corrosion inhibition arabian journal

8/10/2019 Corrosion Inhibition Arabian Journal

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S. A. Umoren and M. M. Solomon

July 2010 The Arabian Journal for Science and Engineering, Volume 35, Number 2A 115

EFFECT OF HALIDE IONS ADDITIVES ON THE

CORROSION INHIBITION OF ALUMINUM IN HCl BYPOLYACRYLAMIDE

S. A. Umoren* and M. M. Solomon

Department of Chemistry, Faculty of Science, University of Uyo, PMB 1017 Uyo, Nigeria

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*Corresponding Author:

E-mail: [email protected]

Phone: +234-802-3144-384______________________________________________________________________________

Paper Received May 19, 2009; Paper Revised August 18, 2009; Paper Accepted October 7, 2009


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The Arabian Journal for Science and Engineering, Volume 35, Number 2A July 2010116

ABSTRACT

The influence of bromide and iodide ions on the inhibitive effect of polyacrylamide (PA) on aluminum corrosion

in HCl solution was studied using weight loss, hydrogen evolution, and thermometric techniques at 30 and 60oC. The

results show that the halide additives synergistically increased the inhibition efficiency of polyacrylamide. The

increase in inhibition efficiency (%I)was found to be more pronounced in I-than Br

-ions. The observed difference

in behavior of both halide additives could be linked to the differences in their atomic radii as well aselectronegativity. The values of synergism parameter (S1)obtained for the halides are greater than unity, suggesting

that the improved inhibition efficiency of polyacrylamide caused by the addition of the halide ions is due to

synergistic effect. Corrosion inhibition could be attributed to adsorption of inhibitor molecules on the Al surface via

physical mechanism. The adsorption process followed the kinetic-thermodynamic model of El-Awady adsorption

isotherm. These results were further corroborated by kinetic and thermodynamic parameters for corrosion and

adsorption processes evaluated from experimental data.

Key words: polyacrylamide, aluminum, corrosion, adsorption isotherms, synergism, halides


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EFFECT OF HALIDE IONS ADDITIVES ON THE CORROSION INHIBITION OF

ALUMINUM IN HCl BY POLYACRYLAMIDE

1. INTRODUCTION

Aluminum is an abundant metallic chemical element which is widely used throughout the world for a wide range

of applications. It relies on the formation of a strong adherent and a continuous film on its surface for corrosion

resistance on exposure to the atmosphere. However, in some instances, aluminum may be exposed to a highconcentration of acids and alkalis, which usually leads to the dissolution of the passive films. It is therefore

necessary for it to be protected from corrosion in these environments. The appropriate way is to isolate the metal

from corrosive agents present in solutions by the use of corrosion inhibitors. Organic compounds containing oxygen,

nitrogen, or sulphur have been found suitable for this purpose, and their action is attributed to adsorption onto the

surface of the metal/solution interface [14]. Polymers, both naturally occurring and synthetic ones, have been

studied as metal corrosion inhibitors. Interest in them arose from their stability in acid media, low cost, and thepresence of multiple adsorption sites in their molecular structure for bonding with a metal surface [5]. Among those

studied to date include polyethylene glycol, polyvinyl alcohol, polyacrylamide, carboxyl methylcellulose, poly

(aminoquinone), polyethylene glycol methyl ether (PEGME), polyvinylpyrrolidone and polyethylenimine, poly

(4-vinylpyridne), and poly (diphenylamine) [613], to mention but a few.

It has been established that the presence of halide ions in solution enhances the inhibition efficiency of most

inhibitors. It is generally accepted that the halide ions facilitate adsorption of organic cations during metal corrosion

by forming intermediate bridges between the metal surface and the positive end of the organic inhibitor. Some

authors have reported the synergistic effect of halide ions in combination with some organic compounds [1419] and

naturally occurring substances [2023]. In each case, the synergistic effect of the halide ions increases in the order

Cl-< Br

-< I

-. This may be attributed to the atomic radii, as well as the electronegativity, which increase in the order

Cl-< Br

-< I

-and Cl

-> Br

-> I

-, respectively, in the halogen series.

