INHIBITORINHIBITOR

IS A CHEMICAL SUBSTANCE THAT, WHEN IS A CHEMICAL SUBSTANCE THAT, WHEN ADDED IN A SMALL CONCENTRATION TO ADDED IN A SMALL CONCENTRATION TO AN ENVIRONMENT, EFFECTIVELY AN ENVIRONMENT, EFFECTIVELY DECREASES THE CORROSION RATEDECREASES THE CORROSION RATE

In the oil extraction and processing industries inhibitors have always been considered to be the first line of defense against corrosion

The efficiency of inhibitor can be The efficiency of inhibitor can be expressed by a measure of this expressed by a measure of this improvement :improvement :

Inhibitor efficiency (%) = Inhibitor efficiency (%) =

[CR[CRuninhibiteduninhibited – CR – CRinhibitedinhibited] ]

CRCRunhibited unhibited or or

[R[Rp,inhibited p,inhibited – R– Rp, uninhibitedp, uninhibited]]

RRp,inhibitedp,inhibited

CR : corrosion rate ; RCR : corrosion rate ; Rp p = polarization = polarization resistanceresistance

x 100%

x 100%

Inhibitor efficiency of Trans-Inhibitor efficiency of Trans-Cinnamaldehyde (TCA)Cinnamaldehyde (TCA)

TCATCA

ppmppmRRpp

ΩΩ. cm. cm22

Corrosion Corrosion current , current , mA.cmmA.cm-2-2

CorrosionCorrosion

Rate, Rate, mm.ymm.y-1-1

Effi-Effi-ciency, ciency,

%%

00 1414 1.551.55 18.018.0 00

250250 3535 0.620.62 7.27.2 6060

10001000 143143 0.1520.152 1.761.76 9090

50005000 223223 0.0970.097 1.131.13 9494

Inhibitors are chemicals that interact with Inhibitors are chemicals that interact with a metallic surface, or the environment this a metallic surface, or the environment this surface is exposed, giving the surface a surface is exposed, giving the surface a certain level of protection.certain level of protection.

Inhibitors often work by adsorbing Inhibitors often work by adsorbing themselves on the metallic surface by themselves on the metallic surface by forming a filmforming a film

Inhibitors slow corrosion process byInhibitors slow corrosion process by:: - - Increasing the anodic or cathodic Increasing the anodic or cathodic

polarization behavior (Tafel slopes)polarization behavior (Tafel slopes) - Reducing the movement or diffusion of - Reducing the movement or diffusion of

ions to the metallic surface ions to the metallic surface - Increasing the electrical resistance to - Increasing the electrical resistance to

the metallic surfacethe metallic surface

CLASSIFICATION OF INHIBITORCLASSIFICATION OF INHIBITOR

Passivating (anodic) inhibitorsPassivating (anodic) inhibitors Cathodic inhibitors Cathodic inhibitors Organic inhibitorsOrganic inhibitors Precipitation inhibitorsPrecipitation inhibitors Volatile corrosion inhibitorsVolatile corrosion inhibitors

Passivating (anodic) inhibitorsPassivating (anodic) inhibitors Passivating inhibitors cause a large anodic Passivating inhibitors cause a large anodic

shift of the corrosion potential, forcing the shift of the corrosion potential, forcing the metallic surface into the passivation metallic surface into the passivation range.range.

There are two types of passivating There are two types of passivating inhibitors:inhibitors:

- oxidizing anions, such as chromate, - oxidizing anions, such as chromate, nitrite and nitrate that can passivate steel nitrite and nitrate that can passivate steel in the absence of oxygen.in the absence of oxygen.

- nonoxidizing ions, such as phosphate, - nonoxidizing ions, such as phosphate, tungstate and molybdate that require the tungstate and molybdate that require the presence of oxygen to passivate the steelpresence of oxygen to passivate the steel

The reduction potential of chromate ion to solid Cr2O3 is possible to increase the corrosion potential of steel into its passivation region but not for molybdate and tungstate.

