform or General attack is the most common form of corrosion. It
is characterized by a chemical or electrochemical reaction in which corrosion
takes place uniformly over the entire exposed surface area of the material. This
type of corrosion is not too great concern from the technical standpoint
because life of equipment can be accurately estimated using simple tests like
immersion of specimen in the fluid of our interest. This form of corrosion can
be prevented or reduced by proper selection of materials, protective coatings,
use of inhibitors and applying cathodic protection to the affected equipment.
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1.1.3.2 Galvanic or Two-metal corrosion
Two dissimilar metals or alloys are exposed to an electrolyte
(corrosive or conductive) a potential difference is developed between these
two. If these two are electrically coupled or placed in contact, this potential
difference causes an electron flow from the more active metal to less active
metal. The more active metal undergoes corrosion (anodic) and the less active
metal is protected from the corrosion (cathodic). Because of the electric
current and two dissimilar metals are involved, this corrosion is called as
Galvanic or Two-metal corrosion.
Galvanic corrosion has several applications e.g.:- Dry cells,
cathodic protection etc., (Fontana 1987). Galvanic corrosion can be
minimized by
1. Coupling of dissimilar metals which are close together in
galvanic series.
2. Use anode area as large as possible.
3. Electrically insulate dissimilar metals from each other.
4. Apply protective coatings on the metal surface.
5. Add inhibitors to reduce the aggressiveness of the environment.
6. Proper designing of metal article.
7. Electrically connect a third metal that is anodic to both metals in
the galvanic contact.
1.1.3.3 Crevice corrosion
Crevice corrosion is an intensive localized corrosion occurs within
crevices and other shielded area on metal surfaces exposed to a corrosive
environment. Crevice corrosion is more likely to occur in holes, gasket
surfaces, lap joints, surface deposits and crevices under bolt and rivet heads
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that retain solutions and take longer time to dry out. This form of corrosion is
sometimes called as deposit or gasket corrosion. The rate of crevice corrosion
is based on some important factors such as lack of oxygen, changes in acidity,
inadequate anodic treatment and decrease or depletion of an inhibitor. Crevice
corrosion can be prevented by the use of high resistance alloy, use of welded
joints instead of riveted or bolted joints, maintaining clean surfaces, use of
solid non-absorbent gaskets and frequent removal of accumulated deposits and
designing containment vessels to avoid stagnant areas and ensure complete
drainage.
1.1.3.4 Pitting corrosion
Pitting corrosion is extremely localized attack that results from
inhomogeneities in metal due to inclusions, coring and distorted zones which
set up difference of potential at localized spots to cause deep isolated hole or
pits. These holes may be smaller or larger in diameter, but in most cases they
are relatively small and because of their smaller size and are often covered by
corrosion products, they are not easily identified. It is very much difficult to
measure quantitatively and compare the extent of pitting because of the
varying depths and number of pits that may occur on identical conditions.
Pitting is an autocatalytic process. This process is self-stimulating and
self-propagating. This corrosion processes usually requires an initiation period.
The period ranges from months to years depending on both the specific metal
and the corrosive. Depth of pitting is some time expressed as pitting factor,
which is the ratio of the depth of deepest pit to the average penetration as
calculated from weight loss.
This form of corrosion can be prevented by removing deposits of
solids from exposed metal surface, by selection of material, by using corrosion
8
inhibitors suitable for the environment and by preventing formation of oxygen
concentration cell (Bonora et al 1974).
1.1.3.5 Intergranular Corrosion
Intergranular corrosion is a localized attack at and adjacent to the
grain boundaries with relatively little corrosion of the grains. In this corrosion
alloy disintegrates and loss its strength. Intergranular corrosion is a
non-uniform corrosion. It can be caused by impurities at the grain boundaries,
enrichment of one of the alloying elements or depletion of one of these
elements in the grain boundaries. The well-known example is the failure of
18-8 stainless steel due to IGC (Sastri 1998). When 18-8 stainless steel is
heated to 950 - 1450°F, depletion of chromium takes place by metallurgical
reaction with carbon, this chromium carbide being insoluble and precipitates
out of the solid solution along the grain boundaries. When carbon content is
higher than 0.02% then the resultant structure is susceptible to intergranular
attack. Stainless steel may be protected from intergranular corrosion by the
following measures: (1) subjecting the sensitized material to a
high-temperature heat treatment in which all chromium carbide particles are
re-dissolved,(2) lowering the carbon content below 0.02 % so that carbide
formation is minimal, and (3) alloying the stainless steel with another metal
such as niobium or titanium, which has a greater tendency to form carbides
than does chromium so that the chromium remains in solid solution.
1.1.3.6 Selective leaching or Parting
Selective leaching is generally found in solid solution alloys. It
occurs when one element or constituent is completely removed by corrosion
processes. Selective leaching is also called as dealloying and parting. The most
common examples are dezincification of brass and graphitization.
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Dezincification of brass is the selective leaching of zinc from yellow brass
leaving porous mass of copper which may be red colour or copper colour.
Graphitization is the selective leaching of iron from grey cast iron leaving
weak, porous and inert graphite. It is a slow process and occurs in relatively
mixed corrosive environments such as soil or water. Selective leaching also
occur with other alloy systems in which aluminium, cobalt, chromium and
other elements are vulnerable to preferential removal. Selective leaching
reduces the mechanical properties of the alloy. The colour and appearance of
the material is also changed. Selective leaching can be minimized by reducing
the aggressiveness of environment, cathodic protection and using less
susceptible alloys.
1.1.3.7 Erosion corrosion
Erosion corrosion usually occurs due to the combined action of
chemical attack and mechanical abrasion or wear as a consequence of fluid
motion. This type of corrosion is associated with systems where the corrosive
fluid having high velocities. Rate of erosion corrosion depends upon the nature
of surface film, corrosion environment, velocity, presence and size of air
bubbles, chemical compositions, suspended solid, corrosion resistance and
metallurgical properties of metals and alloys. This type of attack can be
observed in piping system such as bends, elbows, and tees, valves, pumps,
blowers, centrifugals, impellers, heaters and condensers etc. All types of
equipments exposed to moving fluids are susceptible to erosion corrosion.
Among the available alloys, Ti has been found to possess the
greatest resistance to erosion corrosion and 70: 30 brass is known to have poor
resistance to erosion. Erosion corrosion can be prevented by selection of
materials with better resistance, proper design, change of aggressive
environment, use of surface coating and cathodic protection.
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1.1.3.8 Stress corrosion
Stress corrosion sometimes termed as stress corrosion cracking,
results from the combined action of an applied tensile strength and a corrosive
environment. In this corrosion small cracks are formed first and then propagate
in a direction perpendicular to the stress. The main cause of stress corrosion
cracking depends upon metallurgical factors such as chemical composition of
alloys, preferential orientation of grains, composition and distribution of
precipitates. The source of stress can be applied, residual, thermal, welding or
corrosion products in constructed regions. As stress corrosion crack penetrates
the metal, the cross sectional area decreases and the cracking failure occurs
due to mechanical action. Long-term stress corrosion cracking tests are
necessary to assess the cracking tendencies of alloys. The mechanisms of
stress corrosion can be broadly classified as dissolution based and cleavage
based. Corrosion plays an important role in the initiation of cracks. Stress
corrosion can be minimized by lowering the magnitude of stress, eliminating
the critical environment species, changing of alloy composition, applying
cathodic protection, adding inhibitors and coatings (Lu et al 2005).
1.2 Potential – pH diagrams (E-pH)
Thermodynamic data can be used to map out the occurrence of
corrosion, passivity and nobility of a metal and also it provides a means for
predicting the equilibrium state of a system of specific component.
M.Pourbaix developed the thermodynamic interpretation of corrosion in a
systematic way and presented his results in the form of potential–pH diagrams
otherwise known as Pourbaix diagrams. Pourbaix diagrams are convenient
way of summarizing much thermodynamic data and providing the useful
means of summarizing the thermodynamic behavior of metal and associated
species in given environmental conditions. Pourbaix diagrams are also very
much useful in determining the stable chemical species for metals in contact
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with aqueous solution. Figure 1.1 is a simplified Pourbaix diagram for iron
(Fontana 1987).
Figure 1.1 Potential – pH diagram for iron-water system at 25°°°°C
Potential–pH diagrams are also available for corrosion of metals in
atmosphere. The metal ion concentration in water film is fluctuating and is
generally high. Apart from metal- water system one must consider the
presence of SO42-
, Cl – and NO3– which may be dissolved in the film or
moisture and under certain conditions which may form solid phase complexes
with the corroding metal. This diagram clearly distinguishes the regions of
immunity, corrosion and passivity. But these E / pH diagrams have some
inherent limitations which are
• It does not deal with the effect of impurities or alloying element
present in the metal.
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• In most cases, the measured electrode potential varies with time.
So extreme care is necessary in applying Pourbaix diagrams in
such cases.
• These diagrams give no information on rates of reaction.
• They represent only equilibrium conditions in certain specific
environments and do not take into account the factors such as
velocity and temperature variation during the course of the
process.
• The effect of pH can be varied by the presence of unintentional
or otherwise, of anions such as Cl –, NO3– and complexing ions
which are likely to form.
• Passivity is also depending on solubility, adhesion, cohesion, ion
conductivity and crystal formation of passive layer.
1.3 Kinetics of electrochemical corrosion reaction
Consider an electrochemical reaction, where the reaction takes
place at the metal – solution interface and the rate of reaction is proportional to
current-potential dependence. Over voltage (polarization) η is the potential
change, E - Er, from the equilibrium half-cell electrode potential Er, caused by
a net surface reaction rate for the half-cell reaction was introduced by Nernst
and Caspari. The dependence of η on the current-density for a hydrogen
evolution reaction has been shown to be η = a + b log i by Tafel. Butler gave
a kinetic treatment of reversible electrode in which the concepts of the partial
anodic and cathodic currents related to η through an exponential equation.
1.3.1 Activation controlled corrosion reaction
Activation polarization is generally caused by slow electrode
reaction. The reaction at the electrode requires activation energy in order to
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proceed. The most important example for activation controlled corrosion
reaction is that of hydrogen ion reduction at cathode. The relationship between
current and potential for a corroding system in which anodic reaction is metal
dissolution reaction and cathodic reaction is hydrogen evolution reaction can
be derived by the application of electrochemical kinetic theory.
For the metal dissolution reaction,
� → ��� + 2�� (1.8)
(1.9)
Where Ear is the reversible potential of anodic dissolution reaction, iaº is
exchange current density for anodic reaction and α a βa are the transfer
coefficients of metal dissolution and deposition reaction.
Similarly for cathodic reaction
2H� +2e� → H� ↑ (1.10)
(1.11)
Where is Ecr is the reversible potential of cathodic dissolution reaction, Icº is
the exchange current density for cathodic reaction and α c βc are transfer
coefficients of reduction reaction.
Normally the corrosion potential (Ecorr) will be far away from the
equilibrium potential of reversible reaction. Hence the contribution from the
deposition reaction of metal dissolution reaction and the anodic partial reaction
−
β−−
−α
= )EE(RT
Fexp)EE(
RT
Fexpii r
aar
aao
aa
−
β−−
−α
= )EE(RT
Fexp)EE(
RT
Fexpii r
ccr
cco
cc
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of reduction is negligible. Therefore the net current of the mixed electrode
system is
i = ia - ic (1.12)
(1.13)
At corrosion potential E = Ecorr i = 0
i.e.
for anodic reaction and
i.e for cathodic reaction
Substituting the terms Ear and Ec
r in terms of Ecorr
(1.14)
The above equation can be rewritten in terms of Tafel slopes ba and bc as
(1.15)
since E - Ecorr = η
(1.16)
−
β−−
−α
= )EE(RT
Fexp)EE(
RT
Fexpi r
aar
aao
a
−
α= )EE(
RT
Fexpii r
acorrao
acorr
−
β−= )EE(
RT
Fexpii r
ccorrco
ccorr
−
β−−
−α
= )EE(RT
Fexp)EE(
RT
Fexpii corr
ccorr
acorr
−−−
−=
c
corr
a
corrcorr
b
)EE(3.2exp
b
)EE(3.2expii
η−−
η=
ca
corrb
3.2exp
b
3.2expii
15
The above expression forms the basis of measuring corrosion rate
by electrochemical method.
1.3.2 Diffusion controlled reaction
Concentration polarization or diffusion over potential is the
potential difference of a cathode in absence and presence of external current.
The corrosion process in neutral media consists of metal dissolution reaction
as anodic and oxygen reduction as cathodic reaction. In such cases,
(1.17)
(1.18)
Where D is diffusion coefficient, δ is diffusion layer thickness and Cb is concentration of reduction species.
i = ia – ic
(1.19)
id = limiting diffusion current density
At corrosion potential E = Ecorr, i = 0, therefore icorr = id
It follows that the id is the most significant parameter in the
corrosion reaction in which the cathodic reaction is diffusion controlled and
any factor that increases id will increase the corrosion rate.
−
α= )EE(
RT
Fexpii corr
acorra
δη
== bdc
FDCii
dcorr
a
corra iEERT
Fii −
−= )(exp.α
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1.4 Methods used for studying corrosion rates
1.4.1 Weight loss method
Weight loss methods are commonly used to measure corrosion
rates. A test specimen is polished, cleaned, degreased and weighed. This
specimen is exposed to the corroding medium for a known time and removed.
This treated specimen is cleaned to remove the corrosion products and
weighed. From the weight difference corrosion rate is calculated. To avoid or
minimize the error it is necessary to carry out a control test, irrespective of the
method used to remove corrosion products. The amount of metal dissolved by
the reagent used for cleaning corrosion product must be determined.
1.4.2 Electrochemical methods
Electrochemical methods which can be used for the determination
of corrosion rates have been published by several authors (Chandrasekaran et
al 2005, Gunasekaran and Chauhan 2004, Abdel-Gaber et al 2006 and
Hougaarad et al 1983). The majority of corrosion behavior are due to
electrochemical process ie., they are governed by the law of electrochemical
kinetics. Therefore it is very important to study the electrochemical
characteristics of metals during corrosion tests in order to understand
mechanism and the rate of the most corrosion process.
Tafel polarization method: This method involves the
measurement of over potentials for various current densities and a plot of η
versus log i is made. The slope gives Tafel constants [ba and bc] and the
intercept corresponds to icorr. Disadvantages of this method are
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1. Polarization of several hundred milli volts from open circuit
potential perturbs the system significantly and alters the surface
conditions.
2. This method is applicable to conducting media and for systems
controlled by activation controlled reactions.
3. In actual practice, the linear region does not exceed a decade
due to interference from concentration polarization.
Linear polarization resistance (LPR) method: Stern and Geary
have shown that when the over potential (η) is less than ±20 mV, there is a
linear relationship between current and potential. Measuring (dη / di) η → 0,
the corrosion current can be obtained from
(1.20)
The above equation is valid only for activation-controlled reactions.
Improvements and the simultaneous determination of Tafel slopes and
corrosion current at corrosion potential have been suggested by many authors
(Reeve et al 1973). The graphical method of linear polarization curve analysis
has been obtained by Oldham and Mansfeld (Oldham et al 1973). Mansfeld
developed a computer programme (CORFIT) of quantitative determination of
icorr which requires simultaneous determination of both Rp and B. However
this method requires knowledge of ba and bc.