In our laboratory, a series of reports have been highlighted on the synergistic effect of halide ions on the

corrosion of aluminum in acidic environment using naturally occurring substances [2426] and polymeric materials

in acidic/alkaline media [2730]. As part of our contribution to the growing interest of exploring polymers as

corrosion inhibitors, the present work reports the inhibitive effect of polyacrylamide on aluminum corrosion in HCl,

including the synergistic effect of bromide and iodide ions using gravimetric, hydrogen evolution, and thermometric

techniques at 30 and 60oC.

2. EXPERIMENTAL

2.1. Materials

An aluminum sheet supplied by First Aluminum Company, Nigeria Limited, Port Harcourt, Nigeria with the

following composition (Wt %): Si 0.22851 , Fe 0.57828 , Cu 0.008090 , Mn 0.27315 , Mg 0.0731,

Zn 0.10291, Ti 0.01229 , Cr 0.005732, Ni 0.00548, V 0.01229, Pb 0.07663, and the balance Al was used

for the study. Each sheet was 0.9 mm in thickness and was mechanically press cut into 5 cm x 4 cm coupons. These

coupons were used without further polishing. However, for surface treatment, they were degreased in absolute

ethanol, dried in acetone, and stored in desiccators free from moisture before being used in the corrosion studies.

Polyacrylamide (Hi-tek polymers, Japan) [Mn: 50,000 g mol-1

] was used as the inhibitor in the concentration range of

1 x 10-5 1 x 10-4 M. The hydrochloric acid (HCl) (Sigma-Aldrich) concentration was 0.5M prepared fromanalytical grade reagent. Potassium bromide and iodide (KBr, KI, Sigma-Aldrich) were used in the concentrationrange of 1 5mM. For the systems containing PA and halide mixtures, a fixed concentration of KBr and KI (5mM)

was used while the concentration of PA was varied. All preparations were made using doubly distilled water. The

study was conducted at 30 and 60oC and the temperature was maintained using a thermostated water bath.

2.2. Gravimetric Technique

In the gravimetric measurements, clean weighed aluminum coupons were immersed completely in 250ml beakers

containing 200ml of the corrodent (HCl) and inhibitors with the aid of glass rods and hooks. The beakers wereplaced in a constant thermostated bath maintained at 30 and 60

oC. The coupons were retrieved at 24-hour intervals

progressively for 168 hours (7 days), and immersed in 70% nitric acid for 2 minutes at room temperature. They

were scrubbed with a bristle brush under running water, dried in acetone, and weighed [31].

The differences in weight of the coupons before and after immersion in different test solutions were taken as theweight loss. It was then used to calculate the corrosion rate using the formula [32]


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CRWL(mmy) =87.6W

At (1)

where Wis the weight loss (mg ), is the density of the specimen (gcm-3

),Ais the area of the specimen (cm2), and t

is the exposure time (h).

The inhibition efficiency of PA and PA in combination with halides acting as inhibitor in 0.5 M HCl wascalculated using the following expression:

% 100blank inh

blank

CR CRI x

CR

=

(2)

where CRblank and CRinh are the corrosion rate in the absence and presence of the PA and PA halide mixtures,

respectively.

2.3. Hydrogen Evolution Technique

The apparatus and procedure followed was similar to that previously described [2729]. The gasometricassembly is essentially an apparatus that measures the volume of gas evolved from a reaction system. Itconsists essentially of a two-necked round bottom flask which serves as the reaction medium containing

the corrodent and the metal coupons. Other components are a separating funnel, a burette fitted withtaps, and an outer glass jacket that serves as a water condenser. In this technique, 100mL of different testsolutions were introduced into a reaction vessel, which was connected to a burette through a delivery tube. The

initial volume of air in the burette was recorded. Aluminum sheets of dimensions 5 cm x 4 cm were carefully

dropped into the test solution of HCl and the reaction vessel was quickly closed to avoid any escape of hydrogen gas.

The volume of hydrogen gas evolved from the corrosion reaction was monitored by the depression (in cm3) in the

paraffin oil level. This depression was monitored at fixed time intervals. From the volume of hydrogen gas evolved,

the corrosion rate was computed using the following expression:

t iH

t i

V VCR

t t

=

(3)

where Vtand Viare the volumes of hydrogen evolved at time ttand ti, respectively.

The inhibition efficiency (%I) was calculated using Equation (2).

