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INTRODUCTION TO CORROSION AND CORROSIONINHIBITORS

Chapter I Introduction to Corrosion and Corrosion Inhibitors

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INTRODUCTION TO CORROSION AND CORROSION INHIBITORS

SECTION - I: CORROSION

1.1.1. Introduction to corrosion

Corrosion is an irreversible chemical or electrochemical reaction of a material with

the environment, which usually (but not always) results in a deterioration of the material

and its properties. It is the natural tendency of a material’s compositional elements to

return to their most thermodynamically stable state. For most metallic materials, this

means the formation of oxides or sulfides, or other basic metallic compounds. Most of the

metals used in industries except noble metals are unstable to varying degrees in the natural

environmental conditions due to corrosion. The damage due to corrosion is serious

engineering problem and the national economies have suffered great losses due to

corrosion. Protection of metals and their alloys from corrosion is the main purpose of study

of corrosion in general. Mild steel is one of the well known engineering structural material

used in chemical processing, petroleum production, refining, pipelines, mining,

construction, etc., due to its excellent mechanical properties and low cost. One of its

shortcomings is that it undergoes corrosion in various operating environments such as

addition of acids for the removal of undesirable scale and rust in many industrial

processes. Crude oil is corrosive to mild steel which is widely used in the petroleum

industry. About 25 – 30 % of the total economic losses in the oil and natural gas industries

are due to failure of pipes and other plants resulting from metallic corrosion.

Corrosion intrudes itself into many parts of our lives. The damage it makes is very

clear. The economic costs of corrosion are obviously enormous. Like other natural hazards

such as earthquakes and severe weather disturbances, corrosion can cause dangerous and

expensive damage to infrastructure, waterways, ports, railroads, hazardous materials

storage, drinking water, sewer systems, gas distribution, electrical utilities,

telecommunications, automobiles, ships, aircrafts, mining, petroleum refining, chemical,

petrochemical and pharmaceutical production, pulp and paper, agricultural production,

food processing, electronics, defense, home appliances, gas transmission pipelines and

highway bridges. Losses sustained by corrosion amounts to many billions of dollars

annually. The economic factor is the main object for much of the current research in

corrosion. Another factor is the safety of operating equipments such as pressure vessels,


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boilers, material containers for toxic materials and may be the most important of all is the

equipment for nuclear power plants and disposal of nuclear wastes.

Corrosion problems also occur in the pipe lines due to aggressive nature of the liquid

which carried by them (Figs. 1.1-1.3). These liquids may be petroleum containing water

and sulphur, high saline formation or sea water and pipes used in cooling and heating

systems in many operations. Some pipelines deteriorate slowly and in certain cases

pipeline life span has been reliably calculated to last for many years.

Fig. 1.1: Oil pipeline corrosion.

Fig. 1.2: Gas pipeline corrosion.


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Fig. 1.3: Inner view of corroded pipeline.

Pickling or descaling is the removal of heavy, tightly adhering oxide films such

as stains, inorganic contaminants, rust or scale from ferrous metals, copper and aluminum

alloys (Fig. 1.4) resulting from hot-forming operations, thermal treatments (such as

annealing or hardening) or welding. A solution called pickle liquor which contains

strong acids, is used to remove the surface impurities. It is commonly used to descale or

clean steel in various steel making processes. Many hot working processes and other

processes at high temperatures leave a discoloring oxide layer or scale on the surface. In

order to remove the scale, the work piece is dipped into a vat of pickle liquor [1].

Pickling inhibitors are used to protect metal components from corrosive action

against acid effect while maintaining the pickling performance of the system. So, cleaning

ability of bath system directly intensifies on stains, residuals, tarnish and oxide layers.

Thus, pickling bath solution will have a long-lasting lifetime with inhibitor addition and

acid consumption is appreciably minimized at plants. On the other hand, without inhibitor

usage, the corrosive effect of acid on parts will lead to a remarkable amount of metal loss

during pickling process. In pickling baths, hydrogen gas releases from the surface of the

metal due to the metal-acid interaction. The exposure of hydrogen to the metal potentially

brings a problematic issue known as hydrogen embrittlement where diffusing of hydrogen

through the metal makes it brittle [2]. In the case of inhibitor usage, a barrier is formed on

clean areas of metal surface, so excess hydrogen release due to acid-metal interaction is


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strongly inhibited. Thus, hydrogen embrittlement risk on metal parts is remarkably reduced

with inhibitor addition to pickling baths.

Mild steel is widely used material in pipelines for domestic and industrial water

utilities and heat exchangers due to its good thermal conductivity and mechanical

workability. Scale and corrosion products have a negative effect on heat transfer and they

cause a decrease in heating efficiency of the equipment and transportation of the liquid,

which is why periodic descaling and cleaning using pickling solutions are necessary. Most

pickling inhibitors function by forming an adsorbed film on the metal surface, probably

not more than a monolayer in thickness, which essentially blocks discharge of H+ and

dissolution of metal ions. Dilute sulfuric or hydrochloric acids with pickling inhibitors are

used to clean out steel water pipes clogged with rust or clean boiler tubes encrusted with

CaCO3 or iron oxide scales, and to activate oil underground wells. For example, boiler

scale can be removed by using 0.1% hexamethylenetetramine in 10 % HCl at a maximum

temperature of 70 °C.

Fig. 1.4: Stainless steel sheet before and after pickling.

Cooling and heating water circulation system can present several problems.

Formation of salt deposits and fouling by micro-organisms can appear when water is used

as thermal fluid (Fig. 1.5). These problems can occur jointly and reduce the thermal

efficiency of the circuits with significant economic repercussions. To reduce or eliminate

these problems, feed waters of boilers are treated with inhibitive formulations composed of


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corrosion inhibitors associated with chemical reagents used both to reduce corrosion of the

boiler and auxiliary equipment and to reduce formation of inorganic deposits in the boiler

tubes (scaling), which interfere with heat transfer [3].

Fig. 1.5: Microbial corrosion in pipeline.

In oil production industries, economic losses and ecological damage caused by

corrosion stem from the very large amounts of metal equipment and structures that come

into contact with highly aggressive media. Stratal water content in oil and gas industries

often cause severe problems for steel pipelines of heat exchangers, boilers and condensers

(Figs. 1.6 and 1.7), and it negatively affects metal in oil production, refinery, transportation

and processing operations. The most important tasks in the development of an oilfield are

reliable operation and long life of equipments and pipeline systems. Acidic treatment is

often used to intensify oil recovery and increase the efficiency of oil deposits. Acidic fluids

are very corrosive reagents to steel equipments. Dissolved metal by acidic treatment can

form precipitations of iron oxides or iron sulphides (in the presence of hydrogen sulphide),

which negatively affects the oil production equipments and the quality of the crude oil.

Under such conditions, a technically justified and efficient method of protection is the use

of inhibitors that adsorbed as protective films on the metal to prevent its corrosion. At the

same time, inhibitor injection, especially nitrogen-containing compounds with a diphilic

structure (Example: colloidal cationic surfactants such as amine, imidazoline and their

salts) seem to be one of the most appropriate and most cost-effective methods to solve this


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problem. However, most inhibitor’s evaluations are generally based on the test results

under stagnant or low flow rate (<1 m s-1

) conditions [4].

Fig. 1.6: Corrosion in heat exchanger.

Fig. 1.7: Corrosion in boilers.


