CHAPTER – 1

INTRODUCTION

Carbon steel and mild steel are the most common metallic material being used for

numerous applications in a variety of industries as well as in daily life because of its low cost and

reasonably good strength. Carbon steel and mild steel are used in fabrication of reaction vessels,

storage tanks by industries, which either manufacture or use organic acid as reactant. Stainless

steel is used in the manufactures of large number of utensils, sinks and kitchen appliances.

Among the all inorganic acids, Hydrochloric acid contains aggressive chloride ions and hence it

is the most difficult to handle hydrochloric acid from the standpoints of corrosion. Selection of

material is an important point in handling most corrosive acids like hydrochloric acid, even in

relatively diluted concentrations1 or in process solutions containing appreciates amounts of HCl

acid. Most of severe forms of corrosion problems like pitting and crevice corrosion encountered

involve the inorganic mineral acids or their derivatives. Carbon steel and mild steel corrodes

heavily when it comes in contact with most common inorganic acids.

Corrosion inhibitors are added to the corroding medium in small concentration to control

the corrosion rate of metals and alloys. In general, any phase constituent whose presence is not

essential to the occurrence of an electrochemical process, but leads to a retardation of this

process by modifying the surface state of the metallic material will be called an inhibitor2. In a

sense, an inhibitor can be considered as a retarding catalyst. Corrosion inhibitors which are used

in acidic conditions, have wide application in the industrial fields as an essential components in

acid pickling, acid cleaning, acid desalting (antiscalants) etc. Organic compounds having one or

two hetero atoms having multiple bonds in their molecule which are adsorbed on metal surface,

are commonly used corrosion inhibitors in industry. There are different corrosion inhibitor types

and compositions. Most inhibitor has been developed by empirical experimentation and many

inhibitors are proprietary in nature and thus their composition is not disclosed. Inhibitors are

classified into following types depending upon their mechanism and composition.

1. Adsorption Type Inhibitors: Adsorption type is the largest group of corrosion

inhibitors, which are mainly organic compounds and their derivatives and adsorb on the metal

surface and slow down metals dissolution and also cathodic reduction reactions. In most of the

cases it appears that adsorption inhibitors affect both the anodic and cathodic processes but to

different extent. Ultimately rate of corrosion slow down.

2. Hydrogen- Evolution Poisons: Hydrogen gas is evolved as by product of corrosion. Few

substances having arsenic and antimony ions specifically retard the hydrogen-evolution reaction.

As a result, these substances are very effective in minimizing the rate of corrosion in acidic

solution but are ineffective in environments where after reduction processes such as oxygen

reduction are the rate controlling cathodic reactions.

3. Scavengers: Scavengers selectively removes corrosive reagents from the medium. These

substances act by removing corrosive reagents from solution such inhibitor which are responsible

for the corrosion and will work very effectively in solutions where oxygen reduction is the

controlling corrosion cathodic reaction but there limitation is that they will not be effective in

strong acid solutions.

4. Oxidizers: Few substances like, phosphates, chromates, nitrate and ferric salts also act as

corrosion inhibitors in many different systems. They are specially used to inhibit the corrosion of

metals and alloys that are shows active to passive and passive to active transitions, for example,

iron and its alloys and stainless steels.

5. Vapor-Phase Inhibitors: These are very much similar to organic adsorption inhibitors and

possess a very high vapor pressure. As a consequence, these materials can be used to inhibit

atmospheric corrosion of metals without being placed in direct contact with the metal surface.

The vapor-phase inhibitors are usually only effective if used in closed spaces such as inside

packages or on the interior of machinery during shipment.

Corrosion inhibitors are extensively used in various applications and many industries

are dependent on their successful application. However, the chemical composition of a

majority of corrosion inhibitors is not public knowledge and it is kept secret by chemical

companies3. At present, these is a great deal of information on corrosion inhibitors and there

exist a great need for insight into corrosion inhibitors to enable all who are faced with the task

of selection and use of corrosion inhibitors to have a suitable background knowledge.

1.1 . CORROSION PROCESS IN ACIDIC MEDIUM

The corrosion processes occurring in Carbon steel, stainless steel and mild

steel – HCl system must be understood clearly if inhibitors of low cost and high efficiency are

to be finding out. Corrosion may be defined as the loss of a material useful properties due to its

reaction with its surrounding environment. Corrosion of metals could be considered as extractive

metallurgy in reverse. Extractive metallurgy means extraction of metal from the ore and refining

or alloying the metal for use4. Pure metals are always in a state of high energy and have a natural

tendency to react with the environment to form thermodynamically stable compounds of less

energy.