2.4. Thermometric Technique

The apparatus consists essentially of a two-necked reaction flask to which a thermometer is fitted. The reaction

flask is lagged to prevent heat losses to the surroundings. In this technique, the corrodent (HCl) concentration was

kept at 2M and 50 ml of the test solution was used. Aluminum coupons were completely immersed in HCl without

the inhibitor and with different concentrations of the inhibitors and inhibitorrhalide mixtures. The temperaturechanges with the dissolution of aluminum were followed accordingly at various time intervals by means of a

thermometer (0100oC) measured to the nearest + 0.05

oC.

This method enabled the computation of the reaction number (RN)defined by the equation

(4)

where Tmand Tiare the maximum and initial temperatures, respectively, and tis the time (min) taken to reach the

maximum temperature. The inhibition efficiency (%I)was calculated from percentage reduction in the RN given by

Equation (5)

% 100aq wi

aq

RN RNI x

RN

= (5)

whereRNaqandRNwiare the reaction numbers of in the absence and presence of the additives, respectively.

3. RESULTS AND DISCUSSION

3.1. Gravimetric Measurements

The weight loss method has proved to be useful in monitoring the corrosion of metals in different aqueous mediabecause of its simplicity and reliability. Some authors have reported comparable results between the weight loss

technique and other techniques of corrosion monitoring, such as polarization measurements, hydrogen evolution,

( )/mino m iT TRN C t

=


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thermometric methods, and electrochemical impedance spectroscopy [3336]. The loss in weight of aluminum in 0.5

M HCl in the absence and presence of polyacrylamide, halides, and polyacrylamideehalides mixtures after 168

hours of immersion was evaluated at temperatures of 30 and 60oC. Figure 1 shows the plot of weight loss versus time

for the dissolution of aluminum in the absence and presence of polyacrylamide, halides, and polyacrylamidehalidesmixtures at (a) 30

oC and (b) 60

oC. Inspection of the figure shows a decrease in weight loss of aluminum in the

presence of halides and polyacylamide compared to the blank. Further reduction in weight loss was observed in the

presence of PA in combination with halides with the most significant decrease obtained for PAKI mixtures at both

temperatures. Examination of the figure also reveals that the weight loss of aluminum in the absence and presence of

halides, polyacrylamide, and polyacrylamidehalides mixtures increases with increase in temperature.

Figure 1: Variation of weight loss against time for aluminum corrosion in 0.5M HCl in the absenceand presence of halides, PA and PA halide mixtures at (a) 30 and (b) 60 oC


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The calculated values of corrosion rate and inhibition efficiency for the different systems studied from the weight

loss measurements are presented in Table 1. Results given in the table clearly indicate that the corrosion rate of

aluminum in HCl was found to reduce in the presence of polyacrylamide compared to its absence. This is an

indication that PA afforded the inhibition of acid induced corrosion of aluminum. Further reduction in corrosion ratewas also observed on addition of halide ions to polyacrylamide. It is also seen in the table that corrosion rate in the

presence of PA alone and on addition of the halides decreases with increase in PA concentration but increases with a

rise in temperature. At the temperatures studied, the reduction in corrosion rate in the presence of the halides was

found to be in the order PA + KBr > PA + KI.

Table 1. Calculated Values of Corrosion Rate, Inhibition Efficiency and for Aluminum Corrosion in 0.5M

HCl for Different Systems at 30 and 60oC from Weight Loss Measurements

Systems/concentration Corrosion rate (mm/yr) Inhibition efficiency (I %)