INHIBITING MECHANISM OF NITRITESINHIBITING MECHANISM OF NITRITES The formation of ferric oxides with the The formation of ferric oxides with the

participation of nitrite ions takes place participation of nitrite ions takes place according to the following reactions according to the following reactions (Joseph et al):(Joseph et al):

Formation of a lower oxide:Formation of a lower oxide:

NONO22--+8H+8H+++6e = NH+6e = NH44

+++2H+2H22OO

9 Fe(OH)9 Fe(OH)22 = 3Fe = 3Fe33OO44+6H+6H22O+6HO+6H+++6e+6e

2H2H22O = 2HO = 2H+++ 2OH+ 2OH--

9Fe(OH)9Fe(OH)22+NO+NO22-- = 3Fe = 3Fe33OO44+NH+NH44

+++2OH+2OH--+6H+6H22OO

Formation of a higher oxide:Formation of a higher oxide:

NONO22--+8H+8H+++6e = NH+6e = NH44

+++2H+2H22OO

6Fe(OH)6Fe(OH)22 = 2Fe = 2Fe33OO44+4H+4H22O+4HO+4H+++4e+4e--

2Fe2Fe33OO44+H+H22O=3(O=3(γγ-Fe-Fe22OO33)+2H)+2H+++2e+2e--

2H2H22O = 2HO = 2H+++ 2OH+ 2OH--

6Fe(OH)6Fe(OH)22+NO+NO22--=3(=3(γγFeFe22OO33)+NH)+NH44

+++3H+3H22O+2OHO+2OH--

Sodium nitrite is more effective in suppressing the Sodium nitrite is more effective in suppressing the aggressive properties of chlorides than are benzoate aggressive properties of chlorides than are benzoate and chromate. In the presence of sulfate, nitrate is and chromate. In the presence of sulfate, nitrate is slightly less effective than are chromate and slightly less effective than are chromate and benzoate. benzoate.

Protective properties of sodium nitrite as Protective properties of sodium nitrite as function of sodium chloride concentrationfunction of sodium chloride concentration

CCNaClNaCl

%mass%mass

CCNaNO2NaNO2

%mass%mass

CCNaNO2NaNO2

CCNaClNaCl

T, hrT, hr Pot.Pot.

mVmVState of State of metal metal surfacesurface

0.50.5 0.200.20 0.40.4 20.520.5 -11-11 corrodescorrodes

0.50.5 0.350.35 0.70.7 49.549.5 +271+271 protectedprotected

0.50.5 0.450.45 0.90.9 43.043.0 +285+285 protectedprotected

0.50.5 1.001.00 2.02.0 122.0122.0 +300+300 protectedprotected

Chromate-base inhibitors are the least-Chromate-base inhibitors are the least-expensive inhibitors and were used until expensive inhibitors and were used until recently in a variety of application (e.g. recently in a variety of application (e.g. recirculation cooling systems of internal recirculation cooling systems of internal combustion engines, refrigeration units combustion engines, refrigeration units and cooling towers). Sodium chromate, and cooling towers). Sodium chromate, typically in concentrations of 0.04 to 0.1% typically in concentrations of 0.04 to 0.1% was used for this applications. At higher was used for this applications. At higher temperatures or in fresh water with temperatures or in fresh water with chloride concentration above 10 ppm, chloride concentration above 10 ppm, higher concentration are required. If higher concentration are required. If necessary, sodium hydroxide is added to necessary, sodium hydroxide is added to adjust the pH to a range of 7.5 – 9.5. adjust the pH to a range of 7.5 – 9.5. If the If the concentration of chromate falls below a concentration of chromate falls below a concentration of 0.016% corrosion will be concentration of 0.016% corrosion will be accelerated. accelerated.

Cathodic InhibitorsCathodic Inhibitors

Cathodic inhibitors either slow the Cathodic inhibitors either slow the cathodic reaction itself or selectively cathodic reaction itself or selectively precipitate on cathodic areas to increase precipitate on cathodic areas to increase the surface impedance and limit the the surface impedance and limit the diffusion of reducible species to these diffusion of reducible species to these area.area.

Cathodic inhibitors can provide inhibition Cathodic inhibitors can provide inhibition by three different mechanisms: 1. as by three different mechanisms: 1. as cathodic poisons; 2. as cathodic cathodic poisons; 2. as cathodic precipitates, and 3. as oxygen scavenger. precipitates, and 3. as oxygen scavenger.