The main advantage of Linear Polarization method is that it gives
instantaneous corrosion rate and is extremely helpful in determining the effect
of process change with time on corrosion rate. Based on this method,
corrosion rate measuring meters have been developed. This method is useful
Rp
K
di
d
bb
bbi
ca
ca
corr =×+
×=
η)(303.2
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for periodic monitoring of corrosion process and the small potential scan does
not perturb the system, very low corrosion rates with high accuracy can be
followed. The main limitation is that this method requires a conducting liquid
as a medium.
1.4.3 Coulostatic method (Sato et al 1978)
This method is suited especially for the measurement of corrosion
rate of metals in high resistive media. The polarization resistance (Rp) is
measured from the η-t transient of the electrode on discharging a charged
capacitor (c) through the cell. The electrode potential decays as
ηt=ηt = 0 exp (-t/ Cdl Rp) (1.21)
Where ηt is over potential at any time ‘t’, ηt=0 is over potential immediately
after charging the double layer of the electrode and Cdl is differential capacity
of the double layer.
The plot of log ηt versus time (t) is a straight line and the slope
gives 1 / 2.303 Cdl Rp and the intercept is ηt = 0.
1.4.4 Electrochemical impedance spectroscopic method (EIS)
AC impedance measurements are useful for identification of
phenomena governing the electrochemical systems. EIS technique provides
data on both electrode capacitance and charge transfer resistance, thereby
providing valuable mechanistic information. For this reason, EIS become a
powerful tool in the study of corrosion, semiconductors, batteries,
electroplating, and electro-organic synthesis (Sha Cheng et al 2007, Mansfeld
et al 1982, Lorenz et al 1981, Issaacs et al 1982, Glarum et al 1982, McCann et
19
al 1982, Sluyters et al 1961, Sluyters et al 1962, Glarum et al 1981 and
Franceschetti et al 1982). EIS offers three main advantages over
DC techniques. Very small excitation amplitude of 5–10 mV is applied for the
EIS measurements. This small amplitude causes only minimal perturbation of
electrochemical test system and reduces errors caused by the measurement
technique.
Measurement Accuracy: The measurements are based on the
consideration that an electrochemical cell representing the conducting system
is analogous to an electronic or equivalent circuit consists of an array of
resistors and capacitors. It does not involve any potential scan so that
measurements can be made even in low conducting solutions where DC
techniques are subjected to serious potential–control errors. The measurements
are carried out by studying the response of electrochemical system to an
excitation over a wide range of frequency, may be extended from 10 –1 MHz
depending on the nature of process being studied. In A.C theory E = IZ where
E and I are waveform amplitudes for potential and current, Z is the impedance.
The expression for impedance is
Ztotal = Z′real + Z′′imaginary (1.22)
i.e. impedance can be expressed as a complex numbers, where resistance is the
real component and combined capacitance and inductance is the imaginary
component.
The A.C impedance data can be presented in many forms and each
mode of presentation has its own advantages and disadvantages.
20
Nyquist plot: In this plot, Z′′ is plotted against Z′ at each
excitation frequency and is also known as Cole – Cole plot. From this plot Rs,
Rct and Cdl can be calculated.
The advantages of Nyquist plot are:
1. The plot format make it easy to see the effects of ohmic
resistance
2. The shape of the curve does not change when Rs changes, it is
possible to compare the resistance of two separate experiments
3. It emphasizes the circuit components which are in series such as
ohmic resistance
The disadvantages are:
1. Frequency does not appear explicitly
2. Although the Rs and Rp are already known, but the electrode
capacitance can be calculated only after frequency information
is known.
Bode plot: In this plot, logZ is plotted against frequency. The
main advantage of Bode plot over Nyquist plot is that the frequency appears as
one of the axes and it is easy to understand how the impedance depends on
frequency. From this plot, capacitance value can be calculated using the
following formula,
Z = 1 / Cdl, at ω = 1 (1.23)
Bode plot avoids the longer measurement times associated with low
frequency Rp determinations, furthermore, allows more effective extrapolation
21
data from higher frequency range. In some electrochemical process, there is
more than one rate – determining step. From this plot it is easy to identify the
frequency break points associated with each limiting step. The main
disadvantage is that the shape of the curve changes if the circuit values
changes.
Randels plot: The Randels plot is useful in determining whether
Warburg impedance is a significant component of the equivalent circuit model
(reaction mechanism). For a completely reversible system under pure diffusion
control, the Warburg impedance Zω is
Warburg Impedance (1.24)
Thus the linearity of the Randels plot can be used as a test of
diffusion control, the Warburg impedance coefficient can also be calculated
from the slope.
Faradaic Distortion: On superimposing a sinusoidal alternating
voltage i.e. Em sinωt to the electrode at corrosion potential, harmonic current
components are produced due to the non-linear relationship between current
and potential. Measurements of fundamental (i1), second harmonic (i2) and
third harmonic (i3) current components are made for getting icorr, ba and bc
from the following relationships,
(1.25)
(1.26)
→ω
=ω S2S
Z
2231
21
corr
ii,i248
ii
−=
+=
1
2
corr
1
i
i4
i
i
Em6.4
1
ba
1
22
(1.27)
The advantage of this method is that the measurement of corrosion
current is possible at corrosion potential without the usage of anodic and
cathodic Tafel slopes.
Electrochemical Noise (Dawson et al 1983): Impedance and
polarization analysis are ideal techniques for use along with general corrosion
tests. However, electrochemical noise, i.e. the spontaneous fluctuations in
potential and current, offer some possibilities of assessing localized corrosion.
The corrosion potential noise can be measured either between two identical
corroding electrodes or between a corroding electrode and a low noise
reference electrode (Hiladky et al 1982). Electrochemical noise is random
process comprising initiating events controlled by a poisson’s distribution
(stochastic) coupled to kinetic parameters (deterministic). The noise data can
be presented auto correction functions and spectral density plots, the time
record being transformed to the frequency domain by the Fast Fourier
Transform (FFT) or the Maximum Entropy Method (M.E.M) (Childers et al
1978 and Haykin 1979). This technique is useful in monitoring a number of
practical situations (Dawson et al 1983). Noise analysis and monitoring have
been successfully applied to corrosion involving film breakdown process (e.g)
pitting of stainless steel and aluminium. However general corrosion systems
have not been investigated to the same extent (Al-Zanki et al 1986).
1.5 Corrosion prevention
There are many corrosion prevention methods available and some
of the important corrosion control methods are described as follows:
+=
1
2
corr
1
i
i4
i
i
Em6.4
1
bc
1
23
Proper designing: Proper design should permit least contact
between the structure and corroding agent. It is desirable that the design allows
for adequate cleaning and flushing of critical parts of the equipment. Crevices,
recesses, pockets and sharp corners should be avoided, because they favor the
formation of stagnant areas and accumulation of solids.
Using pure metal: Impurities in a metal cause heterogeneity, which
decreases corrosion resistance of the metal. The corrosion resistance of a metal
can be improved by increasing the purity of the metal.
Using metal alloys: It is common to increase both strength and
corrosion resistance by the use of suitable alloying elements. For example
small amounts of phosphorus and copper improve the resistance of structural
steel to atmospheric corrosion. About 10 % aluminium renders iron extremely
resistant to high temperature oxidation. Intergranular corrosion in stainless
steels can be avoided either by reducing the carbon percentage or by the
addition of titanium or columbium.
Cathodic protection: In general, cathodic protection is an approach
where the metal surface to be protected is made into cathode of a corrosion
cell. Since corrosion and the material loss are occur only at the anode, this
approach protects the metal. There are two types of cathodic protection
method one is sacrificial anodic method and another one is impressed current
technique. In the sacrificial anodic protection method highly active metal or
more anodic metal is connected to the metal surface which is to be protected
from the corrosion. In this cell the more active metal is acting as an anode and
it is corroded thereby it protects the metal surface. In the impressed current
technique, an impressed current is applied in opposite direction to nullify the
corrosion current, and convert the corroding metal from anode to cathode.
24
Modifying the environment: The corrosive nature of environment
reduced thereby corrosion is controlled. This can be done by the removal of
harmful constituents or by the addition of specific substances, which is
neutralize the effect of corrosive constituents of the environments.
Application of protective coatings: Protecting the surface of an
article by applying protective coating is the oldest and most common method
to control corrosion. Protective surface coating includes metallic coatings,
organic coatings, anodizing and ceramic coatings.
Use of inhibitors: This will be discussed in detail in the following
chapter.
1.6 Inhibitors
A corrosion inhibitor is a substance which when added in small
quantities to a corrosive environment, effectively decreases the corrosion of
the metal. In a sense, a corrosion inhibitor can be considered as a retarding
catalyst. Corrosion inhibitors are added to many systems including cleaning
pads, cooling systems, various refinery units, chemical operations, steam
generators, ballast tank, oil and gas production units. Inhibitors function by
� Adsorption as a film on to the surface of the corroding medium.
� Inducing the formation of a thick product.
� Changing the characteristics of environment either by producing
protecting precipitates or by inactivating an aggressive
constituent.
So that it prevent or arrest corrosion processes. It can also interface
with cathodic, anodic or both reactions.
25
1.6.1 Classification of corrosion inhibitors
Inhibitors can be classified on the basis of environments and
reaction mechanism involved during the inhibition.
1.6.1.1 Based on environment
a) Acid inhibitors
This may be classified into
� Inorganic inhibitors
� Organic inhibitors
Inorganic inhibitors: The protection of metal in corrosion medium
with inorganic inhibitors is due to the reduction of electropositive ions and
deposition on the metal surface and lowering of over voltage of the main
cathodic depolarization reaction (Tamashov et al 1967). Based on the
electrode potential ions like Cu, Hg, Ir, Pt, Rh should improve the resistance of
corrosion of titanium and the most effective is platinum due to its low
hydrogen over voltage.
The influence of halide ions on inhibitive effect of Congo red on in
strong acid solutions has been found to be effective inhibitors (Oguzie 2004).
Recently it is shown that the addition of heavy metal ions such as Pb2+, Tl+,
Mn2+, Cd2+ is found to inhibit corrosion of iron in acids. This effect is
attributed due to lower potential deposition of metal ions leading to complete
coverage of added metal on iron surface.
Zinc ions and Nickel ions were also found to reduce the corrosion
rate of Fe and Ti (Shedriks et al 1972). Even 0.2 ppm nickel ions render
passivity of Ti in 3.5% NaCl. This effect of nickel ions may be due to
lowering over voltage of hydrogen on Ti.
26
Organic inhibitors: Usually the corrosion of metals and alloys in
acid solution is very severe and this kind of attack can be inhibited by a large
number of organic substances. In general, nitrogen, oxygen and sulphur
containing compounds with a hydrocarbon part attached to the polar group are
used as inhibitors (Weihua Li et al 2007, popova et al 2006). Many nitro and
nitroso compounds have been tried as corrosion inhibitors. Certain azo
compounds such as cango red dye, methyl orange and methyl red have been
found to be effective inhibitors. Many types of quinine such as anthroquinone
and its derivatives act as oxidative passivators. Organic corrosion inhibitors
can be anodic, cathodic or mixed type depending on its size, carbon chain
length, atomicity, conjugation and nature of bonding atoms.
b) Neutral inhibitors
Inhibitors, which are effective in acidic solutions, do not function
effectively in neutral solutions. In neutral solutions, interaction of inhibitors
with oxide covered metal surfaces and prevention of oxygen reduction
reactions at cathodic sites takes place. Such inhibitors protect the surface
layers from aggressiveness. The first step involves displacement of pre
adsorbed water molecules by inhibitors followed by chemical or
electrochemical reactions at the surface. Hence inhibitors, which block
cathodic reduction of oxygen by restricting diffusion of oxygen to the surface
of metal, are cathodic inhibitors and those which prevent anodic dissolution by
the formation of thin passivating film on metal surface are called anodic
inhibitors.
c) Alkaline inhibitors
The metals, which form amphoteric oxides, are prone to corrosion
in basic solution. Inhibitors in basic solution mainly involve metals like Al,
Zn, Cu, Fe, etc. Many naturally occurring organic compounds are often used
27
as inhibitors for metals in basic solutions e.g. tannin, gelatins, saponin,
agar-agar etc. Compounds such as thiourea, substituted phenols, naphthol,
β-diketone etc. have been used as effective inhibitors in basic solutions due to
the formation of metal complexes. Lysine was also found as effective
inhibitors in 4.0 N sodium hydroxide solutions (Jeyaraj et al 2003).
d) Vapour phase inhibitors
Metals in closed space such as in parcels, during storage undergo
corrosion. This type of corrosion can be prevented by using certain substances
called vapor phase inhibitors (VPI), which are specific in nature. The vapors of
inhibitors form an extremely thin film over certain metal surfaces, especially
those of iron and steel, thereby rendering them passive. The inhibitors are
mostly crystalline solids whose vapor pressure in controlled by the structure of
crystal lattice and character of the atomic bond in the molecule. The protective
vapors expand within the enclosed space until the equilibrium determined by
their partial pressure of the vapor is reached. At higher values of vapor
pressure protected the space reaches saturation stage (Trabanelli 1987). This
method is easy to apply as paper coated VPI for temporary protection. The life
time of the inhibitor is very long.
1.6.1.2 Based on electrode process
a) Anodic inhibitors
An anodic inhibitor increases the anode polarization and hence
moves corrosion potential to the positive direction. A number of inorganic
inhibitors such as orthophosphates, silicates etc fall under anodic type. Even
though anodic inhibitors are widely used, a few of them have some
undesirable property. If such inhibitors are used in very low concentration they
28
cause stimulation of corrosion such as pitting and hence anodic inhibitors are
denoted as dangerous.
b) Cathodic inhibitors
These inhibitors shift the corrosion potential in negative direction.
Here the cations migrate towards cathode surfaces where they are precipitated
chemically or electrochemically and thus block these surfaces e.g. Action of
As3+ and Sb3+ on dissolution of Fe in acids.
c) Mixed inhibitors
These inhibitors retard both anodic and cathodic processes. The
potential is smaller and the direction is determined by the relative size of the
anodic and cathodic sites. Such inhibitors will have the advantage over other
inhibitors as they control both partial corrosion reactions and hence they are
very safe to apply.
1.6.2 Theories of inhibition
a) Adsorption theory
According to adsorption theory, inhibitors are adsorbed on the
metal surface forming a protective layer. The mode of adsorption leads to its
classification as physical and chemical adsorption.
Physical adsorption or Physisorption: The electrostatic attraction
between ions or dipole of the inhibiting species and electrically charged
surface of the metal gives physical adsorption. The charge on the metal surface
depends on free corrosion potential Ecorr and potential with respect to zero
charge (PZC).
29
The potential of metal measured against a reference electrode is
known as zero-charge potential, which is carried out under the condition that
metal is in zero charge. At zero-charge potential, electrodes adsorb substances
dissolved in the electrolyte. At PZC, the net charge on electrode is zero. At
potential more positive and negative than PZC, the electrode is positively and
negatively charged respectively.