30oC 60

oC 30

oC 60

oC

Blank 2.69 5.87 - -

5mM KBr 0.99 3.45 62.9 41.2

5mM KI 0.83 2.57 69.2 56.2

1 x 10-5

M PA 1.32 3.83 51.3 34.83 x 10

-5M PA 1.13 3.15 57.6 46.3

5 x 10-5

M PA 1.08 2.90 60.2 50.6

7 x 10-5

M PA 1.02 2.76 61.8 52.9

1 x 10-4

M PA 0.97 2.55 63.6 56.6

1 x 10-5

M PA + 5mM KBr 0.86 3.17 65.4 45.9

3 x 10-5

M PA + 5mM KBr 0.82 2.98 68.0 49.2

5 x 10-5

M PA + 5mM KBr 0.75 2.73 69.5 52.6

7 x 10-5

M PA + 5mM KBr 0.73 2.72 72.2 53.5

1 x 10-5M PA + 5mM KBr 0.58 2.09 78.4 64.4

1 x 10-5

M PA + 5mM KI 0.69 2.48 74.3 57.8

3 x 10-5

M PA + 5mM KI 0.68 2.36 74.7 59.8

5 x 10-5

M PA + 5mM KI 0.65 2.18 75.8 62.9

7 x 10-5

M PA + 5mM KI 0.61 2.02 77.3 65.6

1 x 10-4

M PA + 5mM KI 0.50 1.78 81.0 69.7

The inhibition efficiency was observed to increase with increase in concentration of polyacrylamide and was

greatly enhanced on addition of the halide ions. The required enhancement of inhibition efficiency of polyacrylamide

on the addition of halide ions was more pronounced with iodide ion compared to bromide ion. This may be attributedto the differences in atomic radii as well as electronegativity of the halide ions, which increases in the order I

-> Br

->

Cl-and Cl

-> Br

-> I

- , respectively. Inspection of the results presented in the table further revealed that inhibition

efficiency decreases with increase in temperature, suggesting a physisorption mechanism.

It is generally accepted that the inhibitive power of polymers are related to cyclic rings and heteroatoms (oxygen

and nitrogen) which are the centers of adsorption. The inhibition of aluminum corrosion by polyacrylamide could be

attributed to adsorption of PA through the oxygen and nitrogen atoms on the aluminum surface, which creates a

barrier isolating the metal from attack by the aggressive anions of the acid. In acid solution, PA may exist in both

protonated and molecular species, which can affect the corrosion process to different extents depending on their

relative proportion. The decrease in inhibition efficiency with increase in temperature observed in this study is an

indication that PA molecules were physically adsorbed onto the metal surface. Physical adsorption is a result of

electrostatic interactions between charged metal surface and protonated species in the bulk solution, while

chemisorption is characterized by adsorption of molecular species (uncharged molecules) on a heterogeneous

surface. It is pertinent to conclude that adsorption of protonated PA species by electrostatic interaction with chloride

ion adsorbed on the aluminum surface is the predominating factor rather than the participation of molecular species

in this study.


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3.2. Hydrogen Evolution Technique

Experiments were also undertaken using the hydrogen evolution technique. Figure 2 shows variation of volume

of H2 evolved with time for aluminum dissolution in 2M HCl in the absence and presence of halides,

polyacrylamide, and polyacrylamidehalides mixtures at (a) 30oC and (b) 60

oC. From the figures, it can be seen that

the volume of H2evolved increases with time. The volume of H2evolved was observed to appreciably reduce in the

presence of polyacrylamide and halides compared to the free acid solution. Further reduction in the volume of H2evolved was observed on the introduction of Br

- and I

- ions into the solution containing PA. The volume of H2

evolved is also seen to increase with increase in temperature. This shows that the polyacrylamide alone and in

combination with halides retards the dissolution rate of aluminum in the acidic solution more at lower temperatures

than at higher temperatures. The figure also revealed that the most remarkable reduction in the volume of H2evolved

was observed with a polyacrylamideiodide ion combination at both 30 and 60oC.

Figure 2: Variation of volume of H2evolved against time for aluminum, corrosion in 2M HClin the absence and presence of halides, PA and PA halide mixtures at (a) 30 and 60oC

The corrosion rates of aluminum in the absence and presence of various additives were calculated using Equation

(4) and the corresponding values at different temperatures are presented in Table 2. The results show that corrosionrates increase with increase in temperature but decrease in the presence of halides, polyacrylamide, and

polyacrylamidehalides mixtures. The inhibition efficiency (%I) obtained from the hydrogen evolution

measurements are also presented in Table 2. From the table, inhibition efficiency is enhanced on the addition of

halide ions to polyacrylamide but decreases with increase in temperature. This is in agreement with the trend

reported for weight loss measurement. The inhibition efficiency increases with increase in the concentration of PA. It

is also found to increase markedly on the addition of Br- and I

- ions, with the most profound effect noticed in the

presence of I-ions.