Atmosphere

Iron ore Pipe Rust

Sheets Hydrated Iron Oxide Soil & Water

Underground

Most of the irons ores contain oxides iron and rusting of steel by water and oxygen results in a

hydrated iron oxide. ‘Rusting’ term is used for iron and steel corrosion, however, many other

metals form their oxides when corrosion occurs.

Corrosion is preferably classified into (1) wet corrosion and (2) Dry corrosion. Dry Corrosion

takes place due to dry atmosphere i.e. in the absence of liquid phase. Vapors and gases of

different chemical species are generally the corrodents. Wet corrosion takes place in presence of

liquid phase mainly water. For the formation of electrochemical cell and electrolyte is must.

Electrolyte generally involves aqueous solutions or electrolytes of acidic solutions and majority

of corrosion occurs is of this type. In another classification most of the corrosion processes are

electrochemical in nature except in some cases such as corrosion due to fused salts and liquid

metals, corrosion in organic reagents, high temperature oxidation etc. where it is chemical in

nature. The requirement of aggressive environment is a must for corrosion to occur. In fact, all

the environments may be corrosive in one way or the other depending upon the corroding

system.

Some examples are air & moisture; freest, distilled, distilled, salt and mine waters; rural,

urban and industrial atmospheres system and other gases such as chlorine, ammonia, hydrogen

sulfide, sulfur dioxide and fuel gases, mineral acids such as hydrochloric, sulfuric and nitric;

MINE

REDUCTION REFINING CASTING ROLLING SHAPING

organic acids such as naphthenic, acetic, and formic alkalis; soil; solvents; vegetable and

petroleum oils and a variety of food products. Generally inorganic materials are more corrosive

than organic. For example, corrosion in the petroleum industry is more due to sodium chloride,

sulfur hydrochloric and sulfuric acids and water than to oil, naphtha or gasoline.

1.1.1. ELECTROCHEMICAL ASPECTS OF CORROSION:

Corrosion of metals in contact with water is electrochemical in nature. The basic

corrosion reaction occurs at the region of the lower potential, which is anode. The metal

dissolution reaction causes formation of metal ions. The anodic oxidation reaction can be

generally represented by

M Mn+ + ne- … (1)

Where, M represents the metal that has been oxidized to its ionic form having a valency of

n+ and the release of n electrons. For the more common heat transfer materials, the individual

reactions are:

Fe Fe2+ + 2e-

Cu Cu+ + e-

Al Al3+ + 3e-

The liberated electrons that migrate through the metal to areas of higher potential are used

in the reduction of other ions or oxygen in the water. Different possible types of reactions that

takes place at cathode are :

O2 + 4H+ + 4e- 2H2O … (2)

O2 + 2 H2O + 4e- 4OH- … (3)

2 H+ + 2e- H2 … (4)

Cu2+ + 2e- Cu

Fe3+ + e- Fe2+ … (5)

Interactions between the products of the anodic and cathodic reactions can occur, forming

solid corrosion products on the metal surface. For example, iron ion coming from the corrosion

the metallic iron will combine with the hydroxyl ions produced from the reduction of dissolved

oxygen:

Fe2+ + 2 OH- Fe (OH)2 … (6)

Ferrous hydroxide is further oxidized to form ferric hydroxide, which is unstable and

subsequently changed to hydrated ferric oxide, common red rust:

Fe (OH)2 + OH- Fe(OH)3 … (7)

2Fe (OH)3 Fe2O3 + 3H2O … (8)

Fe2O3 + x H2O Fe2O3. xH2O (Rust) … (9)

During corrosion, more than one oxidation and one reduction reactions may occur

simultaneously this leads to the faster corrosion rate. Metals have different tendencies to corrode

depending upon the energies associated with the chemical reactions taking place during their

corrosion. This energy known as Gibbs free energy (∆G) is related to the electrode potential by

the relation

-∆ G = n F E … (10)

Where n represents the number of electrons involved in the reaction, E, the electrode

potential and F, the Faraday constant. A large negative value of ∆G indicates a pronounced

tendency for the reaction to proceed whereas a positive value of ∆G indicates that the reaction

has no tendency to proceed. The electrode potential is calculated from the Nernst equation.