The concept of PZC can also be explained by means of synergistic
effect of iron in H2SO4 by means of chloride ion (Rozenfeld 1981).
Chemical adsorption or Chemisorption: It is due to the
interaction between the metal surface and an inhibitor molecule. The adsorbed
molecule is in contact with the surface of the metal. In this, a co-ordinate bond
is involved in the electron transfer from metal inhibitors to metal (Fathy
Bayoumi et al 2005 and Hackerman et al 1962).
Chemisorption is slower compared to electrostatic adsorption
process and it has higher activation energy. It depends upon temperature and
inhibitor efficiency. Chemisorption is specific to certain metals electron
transfer from inhibitor to metal is facilitated by the presence of unshared lone
pair of electrons, π electrons due to multiple bonds and aromaticity.
The strength of the adsorption bond depends upon
i. The electron density of donar atom of functional group
ii. Polarizability of the group
30
b) Film theory
This theory revealed that effective protection of metals by inhibitors
is due to the formation on the metal surface a layer of insoluble or slightly
soluble corrosion products.
c) Hydrogen over voltage theory
This theory postulates that inhibitors, which are adsorbed on the
metal retard either anodic or cathodic or in some cases both the reactions.
This leads to rapid polarization of anodic or cathodic sites, and thus overall
corrosion rate is retarded.
d) Quantum chemical approach
Direct interaction between surface metal atoms and outermost
electrons of organic molecule sometimes leads to chemisorption phenomenon
and thereby cause inhibition. Chemisorption of organic inhibitors may be
taken as linear combination of participating wave function of inhibitor
molecule as well as of surface metal atoms. The binding energy of metal
inhibitor adduct may thus be correlated to the energy difference between
lowest free molecular orbital (LFMO) and highest occupied molecular orbital
(HOMO) of metal atoms and inhibitor molecule respectively.
1.6.3 Effect of inhibitors on corrosion processes
Electrochemical studies have shown that inhibitors in acid solution
may affect corrosion reactions of metal in the following ways.
a) Formation of diffusion layer
The adsorbed inhibitors may form a surface film, which acts as a
physical barrier to restrict the diffusion of ions, or molecules to or from metal
31
surface and so retard the corrosion reactions. This effect occurs particularly
when the inhibitor species are large molecules e.g. proteins such as gelatins,
agar-agar etc. Similar effect also occurs when the inhibitor can undergo
reaction to form a multi molecular surface film e.g. acetylene compounds and
sulphoxides.
b) Blocking of reaction sites
The interaction of absorbed inhibitors with surface metal atoms may
prevent these metal atoms from participating in either anodic or cathodic
reactions of corrosion. The simple blocking effect decreases number of surface
metal atoms at which these reactions can occur, and hence rates of these
reactions are proportional to the extent of adsorption. The mechanism of
reactions is not affected and Tafel slope of polarization curve remains
unchanged. Behavior of this type has been observed for iron in sulphuric acid
solutions containing 2,6-dimethyl quinoline, β-naphthaquinoline or aliphatic
sulphides.
c) Precipitation on the electrode reaction
The electrode reactions of corrosion involve the formation of
adsorbed intermediate species with surface metal atoms e.g. adsorbed
hydrogen atoms in the hydrogen evolution reaction; adsorbed (FeOH) in the
hydrogen evolution reaction, adsorbed (FeOH) in the anodic dissolution of
iron. The presence of adsorbed inhibitors will interface with formation of these
adsorbed intermediates but electrode process may then proceed via alternative
paths through intermediates containing inhibitor. In this process inhibitor
species act in a catalytic manner and remain unchanged, since participation by
inhibitor is generally characterized by a change in Tafel slope observed for the
process.
32
1.7 Adsorption isotherm
In order to gain more information about the mode of adsorption of
inhibitors on metal surface, the experimental data have been tested with
several adsorption isotherms. The fractional surface coverage values (θ) to
different concentrations of inhibitors from capacitance measurements as
described elsewhere to explain the best isotherm to determine adsorption
process. It gives relationship between coverage of an interface with adsorbed
species and concentration of species in solution. Various adsorption isotherms
have formulated and are given in Table1.1 (Damaskin et al 1971).
Table1.1 Adsorption isotherms
S.No. Isotherm Equation
1 Henry βc = θ
2 Freundlich βcn = θ
3 Langmuir βc = θ / 1-θ
4 HFL βc = (θ /1-θ)e(2- θ) / (1-θ)2
5 Frumkin βc = θ /1-θ e (-2aθ)
6 Temkin βc = e(a-θ) / 1-e [-a(1-θ)]
7 Parsons βc = θ / 1-θ e (2-θ)/(1-θ) e (-2aθ)
8 Blomgren-Bockris βc = θ / 1-θ e (pθ3/2 – q θ 3)
9 BDM Log c + log (θ / 1- θ ) = c + βθ3/2
where β = e , a = Interaction parameter a > 0, attraction and a < 0, repulsion
Most of organic inhibitors obey Langmuir (El-Awady et al. 1992),
Temkin, Frumkin and Flory–Huggins isotherm. The adsorption isotherm
relationships of Frumkin are represented in the following equation:
��� � �����– ���� = ��� + 2!" (1.28)
33
Where a is the lateral interaction term describing molecular
interaction in adsorbed layer and heterogeneity of the surface, K is binding
constant (equilibrium constant) of adsorption reactions, C is inhibitor
concentration in bulk of the solution. If this relation gives a straight line, then
Frumkin isotherm is applicable.
El-Awady et al. kinetic model and Flory–Huggins isotherm are
represented according to the following equations:
��� � ����� = ��ℎ
′ + $���� (1.29)
%&'�( = ���) + )����1 − " (1.30)
Where Y is the number of inhibitor molecules occupying one active site,
X the number of adsorbed water molecules replaced by one molecule of
organic inhibitor, K and K' are the binding constants. The above expressions
include equilibrium constant of adsorption process K that is related to standard
free energy of adsorption (∆Gads) by
= �++.+ �)-�−∆/012
3 45⁄ (1.31)
The value of 55.5 is the concentration of water in solution in mole l−1.
For El-Awady et al. model a plot of log [θ/(1−θ)] versus logC give
a straight lines of slops (X) and intercept (K'). In Flory–Huggins isotherm, a
plot of log θ/C versus log [1−θ] gave straight lines of slope (X) and intercepts
(log XK). Obey the Flory–Huggins isotherm. The values of equilibrium
constant, change in free energy ∆Gads and number of active site are calculated
from e slope and intercept values.
34
1.8 Literature review of inhibitors for phosphoric acid
The inhibition effect of benzyltrimethylammonium iodide (BTAI)
on the corrosion of cold rolled steel (CRS) in phosphoric acid produced by
dihydrate wet method process (7.0 M H3PO4) solution was investigated for the
first time by weight loss, potentiodynamic polarization, electrochemical
impedance spectroscopy (EIS) and scanning electron microscopy (SEM)
methods. The results show that BTAI is a good inhibitor, and the adsorption of
BTAI obeys Langmuir adsorption isotherm. Polarization curves show that
BTAI behaves as a mixed-type inhibitor (Xianghong Li et al 2011).
The inhibition of the corrosion of mild steel in hydrochloric acid
solutions by 4-amino-5-phenyl-4H-1, 2, 4-trizole-3-thiol (APTT) inhibitor was
studied using weight loss technique. Basic kinetic parameters of the corrosion
inhibition process were obtained by reaction kinetic equations. Rustles show
that the inhibition increases with increasing of inhibitor concentration
(Ahemed Y. Musa et al 2010). The synergistic inhibition effect of rare earth
cerium (IV) ion and sodium oleate (SO) on the corrosion of cold rolled steel
(CRS) in 3.0 M phosphoric acid has been investigated by weight loss,
potentiodynamic polarization, electrochemical impedance spectroscopy (EIS)
and scanning electron microscope (SEM) methods. The results revealed that
sodium oleate has a moderate inhibitive effect and Ce4+ has a poor effect.
However, incorporation of Ce4+ with SO improves the inhibition performance
significantly and exhibits synergistic inhibition effect. SO acts as a cathodic
inhibitor, while SO/Ce4+ mixture acts as a mixed-type inhibitor (Xianghong Li
et al 2010). The effect of benzimidazole (BI), 2-methyl benzimidazole (2MBI)
and 2-aminobenzimidazole (2ABI) on the corrosion of mild steel was
evaluated in 1 M phosphoric acid at various concentrations using
electrochemical techniques (Electrochemical Impedance Spectroscopy (EIS)
and DC polarization). Inhibition of imidazole derivatives was evaluated at
35
concentrations between 5 × 10−2–10−4 M. It was observed that inhibition
efficiency increased with increasing inhibitor concentration (A. Ghanbari et al
2010).
The effect of 2-phenyl-1-hydrazine carboxamide on the corrosion of
mild steel in phosphoric acid has been studied by weight loss and
electrochemical techniques and results revealed that this compound act as
mixed type inhibitor and increasing the acid concentration increased the metal
corrosion but did not affect the inhibition efficiency (Dadgarinezhad et al
2009). The corrosion inhibition performance of acetylacetonate complexes of
zinc (II), manganese (II), cobalt (II) and copper (II) on the mild steel substrate
in 1M phosphoric acid was studied by DC polarization. It could be seen that
the above complexes decreased corrosion rate of mild steel in phosphoric acid
media due to the adsorption of complexes on metal surface and acted as
cathodic inhibitors. Presence of chloride ions in the electrolyte enhanced
inhibition performance of acetylacetonate complexes due to synergism
(Ghanbari et al 2009). The synergistic inhibition effect of red tetrazolium (RT)
and uracil (Ur) on the corrosion of cold rolled steel (CRS) in 1to10 M
phosphoric acid solution was studied by weight loss and potentiodynamic
polarization methods. The results revealed that RT had a moderate inhibitive
effect and acted as a mixed-type inhibitor in phosphoric acid. For the Ur, it
showed a poor inhibition effect and acted as a cathodic inhibitor (Xianghong
Li et al 2009).
The inhibitive action of mangrove tannins extracted from mangrove
barks and phosphoric acid on pre-rusted steel in 3.5% NaCl solution was
evaluated and the inhibitive efficiency was compared with that of mimosa
tannins. From the electrochemical studies, the inhibition efficiency of
solutions containing 3.0 g/l tannins depended upon the concentration of
36
phosphoric acid added and the pH of the solution. At pH 0.5 and pH 2.0
inhibitions were found to be great with mangrove and mimosa tannins alone,
while at pH 5.5 the addition of phosphoric acid alone gave the highest
inhibition effect (Afidah A. Rahim et al 2008). The influence of chloride ions
on inhibitive performance of cetyl trimethyl ammonium bromide (CTAB) in
1.0 - 4.0M of phosphoric acid for cold rolled steel has been studied using
weight loss and Tafel polarization techniques. The results revealed that a
synergistic effect has been observed for CTAB with NaCl at each acid
concentration (Xueming Li et al 2008).
The corrosion behaviour of graphite and stainless steels in
phosphoric acid solution (40%) was studied by the use of different
electrochemical methods and scanning vibrating electrode technique (SVET).
It was found that the current density measured by polarization curves
increased with the presence of chloride and sulphate ions in the acid solution
inspite of the tested material. A generalized corrosion was occurred on
graphite whereas a localized corrosion was observed for stainless steels. These
results showed that the graphite as component material in some of the
equipments of the phosphoric acid industry (Iken et al 2007).
Corrosion inhibition of mild steel in 0.67M phosphoric acid by
phenacyldimethyl sulfonium bromide and six of its p-substituted derivatives
was studied by different chemical, electrochemical and scanning electron
microscopy techniques. Potentiodynamic polarization curves indicated that the
compounds acted as mixed-type inhibitors and steel dissolution was controlled
by charge-transfer mechanism (Arab et al 2006). Corrosion inhibition of
triazole derivatives (n-PAT) on mild steel in phosphoric acid solution has been
investigated by weight loss and polarization methods. The results indicated
that these compounds acted as mixed-type inhibitors retarding the anodic and
37
cathodic corrosion reactions with emphasis on the former and do not change
the mechanism of either hydrogen evolution reaction or mild steel dissolution
(Wang Lin 2006). The synergistic inhibition effect of 4-(2-pyridylazo)
resorcinol (PAR) and chloride ion on the corrosion of cold rolled steel in
1M phosphoric acid was studied by weight loss and potentiodynamic
polarization method. Results revealed that the single PAR is not an effective
inhibitor for steel corrosion in phosphoric acid, but in the presence of chloride
ion PAR act as a good inhibitor due to the synergism (Libin Tang et al 2006).
Synergistic effect of isopropylamine with Cl – and SO42- on the
inhibition of corrosion of mild steel in phosphoric acid medium was found to
be 92.03% and 97.50 % inhibition efficiency respectively (Chandrasekaran et
al 2005).The effect of some quaternary N-heterocyclic compounds on the
corrosion of mild steel in solutions of phosphoric acid has been investigated.
The results revealed that the studied compounds are good inhibitors for mild
steel corrosion in phosphoric acid solutions. The inhibition efficiency of the
studied inhibitors increased with inhibitor concentration, but decreased with
acid concentration upto critical concentration above which it started to
increase (Noor 2005). The inhibition effect of isopropylmine (PA) on
corrosion of mild steel in phosphoric acid solution was investigated by
polarization technique at different temperature (Chandrasekaran et al 2005).
The effects of single OP and the mixture of various concentrations of OP and
0.1 M NaCl on the corrosion of cold-rolled steel in 1.0-3.5 M phosphoric acid
have been investigated by weight loss method and polarization method. This
study revealed that the cold-rolled steel in phosphoric acid has been more
efficiently inhibited by OP in the presence of NaCl than single OP. The
polarization curves showed that OP acts as cathodic inhibitor, while the
complex of OP and NaCl acts as mixed-type inhibitor which mainly inhibits
the cathodic corrosion of the steel (Li et al 2005). The corrosion rates of steel
38
in concentrated phosphoric acid (1.0 to 11.0M) were determined by the weight
loss method at three temperatures 298, 308 and 323K. Results showed that the
corrosion rate increased with both acid concentration and temperature
(Benabdellah et al 2005).
The effects of single o-phenanthroline and the mixture of various
concentrations of NaCl and 0.0002M o-phenanthroline on the corrosion of
cold rolled steel in 1.0 to 4.0M phosphoric acid have been investigated by
weight loss and polarization methods. The study revealed that the cold rolled
steel in phosphoric acid has been found to be more efficiently inhibited by
o-phenanthroline in the presence of NaCl than single o-phenanthroline at a
relatively higher concentration of NaCl and there was a synergistic effect
between o-phenanthroline and NaCl. (Mu et al 2004). The role of a rust
converter with tannic and phosphoric acids for painted and unpainted steel
with different corrosion and salts contaminating the rust were evaluated
(Ocampo et al 2004). Anodising of commercial pure titanium in phosphoric
acid solution at different concentrations (0.5 to 4M) has also been investigated
by galvanostatic and potentiodynamic polarization techniques. The samples
exhibited a different anodic behaviour and development of gel-like layer
during the formation of thin anodic film on titanium in phosphoric acid
solution by potentiodynamic conditions (Cydzik 2004).