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Table 2. Calculated Values of Corrosion Rate, Inhibition Efficiency for Aluminum Corrosion in 1M HCl

for Different Systems at 30 and 60oC from Hydrogen Evolution Measurements

Systems/concentration Corrosion rate (cm3/min) Inhibition efficiency (I %)

30oC 60

oC 30

oC 60

oC

Blank 3.36 10.23 - -

5mM KBr 2.33 9.55 30.65 6.65

5mM KI 1.23 7.38 62.86 27.86

1 x 10-5

M PA 1.39 6.90 58.63 32.55

3 x 10-5

M PA 0.94 6.70 72.02 34.51

5 x 10-5

M PA 0.78 6.60 76.79 35.48

7 x 10-5

M PA 0.49 5.75 85.42 43.79

1 x 10-4

M PA 0.33 5.02 90.00 50.93

1 x 10-5

M PA + 5mM KBr 0.42 6.06 87.50 40.75

3 x 10-5

M PA + 5mM KBr 0.40 5.48 88.10 46.42

5 x 10-5M PA + 5mM KBr 0.36 5.33 89.29 47.89

7 x 10-5

M PA + 5mM KBr 0.20 4.38 94.04 57.15

1 x 10-5

M PA + 5mM KBr 0.20 3.63 94.04 64.52

1 x 10-5

M PA + 5mM KI 0.37 5.60 88.99 45.26

3 x 10-5

M PA + 5mM KI 0.23 4.80 94.05 53.08

5 x 10-5

M PA + 5mM KI 0.16 4.10 95.24 59.92

7 x 10-5

M PA + 5mM KI 0.13 3.7 96.13 63.39

1 x 10-4

M PA + 5mM KI 0.09 3.18 97.32 68.91

3.3. Thermometric Technique

The dissolution of aluminum in 1M HCl in the absence and presence of halides, polyacrylamide, and

polyacrylamidehalides mixtures was also studied using the thermometric technique. Figure 3 shows the

temperaturetime curve for aluminum corrosion in 2M HCl in the absence and presence of the additives.

Examination of the figure reveals that in the absence of the additives (blank), the temperature increases gradually to

attain a maximum value due to the exothermic corrosion reaction, and gradually decreases again. However, in the

presence of the additives, an interesting behavior was observed in which the temperature accompanying the

dissolution of aluminum was lower compared to the blank and took a longer time to reach the maximum value. This

behavior reflects the high inhibition efficiency (%I) obtained for the PA and PAhalide mixtures toward aluminum

dissolution in the acidic medium.

Figure 3: Variation of temperature with time for aluminum, corrosion in 2M HCl in theabsence and presence of halides, PA and PA halide mixtures


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The calculated values of reaction number (RN) and percentage reduction in reaction number (inhibitionefficiency) for the different systems studied from the thermometric measurement are given in Table 3. It is clear

from the table that reaction number decreased more in the presence of the additives compared to the blank solution.

The table also shows that reaction number decreases with increase in the concentration of the additives. Thepercentage reduction in reaction number (inhibition efficiency) followed the trend reported for weight loss and

hydrogen evolution measurements. It increases with increase in concentration of PA and was appreciably improved

on addition of the halide ions, the iodide ion giving the most remarkable improvement.

Table 3: Calculated Values of Reaction Number and Percentage Reduction in Reaction Number (Inhibition

Efficiency) for Aluminum Corrosion in 1M HCl for Different Systems from Thermometric Method

Systems/concentration Reaction number (oC/min) Inhibition efficiency (I %)

Blank 1.53 -

5mM KBr 0.54 64.71

5mM KI 041 73.01

1 x 10-5

M PA 0.61 60.13

3 x 10-5

M PA 0.53 65.34

5 x 10

-5

M PA 0.48 68.637 x 10

-5M PA 0.43 71.89

1 x 10-4

M PA 0.38 75.16

1 x 10-5

M PA + 5mM KBr 0.45 70.58

3 x 10-5

M PA + 5mM KBr 0.43 71.89

5 x 10-5

M PA + 5mM KBr 0.40 73.86

7 x 10-5

M PA + 5mM KBr 0.38 75.16

1 x 10-5

M PA + 5mM KBr 0.33 78.43

1 x 10-5M PA + 5mM KI 0.23 84.96

3 x 10-5

M PA + 5mM KI 0.21 86.27

5 x 10-5

M PA + 5mM KI 0.20 86.93

7 x 10-5

M PA + 5mM KI 0.19 87.58

1 x 10-4

M PA + 5mM KI 0.19 87.58

3.4. Effect of Halide Ions and Synergistic Consideration

Results obtained from the study show that the inhibition efficiency of PA was remarkably improved on addition