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There are five good reasons to study corrosion:

a) Materials are precious resources of a country. Our material resources of iron,

aluminum, copper, chromium, manganese, titanium, etc., are dwindling fast. Some

days there will be an acute shortage of these materials. An impending metal crisis

does not seem anywhere to be a remote possibility but a reality. There is bound to be

a metal crisis and we are getting the signals. To preserve these valuable resources,

we need to understand how these resources are destroyed by corrosion and how they

must be preserved by applying corrosion protection technology.

b) Engineering knowledge is incomplete without understanding the corrosion. Aero

planes, ships, automobiles and other transport carriers cannot be designed without

any recourse to the corrosion behavior of materials used in these structures.

c) Several engineering disasters such as crashing of civil and military aircraft, naval

and passenger ships, explosion of oil pipelines and oil storage tanks, collapse of

bridges and decks and failure of drilling plat forms and tanker trucks have been

witnessed in recent years. Corrosion has been a very important factor in these

disasters. Applying the knowledge of corrosion protection can minimize such

disasters.

d) Designing the artificial implants for the human body requires a complete knowledge

of the corrosion science and engineering. Surgical implants must be very corrosion-

resistant because of corrosive nature of human blood.

e) Corrosion is a threat to the environment. For instance, water can become

contaminated by corrosion products and unsuitable for consumption. Corrosion

prevention is integral to stop contamination of air, water and soil.

1.1.2. Theories and mechanism of corrosion

1.1.2.1. Local cell theory

According to De la Rive [5] corrosion occurs because of the creation of a large

number of micro electrochemical cells or local cells (Fig. 1.8) at heterogeneities

(impurities, defects, different phases, non-uniform stress distribution etc.,) on the metal

surface. The corrosion is an electrochemical process in which a difference in electrical

potential develops between two metals or between different parts of a single metal. This

voltage can be measured when a metal is electrically connected to a standard electrode.


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The electrical potential of a metal may be more or less than the standard in which

case the voltage expressed as either positive or negative. The difference in potential allows

current to pass through the metal causing reaction at anodic and cathodic sites. Every metal

surface is covered with numerically small anodes and cathodes. These sites usually

developed from the following [6]:

• Stress from welding or others works.

• Compositional differences at the metal surface.

• Surface irregularities from forming, extruding and other metal working operations.

Fig. 1.8: Model for local cell theory of corrosion.

1.1.2.1. Wagner and Traud’s theory

Wagner and Traud [7] have proposed a theory for the corrosion of pure metals.

According to this theory, impurities and other surface heterogeneities are not essential for

corrosion to occur. The necessary condition for corrosion (metal dissolution) to occur is

that, some cathodic reaction should proceed simultaneously on the surface. The impurities

may have formed when the metal is molten and impurities are passed into the surface

during rolling, forming or shaping operations.

Although corrosion is a complicated process, it can be most easily comprehended as

an electrochemical reaction involving the following three steps (Fig. 1.9):

Ionic conductor

Cathodic site Anodic site


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Anode Cathode

Fe+2

Fe+2

+ OH־ OH

- O2 OH

- O2 OH

-

2 e–

Fe(OH)2

a) Loss occurs from that part of the metal called the cathodic area because of the

lower potential at this site. In this case iron is lost to the water solution and

becomes oxidized to Fe2+

ion.

b) As a result of the formation of Fe2+

, two electrons are released to flow through

the steel to the cathodic area.

c) Oxygen in aqueous solution moves to the cathode and completes the electric

circuit by using the electrons that flow to the cathode to form OH- at the surface

of the metal.

Fig.1.9: Reaction occurring during the corrosion of steel.

The reaction takes place as follows:

Anodic reaction:−+ +→ eFeFe

o 22 (1.1)

Cathodic reaction: 0.4:222/1 22 >→++ −−pHOHeOHO (1.2)

In the absence of oxygen, H+ participates in the reaction at the cathode instead of

oxygen and completes the electrical circuit as follows:

↑→+ −+222 HeH (1.3)

Hydroxyl ions will combine with the Fe2+

produced by dissolution of the metal as follows:

( ) ↓→+ −+2

2 2 OHFeOHFe (1.4)


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The ferrous hydroxide produced has a very low solubility and quickly precipitates as a

white flock at the metal – water interface. The floc is then rapidly oxidized to ferric

hydroxide.

( ) ( )3222 424 OHFeOHOOHFe →++ (1.5)

Dehydrolysis of this product leads to the formation of the corrosion products normally seen

on the metal surface (Eqs. 1.6 and 1.7).

( ) OHOFeOHFe 2323 32 +↓→ (1.6)

( ) OHOHFeOOHFe 23 )( +↓→ (1.7)

As solid corrosion products are precipitated at the anode, they may cause the

precipitation of other ions of water. Thus, a corrosion film may show traces of hardness

salts, or such suspended matter as mud, sand, silt clay or microbiological slime. If a porous

film forms over the metal, corrosion can continue because metal ions can penetrate it and

reach the solution interface, but when a tight adherent film is formed, ionic diffusion is

prevented and the metal will no longer dissolve.

1.1.3. Classification of corrosion

There is no universally accepted classification of corrosion, but the following

classification is adapted hereafter [8-10]:

1.1.3.1. Uniform corrosion

This is also called general corrosion. The surface effect produced by most direct

chemical attacks (e.g., as by an acid) is a uniform etching of the metal. Uniform or general

corrosion, which is the simplest form of corrosion, is an even rate of metal loss over the

exposed surface. It is generally thought of as metal loss due to chemical attack or

dissolution of the metallic component into metallic ions. In high-temperature situations,

uniform metal loss is usually preceded by its combination with another element rather than

its oxidation to a metallic ion. Combination with oxygen to form metallic oxides or scale

results in the loss of material in its useful engineering form.


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Some types of uniform corrosion and their description are given below.

• Atmospheric corrosion on steel tanks, steel containers, Zn parts, Al plates, etc.,

• High-temperature corrosion on carburized steels that forms a porous scale of

several iron oxide phases.

• Liquid-metal corrosion on stainless steel exposed to a sodium chloride

environment.

• Molten-salt corrosion on stainless steels due to molten fluorides (LiF, BeF2, etc.,).

• Biological corrosion on steel, Cu– alloys, Zn– alloys in seawater.

• Stray-current corrosion on pipelines near railroad.

1.1.3.2. Galvanic corrosion

Galvanic corrosion is an electrochemical action of two dissimilar metals in the

presence of an electrolyte and an electron conductive path (Fig. 1.10). It occurs, when two

different metallic materials are electrically connected and placed in a conductive solution

(electrolyte), an electric potential exists. This potential difference will provide a stronger

driving force for the dissolution of the less noble (more electrically negative) material. It

will also reduce the tendency for the more noble metal to dissolve. The Precious metals

such as gold and platinum are at the higher potential (more noble or cathodic) end of the

series (protected end), while zinc and magnesium are at the lower potential (less noble or

anodic) end. For example when aluminum alloys or magnesium alloys are in contact with

steel (carbon steel or stainless steel), galvanic corrosion can occur and accelerate the

corrosion of the aluminum or magnesium.

Fig. 1.10: Galvanic corrosion of steel pipe connected to copper connecter.


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1.1.3.3. Localized corrosion

This term implies that, specific parts of an exposed surface area corrode in a suitable

electrolyte. This form of corrosion is more difficult to control than general corrosion.

Localized corrosion can be classified as,

• Crevice corrosion: Crevice corrosion is a localized type of corrosion occurring within

or adjacent to narrow gaps or openings formed by metal-to-metal-to-nonmetal contact. It

results from local differences in oxygen concentrations, associated with deposits on the

metal surface, gaskets, lap joints or crevices under a bolt or around rivet heads where small

amounts of liquid can collect and become stagnant. Crevice corrosion may take place on

any metal and in any corrosive environment. However, metals like aluminum and stainless

steels that depend on their surface oxide film for corrosion resistance are particularly prone

to crevice corrosion, especially in environments such as seawater that contain chloride

ions. The material responsible for forming the crevice need not be metallic. Wood, plastic,

rubber, glass, concrete, asbestos, wax, and living organisms have been reported to cause

crevice corrosion (Fig. 1.11). It is frequently more intense in chloride environments. The

mechanism of crevice corrosion is electrochemical in nature, it requires a prolong time to

start the metal oxidation process, but it may be accelerated afterwards.