The corrosion under heat transfer of 904L stainless steel in
phosphoric acid with silica (SiO2) as inhibitor was examined by
electrochemical and spectroscopic techniques (Bellaouchou et al 2003). The
dielectric properties of anodic film formed on tantalum in dilute phosphoric
acid solution at 20 and 85°C have been investigated by electrochemical
impedance spectroscopy. The slightly higher dielectric constant obtained at
85°C is due to reduced amount of incorporated phosphorus species in the outer
39
layer of anodic films formed at this temperature, together with a reduced
thickness of the outer layer, compared with those of anodic films formed at
20°C (Lu et al 2003). The influence of an aqueous phosphoric acid solution on
the corrosion product of mild steel samples has been carried out using
Mossbauer spectroscopy. It has been observed that the transformation of rust
by phosphoric acid depends strongly on the concentration of the phosphoric
acid. The formation of acid ferric phosphate with 8M concentration of the
phosphoric acid was observed (Nigam et al 1990). Polyaniline (PANI)
coatings were electro synthesized on steel samples (13% and 4.44%Cr) using
sulphuric and phosphoric acids as supporting electrolytes. Protective
properties of PANI coatings in the supporting electrolytes were investigated
(Kraljic et al 2003).
The non-toxic inhibitors are of considerable interest in
investigations into the replacement of hazardous classical molecules. The
action of four amino acids containing sulphur on the corrosion of mild steel in
phosphoric acid solution with and without Cl-, F- and Fe3+ ions has been
studied by both the polarization resistance method and electrochemical
impedance spectroscopy (EIS). Both cysteine and N-acetylcysteine (ACC)
showed higher inhibition efficiency than methionine and cystine (Morad et al
2002). The effect of corrosion in the presence of copper (II) and nitrate ions in
3M phosphoric acid solution at 98oC on the corrosion-electrochemical
properties of 12Kh18N1OT steel was studied. Sodium nitrate and copper
phosphate were used as corrosion inhibitor (Filimonov et al 2002). The
inhibiting action of 2-mercaptobenzimidazole on the corrosion of zinc in
phosphoric acid solution has been investigated by weight loss and polarization
methods. The studies revealed that the inhibitor is effective for the inhibition
of zinc in phosphoric acid solution and retards the anodic and cathodic
corrosion reactions (Wang et al 2002).
40
The electrochemical polarization behavior of Monel (400) in
different compositions of binary and ternary solution mixtures of concentrated
phosphoric, sulphuric, formic and acetic acids has been studied by
potentiostatic polarization technique at 25°C (Singh et al 2001). The
polarization and weight loss studies showed that the 2-mercaptopyrimidine
was effective for the inhibition of low carbon steel over a wide concentration
range of aqueous phosphoric acid solution (Wang 2001). The inhibition
efficiency of 2-chloroethyl phosphoric acid in controlling corrosion of carbon
steel immersed in an aqueous phosphoric acid solution containing 60ppm Cl-,
in the absence and presence of inhibitor has been evaluated. The results
revealed that in the presence of chloride it showed synergistic effect (Amairaj
et al 2001). The inhibition on the corrosion under heat transfer of the stainless
alloy 904L by benzotriazole (BTA) in phosphoric acid composed by 40%
H3PO4+4% H2SO4+300 ppm Cl− has been studied by electrochemical and
spectroscopic techniques. Results obtained by polarization measurements
showed that BTA affects both anodic and cathodic processes. With the
increase in concentration of BTA the corrosion rate decreased and the
inhibition efficiency increased (Bellaouchou et al 2001). The inhibiting action
of 2-mercapton benzimidazole on the corrosion of mild steel in phosphoric
acid solution was investigated by weight loss and polarization methods (Wang
2001a). Weight loss studies showed that both 2-mercaptothiazoline and a cetyl
pyridinium chloride are effective for the inhibition of low carbon steel over a
wide concentration range of aqueous phosphoric acid solution (Wang et al
2001b). The effect of nitrate ions and possible products of their reduction
(ammonium, hydrazine and hydroxylammonium ions) on electrochemical
characteristics of stainless steel were studied in phosphoric acid (Filimonov et
al 2001). Steady-state and electrochemical impedance spectroscopy
measurements have been made on anodic layers on 1050 and 2024T3
aluminium alloys prepared from solutions of phosphoric acid, boric acid and
41
sodium tetraborate, before and after impregnation treatment with zinc
(Dasquet et al 2001).
The erosion-corrosion of an austenitic alloy in a phosphoric acid
media containing sulphide ions has been studied (Bellaouchou et al 2000).
Corrosion resistant low-alloy steel, corrosion resistant iron-titanium alloys
have also been developed which may find use in the hardware of phosphoric
acid making plants and other equipment (Mukherjee et al 2000). The corrosion
inhibition of steel in phosphoric acid by thiosemicarbazide derivatives was
studied by different chemical and electrochemical techniques (Ameer et al
2000). The corrosion resistance of niobium, tantalum and Nb-20, 40, 60 and
80 wt% Ta alloys in 20, 40, 60 and 80 wt% phosphoric acid solutions at
boiling point 150°C and 200°C was evaluated (Robin et al 2000). The
corrosion inhibition of steel in phosphoric acid by thiosemicarbazide
derivatives was studied using different chemical and electrochemical
techniques. Protection efficiency up to 99% was obtained with small amount
(10-4M) of cinnamaldehyde thiosemi-carbazone (CTSCN) (Khamis et al 2000).
Corrosion behavior of Monel in phosphoric acid was studied (Jeyaprabha et al
2000). Weight loss and polarization studies showed that 2-mercapto
benzoxazole was effective for the inhibition of low-carbon steel over a wide
concentration range of aqueous phosphoric acid solutions. The inhibitor
retards the anodic and cathodic corrosion reaction with emphasis on the
former. (Wang et al 2000).
The corrosion of tantalum was investigated in sub- and supercritical
oxidizing solutions of hydrochloric, sulfuric and phosphoric acid between
360°C and 500°C (Friedrich et al 1999). The influence of propargyl alcohol
(PA) on the corrosion behavior of mild steel in phosphoric acid was
investigated. PA inhibits the cathodic corrosion reaction at 40ºC, but
42
accelerated the anodic one without changing the mechanism of both reactions.
Inhibition of mild steel in phosphoric acid by PA is attributed to adsorption of
PA onto steel surface via triple-bond carbon atoms (Morad 1999). Effect of
mineral compounds in phosphoric acid polluted by sulfide ions on corrosion of
nickel was observed (Guenbour et al 1999). The electrochemical behaviour of
316 stainless steel in phosphoric acid solution containing nitrate, dichromate,
tungstate and molybdate anions as inhibitors are presented and discussed
(El-dahan 1999). The inhibiting properties of some organic phosphonium and
ammonium compounds were studied with respect to the corrosion of zinc in
1M H3PO4 solutions (Morad 1999a).
The anodic films formed at 1 mA/cm2 on tantalum in concentrated
phosphoric acid (85%) and sulphuric acid (95%) solutions at 25°C have been
examined directly in the transmission electron microscope employing ultra
micro tomed sections (Shimizu et al 1998). The corrosion rates of AISI 304L
and 316L stainless steels prepared by powder metallurgy (P\M) have been
studied by continuous electrochemical methods in different concentrations of
phosphoric acid solutions at room temperature (298K). For comparison
purposes, a simultaneous study was carried out on similar composition of cast
AISI 304L and AISI 316L stainless steels specially prepared for this study.
The sintered AISI 304L and 316L stainless steels had the highest corrosion
rates, these being much higher in phosphoric acid (Otero et al 1998).
It could be seen from the literature survey that the most of the
inhibitors showed the effective inhibition effect in phosphoric acid medium
and most of the organic inhibitors obeyed Langmuir or Temkin adsorption
isotherm.
43
1.9 Literature review of inhibitors for hydrochloric acid
Corrosion inhibition efficiency of 1-phenyl-3-methyl-5-pyrazolone
(PMP) as corrosion inhibitor on mild steel in acid solution was investigated by
means of weight loss, potentiodynamic polarization curve, electrochemical
impedance spectroscopy (EIS), Raman spectrum and Quantum chemical
method. Weight loss measurements gave an inhibition efficiency of about 32%
in the presence of 1×10-5 M PMP, which increased to about 93% at PMP
concentration of 1×10-3 M. EIS results revealed that PMP took effects
excellently as a corrosion inhibitor for mild steel in 1 M hydrochloric acid
media, and its efficiency attains more than 97.2% at 1×10-3 M at 298 K
(Kun cao et al 2012).
The corrosion inhibition properties of ceftadizime (CZD) for mild
steel corrosion in HCl solution were analyzed by electrochemical impedance
spectroscopy (EIS), potentiodynamic polarization and gravimetric methods.
The results reveal that CZD acts as mixed type inhibitor and it follows
physisorption and chemisorption (Ashish Kumar Singh et al 2011). The
corrosion inhibition properties of disulfiram (DSR) for mild steel in HCl
solution were analyzed by electrochemical impedance spectroscopy (EIS),
potentiodynamic polarization, atomic force microscopy, scanning electron
microscopy and gravimetric methods. Physical adsorption is proposed for the
inhibition and the process followed the Langmuir adsorption isotherm and
kinetic/thermodynamic model of El-Awady et al (Ashish Kumar Singh and
M.A.Quraishi 2011). The inhibition effect of triazolyl blue tetrazolium
bromide (TBTB) on the corrosion of cold rolled steel (CRS) in 1.0 M HCl
solution was investigated for the first time by weight loss, potentiodynamic
polarization curves, and electrochemical impedance spectroscopy (EIS)
methods. The results show that TBTB is a very good inhibitor, and is more
efficiency in 1.0 M HCl. The adsorption of TBTB on CRS surface obeys
44
Langmuir adsorption isotherm. Polarization curves reveal that TBTB acts as a
mixed-type inhibitor in hydrochloric acids (Xianghong Li et al 2011). The
corrosion inhibition and adsorption processes of 1,2-diaminoanthraquinone
(DAQ) on mild steel in HCl was studied at different temperatures (303–333 K)
by means of weight loss measurement and UV-visible spectrophotometric
methods. The results indicate that the studied compound exhibits good
performance as inhibitor for mild steel corrosion in 1 M HCl (N.O.Obei-gbedi
et al 2011). Corrosion inhibition of mild steel in 1M HCl by cefadroxil has
been studied using electrochemical impedance spectroscopy (EIS),
potentiodynamic polarization and weight loss methods. The inhibitor showed
more than 96% inhibition efficiency at optimum concentration of
11.0×10-4 mol l-1 (Sudhish K. Shukla et al 2011).
The inhibition efficiency of imidazole derivatives against mild steel
corrosion in hydrochloric acid were evaluated by weight loss, potentiodynamic
polarization, linear polarization and electrochemical impedance spectroscopy.
The weight loss results showed that these are excellent corrosion inhibitors,
electrochemical polarizations data revealed that the mixed mode of inhibition
and the results of impedance spectroscopy have shown that the change in the
impedance parameters, charge transfer resistance and double layer capacitance
(Niketan S. Patel et al 2010). The inhibition efficiency of DCI (Dicycloimine
hydrochloride) has been evaluated by conventional weight loss method and
electrochemical polarization studies. The results revealed that DCI acts as an
effective inhibitor (around 90% of IE) in hydrochloric acid medium
(Rajalakshmi et al 2010). The corrosion behavior of carbon steel in
1M hydrochloric acid solution in the absence and in the presence of three
compounds of ethoxylated fatty amide was studied by weight loss and
galvanostatic polarization techniques. The inhibition efficiency was found to
45
be increased with increasing inhibitor concentrations, number of ethylene
oxide unit and with decreasing temperature (Zaafarany et al 2010).
The influence of ethylenediamine tetra-acetic acid (EDTA) on the
corrosion of mild steel in 1M hydrochloric acid was investigated by
potentiodynamic polarization and electrochemical impedance spectroscopy
(EIS). The efficiency of EDTA was found to be higher than thiourea for mild
steel in 1M hydrochloric acid medium. (Ahmed Y. Musa 2009). The inhibition
of corrosion of mild steel in hydrochloric acid solutions by 2-benzoylpyridine
(2BP) and pyridoxolhydrochloride (PXO) at 303K, 313K and 323K has been
investigated by weight loss and hydrogen evolution techniques. 2BP exhibited
better inhibition efficiency (78.99%) than PXO (71.93%) (A.O.James et al
2009). The corrosion behavior of 304 SS in 1N hydrochloric acid solution
containing different concentrations of N-benzyl - N′ - phenyl thiourea (BPTU),
at different temperatures was investigated by potentiostatic polarization
technique. The results obtained show that BPTU is an efficient anodic
inhibitor with efficiency of greater than 93% and the inhibition was governed
by physisorption mechanism (Herle.Ramadev et al 2009). The bixin in acidic
media was tested for corrosion inhibition of Ti in 0.1N hydrochloric acid
solution over the temperature range 30o to 40oC by electrochemical methods. It
revealed that bixin act as a corrosion inhibitor in halide medium and protects
the metals from corrosion with great efficiency (Jinendra Singh Chauhan et al
2009). It has been stated that the inhibition action of carmine and fast green
dyes on corrosion of mild steel in 0.5M hydrochloric acid was investigated by
mass loss, polarization and electrochemical impedance (EIS) methods at
300K. The inhibition efficiency of fast green (98 %) is higher than that of
carmine (92 %) and found to be maximum in 1 × 10-3M solution (R.A. Prabhu
et al 2009). 5-allyl-4-phenyl-4H-[1,2,4] triazole-3-thiol of phenyl hydrazides
of fatty acids from neem, rice bran and karanja oils have been synthesized and
46
evaluated as corrosion inhibitors for mild steel in hydrochloric acid solution by
weight loss method. The results revealed that the inhibition efficiency of these
compounds was found to vary with concentration, solution temperature, and
immersion time (Toliwal et al 2009).
Effect of sodium lauryl sulfate, a surfactant on corrosion of mild
steel in hydrochloric acid was studied by weight loss, electrochemical
polarization and metallurgical research microscopy. Results obtained reveal
that SLS is good inhibitor and shows very good corrosion inhibition efficiency
(Atulkumar et al 2008). Inhibitive and absorption properties of sparfloxacin
for the corrosion of mild steel in hydrochloric acid have been investigated
using gravimetric, gasometric and thermometric methods. Inhibition efficiency
of sparfloxacin was increased with increase in concentration of inhibitor
(N.O.Eddy et al 2008). The inhibition effect of N-benzyl - N-phenyl thiourea
on the corrosion of mild steel in 0.01 and 0.5N hydrochloric acid medium has
been investigated by potentostatic polarization studies and revealed that PBTU
is an effective anodic inhibitor and showed above 94% inhibition efficiency
(Divakara shetty et al 2008). The interaction of 1, 2, 3 benztriazole (BTAH) on
austenitic stainless steel in hydrochloric acid medium was studied and results
were compared with reported results. The inhibition efficiency of BTAH was
89.1% (Satpati et al 2008). Effect of inhibitor mixtures (TVE-3A, TVE-3B and
TVE-3C) containing formaldehyde in concentration with phenol or cresol on
corrosion behaviour of N80 steel in 15% hydrochloric acid solution was
investigated. TVE-3B has shown the maximum efficiency of 68% at ambient
temperature, whereas maximum inhibition efficiency shown by TVE-3A and
TVE-3C was found to be 62.2% and 65.7% respectively (Kumar et al 2008).