E = Eo + ]red[

]ox[ln

ZF

RT … (11)

Where

Eo = Standard electrode potential

R = Gas constant (1.98 cal/gm equivalent)

F = Faraday constant (96,500 coulombs / gm equivalent)

T = Absolute temperature

Z = Number of electrons transferred in the reaction

[ox] = represent conc. of species involved in oxidation process ( mol/l)

[red] = represent conc. of species involved in reduction process (mol/l)

At 250C, equation (8) becomes

E = Eo + 0.059 log ]red[

]ox[ ….(11a)

When any metal or alloy is immersed in a conductive corrosive environment, the metallic

surface is divided into areas having different potentials due to the presence of different metallic

phases, grain boundaries, surface conditions, segregates, crystalline imperfections, impurities etc.

which have different electrode potentials6. This difference in potential leads to the formation of

anodic and cathodic sites on the metal where oxidation and reduction reactions occur

respectively. These local sites result in the formation of local action cells on the metallic surface.

Local action cells can also form on the metal surface when there is variation in the concentration,

composition and temperature.

1.1.2. FORMS OF CORROSION:

The corrosion process7 in a cooling water system can take many forms. More common forms of

corrosion that have been observed in cooling water systems are:

1. Uniform or general corrosion

2. Galvanic corrosion

3. Erosion corrosion

4. Crevice corrosion

5. Pitting corrosion

6. Microbial induced corrosion

7. Selective dissolution

8. Stress corrosion cracking

9. Hydrogen damage

1. Uniform Corrosion:

When the metal surface is completely covered with a layer of corrosion product, it is

called uniform or general corrosion. Local anodic and cathodic sites go on changing due to

various mechanical, chemical and metallurgical reasons and metal appears to be uniformly

corroded.

2. Galvanic Corrosion:

Galvanic corrosion is the main form of corrosion when two dissimilar metals comes in

close contact with each other. The metal having more negative potential and placed high in EMF

series will act as anode and corrode. The intensity of the attack is related to the relative surface

areas of the metals in electrical contact. Large cathodic areas coupled to small anodic areas will

aggravate galvanic corrosion and cause severe dissolution of the more active metal. Galvanic

corrosion obviously must be considered while designing the heat transfer equipment and two

dissimilar metals should not be allowed to come in electrical contact. However, in cooling water

systems two dissimilar metals will always be in electrical contact due to presence of water.

3. EROSION CORROSION:

It occurs when corrosive medium is flowing and it is the co-joint action of fluid velocity

and corrosion process. The damage due to erosion corrosion is substantially high in comparison

to the affect of corrosion and fluid velocity separately. The process is usually accelerated when

abrasive solid particles, such as sand are entrained in the water. Because turbulence increases

with velocity, areas having higher water velocity such as bends and inlet-ends are prone to

attack. It can result in grooves, gullies, waves, rounded holes, valleys etc. in pipe lines of cooling

water systems.

4. Crevice Corrosion:

Crevice corrosion is an electrochemical attack that occurs due to the difference in

concentration of corrosive species between a shielded area and its surroundings. Attack usually

occurs in the areas having a small volume of stagnant solution in areas such as tube sheet joints

and supports, stagnant zones, under deposits or tubercles and at the threaded joints. Corrosion is

usually initiated because the oxygen concentration with in the crevice is lower than that of the

surrounding area. Once the corrosion is under way, the area in the crevice or under a deposit

becomes increasingly more aggressive because of pH depression and an increase in electrolyte

concentration.

5. Pitting Corrosion:

Pitting corrosion is one of the most insidious forms of localized corrosion. It takes place

at small discrete areas where overall metal loss is negligible. The pit develops at a localized

anodic site on the surface and continues to grow because of large cathodic area surrounding the

anode. High concentration of metallic chlorides often, develops with in the pit and hydrolyzes to

produce an acidic pH environment. The reaction with in the pit becomes self-sustaining

(autocatalytic) with very little possibility for it to suppress.

6. Microbial Influenced Corrosion (MIC):

MIC is a type of corrosion where microbes play a dominant role. Open recirculating

cooling water system is one of the most favored sites for the growth and proliferation of micro

organisms. Three major class of micro organisms are algae, fungi and bacteria.

Desulfovibrio desulfuricans is an anaerobic bacteria which can convert SO42- or compounds to

H2S. These bacteria are also called as sulfate reducing bacteria (SRB).