Fig. 1.11: Crevice corrosion of mild steel connectors.

• Filiform corrosion: It is basically a special type of crevice corrosion, sometimes termed

"under film" corrosion, is a type of rusting which results in the formation of threadlike


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filaments of corrosion product. This type of corrosion occurs under painted or plated

surfaces when moisture permeates the coating. Lacquers and quick-dry paints are most

susceptible to the problem.

• Pitting corrosion: Pitting corrosion is a form of corrosion often associated with other

types of corrosion mechanisms. It is characterized by a highly localized loss of metal. The

initiation of a pit is associated with the breakdown of the protective film on the metal

surface. (Fig. 1.12). Corrosion products often cover the pits, and may form "chimneys".

Pitting is considered to be more dangerous than uniform corrosion damage because it is

more difficult to detect, predict and prevent. A small and narrow pit with minimal overall

metal loss can lead to the failure of an entire engineering system. Once initiated, both

crevice and pitting corrosion can be explained by differential concentration cells. Cathodic

reactions, i.e., oxygen reduction or hydrogen evolution may start in the crevice or the pits.

Large surface areas will become cathodic and pits or crevices will become anodic and

corrode. Presence of aggressive ions (Cl-, Br

-, F

-, I

-) inducing local attack (dissolution) of

passive film.

Fig. 1.12: Pitting corrosion in mild steel.

• Intergranular corrosion: Intergranular corrosion is a preferential attack on the grain

boundary phases or the zones immediately adjacent to them. Little or no attack is observed

on the main body of the grain. This results in the loss of strength and ductility. The attack

is often rapid, penetrating deeply into the metal and causing failure. When it occurs, the


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surface of the material can appear unattacked, but the mechanical strength of the alloy can

deteriorate slowly or rapidly.

1.1.3.4. Stress corrosion cracking (SCC)

SCC is the growth of cracks in a corrosive environment (Fig. 1.13). It can lead to

unexpected sudden failure of normally ductile metals subjected to a tensile stress,

especially at elevated temperature in the case of metals. SCC is highly chemically specific,

and certain alloys are likely to undergo SCC only when exposed to low chemical

environments. This phenomenon is known as environmentally induced cracking (EIC)

which is divided into the following categories:

• Hydrogen-induced cracking (HIC).

• Corrosion-fatigue cracking (CFC).

Fig. 1.13: Stress corrosion cracking of mild steel.

1.1.3.5. Erosion corrosion

Erosion corrosion is a degradation of material surface due to mechanical action,

often by impinging liquid, abrasion by slurry, particles suspended in fast flowing liquid or

gas, bubbles or droplets, cavitations etc. It is the result of a combination of an aggressive

chemical environment and high fluid surface velocities. This can be the result of fast fluid

flow past a stationary object, such as the case with the oilfield check valve, or it can result

from the quick motion of an object in a stationary fluid, such as when a ship's propeller

churns the ocean.


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1.1.4. Rate of corrosion

As previously mentioned (Wagner and Traud’s theory), three basic steps are

necessary for corrosion to proceed. If any step is prevented from occurring, then corrosion

stops. The slowest step determines the rate of the overall corrosion process. The cathodic

reaction (Eq. 1.3) is the slowest of the three steps involved in the corrosion of steel and this

reaction determines the rate. A large cathodic surface area relative to the anodic area

allows more oxygen, water and electrons to react which increase the flow of electrons from

the anode leading to the increase of corrosion rate more rapidly. Conversely, as the

cathodic area becomes smaller relative to the anodic area, the corrosion rate decreases.

This ratio affects the corrosion rate and plays a significant role in the selection of effective

inhibitors to control corrosion [11].

1.1.5. Factors influencing the corrosion rate

Primary factors influence the corrosion rate is the conditions of the metal surface and

the secondary factors are the nature of the environment [12].

Primary factors

1.1.5.1. Nature of the metal

The tendency of the metal to undergo corrosion is mainly dependent on the nature of

the metal. In general the metal with lower electrode potential have more reactive and more

susceptible for corrosion and metal with high electrode potential are less reactive and less

susceptible for corrosion. For example, metals like K, Na, Mg, Zn etc., have low electrode

potential are undergo corrosion very easily where as noble metals like Ag, Au, Pt have

higher electrode potential, their corrosion rate are negligible. But there are few exception

for this general trend as some metals show the property of passivity like Al, Cr, Ti, Ta etc.,

According to electrochemical series, metal with more positive potential are relatively

stable and those with more negative potential are unstable [13]. If we know the electrode

potentials of metals in some electrolyte, we may predict whether metal would corrode or

not. The electromotive force (E) is the difference between electric potentials of cathodic

and anodic reactions.

E = Ecathodic − Eanodic (1.8)


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This value is related to Gibbs free energy by the equation,

∆G = -nFE (1.9)

∆G is the change of Gibbs free energy of corrosion reaction, n is the number of electrons

taking part in the corrosion reaction and F is Faraday’s constant.

1.1.5.2. Surface state of the metal or nature of the corrosion product

The corrosion product is usually the oxide of the metal, and the nature of the product

determines the rate of further corrosion process. If the oxide layer which forms on the

surface is stoichiometric, highly insoluble and non-porous in nature with low ionic and

electronic conductivity, then that type of layer effectively prevents further corrosion,

which acts as a protective film. For example, Al, Cr, Ti develop such a layer on their

surface and become passive to corrosion, and some metals like Ta, Zr and Mo not only

forms such a protective layers but are capable of self repairing oxide films when it is

damaged. Hence, these are extremely passive metals. If the oxide layer formed on the

metal surface is non-stoichiometric, soluble, unstable and porous in nature and have an

appreciable conductivity, they cannot control corrosion on the metal surface (For Ex: oxide

layer formed on metals like Zn, Fe, Mg etc,).

Secondary factors

1.1.5.3. pH of the medium

In general rate of corrosion is higher in acidic pH than in neutral and alkaline pH.

In case of iron, at very high pH, protective coating of iron oxide is formed which prevents

corrosion, whereas at low pH severe corrosion takes place. But for metals like Al,

corrosion rate is high even at high pH. For metals like Zn, Fe, Mg etc., hydrogen evolution

is thermodynamically favored cathodic reaction, and hence the corrosion of these metals in

acidic medium is therefore highly pH dependent. A decrease in pH facilitates the rate of

hydrogen evaluation and hence increases the corrosion rate. In case where a protective film

is formed on the metal surface, change in solution pH may affect the solubility of the film

and therefore affect the corrosion process. Thus under varying pH conditions of the

medium, a corroding surface may exhibit activity, immunity or passivity [20].


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1.1.5.4. Temperature of the medium

Corrosion is an activation-controlled chemical reaction, the rate of which is

greatly affected by temperature. Usually, corrosion rate increases significantly as

temperature increases. A rule of thumb is that when corrosion is controlled by diffusion of

oxygen, the corrosion rate at given oxygen concentration approximately doubles for every

30 °C rise in temperature. In an open vessel, allowing dissolved oxygen to escape, the rate

increases with temperature to about 80 °C and then falls to a very low value at the boiling

point. The lower corrosion rate above 80 °C is related to a marked decrease of oxygen

solubility in water and this effect eventually overshadows the accelerating effect of

temperature alone. In a closed system, on the other hand, oxygen cannot escape, and the

corrosion rate continues to increase with temperature until all the oxygen is consumed.