of Br-and I

-ions. For instance, inspection of Table 1 reveals that in the presence of the highest concentration of PA

studied (1x10-4

M), the inhibition efficiency is 63.7% at 30oC. On addition of 5mM KBr and KI, the inhibition

efficiency increases to 78.4% and 81%, respectively. Similar observations, which were ascribed to synergistic effect,

had been reported [16,18,23] by other authors. The role of the anions in improving the inhibition efficiency oforganic compounds has been explained as due to the specific adsorption of these anions, which then creates an

excess negative charge towards the solution and favors more adsorption of the organic cations [37]. Other authors

opined that the feasible adsorption of organic cations in the presence of halide ions may be due to the formation of an

intermediate bridge, the negative ends of the halide metal dipoles being oriented towards solution [38]. The

improved inhibition efficiency of PA in the presence of the halide ions increases in the order I- > Br-. This may be

explained on the basis that iodide ions play an essential role in reducing the repulsive forces between PA polycation

head groups and stabilized physical adsorption, so a close packed layer at the metal surface may be formed [39]. Thestrong chemisorption of iodide ions on the metal surface is responsible for the synergism effect of iodide ion in

attraction with cations. In addition to electrostatic effect, some covalent bonding to the metal must be involved. The

large size and ease of polarizability of iodide facilitate the electron pair bonding. In short, the adsorption of inhibitors

between the metal and solution interface is usually accepted as the formation of electrostatic or covalent bonding

between the adsorbates and metal surface atoms. In other words, halide ions will adsorb on the metal surface by

creating oriented dipoles with their negative ends towards solution, thus increasing the adsorption of the organiccations on the dipoles, and as a result, a positive synergistic effect arises.


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The synergistic parameter (S1) was evaluated from inhibition efficiency values obtained from the three methodsusing the relationship initially given by Aramaki and Hackermann and reported elsewhere [18,20]:

1 21 '

1 2

1

1

IS

I

+

+

=

(6)

where I1+2 = I1 + I2, I1 is inhibition efficiency of the halides, I2 is the inhibition efficiency of inhibitor(polyacrylamide), andI

1+2is measured inhibition efficiency for the inhibitor in combination with the halides.

The values obtained are listed in Table 4. S1approaches 1 when no interaction between the inhibitor compounds

exist, while S1>1 points to a synergistic effect. In the case of S1< 1, the antagonistic interaction prevails, which maybe attributed to competitive adsorption. The results in the table show that the synergistic parameter (S1) for the

halides is greater than unity, suggesting the enhanced inhibition efficiency caused by the addition of halide ions to

the polyacrylamide is due to synergistic effect.

Table 4. Synergism Parameter for the Halides from the Three Methods at 30oC

Halides Synergism parameter (S1)

Weight loss Hydrogen evolution Thermometric

KBr 1.88 1.28 1.81

KI 1.89 1.89 1.82

3.5. Adsorption Isotherm

As far as the inhibition process is concerned, it is generally assumed that the adsorption of the inhibitors at the

metal-aggressive solution interface is the first step in the inhibition mechanism [40]. Considering the dependence of

inhibition efficiency on inhibitor concentration, as shown in Table 1, it seems probable that the inhibitor acts by

adsorbing and blocking the available active center on the aluminum surface. In other words, the inhibitor decreasesthe active center for aluminum dissolution. The values of degree of surface coverage from weight loss

measurements, (= %I/100), assuming a direct relationship between surface coverage and inhibition efficiency fordifferent concentrations of inhibitor (PA) and inhibitorhalide mixtures at 30 and 60

oC studied, have been used in

explaining the adsorption process. Langmuir adsorption isotherm was tested for its fit to the experimental data. The

characteristic of Langmuir adsorption isotherm is given by the equation

1

ads

C CK

= + (10)