Some cathodic inhibitors, such as Some cathodic inhibitors, such as compounds of arsenic and antimony, work compounds of arsenic and antimony, work by making recombination of hydrogen more by making recombination of hydrogen more difficult. These substances are very difficult. These substances are very effective in acid solutions but are ineffective effective in acid solutions but are ineffective in environments where other reduction in environments where other reduction processes such as oxygen reduction are the processes such as oxygen reduction are the controlling cathodic reactions.controlling cathodic reactions.

Other cathodic inhibitors, ions such as Other cathodic inhibitors, ions such as calcium, zinc, or magnesium, may be calcium, zinc, or magnesium, may be precipitated as oxides to form a protective precipitated as oxides to form a protective layer on the metal. layer on the metal.

Oxygen scavengers help to inhibit corrosion Oxygen scavengers help to inhibit corrosion by preventing cathodic polarization caused by preventing cathodic polarization caused by oxygen. Examples of this type of by oxygen. Examples of this type of inhibitors are sodium sulfite and hydrazine. inhibitors are sodium sulfite and hydrazine.

They remove dissolved oxygen from They remove dissolved oxygen from aqueous solutions;aqueous solutions;

2 Na2 Na22SOSO33 + O + O2(dissolved ox.)2(dissolved ox.) = 2Na = 2Na22SOSO44

NN22HH44 + O + O22 = N = N22 + 2H + 2H22O O

These inhibitors will work effectively These inhibitors will work effectively in solutions where oxygen reduction in solutions where oxygen reduction is controlling the cathodic process is controlling the cathodic process but will not effective in acid solution.but will not effective in acid solution.

Organic InhibitorsOrganic Inhibitors

Both anodic and cathodic effects are sometimes Both anodic and cathodic effects are sometimes observed in the presence of organic inhibitors, but observed in the presence of organic inhibitors, but as general rule, organic inhibitors effect the entire as general rule, organic inhibitors effect the entire surface of corroding metal present in sufficient surface of corroding metal present in sufficient concentration.concentration.

Organic inhibitors, usually designated as film Organic inhibitors, usually designated as film forming, protect the metal by forming hydrophobic forming, protect the metal by forming hydrophobic film on the metal surface. Their effectiveness film on the metal surface. Their effectiveness depends on the chemical composition, their depends on the chemical composition, their molecular structures, and their affinities for the molecular structures, and their affinities for the metal surface. Because film formation is an metal surface. Because film formation is an adsorption process, the temperature and pressure adsorption process, the temperature and pressure in the system is the important factors.in the system is the important factors.

Organic inhibitors will adsorbed according to the Organic inhibitors will adsorbed according to the ionic charge of inhibitors and the charge of the ionic charge of inhibitors and the charge of the surface. surface.

Cationic inhibitors, such as amines, or anionic Cationic inhibitors, such as amines, or anionic inhibitors such as sulfonates, will be adsorbed inhibitors such as sulfonates, will be adsorbed preferentially depending on whether the metal is preferentially depending on whether the metal is charge negatively or positively.The strength of charge negatively or positively.The strength of adsorption bond is the dominant factor for soluble adsorption bond is the dominant factor for soluble organic inhibitors.organic inhibitors.

These materials build up a protective film of These materials build up a protective film of adsorbed molecules on the metal surface, which adsorbed molecules on the metal surface, which provides a barrier to the dissolution of the metal provides a barrier to the dissolution of the metal in the electrolyte. Because the metal surface in the electrolyte. Because the metal surface covered is proportional to the inhibitors covered is proportional to the inhibitors concentrate, the concentration of inhibitor in the concentrate, the concentration of inhibitor in the medium is critical. For any specific inhibitor in medium is critical. For any specific inhibitor in any given medium there is an optimal any given medium there is an optimal concentration.concentration.