When corrosion is accompanied by hydrogen evolution, the corrosion rate is more than

double for every 30 °C rise in temperature [14]. In general, as temperature rises, diffusion

increases, and both viscosity and over-voltage decrease causing depolarization by

hydrogen evolution. Increased diffusion enables more dissolved oxygen to react with

cathodic surface, thereby depolarizing the corrosion cell. A decrease in viscosity aids

depolarization mechanism, and it favors the solution having atmospheric oxygen and

enhances hydrogen evolution. In a domestic water system, an increase in temperature from

25 oC to 75

oC (Fig. 1.14) may increase the corrosion as much as 400 percent. An increase

in temperature is normally expected to speed up a chemical reaction according to

thermodynamic considerations [15].

Fig. 1.14: Effect of temperature on the corrosion rate of low carbon steel in tap water.

At constant O2

concentration


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1.1.5.5. Effect of dissolved oxygen

Dissolved oxygen plays a very important and complicated role in the corrosion of

metals. Oxygen takes part in cathodic processes on the metal surface in neutral, alkaline

and acidic media. If dissolved oxygen is absent in water, corrosion diminishes nearly to

zero in neutral and alkaline solutions. If the concentration of dissolved oxygen increases,

corrosion accelerates as a result of oxygen participation in the cathodic processes. In the

presence of oxygen, depolarization of the cathodic products takes place according to Eqs.

(1.1) and (1.2). In most situations, depolarization by oxygen that tend to control the rate at

which the iron corrodes. However, Eq. (1.2) is generally so rapid that oxygen concentration

at the cathodic surface approaches zero. Therefore, rate of oxygen depolarization depends

on the rate of diffusion of oxygen through the resistant film at the surface of the metal.

Ferrous ion is converted to the ferric state by further oxidation, and most ordinary rust is

comprised of the hydrated ferric oxide. Frequently, a black layer of magnetic hydrous

ferrous ferrite (Fe3O4. nH2O) forms between Fe2O3 and FeO. Hence, it is considered that,

rust film normally consists of three layers of iron oxide in different states of oxidation.

Although increase in oxygen concentration at first accelerates the corrosion of iron, it is

found that, beyond a critical concentration, the corrosion rate drops again to a low value. If

we inject more and more oxygen in water, under some particular conditions (in water of

high purity) and at high temperature may result in the formation of a passive protective

dense film composed of metal oxides on the metal surface and corrosion would decrease

[16]. The injection of oxygen in water is one of the corrosion control methods at power

stations.

Cohen [17] reported that the corrosion rate in the presence of oxygen is 65 times the

rate in the absence of oxygen. Whitman [18] stated that the corrosion rate showed increase

at higher velocity due to an increase in oxygen diffusion and breaking down of the

protective films on the metal surfaces. Frese [19] showed that, iron tends to become

passive with high oxygen. Fig. 1.16 shows the effect of oxygen concentration on the

corrosion of low carbon steel in tap water at different temperatures.


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Fig. 1.16: Effect of oxygen concentration on the corrosion of low carbon steel in tap water

at different temperatures.


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SECTION - II: CORROSION INHIBITORS

1.2.1. Introduction to corrosion inhibitors

A corrosion inhibitor is a chemical substance which, when added in small

concentrations to an environment, minimizes or prevents the rate of corrosion.

Concentrations of corrosion inhibitors can change from 1 to 15,000 ppm (0.0001 to 1.5 wt

%). Corrosion inhibitors can be solids, liquids and gases, and can be used in solid, liquid

and gaseous media. Solid media can be concrete, coal slurries or organic coatings (paints).

Liquids may be water, aqueous solutions or organic solvents. A gaseous medium is an

atmosphere or water vapor. Corrosion inhibitors are selected on the basis of solubility or

dispersibility in the fluids which are to be inhibited. For instance, in a hydrocarbon system,

a corrosion inhibitor soluble in hydrocarbon is used. Two phase system composed of both

hydrocarbons and water is utilized as oil soluble water-dispersible inhibitors.

Corrosion inhibitors have been found to be effective and flexible means of

corrosion mitigation. The use of chemical inhibitors to decrease the rate of corrosion

processes is quite varied. Corrosion inhibitors are used in oil and gas exploration and

production, petroleum refineries, chemical manufacturing, heavy manufacturing, water

treatment and product additive industries [20]. In the oil extraction, processing and

chemical industries, inhibitors have always been considered to be the first line of defense

against corrosion. Several scientific studies have been recently reported to the subject of

corrosion inhibitors [21-24].

Inhibitors find major use in closed environmental systems that have good circulation

so that an adequate and controlled concentration of inhibitor is ensured. Such conditions

can be met, for instance in cooling water recirculating systems, oil production, oil refining

and acid pickling of steel components. Inhibitors can be organic or inorganic compounds

and they are usually dissolved in aqueous environments. The organic inhibitors include

amines, heterocyclic nitrogen compounds, and sulfur compounds such as thioethers,

thioalcohols, thioamides, thiourea and hydrazine. Many inorganic inhibitors are nowadays

largely replaced by organic inhibitors due to their toxicity. Thus practical criteria for the

selection of corrosion inhibitors from the great variety of inorganic and organic substances

with inhibiting properties are not only their protection efficiency but also safety of use,


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economic constraints and compatibility with other chemicals in the system and

environmental concerns [25].

In order to avoid or reduce the corrosion of metallic materials, inhibitor used in

cooling system must satisfy the following criteria [26]:

a) It must give good corrosion protection at a very low concentration of inhibitor.

b) It must protect all exposed materials from the attack of corrosion.

c) It must remain efficient in extreme operating conditions (higher temperature and

velocity).

d) In case of an under or over dosage of inhibitor, corrosion rate should not increase

drastically.

e) The inhibitor or reaction products of the inhibitor should not form any deposits on

the metal surface particularly at locations where heat transfer takes place.

f) It should suppress both uniform and localized corrosion.

g) It should have long range effectiveness.

h) It should not cause toxicity and pollution problems.

1.2.2. Mechanism of corrosion inhibition

Many corrosion inhibitors can form protective films on the metal surface and

diminish possible contact with aggressive components. In order to protect metals from

corrosion, inhibitors must reach the surface of metals and react with the products of

electrochemical reactions or be adsorbed. The protective mechanisms of anodic, cathodic

and adsorbing inhibitors are different. The protective mechanism of anodic inhibitors

(phosphates, carbonates, molybdates and nitrites) is based on the reaction with the metal

surface and the formation of passive layers of oxides, hydroxides or salts. These inhibitors

significantly influence the corrosion potentials of the protected metals. The protective

mechanism of cathodic inhibitors is generally based on the reaction with the products of a

cathodic electrochemical reaction (OH−) [27-29]. For example, Zn

2+ reacts with OH

− with

the formation of insoluble Zn(OH)2 at cathodic sites of metallic surfaces. Organic

inhibitors are adsorbed on the metal surface and the presence of polar groups such as CN,

CS and CO in organic molecules with free electrons on N, S, O and P atoms promote their

adsorption on metallic surfaces. The mechanism of adsorption may be physical or

chemical. When weak Coulomb forces are formed between atoms of inhibitors and


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metallic atoms, the adsorption is physical. If strong chemical bonds are formed between

atoms of inhibitors and metallic atoms, the chemisorption occurs. Organic inhibitors are

sometimes called non-passivating types. They nearly have no influence on the corrosion

potential of metals.