10H+ + SO42- + 4Fe 4Fe2+ + H2S + 4H20

H2S + Fe2+ FeS + 2H+

Carbon steel is severely attacked and sulfide ions can lead to hydrogen damage. Aerobic sulfur

bacteria, thiobacillus can oxidise sulfur, sulfides or sulfates to H2SO4 and can cause thinning of

carbon steels. Both desulfovibro and thiobacillus can coexist in close proximity in cooling water

systems. Nitrifying bacteria can oxidize ammonia to nitrate which decreases pH and promotes

corrosion.

NH3 + 2O2 HNO3 + H2O

Slime forming bacteria form dense sticky biomasses that impede water flow. The primary

mechanism of MIC of metal surface involves the creation within the biofilms of local

physiochemical corrosion cells. Biofilms can develop on any metallic surface and no metal/alloy

used in cooling water services is immune to MIC.

7. Selective Dissolution:

Selective dissolution is the preferential removal of an element from an alloy by corrosion.

Selective removal of zinc from brass leaving behind a sponge mass of copper is a prime example

of this form of attack. The removal of zinc can be uniform or localized. In general, a uniform

type of de-zincification is observed in brasses having high zinc content. A localized type of

dezincification is commonly observed in low zinc brasses in neutral, alkaline or mildly acidic

environments. A similar form of attack has also been observed with other alloys in which

aluminum, graphite, chromium etc. are selectively removed.

8. Stress Corrosion Cracking:

Stress corrosion cracking refers to cracking caused by the simultaneous presence of

tensile stress and a specific corrosive medium. During stress corrosion cracking, the metal may

be virtually uncorroded over most of the surface, but fine cracks progress through the metal. This

type of cracking has serious consequences because it can occur at stresses with in range of

typical design stress. The important factors, which affect this type of damage are temperature,

water composition, composition of the metal, stress and micro structure of the metal. Stress

corrosion cracks have the appearance of a brittle mechanical fracture although they result due to

corrosion. Intergranular stress corrosion cracking occurs along grain boundaries. In general,

cracking occurs perpendicular to the applied stress. The type of crack, either single or branching

depends up on the structure and composition of the metal and composition of the cooling water.

9. Hydrogen Damage:

Nascent hydrogen produced on the metal surface either at high temperature or due to

corrosion reaction in low pH water may cause hydrogen damage to the metallic materials. It

occurs most often in high strength steels, primarily quenched, tempered steels and precipitation

hardened steels. Nascent hydrogen has the capability to penetrate the bulk of metal and is trapped

at carbide precipitation, dislocations etc. It changes into molecular hydrogen and is now not able

to diffuse through metal. Pressure of hydrogen gas goes on building up and can rupture steel of

any known strength. The Hydrogen when present in steel always creates problem like it reduces

the tensile ductility, leads to premature failure under static load which directly or indirectly

depends upon the stress and time.

1.1.3. KINETIC ASPECTS OF CORROSION:

The kinetics of electrochemical reaction is based largely on the mixed potential theory of

the electrode kinetics as stated by Wagner and Traud8. The theory is based on the simple

assumption i.e. (a) the kinetics of various potential reactions can be treated separately and (b) no

net current flows from the electrode which is in equilibrium or at steady state. The condition of

no net current flow means the total rate of reduction must be equal to the total rate of oxidation

on the electrode surface if the electrode is at steady state or at equilibrium. The equilibrium is

dynamic in nature. Reactants and products jump back and forth at a very real rate. For an

electrochemical reaction this equilibrium exchange rate is called exchange current density, io.

When a reaction is forced away from equilibrium, at which the reaction is occurring, the

magnitude by which the potential changes, is the over voltage.

η = Eeq – Ei … (16)

Where, η = Over voltage

Eeq = Equilibrium potential

Ei = Polarized or current flowing potential

The over voltage, exchange current density and the rate of various partial processes can be

related in the form of a chemical rate equation:

→i = io exp

a

2.3

βη

… (17)

←i = io exp

c

2.3–

βη

… (18)

Where →i = Anodic current density

←i = Cathodic current density

βa and βc are anodic and cathodic Tafel constants respectively.