Precipitation InhibitorsPrecipitation Inhibitors Precipitation-inducing inhibitors are film forming Precipitation-inducing inhibitors are film forming

compounds that have general action over the compounds that have general action over the metal surface, blocking both anodic and cathodic metal surface, blocking both anodic and cathodic sites indirectly. Precipitation inhibitors are sites indirectly. Precipitation inhibitors are compound that cause the formation of compound that cause the formation of precipitates on the surface of the metal, thereby precipitates on the surface of the metal, thereby providing protective layer. Hard water that is providing protective layer. Hard water that is high in calcium and magnesium is less corrosive high in calcium and magnesium is less corrosive than soft water because of the tendency of the than soft water because of the tendency of the salts in the hard water to precipitate on the salts in the hard water to precipitate on the surface of the metal and form a protective film.surface of the metal and form a protective film.

The most common inhibitors in this category are The most common inhibitors in this category are the silicates and the phosphates, i.e. sodium the silicates and the phosphates, i.e. sodium silicate is used in many domestic softeners to silicate is used in many domestic softeners to prevent the occurrence of rust water. In aerated prevent the occurrence of rust water. In aerated hot water systems, sodium silicates protect steel, hot water systems, sodium silicates protect steel, copper and brass. copper and brass.

However the protection is not always reliable and However the protection is not always reliable and depends heavily on pH and a saturation index depends heavily on pH and a saturation index that is influenced by water composition and that is influenced by water composition and temperature. Phosphates also require oxygen for temperature. Phosphates also require oxygen for effective inhibition. Silicates and phosphates do effective inhibition. Silicates and phosphates do not afford the degree of protection provided by not afford the degree of protection provided by chromates and nitrites; however they are very chromates and nitrites; however they are very useful in situations where nontoxic additive are useful in situations where nontoxic additive are required.required.

Volatile Corrosion InhibitorsVolatile Corrosion Inhibitors Volatile corrosion inhibitors (VCIs), also vapor Volatile corrosion inhibitors (VCIs), also vapor

phase inhibitors (VPIs), are compounds transported phase inhibitors (VPIs), are compounds transported in a closed environment to the site of corrosion by in a closed environment to the site of corrosion by volatilization from a source. In boilers, volatile volatilization from a source. In boilers, volatile basic compounds, such as morpholine or hydrazine basic compounds, such as morpholine or hydrazine , are transported with steam to prevent corrosion , are transported with steam to prevent corrosion in condencer tubes by neutralizing acidic carbon in condencer tubes by neutralizing acidic carbon dioxide or by sifting surface pH toward less acidic dioxide or by sifting surface pH toward less acidic and corrosive values.and corrosive values.

In closed vapor spaces, such as shipping In closed vapor spaces, such as shipping containers, volatile solids such as salts of containers, volatile solids such as salts of dicyclohexylamine, cyclohexylamine, and dicyclohexylamine, cyclohexylamine, and hexamethylene amine are used. On contact with hexamethylene amine are used. On contact with the metal surface, the vapor of this salt condenses the metal surface, the vapor of this salt condenses and hydrolyzed by any moisture to liberate and hydrolyzed by any moisture to liberate protective ions. protective ions.

It is desirable, for an efficient VCI, to provide It is desirable, for an efficient VCI, to provide inhibition rapidly and to last for long periods. inhibition rapidly and to last for long periods. Both qualities depend on the volatility of these Both qualities depend on the volatility of these compounds, fast action wanting high volatility, compounds, fast action wanting high volatility, whereas enduring protection requires low whereas enduring protection requires low volatility. volatility.

The majority of inhibitor applications for aqueous, The majority of inhibitor applications for aqueous, or partly aqueous, systems are concerned with or partly aqueous, systems are concerned with four main types of environment:four main types of environment:

Aqueous solution of acids as used in metal-Aqueous solution of acids as used in metal-cleaning processes such as pickling for removal of cleaning processes such as pickling for removal of rust or mill scale during the production and rust or mill scale during the production and fabrication of metals or in the postservice fabrication of metals or in the postservice cleaning of metal surfaces.cleaning of metal surfaces.

Natural waters, supply waters, and industrial Natural waters, supply waters, and industrial cooling towers in near-neutral pH range (5 to 9)cooling towers in near-neutral pH range (5 to 9)

Primary and secondary productions of oil and Primary and secondary productions of oil and subsequent refining and transport process.subsequent refining and transport process.

Atmospheric or gaseous corrosion in confined Atmospheric or gaseous corrosion in confined environments, during transport, storage, or any environments, during transport, storage, or any other confined operation.other confined operation.