Organic compounds containing multiple bonds, especially triple bonds are effective

inhibitors. The choice of effective inhibitors is based on their mechanism of action and

their electron donating capability. The inhibiting ability of the inhibitor is supported by

molecular structure of the adsorption active sites with lone pair and/or p - orbitals such as

heterocyclic rings containing sulphur, oxygen, phosphorus and/or nitrogen atoms [30-32].

They have an ability to accept or donate electrons in order to be adsorbed on metallic

surfaces by electrostatic interaction between the unshared electron pair of corrosion

inhibitor and metal. These inhibitors are usually adsorbed on the metal surface by the

formation of a coordinate covalent bond (chemical adsorption) or the electrostatic

interaction between the metal and inhibitor (physical adsorption). The adsorbed inhibitors

then acts to retard the cathodic and/or anodic electrochemical reactions. Inhibitors in acid

solutions can interact with metals and affect the corrosion reaction in a number of ways,

some of which may occur simultaneously. It is very difficult to assign a single general

mechanism of action to an inhibitor because the mechanism may vary with experimental

conditions. Thus, the action of an inhibitor depends on its concentration, the pH of the

acid, the presence of other species in the solution, the extent of reaction to form secondary

inhibitors and the nature of the metal. The mechanism of action of inhibitors with the same

functional group may additionally differ with factors such as the effect of the molecular

structure and the electron density. Inhibition usually results from one or more of the

following mechanisms [33-34].

• Adsorption of corrosion inhibitors onto metals: The inhibitive performance is

usually depends on the fraction of the surface covered (θ) with adsorbed inhibitor.

But, at low surface coverage (θ < 0.1), the effectiveness of adsorbed inhibitor species

in retarding the corrosion reactions may be greater than at high surface coverage.

• Presence of surface charge on the metal: Adsorption of inhibitor on the metal

surface may be due to electrostatic force of attraction between ionic charges or dipoles

of the adsorbed species and the electric charge on the metal at the metal/solution

interface.


23

• Effect of functional group and structure of the inhibitor: Inhibitors can also bond

to metal surfaces by electron transfer to the metal to form a coordinate type of bond.

Usually, the above process is favored when the metal contain vacant electron orbitals

of low energy such as transition metals. Electron transfer from the adsorbed species is

favored by the presence of relatively loosely bound electrons. Example: Anions and

neutral organic molecules containing lone pair electrons or electron systems

associated with multiple bonds especially triple bonds or aromatic rings. The electron

density at the functional group is directly proportional to the inhibitive efficiency in a

series of related compounds.

• Interaction between inhibitor and water molecules: Adsorption of inhibitor

molecules is the displacement reaction involving removal of adsorbed water

molecules from the metal surface. During the adsorption of an inhibitor molecule, the

change in interaction energy with water molecules in passing from the dissolved to the

adsorbed state forms an important part of the free energy change on adsorption. This

increases the solvation energy of the inhibitor species, which in turn related with size

of the hydrocarbon portion of an inhibitor molecule. Thus, increasing size leads to

decreasing solubility and increasing adsorption ability. This is consistent with the

increasing inhibitive efficiency observed at constant concentrations with increasing

molecular size in a series of related compounds.

• Interaction of adsorbed inhibitor species: Lateral interactions between adsorbed

inhibitor species may become significant as the surface coverage and hence the

proximity of the adsorbed species increases. These lateral interactions may be either

attractive or repulsive. Attractive interactions occur between molecules containing

large hydrocarbon components (e.g., n-alkyl chains). As the chain length increases,

the increasing Van der Waals attractive force between the adjacent molecules leads to

stronger adsorption at high coverage.

• Reaction of adsorbed inhibitors: In some inhibitors, the adsorbed corrosion inhibitor

may react usually by electrochemical reduction to form a product that may also exhibit

inhibitive action. Inhibition due to the added substance is called primary inhibition and

that due to the reaction product is secondary inhibition. In these cases, the inhibitive

efficiency may increase or decrease with time, it depends on whether the secondary

inhibition is more or less effective than the primary inhibition. For example,

sulfoxides can be reduced to sulfides which are more efficient inhibitors.


24

• Formation of a diffusion barrier: The absorbed inhibitor may form a surface film

that acts as a physical barrier to limit the diffusion of ions or molecules to or from the

metal surface, and hence retard the rate of corrosion reactions. Generally, this effect

occurs when the inhibitor species are large molecules (e.g., proteins such as gelatin or

agar agar, polysaccharides such as dextrin or compounds containing long hydrocarbon

chains). A surface film of these types of inhibitors affects both anodic and cathodic

reactions.

• Participation in the electrode reactions: Sometimes corrosion reactions involve the

formation of adsorbed intermediate species with surface metal atoms (e.g., adsorbed

hydrogen atoms in the hydrogen evolution reaction and adsorbed (FeOH)2 in the

anodic dissolution of iron). The adsorbed inhibitors will hinder the formation of these

adsorbed intermediates, but the electrode processes may then proceed by alternative

paths through intermediates containing the inhibitors. In these processes, the inhibitor

species act like catalyst and remain unchanged. Such action of inhibitor is generally

characterized by an increase in the Tafel slope of the anodic dissolution of the metal.

Inhibitors may also retard the rate of hydrogen evolution on the metals by affecting

the mechanism of the reaction by increasing the Tafel slopes of cathodic polarization

curves. This effect has been observed on iron in the presence of inhibitors such as

phenylthiourea, acetylenic hydrocarbons, aniline derivatives, benzaldehyde derivatives

and pyrilium salts.

• Alteration of the electrical double layer: The adsorption of ions or species that can

form ions on metal surfaces will change the electrical double layer at the

metal/solution interface, and this will affect the rates of the electrochemical reactions.

The adsorption of cations such as quaternary ammonium ions and protonated amines

makes the potential more positive in the plane of the closest approach to the metal ions

from the solution. This positive potential shift hinders the discharge of the positively

charged hydrogen ions. On the other hand, the adsorption of anions makes the

potential more negative on the metal side of the electrical double layer, and this will

tend to accelerate the rate of discharge of hydrogen ions. This effect has been

observed with sulfosalicylate ions and the benzoate ions.


25

1.2.3. Classification of corrosion inhibitors

A common classification of inhibitors is based on their effects on the electrochemical

reactions involved in the corrosion process [35-40].

1.2.3.1. Passivating inhibitors

Passivating inhibitors cause a large anodic shift of the corrosion potential and forcing

the metallic surface into the passivation range. There are two types of passivating

inhibitors.

a) The oxidizing anions such as chromates, nitrites and nitrates that can passivate steel in

the absence of oxygen.

b) The non-oxidizing ions such as phosphates, tungstates and molybdates that require the

presence of oxygen to passivate steel.

In general, passivation inhibitors can actually cause pitting and accelerate corrosion

when concentrations fall below minimum limits. For this reason, it is essential to monitor

the inhibitor concentration.

1.2.3.2. Volatile inhibitors

Volatile corrosion inhibitors (VCIs) also called vapor phase inhibitors (VPIs) are

compounds transferred in a closed environment to the site of corrosion by volatilization

from a source. In boilers, volatile basic compounds such as morpholine or hydrazine are

transported with steam to prevent corrosion in condenser tubes by neutralizing the acidic

carbon dioxide or by shifting the surface pH towards less acidic. If the corrosion product is

volatile, it volatilizes as soon as it is formed, thereby leaving the underlying metal surface

exposed for further attack. This causes rapid and continuous corrosion leading to excessive

corrosion. For example, molybdenum oxide (MoO3), the oxidation corrosion product of

molybdenum is volatile. In closed vapor process (shipping containers), volatile solids such

as salts of dicyclohexylamine, cyclohexylamine and hexamethylene amine are used as

volatile corrosion inhibitors.