The plot of C/as a function of C (inhibitor concentration) is shown in Figure 4. Although the figure gives alinear plot with high correlation coefficient (0.99), the deviation of the slopes from unity (Table 4) (for ideal

Langmuir isotherm) can be attributed to the molecular interaction among the adsorbed inhibitor species, a fact that

was not taken into consideration during the derivation of Langmuir equation. This deviation necessitated the fitting

of the experimental data into El-Awadys kinetic thermodynamic model. The isotherm is expressed as

log1

Log LogK y C

= +

(11)

where y is the number of inhibitor molecules occupying one active site, is the surface coverage, C is the

concentration , K is the equilibrium constant of the adsorption process, and Kads = K

1/y

where 1/y represents thenumber of active sites of the metal surface occupied by one molecule of inhibitor [41]. Kads is related to the free

energy of adsorption Gadsby the equation

1exp

55.5

o

adsads

GK

RT

=

(12)

where R is the molar gas constant, T is the absolute temperature, and 55.5 is the concentration of water in solution

expressed in mol dm-3

.


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Figure 4: Langmuir adsorption isotherm plot for aluminum corrosion in 0.5M HCl

for PA and PA halide mixtures at different temperatures

Figure 5 shows the plot of El-Awadys kinetic thermodynamic adsorption isotherm model. Linear plots were

obtained indicating that the adsorption of PA alone and in combination with halide ions can be approximated by El-

Awadys kinetic thermodynamic model. Adsorption parameters derived from the model are given in Table 5.

Results in the table show that the values of K are very low and decrease with increasing temperature, suggesting

physical adsorption of the PA and PAhalides on the aluminum surface. The low values of K also indicate weak

interaction between the adsorbed species. The values of 1/yobtained are more than unity, indicating that the PA and

PAhalides are attached to more than one active site on the aluminum surface. These results may suggest that the

inhibitor was vertically adsorbed on the aluminum surface while the occurrence of flat adsorption by the studied

compound (PA) may be completely ruled out [41].

Table 5. Adsorption Parameters from Langmuir and El-Awady et al.Isotherms for Aluminum

in 0.5M HCl Containing PA and PA Halide Mixtures

Systems Temperature (oC) Langmuir El-Awady

Slope R2 G

o

ads

(kJ/mol

K 1/y R2

PA 30 3.35 0.99 -5.09 1.48 14.71 0.90

60 4.25 0.99 -4.22 0.50 11.11 0.89

PA + KBr 30 2.79 0.99 -5.16 1.56 15.63 0.92

60 3.28 0.99 -5.15 1.04 10.42 0.95

PA + KI 30 2.71 0.99 -5.69 2.49 25.00 0.84

60 3.09 0.99 -4.29 0.53 15.87 0.91

0

5

10

15

20

25

1 3 5 7 10

C x 10-5 (M)

C/x10-5

(M)

PA (30oC)

PA (60oC)

PA + KBr (30oC)

PA + KBr (60oC)

PA + KI (30oC)

PA + KI (60oC)

0

5

10

15

20

25

1 3 5 7 10

C x 10-5 (M)

C/x10-5

(M)

PA (30oC)

PA (60oC)

PA + KBr (30oC)

PA + KBr (60oC)

PA + KI (30oC)

PA + KI (60oC)


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Figure 5: El-Awady adsorption isotherm plot for aluminum corrosion in 0.5M HClfor PA and PA halide mixtures at different temperatures

3.6. Kinetic and Thermodynamic Studies

The mode of inhibitor adsorption on the metal surface can be probed by varying the temperature during the

corrosion process. The use of two temperatures to establish the mode of inhibitor adsorption on the surface of a

corroding metal has been reported by some authors [42 44] and has been found to be adequate. The values of the

activation energy Eawere calculated using the Arrhenius equation

2

1 1 2

1 1log

2.303

aCR E

CR R T T

=

(13)

where CR1and CR2are the corrosion rates at temperatures T1and T2, respectively. The values obtained are presented

in Table 6. The result shows that Ea increases markedly in the presence of polyacrylamide and polyacrylamidehalide mixtures compared to the blank. The higher value of Ea in the presence of polyacrylamide and

polyacrylamidehalide mixtures compared to the blank is attributed to physical adsorption mechanism [45]. As a

consequence, corrosion inhibition is assumed to occur primarily through physical adsorption of the inhibitor

molecules on the aluminum surface, giving rise to the deactivation of these surfaces to hydrogen atom

recombination. A similar result has been reported in our earlier publication [42]. An estimate of heat of adsorption