For corrosion and electrochemical studies, above equations are usually written in

logarithmic form9 and called Tafel equation10.

oo i

iloga

i

ilogc β=β−=η … (19)

The magnitude of the applied current density required to maintain a given over voltage

can be calculated by:

←→−= iiiappl. … (20)

Fig . (1.1) shows the polarization diagram for a corroding metal M and may demonstrate

the application of mixed potential theory in an appropriate manner. Cathodic process being

hydrogen evolution and the whole surface may be considered as anode if the attack is of general

uniform nature. When the metal is not corroding, it is at reversible equilibrium having potential

E (M+/M) with exchange current density io (M+/M). If on the other hand, the non-corroding

electrodes were saturated with hydrogen gas at unit activity and pressure, it will assume the

equilibrium electrode potential E (H+/H2) with the exchange current density io (H+/H2).

1.1.4. POLARISATION PHENOMENON:

Polarization can be defined as the displacement of equilibrium electrode potential value

resulting from a net current flow. The measured potential of such an electrode is shifted to an

extent that depends on the magnitude of the external current and its direction. The direction of

potential change is always such as to oppose the shift from equilibrium and hence to oppose the

flow of current, whether the current is impressed externally or is of galvanic origin. When

current flows, the anode potential always shifts in the cathodic direction and that of cathode in

the anodic direction, decreasing the difference of potential. The extent of potential change caused

by net current flow to or from an electrode, measured in volts, is called polarization.

Electrochemical polarization is divided into three types: activation, concentration and resistance

polarization.

Activation Polarization:

Electrochemical reactions, which are controlled by a slow step in the reaction sequence,

undergo activation polarization or stated in other way the reaction must have an extra energy, an

activation energy, in order to proceed to form products.

This is easily illustrated by considering hydrogen ion reduction at a cathode, 2H+ 2e- → H2, the

corresponding activation polarization term being called hydrogen over voltage. The rate at which

hydrogen ions are reduced to hydrogen gas will be a function of several factors including the

speed of electron transfer to the hydrogen ion at the metal surface. Thus, there is an inherent rate

for this reaction depending on the particular metal, hydrogen ion concentration and the

temperature of the system. In fact, there are wide variations in the ability of various metals to

transfer electrons to hydrogen ions and as a result the rate of hydrogen evolution from different

metals surface is observed to be quite different. Pronounced activation polarization also occurs

with discharge of OH- ions at the anode in alkaline solutions accompanied by oxygen evolution.

4 OH- → O2 + 2H2O + 4e- … (21)

The activation polarization associated with the above reaction is known as oxygen over voltage.

Activation polarization also occurs during metal ion deposition or dissolution (corrosion).

Concentration Polarization:

Electrochemical reactions as a result of the concentration change in solution adjacent to

the electrode surface is referred as concentration polarization of diffusion over potential. It is

obvious from Nernst equation that the potential of an electrode depends upon the concentration

of ions in electrolyte in its immediate vicinity. One can consider, for example, the reduction of

Mn+ ion in a solution in which its activity is given by aMn+ According to the Nernst equation the

potential E1 in the absence of external current is

E1 = E0 + 0.059/n log aMn+ … (22)

If an external current is made to flow so as to accelerate the reduction rate, Mn+ ions are

depleted in the vicinity of the electrode due to their reduction at electrode and fresh Mn+ ions

tend to reach the electrode by ionic migration, diffusion and agitation of bulk electrolyte

Ionic migration and diffusion mode of transfer inevitably occur while the agitation of

bulk electrolyte can be controlled. At very low reduction rate, migration of ions is sufficient to

maintain the concentration level in vicinity of the electrode but at higher rate of reduction,

concentration of these ions changes to say Msn+, therefore, the potential E2 in this condition is

given by

E2 = E0 + 0.059/n log aMsn+ … (23)

The difference in potential E1 - E2 is known as concentration polarization or concentration over

potential at the cathode and is given by :

ηcc = E1 – E2 = 0.059/n log aMn+ / aMs

n+ … (24)

Since the aMsn+ is less than the aM

n+, the potential of polarized electrode, E2, is less noble

than the unpolarized electrode potential, E1. It is predominated when the concentration of active

species is low. If corrosion is controlled by concentration polarization then any change which

increases the diffusion rate of the active species will increase corrosion rate. In such a system,

stirring of liquid would tend to increase the corrosion rate of metal.

Resistance Polarization:

During the measurement of polarization there is also an ohmic potential drop through

either a portion of electrolyte surrounding the electrode or through a metal reaction product film

on the surface or both. This contribution to polarization is equal to IR where I is the current

density and R represents the value of path resistance in ohm. Resistance polarization may be

written as :

Rη = RI … (25)

Where, R is film resistance for all electrode surfaces in ohm and I is current in ampere.