1.2.3.3. Cathodic inhibitors

Cathodic inhibitors act by either slowing the cathodic reaction itself or selectively

precipitating on cathodic areas to limit the diffusion of reducing species to the surface. The


26

rates of the cathodic reactions can be reduced by the use of cathodic poisons. Cathodic

inhibitors reduce corrosion by slowing the reduction reaction rate of the electrochemical

corrosion cell. For example, calcium, magnesium and zinc ions will precipitate as

hydroxides on cathodic sites as the local environment becomes more alkaline due to the

reduction reaction at these sites. Cathodic inhibitors are effective when they slow down the

cathodic reaction rate. Arsenic, bismuth and antimony are referred to as cathodic poisons,

which reduce the hydrogen reduction reaction rate and thus lower the overall corrosion

rate. Other cathodic inhibitors remove reducible species from the environment.

1.2.3.4. Anodic inhibitors

Anodic inhibitors usually act by forming a protective oxide film on the surface of

the metal causing a large anodic shift of the corrosion potential. This shift forces the

metallic surface into the passivation region. They are also sometimes referred to as

passivators. Chromates, nitrates, tungstate, molybdates are some examples of anodic

inhibitors. Although, this type of control is affected, yet it may be dangerous since severe

local attack can occur, if certain areas are left unprotected by depletion of the inhibitors.

1.2.3.5. Mixed inhibitors

Some substances inhibit corrosion by reducing simultaneously the rate of the anodic

and cathodic reactions involved in the corrosion process and are therefore called mixed

inhibitors. They are typically film forming compounds that cause the formation of

precipitates on the surface blocking both anodic and cathodic sites indirectly. Anodic

inhibitors are, for the most part, dangerous inhibitors, especially if its concentration is too

less. But cathodic inhibitors are generally safe. Mixed inhibitors are less dangerous than

pure anodic inhibitors, and in number of cases they may not increase the corrosion

intensity. The most common inhibitors of this category are the silicates and the phosphates.

1.2.3.6. Synergistic inhibitors

It is very rare that a single inhibitor is used in systems such as cooling water

systems. More often, a combination of inhibitors (anodic and cathodic) is used to obtain

better corrosion protection properties. The blends which are produced by mixing of multi-

inhibitors are called synergistic blends. Examples include chromate-phosphates,

polyphosphate-silicate, zinc-tannins, zinc-phosphates.


27

1.2.3.7. Precipitation inhibitors

Precipitation inhibitors are compounds that cause the formation of precipitates on

the surface of the metal, thereby providing a protective film. Hard water that is high in

calcium and magnesium is less corrosive than soft water because of the tendency of the

salts in the hard water to precipitate on the surface of the metal and form a protective film.

The most common inhibitors of this category are the silicates and the phosphates. Sodium

silicate, for example, is used in many domestic water softeners to prevent the occurrence

of rust. In aerated hot water systems, sodium silicate protects steel, copper and brass.

1.2.3.8. Green corrosion inhibitors

There is no clear and accepted definition of “environmentally friendly” or “green”

corrosion inhibitors. In practice, corrosion inhibition studies have become oriented towards

human health and safety considerations. For this purpose, recently the researchers have

been focused on the use of eco-friendly compounds such as plant extracts which contain

many organic compounds. Amino acids, alkaloids, pigments and tannins are used as green

alternatives for the toxic and hazardous compounds. Due to biodegradability, eco-

friendliness, low cost and easy availability, the extracts of some common plants and plant

products have been studied as corrosion inhibitors for various metals and alloys under

different environment [41].

1.2.4. Adsorption

Adsorption is a process that occurs when a gas or liquid solute accumulates on the

surface of a solid or a liquid (adsorbent), forming a molecular or atomic film (the

adsorbate). It is different from absorption, in which a substance diffuses into a liquid or

solid to form a solution. The term sorption encompasses both processes, while desorption

is the reverse process. The adsorption of ions or neutral molecules on bare metal surfaces

immersed in solution is determined by the mutual interactions of all species present at the

phase boundary. These include electrostatic and chemical interactions of the adsorbate

with the surface, adsorbate–adsorbent and adsorbate–solvent interactions. Adsorption is

operative in most natural physical, biological and chemical systems, and is widely used in

industrial applications such as activated charcoal, synthetic resins and water purification.

There are two types of adsorption depending on the nature of forces involved [44, 42]. (a)

Physisorption or physical adsorption is a type of adsorption in which the adsorbate adheres


28

to the surface only through Van der Waals (weak intermolecular) interactions, which are

also responsible for the non-ideal behaviour of real gases. (b) Chemisorption is a type of

adsorption whereby a molecule adheres to a surface through the formation of a chemical

bond. The factors that influence the adsorption of inhibitor ions on metal surfaces are [43]:

a) Surface charge on the metal: Adsorption may occur due to the electrostatic attractive

forces between the ionic charges or dipoles on the adsorbed inhibitor and the electric

charge on the metal at the metal-solution interface.

b) The functional groups and structure of inhibitor: Inhibitors can bind to the metal

surface by electron transfer and form a coordinate type of linkage leading to strong

binding and effective inhibition. Species containing relatively loosely bound electrons

in anions, neutral molecules, lone pair of electrons, π – electron systems associated

with triple bonds or organic ring systems and the functional groups containing

elements of group V or VI of the periodic table favour facile electron transfer and

stronger bond formation and hence effective inhibition. The tendency to form stronger

coordinate bond increases with decreasing electronegatively and follows the order O <

N < S < P.

c) The interaction between adsorbed inhibitor species (synergism and antagonism):

Adsorbed species may enter into various interactions on the surface of an electrode

that may significantly influence their inhibitive properties and the mechanism of their

action.

d) Reaction of adsorbed inhibitors: The adsorbed inhibitor species may react usually

by electrochemical reduction to form a product which is also inhibitive. Inhibition due

to the added substance is termed as primary inhibition and that due to reaction

products as secondary inhibition.

Adsorption is usually described through isotherms, i.e., the amount of adsorbate on

the adsorbent as a function of its pressure (if gases) or concentration (if liquids) at constant

temperature. The first mathematical fit to an isotherm was published by Freundlich and

Küster is a purely empirical formula for gaseous adsorbates. Irving Langmuir published a

new isotherm model for gases adsorbed on solids which retained his name. It is a semi-

empirical isotherm derived from a proposed kinetic mechanism. It is based on four

assumptions:


29

1. The surface of the adsorbent is uniform.

2. Adsorbed molecules do not interact.

3. All adsorption occur through the same mechanism.

4. At the maximum adsorption, only a monolayer is formed.

These assumptions are seldom true. There are always imperfections on the surface,

and the adsorbed molecules are not necessarily inert. The mechanism is not clearly same

for the very first molecules to adsorb as for the last. The fourth assumption is the most

troublesome as frequently more molecules will adsorb on the monolayer. This problem is

addressed by the BET isotherm for relatively flat (non-microporous) surfaces.

Nevertheless, the Langmuir isotherm is the first choice for most models of adsorption and

has many applications in surface kinetics (usually called Langmuir-Hinshelwood kinetics)

and thermodynamics.

Various adsorption isotherms were found to describe the adsorption of inhibitors on

metal surface such as Langmuir [44 – 46], Temkin [47–49], Frumkin [50 – 52], Flory–

Huggins [53 – 55], Dhar–Flory–Huggins and Bockris–Swinkels [56], Freundlich [56, 57].