The inhibition effect of 4-[(E)-(phenylimino)methyl]phenol (PIP),
4-[(E)-(4-flourophenylimino)methyl]phenol(FIP), 4-[(E)-(4-chloro phenyl
47
imino) methyl]phenol(CIP), 4-[(E)-(4-bromophenylimino)methyl]phenol(BIP)
and 4-[(E)-(4-nitrophenylimino)methyl]phenol(NIP) on the corrosion of mild
steel in hydrochloric acid was investigated. Results reveal that all the imines
are efficient inhibitors for corrosion of mild steel in hydrochloric acid. The
order of inhibition efficiency is PIP > FIP > CIP > BIP > NIP (A.V.Shanbhag
et al 2007). The inhibition efficieny of methoxy phenol (MPH) and noyl
phenol (NPH) on corrosion of N80 steel in 15% hydrochloric acid has been
studied and found that the maximum inhibition efficiency is about 83 and
78% at 75mM inhibitor concentration respectively (Vishwanatham et al 2007).
Four gemini surfactants namely N-trimethyl butane-diyl-1,2-ethane-bis-
ammonium bromide (BEAB), N-hexane-diyl-1,2- ethane-bis-ammonium
bromide (HEAB), N-dodecane-diyl-1,2- ethane-bis-ammonium bromide
(DDEAB) and N-hexadecane-diyl-1,2- ethane-bis-ammonium bromide
(HDEAB) were synthesized and their influence on the inhibition of corrosion
on mild steel in 1N hydrochloric acid was investigated. Results revealed that
all the compounds were mixed type inhibitors and found to inhibit the
corrosion of mild steel by blocking the active sites of the metal (Sharma et al
2007).
The effects of 2-hydroxy-1-naphthaldehyde glycine (HNG) and
2-hydroxy-1-naphthaldehyde (HN) on the corrosion of mild steel in
hydrochloric acid have been studied. Weight loss measurements reveal that
HNG exhibits higher inhibition efficiency than HN (B.I.Ita et al 2006). Five
heterocyclic compounds having a five atom ring fused with the benzene ring
(indole, benzimidazole, benzotriazole, benzothiazole and benzothiadiazole)
were investigated as corrosion inhibitors for mild steel in 1N hydrochloric acid
by impedance and polarization resistance methods. Four of these compounds
exhibit inhibition properties, while one of them, benzothiadiazole, stimulates
the corrosion process (Popova et al 2006). The effect of three Schiff base
48
compounds namely, (E)-2-(1-(2-(2-hydroxyethylamino) ethylimino)
ethyl)phenol (I), 2,2΄-(1E,10E)-1,10-(2,2΄-azanediylbis (ethane-2,1-diyl) bis
(azan-1-yl-1-ylidene) bis (ethan-1-yl-1-ylidene) diphenol (II) and 2,2΄-
(2E,12E)-3,6,9,12-tetraazaetra deca-2,12-diene-2,13- diyl) diphenol (III) on
the corrosion behavior of steel in 2M hydrochloric acid solution has been
investigated at 298 K . Results proved that the compound III to be the best
inhibitor with a mean efficiency of 93% at 10 ×-2 M additive concentration
(Kaan C Emregul et al 2006).
The inhibition effect of N-cyclohexyl-N’-phenyl thiourea (CPTU)
on the corrosion of mild steel in 0.01 and 0.1 N hydrochloric acid has been
investigated by potentiostatic polarization technique. Results obtained reveal
that CPTU is an efficient anodic inhibitor for mild steel in hydrochloric acid
(S.Divakara Shetty et al 2005). Electrochemical studies of the inhibition and
the effect of a series of semicarbazides and thiosemicarbazides on the
corrosion of iron in 1M hydrochloric acid solution were performed and the
results were found to be 4-ethyl-3-thiosemicarbazide (ETSC) as the better
effective inhibitor than 4-allyl-3-thiosemicarbazide (ATSC) (Nahle .A et al
2005). The inhibitive action of 5-membered heterocycles (Thiophene and
Furan) on the corrosion of brass in 1M hydrochloric acid indicated that the
inhibiting effect grows with rise in the temperature 293K to 333K. The
maximum inhibition efficiency was found to be 98% for thiopene and 95% for
Furan at 1×10-1
M concentration (Adeyemi .O.O et al 2005). The corrosion
inhibitive effect of poly (p-Anisidine) on iron in 1M hydrochloric acid was
studied by electrochemical techniques has shown a remarkable performance of
inhibition efficiency when compared with monomer (Manivel et al 2005).
Studies have been conducted on the inhibitive effect of triethylene tetramine
(TETA) and hexamethylene tetramine (HMTA) for mild steel in 1M HCl in
the concentration range of 10-6 to 10-2 M by weight loss, D.C polarization
49
method and A.C impedance spectroscopy. Results indicated that the inhibition
efficiency of the hexamethylene tetramine was less when compared to that of
triethylene tetramine (Sathiyanarayanan et al 2005). The efficiency of
benzylidene-pyrimidin-2-yl-amine (A), (4-methyl-benzylidene)-pyrimidine-2-
yl-amine (B) and (4-chloro-benzylidene)-pyrimidine-2-yl-amine as corrosion
inhibitors for mild steel in 1M hydrochloric acid have been determined by
weight loss measurement and electrochemical polarization method. The results
showed that these inhibitors have a good corrosion inhibition efficiency even
at very low concentrations and the inhibition efficiency followed the order
C > B > A (Ashassi-Sorkhabi et al 2005). Electrochemical measurements
were performed to investigate the effectiveness of the cationic surfactant;
1-dodecyl-4-methoxy pyridinium bromide as corrosion inhibitor for mild steel
in 2M hydrochloric acid solution. The inhibition efficiency (93.3%) was found
to increase as the concentration of the surfactant increased until critical micelle
concentration (300 ppm) was reached (Migahed 2005). The inhibition effect of
N, N′-bis(salicylidene)-2-hydroxy-1,3-propanediamine (LOH) and N, N′-bis(2-
hydroxyacetophenylidene)-2-hydroxy-1,3-propanediamine (LACOH) in
2 M hydrochloric acid medium on mild steel with known compound has been
investigated at 303K (Emregul et al 2005).
Compounds such as 2,5-Bis (4-dimethylaminophenyl)-1,3,4-
oxadiazole (DAPO) and 2,5-bis(4-dimethylaminophenyl)-1,3,4-thiadiazole
(DAPT) have been synthesized and their inhibiting action on the corrosion of
mild steel in 1M hydrochloric acid at 30°C has been investigated by various
corrosion monitoring techniques. DAPO was found to be more efficient in
1M hydrochloric acid medium and potentiostatic polarization studies proved
that both are behaving as mixed-type inhibitors in 1M hydrochloric acid
(Bentiss et al 2004). New and effective aldimine types of corrosion inhibitors
namely N-methylidene octylamine (MOA), N-ethylidene octylamine (EOA)
50
and N-propylidene octylamine (POA) have been synthesized. Their inhibition
efficiency was investigated for the corrosion of mild steel in 1M hydrochloric
acid solution and it was found that the decrease in corrosion rate was in the
order POA > EOA > MOA (Subramania et al 2004). The effect of 3-amino-1,
2, 4-triazole (ATR) as a corrosion inhibitor for mild steel in acid media has
been investigated and reported that this compound inhibits mild steel corrosion
by affecting both cathodic and anodic reactions (Garcia-Ochoa et al 2004).
The effect of isomers of 3-pyridyl-substituted 1, 2, 4- and 1, 3, 4-thiadiazoles
(3-PTHD and 3-PTH) on the corrosion of mild steel in acid medium has been
investigated. A comparison of the results showed that 3-PTHD was the best
inhibitor in acid medium. It was found to behave better and polarization curves
indicated that 3-PTH and 3-PTHD are mixed type inhibitors in 1M HCl
medium (Bentiss et al 2004).
Three different water-soluble surfactants based on maleic
anhydride-oleic acid adduct (MO) were synthesized, such as triethanol
ammonium salt of MO adduct (TEASMO), triethanolamine ester of MO
adduct (TEAEMO) and polyoxyalkylated MO adduct (POAMO-23). The data
showed that TEASMO exhibited minimum inhibitive efficiency (65%) and on
the other hand, the maximum inhibitive efficiency was noticed (95%) with
POAMO-23 (Osman et al 2003). The inhibition efficiency of benzimidazole
derivatives were examined by corrosion monitoring techniques. Results
revealed that all the five diazoles studied have well pronounced inhibiting
properties in corrosion of mild steel in hydrochloric acid (Popova et al 2003).
The inhibition effect on the corrosion of mild steel in 1M HCl by
3,5-di(m-tolyl)-4-amino-1,2,4-triazole (m-DTAT) and 3,5-di(m-tolyl)-4H-
1,2,4-triazole (m-DTHT) has been investigated at 30ºC by electrochemical and
weight loss measurements. Polarization curves revealed that m-DTHT is a
51
mixed type inhibitor whereas m-DTAT is a cathodic type inhibitor. Inhibition
efficiency up to 95% for m-DTAT and 91% for m-DTHT were obtained
(El Mehdi et al 2003). 5-Bis (n-methoxyphenyl)-1, 3, 4-oxadiazoles is used as
corrosion inhibitors in acidic medium. Correlation between inhibition
efficiency and chemical structure had been studied by weight loss
measurement and electrochemical methods. The results showed that these
inhibitors showed a good corrosion inhibition effect even at very low
concentrations (Bentiss et al 2002).
The corrosion inhibition characteristics of 2-amino thiophenol
(ATP) and 2-cyanomethyl benzothiazole (CNMBT) on two types of steel in
1M hydrochloric acid medium were investigated at different temperatures
(25, 30, 35, 40 and 50°C). Results were correlated to the chemical structure of
the inhibitors. The inhibition efficiency of CNMBT was found to be higher
than that of ATP (Abd El-Rehim et al 2001). The influence of some organic
acid hydrazides namely salicylic acid hydrazide (SAH), anthranilic acid
hydrazide (AAH), benzoic acid hydrazide (BAH) and cinnamic acid hydrazide
(CAH) on the corrosion inhibition of mild steel in the presence of 1N HCl was
studied. The potentiodynamic polarization studies indicated that all the
hydrazides except SAH are mixed type inhibitors (Quraishi et al 2001c). The
effects of benzoin (BN), benzil (BL), benzoin-(4-phenyl thiosemi- carbazone)
(BN4PTSC) and benzil-(4-phenyl thiosemicarbazone) (BL4PTSC) on the
corrosion of mild steel in hydrochloric acid have been studied. The results
revealed that BN exhibited higher inhibition efficiency than BN4PTSC, BL
and BL4PTSC and chemical adsorption mechanism has been proposed for the
action of inhibitors (Ita et al 2001). The inhibiting effects of quinoline,
8-hydroxyquinoline, benzo(f)quinoline, quinoline-2-thiol, triphenylbenzyl, and
tetrabenzyl phosphonium chloride on the corrosion of mild steel (0.26 wt-%C)
52
in de-aerated 3M hydrochloric acid solution have also been studied (Abdel-Aal
et al 2001).
The inhibition effect of 3, 6-bis (2-methoxyphenyl)-1, 2-dihydro-1,
2, 4, 5-tetrazine (2-MDHT) on the corrosion of mild steel in acidic media has
been investigated. 2-MDHT is able to reduce the corrosion of steel more
effectively in 1M hydrochloric acid. Potentiodynamic polarization studies
showed that 2-MDHT is acting as a mixed-type inhibitor in 1M hydrochloric
acid (Elkadi et al 2000). The corrosion inhibition behavior of some substituted
dithiobiurets namely 1,5-diphenyl- 2, 4-dithiobiuret (DPDTB), 1-tolyl-5-
phenyl-2,4-dithio biuret (TPDTB),1- anisidy - l - 5 - phenyl - 2, 4 -
dithiobiuret (APDTB),1-chorophenyl-5-diphenyl-2,4-dithiobiuret (CPDTB)
were studied in 1 to 5 M hydrochloric acid on mild steel. Among the
compounds studied APDTB exhibited the best performance giving more than
98% inhibition efficiency (IE) in hydrochloric acid solutions. DPDTB and
CPDTB were found to reduce hydrogen permeation through mild steel in
hydrochloric acid solutions (Quraishi et al 1999). 2-undecane-5-mercapto-1-
oxa-3, 4-diazole (UMOD), 2-heptadecene-5-mercapto-1-oxa-3, 4-diazole
(HMOD) and 2-decene-5-mercapto-1-oxa-3, 4-diazole (DMOD) were synthesized
in the laboratory and their influence on the inhibition of corrosion of mild steel
in 1N hydrochloric acid was investigated by weight-loss and potentiodynamic
polarization techniques (Ajmal et al 2000). Hexylamine and dodecylamine
were investigated as inhibitors on mild steel corrosion in hydrochloric acid
corrosion (Bastidas et al 2000). The comparative study of corrosion inhibition
of triazole derivatives indicated that the efficiency of the 4-aminotriazole was
greater than that of the 4H-triazole (Bentiss et al 2000).
Sulphamethoxazole was tested as a corrosion inhibitor for mild steel
in 1.0M HCl solution. The results showed that sulphamethoxazole was found
53
to be an effective inhibitor for mild steel in this medium. The protection
efficiency increases with increasing inhibitor concentration (5×10-5 to 1×10-3
M) but decreases with increasing temperature (30 to 60°C) (Foad El Sherbini,
1999a). The effect of addition of 2[5-(2-pyridyl)-1, 2, 4-triazol-3-yl] phenol
(PPT) on mild steel dissolution in 1M hydrochloric acid was studied. The
results showed that PPT revealed a good corrosion inhibition effect and
potentiodynamic polarization studies indicated that PPT is a mixed-type
inhibitor (Bentiss et al 1999a). It has been observed from weight loss and
hydrogen gas evolution measurements that 4-phenylsemicarbazide (4PSC) and
semicarbazide (SC) actually have very significant effects on the corrosion of
mild steel in hydrochloric acid. 4PSC and SC tend to inhibit the corrosion of
mild steel in hydrochloric acid to a remarkable extent, with 4PSC exhibiting a
maximum inhibition efficiency (82%) than that of SC (66%) (Ita et al 1999).