If the resistance of electrolyte is so high that the resultant current is insufficient to

appreciably polarize anode or cathode, the corrosion is said to be under resistance control.

1.1.5. CORROSION PROTECTION:

Although corrosion is unavoidable, its cost can be reduced to a great extent by adopting

protective methods. Several methods, given below are frequently used to minimize corrosion or

to provide protection against corrosion in cooling water systems.

1. Use of proper design

2. Selection of the proper metal or alloy for a particular corrosive system

(a) Change of composition

(b) Change of microstructure

(c) Elimination of tensile stresses

(d) Introduction of surface compressive stresses

3. Alteration of environment

(a) Lowering temperature

(b) Decreasing velocity

(c) Changing concentration

4. Electrochemical protection

(a) Cathodic protection

(b) Anodic protection

5. Use of coatings to separate metal from corrosive environment

(a) Metallic coatings

(b) Non-metallic coatings

6. Use of corrosion inhibitors (Chemical Treatment)

After the selection of metallic materials, designing and installation of the cooling water

system, chemical treatment of the water is the only possibility to control the corrosion problems

in the cooling water systems.

Selection of corrosion Inhibitors depends upon the following.

1. CORRODING SYSTEM PARAMETERS

Nature and location of corrosion

Composition and microstructure of metallic material

Composition and physical properties of corrosive water

Velocity of water or flow parameters

Temperature

Duration and cycle of operation

Composition and solubility of corrosion products

2. INHIBITORS’ PARAMETERS

Cost of corrosion inhibitor and its availability

Solubility in the corroding system

Compatibility with other constituents of corroding system

Depletion mechanism of inhibitor

Physical properties of inhibitor

Toxicity of inhibitor

Other properties such as emulsifying, demulsifying, foaming or defoaming, surface wetting

etc.

A corrosion inhibitor is a chemical formulation which when added in small concentration

to the corroding system causes a substantial reduction in the rate of corrosion of metal either by

reducing the probability of its occurrence or by reducing the rate of attack or by both. It should,

however, be noted that the presence of a corrosion inhibitor is not essential to the occurrence of

an electrochemical process, but leads to the reduction in the oxidation rate of the metal-surface

and it generally implies adsorption, the formation of "surface compounds", or a reaction between

the metallic material and the inhibitor with separation of the corrosion products at the contact

surface of the metallic material with the electrolytic conductor. Inhibitors are always used in

such a manner that they do not interfere with properties of the systems and are compatible with

the corroding system. An inhibitor useful for a particular corrosion system may be harmful to

another under certain situations. The size of inhibitor molecule, molecular weight and shape of

the molecule has been found to influence the efficiency of organic inhibitors markedly.

After adsorbing on the metal surface, inhibitor can effect in many ways. It may form a

physical barrier for the diffusion of reactant, reduce metal reactivity by adsorbing on active sites,

change the surface potential, incorporate itself into electrical double layer, form a complex with

the metal and may increase the stability of natural oxide / hydroxide film present on the metal

surface. Inhibitor action is selective and depends on the nature of the metal to be protected, its

composition and metallurgical treatment. An inhibitor which is quite effective for one metal

may not be satisfactory for the other. When a multi – metal system is to be protected, a mixture

of inhibitors is required to protect all the metals. Some inhibitors may show protection towards

more than one metal. Inhibitors may be classified into several groups i.e. passivators, inorganic

precipitators, vapour phase inhibitor, neutralizers, adsorbents or adsorption inhibitors and

scavengers. Inhibitors can also be classified as anodic, cathodic and mixed type based on their

mechanism of inhibition.

Passivators:

Passivators are anodic inhibitors that shift the corrosion potential of the metal sharply in

the positive (noble) direction. These inhibitors deactivate anodic sites on the metal surface by

causing the local current density to exceed the amount needed for passivation. These are the

substances which when added to an environment retard corrosion but do not interact directly

with the metal surface.

Passivators slow down corrosion by several mechanisms:

� They stabilize the passive film thus reducing the corrosion rate.

� They repassivate the metal if the film is damaged.

� They assist in film repair by forming insoluble compounds that plug pores in the film.

� They prevent adsorption of aggressive anions, such as chloride, by competitive

adsorption of inhibitive anions.