1.2.5. Polarization

Polarization [42] is defined as the departure of the electrode potential from their

equilibrium values that is from open circuit potential (OCP) causing a decrease in the

current density. Corrosion process on a metal surface or when two metals are in contact is

due to the current flowing from anodic area to cathodic area. The open circuit potential

difference between the anodic and cathodic areas determines the direction of current flow.

However, the magnitude of the current is controlled by polarization characteristics of the

electrodes. An important factor that controls the polarization is the concentration of the

corrosive species. When the corrosion reaction progresses, the concentration of the

electrolyte species changes (gets depleted at the cathode and accumulated at the anode) in

the vicinity of each electrode. The greater the polarization of the electrodes (cathodic or

anodic), the smaller will be the corrosion current, and the open circuit potential difference

being high. If the anode alone undergoes polarization, the rate of corrosion is controlled by

anodic polarization as shown in Fig. 1.17a, where the corrosion current is largely

determined by the anode [57]. Fig.1.17b shows the cathodic polarization where the cathode

alone undergoes polarization. Here the cathodic polarization curve is much steeper and the


30

corrosion current is under cathodic control. If both electrodes undergo polarization, it is

said to be under mixed control as shown in Fig.1.17c.

Fig. 1.17: Evans polarization diagrams: (a) anodic control (b) cathodic control (c) mixed

control.

1.2.6. Electrochemical impedance spectroscopy

Electrochemical Impedance Spectroscopy (EIS) is a powerful rapid and accurate

nondestructive method for the evaluation of wide range of materials. Electrochemical

methods based on alternating currents can be used to obtain insights into corrosion

mechanisms and to establish the effectiveness of corrosion control methods such as

inhibition and coatings. In an alternating current circuit, impedance determines the

amplitude of current for a given voltage. Impedance is the proportionality factor between

voltage and current. In electrochemical impedance spectroscopy (EIS), the response of an

electrode to alternating potential signals of varying frequency is interpreted on the basis of

circuit models of the electrode/electrolyte interface [42]. The simplest model for

characterizing the metal – solution interface includes the three essential parameters, Rs (the

solution resistance), Cdl (the capacitance of the double layer) and Rp (the polarization

resistance). When direct current measurements are carried out (i.e., frequency is zero), the

impedance of the capacitor approaches infinity. In parallel electrical circuits, the circuit

with the smallest impedance dominates, with the result that, under these conditions, the

sum of Rs and Rp is measured.

When compared to other techniques for corrosion evaluation, EIS has several

advantages:

• It gives kinetic information on the corrosion processes. The use of AC signals allows

the separation between the resistances of charge transfer resistance of the coating

itself and of the solution.

Icor Icor Icor

Ec

Ecor

Ec

Ecor

Ec

Ecor


31

• It gives mechanistic informations. This is based on the use of ‘‘equivalent circuits’’

which are electronic circuits whose response is identical to that of the cell under

study.

• It provides information on the properties of the coating itself, namely its resistance

and capacitance. The changes in these properties have been associated with the loss

of protective properties.

1.2.7. Quantum chemical calculations

Quantum chemical methods are very useful in determining the molecular structure as

well as elucidating the electronic structure and reactivity [58]. The selection of an effective

and appropriate inhibitor for the corrosion of metals has been widely carried out based on

empirical approach [59-60]. Recently, quantum chemical calculations have been

performed enormously to complement the experimental evidence. Quantum chemical

methods and molecular modeling techniques enable the definition of a large number of

molecular quantities characterizing the reactivity, shape and binding properties of a

complete molecule as well as of molecular fragments and substituents. The quantum

chemical calculations have been applied widely to compute electronic properties possibly

relevant to the inhibition action. Knowing the orientation of the molecule, favorable

configurations, atomic charges, steric and electronic effects would be useful for better

understanding of the inhibitor performance. It has been stated that the experimental data

can be correlated well with quantum chemical parameters such as electron density,

ionization potential, total energy, dipole moment, charge density, highest occupied

molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies

and the gap between them.


32

SECTION - III: ANTIOXIDANT ACTIVITY AND CORROSION

INHIBITION

An antioxidant is a molecule capable of slowing or preventing the oxidation of

other molecules. Oxidation is a chemical reaction that transfers electrons from a substance

to an oxidizing agent. The term antioxidant was used to refer specifically to a chemical

that prevent the consumption of oxygen. In the late 19th and early 20th century extensive

study was devoted to the uses of antioxidants in important industrial processes such as the

prevention of metal corrosion. It was found that these antioxidants can act as good

corrosion inhibitors and their efficiencies depend principally on the substituted functional

groups. Organic molecules of this type can adsorb on the metal surface and form a bond

between their N-electron pair and/or π-electron cloud and the metal surface, thereby

reducing the corrosion in different aggressive solutions.

Antioxidants (free radical scavengers) are widely applied in the coating industry to

prevent thermal and photochemical degradation of organic coatings by inactivating the free

radicals generated by these processes. Consequently, this study will address the application

of free radical scavengers in relation to cathodic delamination of organic coatings

(primers) and the steel surface [61].

Antioxidants from different origin have high bioavailability, therefore high

protective efficiency against free radicals [62]. Free radicals and singlet oxygen scavengers

(antioxidants) were found to have metal and alloy corrosion inhibition character, which

depend to a greater extent on the structural feature of the antioxidant added and to its

accepting - donating hydrogen or electron behaviors [63]. Greater antioxidant activity and

corrosion inhibition behaviour of molecules is linked to the electron donating effect of the

different groups attached to aromatic ring, which increases the electron density on the

benzene ring. The increasing delocalization of electron density in the molecule makes

more reactive towards scavenging reactive oxygen as well as inhibiting corrosion process.

The adsorption of inhibitor molecules is further stabilized by participation of π electrons of

benzene ring. Electronegative oxygen, sulfur and nitrogen atoms present in compounds

facilitate more efficient adsorption of the molecules on mild steel surface. Reduction of

oxygen availability in the corroding system and the presence of a barrier between the

electrode surface and oxygen in the medium retard the rate of metal corrosion [64].


33

Synthetic and natural antioxidants are free radical scavengers containing some N-

heterocyclic compounds and their derivatives which possess compact structures and high

thermal stabilities. They have been widely used as lubricating additives because they

possess excellent antioxidant and anticorrosion properties. The experimental results

demonstrates that antioxidants (free radical scavengers) containing hetero atoms, not only

accept the electron to terminate the oxygen radical reaction, but also donate electron to

form a stable chemical adsorption film on metal surface in order to prevent the corrosion

of metals. A series of N-containing compounds and their derivatives such as benzotriazole

derivatives, 1, 3, 4-thiadiazole derivatives, thiazole derivatives, oxazoline, thiazoline and

imidazoline derivatives were reported as excellent multifunctional lubricating additives

[65]. Christ Tamborski et al. [66] studied antioxidant and anticorrosion properties of new

aromatic phosphine compounds. The antioxidant and anticorrosion property of the some

natural products has been investigated by Shanab and coworkers [67].


34

SECTION - IV: CORROSION INHIBITION STUDIES ON MILD

STEEL: A REVIEW

1.4.1. Organic compounds as corrosion inhibitors

The mild steel corrosion inhibition by organic compounds has been widely studied

using different organic moieties. The use of organic inhibitors for preventing corrosion is a

promising alternative solution. Several studies reported that the adsorption of organic

compounds mainly depends on some physicochemical properties of the molecule such as

functional groups, possible steric effects and electron density of donor atoms. The choice

of effective inhibitors is based on their mechanism of action and their electron donating

capability. The inhibiting ability of an inhibitor is supported by molecular structure of the

adsorption active sites and/or p - orbitals such as heterocyclic rings containing sulphur,

oxygen, phosphorus and/or nitrogen atoms. They have an ability to accept or donate

electrons in order to be adsorbed on metallic surfaces by electrostatic interaction between

the unshared electron pair of corrosion inhibitor and metal.