Four nitrogen substituted thiobisformamidines – phenylthiobis-formamidines
(PTBF), tolyl thiobis formamidines (TTBF), anisidyl thiobisformamidines
(ATBF), and 4-chlorophenyl thiobisformamidines (CPTBF) were synthesized
and their corrosion inhibiting behavior for mild steel in 1M, 3M, and 5M
hydrochloric acid was studied (Ajmal et al 1999). The effect of urea (U),
thiourea (TU), acetamide (A), thioacetamide (TA), semicarbazide (SC),
thiosemicarbazide (TSC), methoxybenzaldehyde thiosemicarbazone (MBTSC),
2-acetylpyridine- (4-phenyl) thiosemicarbazone (2AP4PTSC), 2-acetylpyridine-(4-
methyl)thiosemicarbazone (2AP4MTSC), benzoin thiosemicarbazone
(BZOTSC) and benzil thiosemicarbazone (BZITSC) were investigated for
mild steel corrosion in hydrochloric acid The results (at 308ºC and 408ºC)
indicated that the order of efficiency of the thiocompounds in solution and the
extent of their tendency to adsorb on mild steel surfaces are as follows: TSC >
TU > TA, whereas for the thiosemicarbazone derivatives, the order is
BZOTSC > BZITSC > MBTSC > 2AP4MTSC < 2AP4PTSC. Physical
54
adsorption mechanism has been proposed for all the inhibitors except MBTSC,
BZITSC and BZOTSC, which are chemically adsorbed (Ebenso et al 1999).
The inhibition efficiency of the formulation consisting of
1-hydroxyethane-1, 1-diphosphonic acid (HEDP), molybdate and Zn2+ in
controlling the corrosion of mild steel in neutral, aqueous environment
containing 60ppm Cl- has been evaluated (Apparao et al 1998). The inhibiting
effect of cationic surfactant N, N, N-dimethyl 4-methyl benzyl dodecyl
ammonium chloride on mild steel in hydrochloric acid solutions was
investigated (El-Dahan et al 1998).
The effect of α-pyridoin and 2, 2'-pyridiI on the corrosion behaviour
of mild steel in hydrochloric acid solution has been studied. Weight loss and
hydrogen evolution measurements revealed that α-pyridoin gives a better
inhibition effect than 2,2'-pyridil (Ita et al 1997). The inhibiting action of
linear and cyclic thiocarbamides on mild steel corrosion in lM hydrochloric
acid is examined. The thiocarbamides studied are adsorbed through the S-atom
which is the adsorption centre, forming a donor-acceptor bond between the
unpaired electrons of the S-atom and the positive active centre of the metal
surface (Stoyanova et al 1997). The influence of N-heterocyclics viz.
imidazole (IA), benzimidazole (BIA) and 2-methyl imidazole (MIA) on the
corrosion and hydrogen permeation through mild steel in 1N hydrochloric acid
has been investigated (Muralidharan et al 1997).
This literature survey gives the clear idea about various inhibitors
used for mild steel corrosion in HCl environment. Almost all inhibitors shows
better inhibition efficiency in the room temperature, very few only inhibits
effectively in the higher temperature.
55
1.10 Literature review of inhibitors for sulphuric acid
The inhibitive effect of 4-Aminoantipyrine (4-AAP) on the
corrosion of mild steel in 0.5 M H2SO4 solution at 303 – 323 K was studied by
weight loss measurement as well as computational techniques. Results
obtained showed that 4-aminoantipyrine (4-AAP) is a good inhibitor for the
corrosion of mild steel in sulphuric acid solution. The inhibition efficiency
increased with increase in concentration of 4-AAP and also synergistically
increased in the presence of KI and KSCN, but decreased with temperature
(Acha U. Ezeoke et al 2012).
The inhibition effects of 2-amino-5-mercapto-1,3,4-thiadiazole
(2A5MT) and 2-mercaptothiazoline (2MT) on mild steel corrosion in
1 M H2SO4 were studied with potentiodynamic polarization, linear
polarization resistance and electrochemical impedance spectroscopy
techniques. It was shown that both 2A5MT and 2MT act as good corrosion
inhibitors for mild steel protection (Ali Doner et al 2011).
The corrosion and corrosion inhibition effect of carboxymethyl
cellulose (CMC) for mild steel in sulphuric acid medium was investigated
using weight loss and hydrogen evolution techniques at 30–60 0C. The effect
of addition of halide ions (Cl-, Br-, and I-) was also studied. It was found that
CMC functions as an inhibitor for acid induced corrosion for mild steel.
Inhibition efficiency increases with increase in immersion time but decreases
with increase in temperature (S.A.Umoren et al 2010). Chemical methods
were used to assess the inhibitive and adsorption behaviour of carboxymethyl
cellulose (CMC) for mild steel in H2SO4 solution at 30–60 °C. Results
obtained show that CMC act as inhibitor for mild steel in H2SO4. The
inhibition efficiency was found to increase with increase in CMC
concentration but decreased with rise in temperature (M.M.Solomon et al
56
2010). Ketoconazole (KCZ) has been evaluated as a corrosion inhibitor for
mild steel in aerated 0.1 M H2SO4 by gravimetric method. The effect of KCZ
on the corrosion rate was determined at various temperatures and
concentrations. The inhibition efficiency increases with increase in inhibitor
concentration but decrease with rise in temperature (I.B.Obot et al 2010).
The corrosion inhibition of stainless steel with different
concentrations of 1-methyl-3-pyridine-2-YI-thiourea (MPT) in sulphuric acid
was investigated by potentiostatic polarization measurements. MPT acts as
efficient inhibitor in sulphuric acid (S.M.A Hosseini et al 2009). The corrosion
inhibition of mild steel in sulphuric acid in the presence of methocarbamol was
studied using thermometric and gasometric (hydrogen evolution) methods.
The study revealed that the methocarbamol lowered the corrosion rate of mild
steel in sulphuric acid medium and showed the maximum inhibition efficiency
of methocarbamol under increased level of concentration with decreased level
of temperature. The phenomenon of physical adsorption is proposed from the
thermodynamic parameters (Ebenso et al 2009). The inhibition effect of cetyl
trimethylammonium bromide (CTAB) on acid corrosion of mild steel in
sulphuric acid at different temperatures has been investigated. It showed the
maximum inhibition efficiency 92% at room temperature at 10-3 mol/lit, and
any increase in temperature decreased the inhibition efficiency (Mukta
Sharama et al 2009). Inhibitive and adsorption properties of penicillin G for
the corrosion of mild steel was investigated using gasometric and
thermometric methods. Penicillin- G was found to inhibit the corrosion of mild
steel in sulphuric acid. Inhibition efficiencies of penicillin- G increased as the
concentration of penicillin-G increases but decreased with increase in
temperature. (Eddy et al 2009). The inhibitor effect of naturally occurring
biological molecule caffeic acid on the corrosion of mild steel in 0.1M H2SO4
was investigated by weight loss, potentiodynamic polarization,
57
electrochemical impedance and Raman spectroscopy. The different techniques
confirmed the adsorption of caffeic acid onto the mild steel surface and
consequently effects inhibition of the corrosion process. Caffeic acid acts by
decreasing the available cathodic reaction area and modifying the activation
energy of the anodic reaction (De Souza et al 2009).
The corrosion inhibition of mild steel in 1 M sulphuric acid by
polyvinyl pyrrolidone (PVP) and the synergistic effect of iodide ions were
investigated using weight loss and hydrogen evolution methods in the
temperature range of 30-60 0C. The corrosion rates of mild steel decreased
with the increasing concentration of PVP, while the inhibition efficiency
increased. The inhibition efficiency of PVP decreased with rise in temperature
(S.A.Umoren et al 2008). The inhibitory effect of diethanolamine (DEA) on
corrosion of mild steel in 0.5 M sulphuric acid was investigated by various
corrosion monitoring techniques. The inhibition efficiency varied in the range
of 88.7% to 55.3 % for a concentration range of 10-3M to 10-7M at 303 K
respectively (Ramananda Singh et al 2008). The corrosion inhibition of mild
steel in 2M sulphuric acid using Alizarin yellow GG (AYGG) (an azo dye) in
the presence of iodide ions was studied at 30 – 60oC by using weight loss and
hydrogen evolution methods. It has been observed that the inhibition
efficiency increased with increase in concentration of AYGG and decreased
with increase in temperature. The inhibition efficiency of AYGG
synergistically increased on addition of KI (Ebenso et al 2008).
The corrosion inhibition of mild steel in one normal sulphuric acid
solution by polyethylene glycol methyl ether (PEGME) has been studied using
electrochemical polarization. Polyethylene glycol methyl ether is a very
effective corrosion inhibitor for mild steel in sulphuric acid medium
(A.K.Dubey et al 2007). Inhibitive action of cetyl pyridinium bromide (CPB)
58
on the mild steel corrosion in sulphuric acid was investigated by
electrochemical polarization techniques. The results revealed that this inhibitor
inhibit more effectively the mild steel corrosion in sulphuric acid (A.K.Dubey
et al 2007). Three new Schiff bases, N,N΄-ethylen-bis (salicylidenimine) [S1],
N,N΄ -isopropylien-bis (salicylidenimine) [S2], and N-acetylacetone imine,
N-(2-hydroxybenzophenone imine) ortho-phenylen [S3] have been
investigated as corrosion inhibitors for mild steel in 0.5M sulphuric acid by
Tafel polarization and electrochemical impedance spectroscopy (EIS). The
three Schiff bases functioned as good inhibitors reaching inhibition
efficiencies of 97–98% at 300 ppm concentration and S2 showed better
efficiency than the other two Schiff bases (Hosseini et al 2007). The efficiency
of hexamethylenetetramine (HMTA) as corrosion inhibitor for steel in de-
aerated acid solution has been determined by electrochemical studies. It was
found that the HMTA acts a good corrosion inhibitor and also inhibits the
corrosion by controlling both the anodic and cathodic reactions in 0.1M
sulphuric acid (Bayol et al 2007).
The corrosion rates in the presence of 2,5-bis(4-methoxyphenyl)-
1,3,4-oxadiazole (4-MOX) as a steel corrosion inhibitor in 0.5M sulfuric acid
were measured by weight loss method over the range of temperatures from
303 to 343K. Results revealed that 4-MOX performed excellently as a
corrosion inhibitor for mild steel in sulfuric acid media and its efficiency
attained more than 96.19 % at 8 ×10-4
M at 333K (Bouklah et al 2006). 2,2΄-
Dithiobis(3-cyano-4,6-dimethyl pyridine) was found to be an efficient
inhibitor for the corrosion of mild steel in 1 to 5M sulfuric acid solutions at
35 to 50ºC. The inhibition efficiency is slightly increased or remained
approximately unchanged irrespective of the acid concentration or the solution
temperature. The inhibition action is attributed to chemisorption of the
heterocyclic compound on the steel surface by blocking its active sites (Morad
59
et al 2006). Inhibition of mild steel corrosion in de-aerated 0.5M sulfuric acid
solution containing various concentrations of indole-5-carboxylic acid was
studied over the temperature range from 25 to 55ºC. It gave the highest
inhibition efficiency of 92% at 4×10-3M of indole-5-carboxylic acid and
behaved as mixed-type inhibitor (Quartarone et al 2006). The inhibiting action
of hexadecylpyridinium bromide (HDPB) on mild steel in 0.5M sulfuric acid
solution in the temperature range of 30 to 60ºCwas studied using
potentiodynamic technique and weight loss measurement. Results showed that
the inhibition efficiency increased with increase in temperature and maximum
range of inhibition efficiency of HDPB was obtained around 97% at 1×10-3 M
at 333K (Mahmoud M. Saleh 2006).
Benzyl triphenyl phosphonium bromide (BTPPB) has been
evaluated as a corrosion inhibitor for mild steel in aerated 0.5M sulfuric acid
solution by galvanostatic polarization and potentiostatic polarization methods.
The results of mass loss and potentiodynamic polarization measurements on
the corrosion inhibition of mild steel in 0.5 M sulphuric acid in the
temperature range of 30-60 0C using sodium naphthalene disulphonic acid
(NDSA) as an inhibitor reveals that inhibition efficiency increased with the
increase in concentration of NDSA. The adsorption of inhibitor followed
Flory-Huggins adsorption isotherm (Fathy M.Bayoumi et al 2005). The effect
of formazones as a new class of corrosion inhibitor on mild steel in sulphuric
acid has been investigated by various techniques. The results revealed that the
compounds gave a maximum inhibition efficiency of 98-99% at 0.5 to 10 M
concentrations of inhibitor (Selvaraj et al 2005). The inhibitive capabilities of
some organic dyes namely; safranine-O (SO), thymol blue (TB) and
fluorescein-Na (F-Na) on the electrochemical corrosion of mild steel in
sulphuric acid solution was rapidly assessed using the gasometric technique.
The results indicated that all the compounds act as inhibitor in the acidic
60
medium and inhibition efficiency increased with concentration for SO and TB
but decreased with concentration for F-Na (Ebenso et al 2005). The influence
of some substituted dianils on corrosion of mild steel in 1N sulphuric acid has
been studied. Methylene blue dye (MB) was investigated as a corrosion
inhibitor for mild steel in 2 M sulphuric acid solution in the presence of halide
additives namely KCl, KBr and KI. It was found that in the presence of halide
additives the inhibition efficiency increased. (Oguzie et al 2005). Nasser
(2005) results revealed that the some aliphatic sulphides inhibit the corrosion
of mild steel in 1M sulphuric acid in the order: Ethyl butyl sulphide > Allyl
butyl sulphide > Ethyl propylsulphide > Diethyl sulphide > Ethyl methyl
sulphide.
The effect of thiourea (TU), methylthiourea (MTU) and
phenylthiourea (PTU) on the corrosion behaviour of mild steel in 0.1 M
solution of sulfuric acid has been investigated in relation to the concentration
of thioamides (–CS–NH2). Their inhibition efficiencies increased in the order
of phenylthiourea > methylthiourea > thiourea (Ozcan et al 2004). An anionic
surfactant [p-myristyloxycarbonyl methoxy-p-sodiumcarboxylate-azobenzene]
was prepared. The inhibition efficiency of this surfactant has been studied by
both chemical and electrochemical techniques at 25ºC. The inhibition
efficiency reached to an extent of 95.82% at 10−4M of the inhibitor (Migahed
et al 2004). Gravimetric method was used to study the inhibitory properties of
indigo dye during corrosion of mild steel in aerated sulphuric acid solutions in
the range 30°–50°C. The effect of halide salts KCl, KBr and KI was also
investigated. (Oguzie et al 2004). The addition of halide salts synergistically
increased the inhibition efficiency of indigo in the order KI >KBr >KCl. The
inhibitive action is probably based on the adsorption ability of the polar N or O
atom in the inhibitor which is bonding to the metal surface by chemical
adsorption mechanism (E.E. Oguzie et al 2004).
61
Macrocyclic compounds constitute a potential class of inhibitors.
The influence of some substituted dianils on corrosion of mild steel in 1N HCl
and 1N H2SO4 has been studied using weight loss and electrochemical
techniques. All the five compounds studied, 1,4-dicinnamyledene
aminophenylene (DCAP) showed the best performance. The inhibition
efficiency values of aromatic dianils showed that at a common concentration
of 500 ppm follow the order DCAP > DBAP > DDAP > DVAP > DSAP
(Quraishi et al 2003). The inhibition effects of sodium
dodecylbenzenesulphonate (SDBS) and hexamethyl enetetramine (HA) on the
corrosion of mild steel in sulphuric acid solution have been studied (Hoesseini
et al 2003a). The inhibition effects of 3,5 bis (2-pyridil) 4-amino 1,2,4 triazole
(NBTA) and 1-10 phenantrolin (PHEN) on corrosion of mild steel in acid
solution was studied (Arshadi et al 2002). A macrocyclic compound namely
2,3,9,10-tetraphenyl-6,13-dithia-1,4,5,7,8,11,12,14-octaaza-cyclotetradeca
-1,3,6,8, 10,13-hexaene (PTAT) was also synthesized to study the corrosion
inhibitive effect on pickling of mild steel in 20% sulphuric acid at 95°C
(Quraishi et al 2001a).