The passivating type of inhibitors are mainly inorganic oxidizing chemicals, e.g.

chromate, nitrate, nitrite etc. and inorganic non-oxidizing chemicals e.g. phosphate, tungstate,

molybdate etc. Passivators are dangerous if not used in sufficient concentration, because a metal

almost completely passivated has a very large cathode/anode area ratio that will concentrate all

attack on unpassivated regions to create pits.

Inorganic Precipitation Inhibitors:

Phosphates are the most widely used precipitation inhibitors, which precipitate ferrous

and ferric phosphates (FeHPO4 and FePO4) on carbon steel. Bicarbonate ions (HCO3) form

insoluble carbonate in alkaline solution. Zinc salts are also examples of this type of inhibitors.

Oxygen Scavengers:

Scavengers eliminate dissolved oxygen from a closed recirculating cooling water system

with neutral or alkaline pH. Due to non-availability of oxygen for cathode reaction, corrosion

stops. Sulphite is a common, inexpensive scavenger widely used in closed recirculating cooling

water systems. Scavengers remove oxygen, not H+ ions and so are ineffective in acids. Some

other examples of scavengers are phosphites, hydrazine, morpholine, isopropyl hydroxyl amine,

hydroquinone, diethyl hydroxy amine (DEHA)11 etc.

Neutralizers:

Neutralizers reduce corrosion by reducing the concentration of H+ ions in solution.

Neutralizers are even added to neutral cooling water of pH 7.0 because the H+ ions concentration

is increased at high temperatures due to the presence of CO2 which forms carbonic acid (H2CO3).

Some common neutralizers are borates, alkalis, amines, ammonia etc.

When an inhibitor is added to a cooling water system, adsorption of the inhibitor

molecule at the metal-solution interface occurs and this is accompanied by a change in potential

difference between the metal electrode and the solution interface. The two main types of

adsorption of an organic inhibitor on a metal surface are physical or electrostatic and

chemisorption depending upon the type of interaction involved. For determining such types of

interaction, adsorption kinetics, the heat of adsorption or the reversibility and specificity of the

bond established are examined. Also, the adsorption of inhibitors is governed by the residual

charge on the surface of the metal and by the nature and chemical structure of inhibitor molecule.

Physical Adsorption:

Physical adsorption is due to weak electrostatic attraction between the inhibiting ions or

diploes and the electrically charged surface of the metal. The inhibiting species adsorbed on the

metal due to electrostatic forces can also be desorbed easily. Infact, the ions are not in direct

physical contact with the metal. A layer of water molecules separates the metal from the ions.

The physical adsorption process has low activation energy and is relatively independent of

temperature.

Chemical Adsorption:

Chemical adsorption or chemisorption is probably the most important type of interaction

produced by charge sharing or charge transfer between the metal surfaces and an inhibitor

molecule. The adsorbed species is in contact with the metal surface. A coordinate type of bond

involving transfer of electrons from inhibitor to the metal is assumed to take place in the

process12. An opposing view that there is necessarily no chemical bond between the metal and

the adsorbed species is held by Bockris13. Chemisorption process is slower than electrostatic

sorption and has higher activation energy. It improves significantly with temperature.

Chemisorption of organic molecules is specific for certain metals and is not completely

reversible.

1.1.6. FACTORS INFLUENCING CORROSION INHIBITION EFFICIENCY

Availability of π Electron Density:

The electron density of the organic function that can be considered as the reaction centre for the

establishment of the adsorption bond of obviously important since it is possible to assume a bond

of the Lewis acid base type15, generally with the inhibitor as the electron donor and the metal as

the electron acceptor. Availability of π electrons due to the presence of the multiple bonds or

aromatic rings in the inhibitor molecule would facilitate electron transfer from the inhibitor to the

metal. In general, the organic inhibitors used have reactive functional groups which are the sites

for the chemisorption process that is why most organic inhibitors are compounds with at least

one polar function, having atoms of nitrogen, sulphur, oxygen and in the some cases selenium

and phosphorus. This polar function is regarded as the reaction centre for the establishment of

the chemisorption process. In such a case the adsorption bond strength is determined by the

electron density of the atom acting as the reaction centre and by the polarisability of the function.

A more general interpretation of the importance of the electron density in chemisorpation of

organic substances in relation to inhibition phenomenon has been attempted by Donahue16-18,

who pointed out the changes in the corrosion current densities of iron immersed in H2SO4

containing various inhibitors as a function of Hammett’s σ constant or of Taft’s σ* constant. A

similar interpretation of the organic inhibitors has been given by Grigoryev and Osipov19.