(Qads) was obtained from the trend of surface coverage with temperature as follows:

12 1 1 2

2 1 2 1

2.303 log log1 1

Xads

T TQ R x kJmol T T

=

(14)

where 1and2 are are degrees of surface coverage at temperature T1 and T2,respectively, and R is the molar gas

constant. The calculated values of heat of adsorption are given in Table 6. From the table, it is clear that Q adsvalues

are negative. The negative values of Qadsshow that the adsorption and, hence, inhibition efficiencies decreased with

a rise in temperature, thus also supporting the proposed physisorption mechanism. The free energy of adsorption was

obtained from the relationship expressed in Equation (12) and the calculated values are given in Table 5. From the

table, it is seen that the values of Goadsincreased with an increase in temperature, a phenomenon which indicatesthat the adsorption of the inhibitor onto the aluminum surface was unfavorable with increasing experimental

temperatures as a result of desorption of adsorbed inhibitor from the metal surface. The values of Goadsare negative,which reveals the spontaneity of the adsorption process and the stability of the adsorbed layer on the aluminum

surface. Since the values of Goadsof 40 kJmol-1

are usually accepted as a threshold value between chemisorption

and physiosorption, the obtained values of Goadsfor the studied inhibitor are below 40 kJmol -1, which is consistentwith electrostatic interactions between charged molecules and charged metal, which are indicative of physiosorption.

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-5 -4.5 -4.3 -4.1 -4.0

Log C

Log(/1-)

PA (30oC)

PA (60oC)

PA + KBr (30oC)

PA + KBr (60oC)

PA + KI (30oC)

PA + KI (60oC)

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-5 -4.5 -4.3 -4.1 -4.0

Log C

Log(/1-)

PA (30oC)

PA (60oC)

PA + KBr (30oC)

PA + KBr (60oC)

PA + KI (30oC)

PA + KI (60oC)


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Table 6. Calculated Values of Activation Energy (Ea) and Heat of Adsorption (Qads)

for Aluminum in 0.5M HCl for the Different Systems

Systems/concentration Activation energy (Ea) (kJ/mol) Heat of adsorption (Qads) (kJ/mol)

30 60oC 30 60

oC

Blank 26.68 -5mM KBr 44.29 -24.47

5mM KI 43.20 -18.48

1 x 10-5

M PA 43.90 -29.82

3 x 10-5

M PA 43.33 -23.06

5 x 10-5

M PA 43.37 -22.58

7 x 10-5

M PA 43.43 -23.05

1 x 10-4

M PA 43.06 -21.37

1 x 10-5

M PA + 5mM KBr 44.15 -27.49

3 x 10-5M PA + 5mM KBr 43.93 -25.57

5 x 10-5

M PA + 5mM KBr 43.45 -24.82

7 x 10-5

M PA + 5mM KBr 43.50 -21.93

1 x 10-5

M PA + 5mM KBr 43.16 -19.31

1 x 10-5

M PA + 5mM KI 44.28 -2.25

3 x 10-5

M PA + 5mM KI 43.57 -21.69

5 x 10-5

M PA + 5mM KI 43.42 -20.89

7 x 10-5

M PA + 5mM KI 42.89 -18.91

1 x 10-4

M PA + 5mM KI 42.76 -16.86

4. CONCLUSIONS

Polyacrylamide was found to be an efficient inhibitor of aluminum corrosion in HCl at lower temperatures.

Inhibition efficiency increased with increase in the concentration of polyacrylamide and synergistically increased on

addition of halide (Br- and I

-) ions with the most profound effect obtained with iodide ion. The physisorption

phenomenon is proposed from the kinetic and thermodynamic parameters (Ea, Goads, and Qads) obtained. The

adsorption process for polyacrylamide and polyacrylamidehalide mixtures followed the kinetic-thermodynamic

model of El-Awady et al. The synergistic parameter evaluated was found to be greater than unity, which indicates

that the phenomenon of synergism exists between PA and halide ions.

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