The corrosion and inhibition behaviors of mild steel in aerated
sulphuric acid in the presence of propargyl alcohol (PA) and potassium iodide
were investigated by electrochemical methods. The experimental results
suggested that the presence of iodide ions in the solutions stabilized the
adsorption of PA molecules on the metal surfaces and improved the inhibition
efficiency of PA (Feng et al 1999). Effectiveness of quaternary ammonium salt
used as corrosion inhibitor on mild steel in 5% sulphuric acid solution at
temperatures between 30° and 60°C (Dahan et al 1999). The inhibition of mild
steel corrosion in 0.5M sulphuric acid with simple amines (cyclohexylamine,
pyridine, triethylamine) was investigated by DC polarization, electrochemical
impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS).
62
The results indicated that a strong dependence of the inhibition performance
on the nature of the metal surface, in addition to the structural effects of
amines (Li et al 1997).
Inhibitor survey for the mild steel corrosion reveals that all the
inhibitors shows very good inhibition efficiency for the mild steel in H2SO4
environment.
1.11 Literature review of synthesized inhibitors for acid medium
The corrosion rates in the presence of new synthesized pyridazine
derivatives (P1, P2, P3 and P4) as a steel corrosion inhibitors in
1 M hydrochloric acid, were measured by the weight loss method, in the range
of temperatures from 303 to 353 K. Results obtained revealed that the
inhibition efficiency of these compounds decreases markedly with increasing
temperature and its value reaches 48.5% at 353 K at 10-3M (B.Zerga et al
2012). The effect of adding new pyridazine derivatives, ethyl[4-(2-chloro
benzyl)-3-methyl-6-oxopyridazin-1(6H)-yl]acetate (P1), ethyl [4-(2-
chlorobenzyl) -3-methyl-6-thioxopyridazin-1(6H)-yl] acetate (P2), 5-(2-
chlorobenzyl)-2-(2-hydroxyethyl)-6-methylpyridazin-3(2H)-one (P3) and 5-
(2-chlorobenzyl)-2-(2-hydroxyethyl)-6-methylpyridazine-3(2H)-thione(P4), on
the electrochemical behaviour of steel in molar hydrochloric acid was
investigated by using weight loss method, potentiodynamic polarization and
electrochemical impedance spectroscopy (EIS) measurements. Results
reported in this study show that the addition of these compounds inhibits the
corrosion of steel and the extent of inhibition depends upon the type and
concentration of the pyridazine compounds ( B.Zerga et al 2012).
The inhibition effect of synthesized 1-(6-ethoxy-6-oxohexyl)
pyridazinium bromide(I), 1-benzylpyridazinium bromide (II), 1-phenethyl
63
pyridazinium bromide (III) and 1-(4-phenoxybutyl)pyridazin-1-ium bromide
(IV) on the corrosion of mild steel in hydrochloric acid was investigated by
gravimetric method. The inhibition efficiency was increased with increase in
concentration of all four inhibitors and also synthesized inhibitors acts as a
mixed type inhibitor (M.Messali et al 2011). The inhibition effect of a new
synthesized organic inhibitor, namely 1-(3-Nitrobenzylidene)Thiosemi
carbazide (A) on the corrosion of mild steel in 0.5 M sulphuric acid have been
investigated at room temperature using weight loss, electrochemical
impedance spectroscopy (EIS) and Tafel polarization measurements. The
inhibition efficiencies obtained from all methods employed are in good
agreement with each other. The obtained results show that compound (A) is a
very good inhibitor with efficiency of 98% at 100 ppm additive concentration
in acid solution (Athareh DADGARINEZHAD et al 2011). Schiff bases
derived from condensation reaction of Acrolein with 2-aminophenol (SB1),
cinnamaldehyde with 2-aminophenol (SB2) and cinnamaldehyde with
phenylene diamine (SB3) were prepared. These schiff bases identified by
UV-Vis, IR, CHN and H1NMR.The study also included the use of these schiff
bases as inhibitors for corrosion of carbon steel in acidic media 0.5 N HCl.
The rate corrosion was measured by electrochemical and weight loss methods
and it was found that their results are in agreement between them. The results
indicated that these schiff bases inhibited the corrosion efficiently
(Mohammed Qasim Mohammed et al 2011). The inhibition effect of triazolyl
blue tetrazolium bromide (TBTB) on the corrosion of cold rolled steel (CRS)
in 1.0 M HCl and 0.5 M H2SO4 solution was investigated by weight loss,
potentiodynamic polarization curves and electrochemical impedance
spectroscopy (EIS) methods. The results show that TBTB is a very good
inhibitor (Xianghong Li et al 2011). 1, 3-Bis-(morpholin-4-yl-phenyl-methyl)-
thiourea (MBT) was synthesized and their influence on the inhibition of
corrosion on mild steel in various hydrochloric acid concentrations has been
64
investigated by weight loss, potentiodynamic polarization, electrochemical
impedance (EI), Tafel polarization, and scanning electron microscope (SEM)
and FT-IR methods. The result of weight loss study shows that the corrosion
inhibition efficiency (IE) is directly proportional to the concentration of the
inhibitor and inversely proportional to the temperature (Devaraj Karthik et al
2011).
Acenaphtho [1,2-b] quinoxaline (AQ) was tested as a novel
corrosion inhibitor for mild steel in 0.5 M H2SO4 solution using chemical
technique at 30 °C. AQ acts as an effective inhibitor for mild steel in acidic
medium. Inhibition efficiency increased with increasing concentration of AQ
(I.B. Obot et al 2010). The corrosion inhibition of mild steel in 0.5 M sulfuric
acid solutions by some new synthesized organic compounds namely (E)-2-
acetyl-3-(butyl amino)-N-phenyl buten-2-thioamide (compound A), (E)-3-(4-
(dimethyl amino) phenyl amino)-2-acetyl-N-phenyl buten-2-thioamide
(compound B) and (E)-3-(2,3-dimethyl phenyl amino)-2-acetyl-N-phenyl
buten-2-thioamide (compound C) was investigated using weight loss and
potentiostatic polarization techniques. These measurements reveal that the
inhibition efficiency obtained by these compounds increased by increasing
their concentration. The inhibition efficiency follows the order A > B > C
(S.M.A. Hosseini et al 2010). The influence of inhibitor concentration and
temperature on the corrosion behaviour of steel in molar HCl solution has been
investigated by weight loss method. Results obtained show that the inhibitory
effect of 2-phenylthieno (3, 2-b) quinoxaline (P4) increases with increasing P4
concentration to attain the highest value (95%) at 5×10-4M (El Ouali et al
2010). Newly synthesized compounds such as benzoic-triazole derivative 3,
5-dimethylbenzoicacid [1, 2, 4] triazol-1-ylmethyl ester (DBT) were also
investigated. The results revealed that DBT was an excellent inhibitor and
exhibited mixed-type character (Zhihua Tao et al 2010). Sulphaniilic acid and
65
sulphanilamide Schiff bases have been synthesized and evaluated as inhibitors
for mild steel in sulphuric acid by weight loss and electrochemical methods.
The inhibition efficiency increases with increase in concentration of inhibitor
and decreases with temperature (S.Chitra et al 2010). Four heterocyclic
compounds, namely 4- phenyl-5-acetyl/carbethoxy-3-methyl-6-hydroxy-6-
methyl-4,5,6,7-tetrahydro-2,1-benzoisoxazole and benzopyrazole (BIS1, BP1
and BIS2, BP2) were synthesized and their influence on the inhibition of
corrosion of mild steel in sulphuric acid was investigated by means of weight
loss, potentiodynamic polarization, electrochemical impedance and scanning
electron microscopy. All the four inhibitors exhibit excellent inhibition
efficiency towards corrosion of mild steel in sulphuric acid (K. Parameswari et
al 2010).
The inhibition effect of Bis (benzimidazol-2-yl) disulphide
(BIMDS) on corrosion behavior of mild steel (MS) in 1.0 M HCl and
0.5 M H2SO4 was studied using different techniques. These studies have
shown that studied compound is a good inhibitor for MS in 1.0 M HCl and
0.5 M H2SO4 solutions. Inhibitor showed better performance in 0.5 M H2SO4
solutions than 1.0 M HCl (Ishtiaque Ahamad et al 2009). 3-{[8-
trifluoromethyl)quinoline-4-yl]thio}-N’-(2,3,4-trihydroxybenzylidene)propano
hydrazide (TQTHBH) was synthesized, characterized and tested as a corrosion
inhibitor for mild steel in hydrochloric acid and sulphuric acid solutions using
weight loss, electrochemical impedance and potentiodynamic polarization
methods. The newly synthesized TQTHBH acts as good corrosion inhibitor for
mild steel in acid medium (V. Ramesh Saliyan et al 2009).
A new corrosion inhibitor N-(3,4-dihydroxybenzylidene)-3-{[8-
(trifluoromethyl) quinolin-4-yl]thio} propane hydrazide (DHBTPH) was
synthesized, characterized and tested as a corrosion inhibitor for mild steel in
66
sulphuric acid (0.5M, 1M) solutions using weight-loss method,
electrochemical methods. The results showed that DHBTPH is a very good
inhibitor for mild steel in acidic medium and the inhibition efficiency was
found to be in the decreasing order 0.5M H2SO4 > 1M H2SO4 (Ramesh Saliyan
et al 2008). The cycloaddition reactions of the cyclic nitrones 1-pyrroline 1-
oxide and 3,4,5,6-tetrahydropyridine 1-oxide with alkenes, 11-phenoxy-1-
undecene and 11-p-methoxyphenoxy-1-undecene, afforded cycloaddition
products (bicyclic isoxazolidines) in excellent yields. One of the cycloadducts
on reaction with propargyl chloride and ring opening with zinc in acetic acid
afforded quaternary ammonium salt and aminoalcohol, respectively. All the
new inhibitor molecules in the presence of 400 ppm at 60 °C achieved
inhibition efficiencies, determined by gravimetric method, in the range
99–99.6% and 85–99% for mild steel in 1 M HCl and 0.5 M H2SO4 (S.A. Ali
et al 2008). The newly synthesized 2-(Alkylsulfanyl)-N-(pyridin-2-yl)
acetamide derivatives were tested as a corrosion inhibitor on steel in acidic
mediums. All tested inhibitors showed promising inhibition efficiencies
(A. Yıldırım et al 2008). Quinolin-5-yl methylene-3- { [ 8- ( tri fluoro methyl )
quinolin-4-yl]thio}propanohydrazide (QMQTPH) was synthesized,
characterized and tested as a corrosion inhibitor for mild steel in 1 M and 2 M
HCl solution using potentiodynamic polarization and electrochemical
impedance spectroscopy (EIS). The results showed that QMQTPH is an
excellent inhibitor for mild steel in acid medium (V. Ramesh Saliyan et al
2008).
The effect of three Schiff base compounds with increasing number
of coordination sites, namely, 2-{(E)-[(2-hydroxyethyl)imino]methyl} phenol
(I), 2-[(E)-({2-[(2-hydroxyethyl)amino]ethyl}imino)methyl]phenol (II) and
2,2′-{iminobis[ethane-2,1-diylnitrilo(E)methylidene]}diphenol (III) have been
investigated at 298 K by weight loss measurements, potentiodynamic
67
polarization and electrochemical impedance spectroscopy (EIS) methods. The
inhibition efficiencies obtained from all methods employed are in good
agreement. Results show compound III to be the best inhibitor with a mean
efficiency of 93% at 10−2 M additive concentration. Studies showed all three
compounds to act as mixed type inhibitors (Canan Kustu et al 2007). The
inhibition effect of synthesized 4-[(E)-(Phenylimino) methyl] phenol (PIP),
4-[(E)-(4-fluorophenylimino) methyl] phenol (FIP), 4-[(E)-(4-chloro
phenylimino) methyl] phenol (CIP), 4-[(E)-(-4-bromophenylimino) methyl]
phenol (BIP) and 4-[(E)-(-4-nitro phenylimino) methyl ]phenol (NIP) on the
corrosion of mild steel in hydrochloric acid was studied by weight loss and
electrochemical techniques (Shanbhag et al 2007). Three triazole derivatives
(4 – chloro – acetophenone –O - 1′- (1′,3′,4′-triazolyl) – metheneoxime
(CATM), 4 – methoxyl – acetophenone - O- 1′- (1′,3′,4′-triazolyl) -
metheneoxime (MATM) and 4 – fluoro – acetophenone - O- 1′- (1′,3′,4′-
triazolyl) - metheneoxime (FATM)) have been synthesized as new inhibitors
for the corrosion of mild steel in acid media. The inhibition efficiencies of
these inhibitors were evaluated by means of weight loss and electrochemical
techniques such as electrochemical impedance spectroscopy (EIS) and
polarization curves. The inhibition efficiency of all three inhibitors increased
with increase in concentration of inhibitors (Weihua Li et al 2007). The
inhibition effect of a new bipyrazole derivative namely N-benzyl-N,N-bis[(3,5-
dimethyl-1H-pyrazol-1-yl)methyl]amine (BBPA) on the corrosion of steel in
1 M HCl is studied at 308 K. Weight-loss measurements, potentiodynamic
polarization, linear polarization and impedance spectroscopy (EIS) methods
were used. Results show that BBPA is a good inhibitor and inhibition
efficiency reaches 87% at 5×10− 4 M (K. Tebbji et al 2007). This study
examined the use of 4H-triazole derivatives namely 3,5-diphenyl-4H-1,2,4-
triazole (DHT), 3,5-bis(4-pyridyl)-4H-1,2,4-triazole (4-PHT) and 3,5-bis(4-
methyltiophenyl)-4H-1,2,4-triazole (4-MTHT) as inhibitor for corrosion and
68
dissolution protection of mild steel in normal hydrochloric acid solution. The
experimental results revealed that 4-MTHT was the best effective inhibitor
(99.6%) at 5×10-4
M and the inhibition efficiency was found to be in the order:
4-MTHT > 4-PHT > DHT (Bentiss et al 2007).
The inhibiting action of newly synthesized 2,2′-Dithiobis(3-cyano-
4,6-dimethylpyridine (PyS)2 on the corrosion of mild steel in 1–5 M H2SO4
solutions at 35–50 °C has been investigated by polarization resistance (Rp),
polarization curves and electrochemical impedance spectroscopy (EIS). (PyS)2
showed excellent performance and its efficiency did not affect either by
increasing the acid concentration or rise of temperature (M.S. Morad et al
2006). Some S-containing newly synthesized thio compounds have been tested
as inhibitors for the corrosion of steel in 1 M HCl solution. Weight loss
measurements, potentiodynamic polarization and impedance spectroscopy
(EIS) methods are used. The inhibiting action increases with the concentration
of the compounds tested (M. Elayyachy et al 2006). The inhibiting action of a
Schiff base 4-[(4-hydroxy-3-hydroxymethyl-benzylidene)-amino]-1,5-
dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (phv), derived from 4-amino-
1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (phz) and 4-hydroxy-3-
methoxy-benzaldehyde (vn), towards the corrosion behavior of steel in
2 M HCl solution has been studied using weight loss, polarization and
electrochemical impedance spectroscopy (EIS) techniques. The inhibitor
efficiencies calculated from all the applied methods were in agreement and
were found to be in the order: phv > phz > vn (Kaan C. Emregül et al 2006).