Structure of Inhibitor Molecules :

Inhibition efficiency can be increased by regular and systematic variations of the molecular

structure of inhibitor. The corrosion inhibition efficiency of cyclic amines was found to be

greater than aliphatic amines in the corrosion of iron in acidic solutions and this has been

attributed to the greater availability of the lone pair of electrons on the basic nitrogen atoms due

to the increase in C-N-C bond angle from 109o (sp3 hybridization ) to 120o (sp2 hybridization)20.

The electron density on the donor atom of an inhibitor molecule which is comprised of aromatic

and heterocyclic structures can be enhanced by introducing electron donating substitutes in

suitable positions in the ring structure, facilitating conjugation and hence the strength of

adsorption bond.

This principle of increasing the electron density at the donor atom and hence corrosion

inhibition efficiency has been applied to aromatic acids21-24, aromatic amines and aromatic

nitriles. In thiophene derivatives, substituents increased the dipole moment of the molecule

thereby causing an increase in adsorption and inhibition efficiency. The molecular area,

molecular weight and the molecular configuration have been found to have profound effect on

corrosion inhibition efficiency or organic inhibitors.

Synergistic Effect :

The surface of the iron can be assumed to be positively charged in sulphuric acid and the organic

catonic inhibitors are weakly adsorbed. However, if halide ions are introduced into the solution,

the situation is changed abruptly. For example, Iofa and his co-workers25 showed that if iodide,

bromide or chloride ions are added to the electrolyte along with organic cations, it may greatly

enhance the inhibition action of the organic compounds. The addition of tetra isoamyl

ammonium sulphate to sulphuric acid only slightly affects the kinetics of the electrode reactions

of hydrogen deposition and ionization of iron, whereas the additional introduction of 0.001 N K1

into the electrolyte greatly retards the cathodic and anodic reactions. Similar results were

obtained by Losev26. Thus halide ions alter the properties of the surface so that the adsorption of

organic cautions on it becomes possible. This, in turn, gives rise to significant kinetic effects,

ensuring that the combined action of the inorganic anions and organic cations will be much more

effective than the inhibition action of each additive separately. The synergistic effect can also be

explained taking adsorption phenomenon into account. For high degree of coverage of a surface

with an adsorbate, columbic forces of repulsion may appear between the adsorbed cations. If

both cations and anions are adsorbed, electrostatic forces of attraction will apparently be created

between oppositely charged ions, making the film thicker.

Temperature :

Increasing temperature increases rates, diffusion and the rate of dissolution of gases in water. It

also increases the ionization of water and lowers its pH. For passivated metals, the protective

increases. It becomes more easily damaged by CI ions and thus more susceptible to pitting. At

some point the film breaks down entirely and the metal becomes transpassive, corroding at a

very high rate. As the temperature increases the desorption rate, inhibitors provide less protection

at higher temperature and higher concentrations of inhibitor become necessary by Brasher and

Mercer27-29.

In some very rare cases it has been observed that efficiency of the inhibitor increases with

increase of temperature. It will happen only in cases where adsorption is endothermic in nature.

pH of the System :

All the inhibitors show greater efficiency in their own specific pH range. Beyond this pH range,

the inhibitive efficiency decreases or in some cases the inhibitor becomes ineffective.

Concentration of Inhibitors :

All inhibitors are required to be present above a certain minimum concentration. In most of the

systems, corrosion rate in presence of insufficient inhibitor concentration may be more severe

than in complete absence of inhibitor concentration. In initial stage of use, inhibitor

concentration may fall off rapidly due to formation of protective film or its reaction with

contaminants present in the system and hence the initial concentration of inhibitor is always

higher than that maintained during long use. With the help of various adsorption isotherms

explained below, the metal-surface coverage using different concentrations of various inhibitors

can be studied at particular temperature30-31.

References:

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(1992)

2. Shastri V.S, Corrosion Inhibitors, John Wiley and sons, Chickester

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New York.

4. Mars G. Fontana, Corrosion Engineering Mcgraw Hill Book Company

(1987) 3rd edition.

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(1996)

6. Bhattacharya G.S, Proc. Int. Conf. on corrosion CORCON 97, p. 1052.

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Bockris J. O’M and Conway B.E, Butterworths, London 3(1984).