Selected oxa-diazoles of fatty acids; namely 2-hepta decene-5-mercapto-l-oxa-
3,4-diazole (HMOD); 2-undecane-5-mercapto-l-oxa-3,4-diazole (UMOD); and
2-decene-5-mercapto-l-oxa-3,4-diazole (DMOD), were synthesized. The
inhibition efficiency of these compounds was found to vary with
concentration, immersion time and temperature. All these compounds showed
69
good inhibition efficiency and act as mixed type inhibitors (Quarishi et al
2006). Imidazolines and amidic precursors were synthesized with good yields
through an optimized process. These compounds were evaluated as corrosion
inhibitors in an aqueous solution of 1M hydrochloric acid by gravimetric and
polarization techniques. The results indicated that the inhibition efficiency of
these compounds depended on the molecular structure and concentration of
inhibitor (Olivares-Xometl et al 2006). The corrosion inhibition of mild steel
in 0.5 M hydrochloric acid solutions by some new hydrazine carbodithioic
acid derivatives namely N΄-furan-2-yl-methylene-hydrazine carbodithioic acid
(A), N΄-(4-dimethylamino-benzylidene)-hydrazine carbodithioic acid (B) and
N΄-(3-nitro-benzylidene)-hydrazine carbodithioic (C) were also studied. The
measurements showed that the inhibition efficiency obtained by these
compounds increased by increasing their concentration. The inhibition
efficiency was found to follow the order: C > B > A (K.F. Khaled et al 2006)
A potential class of heterocyclic inhibitors namely 2-amino -1,3,4-
thiadiazole(AT), 5-methyl-2-amino-1,3,4- thiadiazole(MAT), 5-ethyl -2-
amino-1,3,4- thiadiazole (EAT)and 5-propyl -2-amino-1,3,4-thiadiazole
(PAT) were synthesized and their influence on inhibition of mild steel
corrosion in 1N hydrochloric acid was investigated by potentiostatic
polarization technique. It could be seen from the result that all the thiadazoles
exhibited good inhibition efficiency and act as mixed type inhibitor (Quraishi
et al 2005). The influence of some new bipyrazole derivatives synthesized in
the laboratory on the corrosion inhibition of mild steel in 1 M hydrochloric
acid solution was studied. It was found that ethyl 4-[bis (3, 5-dimethyl-1H-
pyrazol-1-yl) methyl)] amino benzoate (bipyrazol-4) was the best inhibitor
(Elayyachy et al 2005). Inhibitory effect of some new synthesized bipyrazole
compounds, namely, N,N-bis[(3,5-dimethyl-1H-pyrazol-1-yl)methyl]-N-(4-
methylphenyl)amine(P1) and methyl-1-[((methylphenyl) {[3-(methoxycarbony
70
l)-5-methyl-1H-pyrazol-1-yl] methyl} amino)methyl]-5-methyl-1H-pyrazole-
3-carboxylate (P2) on corrosion of pure iron in 1 M HCl solution has been
studied using chemical technique as weight loss and electrochemical
techniques as potentiodynamic polarization, linear polarization and
impedance. The inhibition efficiencies obtained from gravimetric, cathodic
Tafel plots, linear polarization resistance and EIS methods are in good
agreement. The results obtained reveal that these compounds are efficient
inhibitors (A. Chetouani 2005). A triazole-based cationic gemini surfactant,
3,5-bis(methylene octadecyl dimethylammonium chloride)-1,2,4-triazole (18-
triazole-18) has been synthesized, and its effect on corrosion inhibition of A3
steel in 1 M HCl has been studied using the weight-loss method. The result
showed that 18-triazole-18 acted as an excellent inhibitor in 1 M HCl (Ling-
Guang Qiu et al 2005). The influence of diethyl pyrazine-2,3-dicarboxylate
(P1) on the corrosion of steel in 0.5 M H2SO4 solution has been studied by
weight loss measurements, potentiodynamic polarization and linear
polarization resistance (Rp) and impedance spectroscopy (EIS) methods. The
inhibiting action increases with the concentration of pyrazine compound to
attain 82% at 10−2 M (M. Bouklah et al 2005). The efficiency of benzylidene-
pyrimidin-2-yl-amine (A), (4-methyl-benzylidene)-pyrimidine-2-yl-amine (B)
and (4-chloro-benzylidene)-pyrimidine-2-yl-amine, as corrosion inhibitors for
mild steel in 1 M HCl have been determined by weight loss measurements and
electrochemical polarization method. The results showed that these inhibitors
revealed a good corrosion inhibition even at very low concentrations
(H. Ashassi-Sorkhabi et al 2005).
The effect of some mercapto-triazole derivatives synthesized in the
laboratory containing different hetero atoms and substituents in the organic
structures on the corrosion and hydrogen permeation of mild steel in 1M HCl
was investigated. It has been reported that all the mercapto-triazole derivatives
71
perform excellently as corrosion inhibitors for mild steel in 1M HCl and
behaved as mixed-type inhibitors (Hui-Long Wang et al 2004).
The inhibition of the corrosion of mild steel in 1 M HCl by 3,5-
di(m-tolyl)-4-amino-1,2,4-triazole (m-DTAT) and 3,5-di(m-tolyl)-4H-1,2,4-
triazole (m-DTHT) has been investigated at 30 °C using electrochemical and
weight loss measurements. Polarization curves reveal that m-DTHT is a mixed
type inhibitor whereas m-DTAT is a cathodic type inhibitor. Inhibition
efficiencies up to 95% for m-DTAT and 91% for m-DTHT can be obtained
(B El Mehdi et al 2003). A new corrosion inhibitor namely 4-amino-3-butyl-5-
mercapto-1,2,4-triazole (ABMT) has been synthesized and its inhibitive
performance towards the corrosion of mild steel in 1 N sulphuric acid (H2SO4)
investigated by weight loss and potentiodynamic polarization techniques.
Potentiodynamic polarization measurements clearly reveal that the
investigated inhibitor is of mixed type (M.A Quraishi et al 2003). A mercapto-
triazole compound, namely 4-salicylideneamino-3-phenyl-5-mercapto-1,2,4-
triazole (SAPMT), was synthesized and its inhibition effect on the corrosion of
mild steel in 1.0 M hydrochloric acid (HCl) solution was investigated by
weight loss and electrochemical techniques. Results obtained revealed that
SAPMT performed excellently as a corrosion inhibitor for mild steel in HCl
solution (Hui-Long Wang et al 2003). Three heterocyclic compounds namely
3-anilino-5-imino-4-phenyl-1, 2,4-thiadiazoline (AIPT), 3-anilino-5-imino-4-
tolyl-1, 2,4-thiadiazoline (AITT), and 3-anilino-5-imino-4-chlorophenyl-1,
2,4-thiadiazoline (AICT) were synthesized and their corrosion inhibition effect
on mild steel in 1M hydrochloric acid was investigated (Quraishi et al 2003b).
Compounds such as 2,3,9,10-tetramethyl-6,13-dithia-1,4,5,7,8,11,12,14-
octaaza-cyclotetradeca-1,3,6,8,10,13-hexaene (MTAH) were synthesized to
study the corrosion inhibitive effect on pickling of mild steel (MS) in 20%
sulphuric acid at 95°C. MTAH exhibited inhibition efficiency of 95% and the
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efficiency further increased in the presence of KI due to synergistic effect
(Quraishi et al 2003).
A new corrosion inhibitor namely 4-amino-3-butyl-5-mercapto-1, 2,
4-triazole (ABMT) has been synthesized and its inhibitive performance
towards the corrosion of mild steel in 1N sulphuric acid was investigated by
weight loss and potentiodynamic polarization techniques. The inhibition
efficiency of ABMT was 99% at 1000 ppm and the inhibitor is of mixed type.
(Quraishi et al 2002).
A new corrosion inhibitor namely 4-salicylideneamino-3-
hydrazino-5-mercapto-1, 2, 4-triazole (SAHMT) has been synthesized and its
influence on corrosion inhibition of oil-well tubular steel (N-80) and mild steel
in 15% hydrochloric acid solution under boiling condition has been studied by
weight loss method. Potentiodynamic polarization measurements clearly
revealed that the investigated inhibitor is of mixed type and inhibits the
corrosion of both the steels by blocking the active site of the metal (Quraishi et
al 2001). Three different long chain fatty acid oxadiazoles namely 2-undecane-
5-mercapto-1-oxa-3, 4-diazole (UMOD), 2-heptadecene-5-mercapto-1-oxa-3,
4-diazole (HMOD) and 2-decene-5-mercapto-1-oxa-3, 4-diazole (DMOD)
were synthesized in the laboratory and were evaluated as corrosion inhibitor
for mild steel in 15% hydrochloric acid at 105±2°C by weight loss method.
UMOD was found to be the best corrosion inhibitor. It exhibited 94%
inhibition efficiency for N-80 steel and 72% inhibition efficiency for mild
steel (Quraishi et al 2001b). New class of corrosion inhibitor such as namely
2,5-bis (4-dimethylaminophenyl)-1,3,4-thiadiazole (DAPT) was synthesized
and its inhibiting action on the corrosion of mild steel in 1M hydrochloric acid
at 30°C was investigated (Bentiss et al 2001).
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The inhibition effect of 3, 6-bis (2-methoxyphenyl)-1, 2-dihydro-
1,2,4,5-tetrazine (2-MDHT) on the corrosion of mild steel in acidic media has
been investigated by weight loss and various electrochemical techniques.
Results obtained reveal that this organic compound is a very good inhibitor. 2-
MDHT is able to reduce the corrosion of steel more effectively in 1 M HCl
than in 0.5 M H2SO4 (L. Elkadi et al 2000). The influence of 3-methyl-2,6-
diphenyl piperidin-4-one (MDPO) and 2-phenyl decahydroquinoline-4-one
(PDQO) synthesized in the laboratory and corrosion inhibition of mild steel in
sulphuric acid has been studied using weight loss and various electrochemical
AC and DC monitoring techniques. Both the compounds inhibit excellently
the corrosion of mild steel in sulphuric acid (S.Muralidharan et al 2000).
A new corrosion inhibitor, namely, 3,5-bis (2-thienyl)-4-amino-
1,2,4-triazoles (2-TAT) has been synthesized and its inhibiting action on the
corrosion of mild steel in acid baths (1 M HCl and 0.5 M H2SO4) has been
investigated by various corrosion monitoring techniques, such as corrosion
weight loss tests and electrochemical impedance spectroscopy. 2-TAT is able
to reduce the steel corrosion more effectively in 1 M HCl than in 0.5 M H2SO4
(F. Bentiss et al 1999).
A new class of corrosion inhibitors, namely,3,5-bis(n-pyridyl)-4-
amino-1,2,4-triazoles has been synthesized and its inhibiting action on the
corrosion of mild steel in 1M hydrochloric acid has been studied (Mernari et al
1998). Attempts were made to develop effective corrosion inhibitors of
synthesized four macrocyclic compounds by condensing o-ethylene diamine
and o-phenylene diamine with ethylacetoacetate and succinic acid. Their
inhibiting action was evaluated on corrosion of mild steel in hydrochloric acid
by weight loss and potentiodynamic polarization methods (Rawat et al 1998).
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This literature survey shows that numerous synthesized organic
compounds were used as corrosion inhibitor in the acid environment. Very few
mannich bases was synthesized and used as the corrosion inhibitor for mild
steel in the acid environment.
1.12 Scope of the investigation
Mild steel is one of the well-known materials used in chemical and
allied industries for handling of acids, alkali and salt solutions due to its low
cost and easy availability for fabrication of reaction vessels, tanks and pipes
etc. But its susceptibility to corrosion in acid medium is the major obstacles
for its use on larger scale (S. Bilgic et al 2001).
Mineral acids are mainly used in chemical industry particularly in
pickling process to remove oxide scale from metal surface at elevated
temperature (H. P. Sachin, et al 2009). Almost all the industries find it difficult
to control corrosion of mild steel in mineral acid environment.
The use of organic inhibitors is one of the practical methods used to
control dissolution of metal in acid medium. The corrosion inhibition of these
compounds is attributed to their molecular structure. Adsorption of these
compounds over metallic surface is featured by its planarity and lone pair of
electrons present in hetero atom. The inhibition of corrosion process is mainly
decided by the formation of donor-acceptor surface complex between free or π
electrons of an inhibitor and vacant d orbital of metal atom (J.L Mora-
Mendoza et al 2002).
A survey of literature reveals that the selection of inhibitor is mostly
based upon the type and number of hetero atoms like N, O and S present in
organic compounds. The type and amount of hetero atoms present and the
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length of compound decides the inhibition efficiency. Many of the available
organic compounds were used as corrosion inhibitors for mild steel corrosion
(K.Parameshwari et al 2010). Some of the newly synthesized organic
compounds were used as corrosion inhibitor for mild steel corrosion (Athareh
dadgarinezhak et al 2011 and Nada F. Atta et al 2010) and also newly
synthesized mannich base was used as corrosion inhibitor for mild steel
corrosion (Duan Xiao-yun et al 2008).
The literature survey reveals that the most of available and
synthesized organic compounds inhibit mild steel corrosion at room
temperature only. It is very much essential to find out suitable inhibitor to
inhibit mild steel corrosion at elevated temperature.
In the present investigation three mannich bases are proposed to be
synthesized by using cyclohexylamine, formaldehyde and urea/acetamide/
acetone. Synthesis of mannich base has increase the length of parent
compound and hence hetero atoms present in the structure. The following
synthesized mannich bases are going to be tested as corrosion inhibitor to find
out the inhibition efficiency of inhibitors on mild steel corrosion in acid
medium at elevated temperature.
� Aminocyclohexane-N’-methylurea.
� Aminocyclohexane-N’-methylacetamide.
� Aminocyclohexane-N’-methylacetone.
The main objectives of the investigations are
1. Characterization of the synthesized mannich bases using FT-IR
and NMR techniques.
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2. To evaluate the inhibition efficiency of synthesized mannich
bases on mild steel corrosion in 1 N HCl and 1 N H2SO4
medium.
3. To study the adsorption isotherms of synthesized mannich bases
on mild steel corrosion in 1 N HCl and 1 N H2SO4 medium.
4. To study the activation and adsorption parameters of these
inhibitors in 1 N HCl and 1 N H2SO4 environment.
5. To study the morphology of the inhibited mild steel surface.
It is proposed to carry out the characterization of synthesized
mannich bases for structural elucidation. It is planned to carry out a study on
synthesized mannich bases for mild steel corrosion in 1 N HCl and 1 N H2SO4
environments by weight loss, potentiodynamic polarization and AC impedance
methods to evaluate the effectiveness of inhibitors. It is also planned to carry
out the surface examination studies by FT-IR spectral analysis and scanning
electron micrograph (SEM).