Abboud et al. [68] synthesized and studied the inhibition of mild steel corrosion in

acidic medium by 2, 2'-bis(benzimidazole) using various corrosion monitoring techniques.

The effect of the Schiff base, N, N'-bis (salicylaldehyde)-1,3-diaminopropane and its

corresponding cobalt complex on the corrosion behaviour of steel in 1 M sulphuric acid

solution were investigated by Abdel-Gaber et al [69]. The effect of 4-(2'-amino-5'-

methylphenylazo) antipyrine (AMPA) on the corrosion of mild steel in a 2 M HCl solution

was studied by Abd El Rehima et al [70]. Ganesha Achary et al. [71] reported that, two

quinoline derivatives namely 8-hydroxy quinoline (HQ) and 3-formyl 8-hydroxy quinoline

(FQ) are good corrosion inhibitors on mild steel in hydrochloric acid solution. The role of

some new thiosemicarbazide derivatives as corrosion inhibitors for carbon steel in 2 M

HCl was investigated by Badr et al [72]. The new isoxazolidines has been synthesized and

its influence on corrosion inhibition of mild steel in 1M hydrochloric acid solution has

been studied by Ali et al [73]. A series of new thiazole derivative has been synthesized and

investigated as corrosion inhibitors for carbon steel in 2 M HCl solutions by Al-Sarawya et

al [74]. Mohammed et al. [75] evaluated the inhibiting affects of the newly synthesized

glycine derivative, 2-(4-(dimethylamino) benzylamino)acetic acid hydrochloride on the

corrosion of mild steel in concentrated H2SO4 solutions using different electrochemical

methods.


35

Shassi-Sorkhabi et al. suggested that, the newly synthesized compound namely 3H-

phenothiazin-3-one -7-dimethylamin acts as corrosion inhibitor on mild steel in 1 M HCl

solution [76]. Corrosion inhibitive effect of indole-3-acetic acid on the corrosion of mild

steel in 0.5M HCl medium was reported by Gulsen avci et al [77]. Experimental and

theoretical investigation of three newly synthesized organic compounds on mild steel

corrosion in sulfuric acid medium was reported by Bahrami and co-workers [78]. Fathy et

al. investigated the corrosion inhibition of mild steel using naphthalene disulfonic acid

[79]. The use of electrochemical and theoretical methods on the corrosion inhibition

mechanism of mild steel by three Schiff bases 2-{[(2-sulfanylphenyl)

imino]methyl}]phenol,2-{[(2)-1-(4 methylphenyl) methylidene] amino}-1-benznethiol and

2-[(2-sulfanylphen-yl)ethanimidoyl)]phenol on the corrosion of mild steel in 15 %

hydrochloric acid solution has been reported by Behpour and co-workers [80]. Ayse ongun

yuce et al. evaluate the adsorption and corrosion inhibition effect of 2-thiohydantoin on

mild steel in 0.1 M HCl solution [81].

Adsorption and inhibitive properties of some new mannich bases of isatin

derivatives on the corrosion of mild steel in acidic media has been studied by Ishtiaque et

al [82]. Kinetics of mild steel corrosion in aqueous acetic acid solutions has been studied

by Singh and Mukherjee [83]. Doner et al. reported the corrosion inhibitive behaviour of

some thiazoles on mild steel using experimental and theoretical studies [84]. The corrosion

inhibition effect of 3-[(2-hydroxy-benzylidene)-amino]-2-thioxo-thiazolidin-4-one on the

corrosion of mild steel in the 0.5 M H2SO4 medium has been investigated by Ali Doner

and coworkers [85]. The corrosion inhibition mechanism of some amino acids on the mild

steel was explored by experimentally and theoretically by Eddy [86]. Adsorption and

corrosion inhibitive properties of 2-amino-5-mercapto-1, 3, 4-thiadiazole on mild steel in

hydrochloric acid medium has been studied by Solmaz [87].

Some new triazole derivatives as inhibitors for mild steel corrosion in acidic

medium were reported by Wei-hua [88]. Novel Schiff base-based cationic gemini

surfactants, synthesis and their effect on corrosion inhibition of carbon steel in

hydrochloric acid solution was reported by Hegazy [89]. Synthesis and characterization of

some amino acid derived Schiff bases bearing nonionic species as corrosion inhibitors for

carbon steel in 2N HCl was studied by Negm and Zaki [90]. Corrosion monitoring of mild

steel in sulphuric acid solutions in the presence of some thiazole derivatives - molecular

dynamics, chemical and electrochemical studies has been studied by Khaled and Amin


36

[91]. Mohana and Badiea [92] has investigated the effect of temperature and fluid velocity

on corrosion mechanism of low carbon steel in presence of 2-hydrazino-4, 7-

dimethylbenzothiazole in industrial water medium. Benzimidazole and its derivatives as

corrosion inhibitors for mild steel in 1M HCl solution has been studied by Aljourani et al.

[93].

Tris-hydroxymethyl-(2-hydroxybenzylidenamino)-methane was synthesized and its

anticorrosive property for cold rolled steel in hydrochloric acid has been studied by Qing

et al [94]. The effect of some triazole derivatives against the corrosion of mild steel in 1 M

hydrochloric acid was studied by Zhanga et al [95]. Quantum chemical study of the

inhibition of the corrosion of mild steel in H2SO4 by some antibiotics was studied by

Nabuk et al [96]. Some Schiff base compounds containing disulfide bond were analyzed as

corrosion inhibitors for mild steel in HCl [97]. Substitutional adsorption isotherms and

corrosion inhibitive property of some oxadiazol-triazole derivative in acidic solution was

analyzed by Zhihua et al [98]. Gopi et al. [99] studied the triazole derived Schiff bases as

corrosion inhibitors for mild steel in hydrochloric acid medium. 1,7-Dimethyl-2-propyl-

1H,3H-2,5-bibenzo[d] imidazole as a corrosion inhibitor of mild steel in 1 M HCl has been

investigated by Patel et al [100]. Experimental and theoretical study on the inhibition

performance of triazole compounds for mild steel corrosion has been examined by Ahmed

et al [101].

Ramazan [102] investigated the inhibition effect of 5-((E)-4-phenylbuta-1,3-

dienylideneamino)-1,3,4-thiadiazole-2-thiol Schiff base on mild steel corrosion in

hydrochloric acid. The effectiveness of some diquaternary ammonium surfactants as

corrosion inhibitors for carbon steel in 0.5 M HCl solution has been studied by Negm et al

[103]. Experimental and molecular dynamics studies on the corrosion inhibition of mild

steel by 2-amino-5-phenyl-1,3,4-thiadiazole has been investigated by Yongming et al

[104]. Obot and Obi-Egbedi [105] studied the adsorption properties and inhibition of

ketoconazole on mild steel corrosion in sulphuric acid solution. Carbon steel corrosion

inhibition in hydrochloric acid solution using a reduced Schiff base of ethylenediamine

was studied by Adriana et al [106]. Inhibitive effect of diethylcarbamazine on the

corrosion of mild steel in hydrochloric acid has been investigated by Singh and Quraishi

[107]. Fekry and Ameer reported the corrosion inhibition of mild steel in 1 M H2SO4

media using newly synthesized heterocyclic organic molecules [108].


37

Three compounds of N-alkyl-sodium phthalamates were synthesized and evaluated

the corrosion inhibition efficiency for carbon steel in 0.5 M aqueous hydrochloric acid by

Eugenio et al [109]. Fouda and coworkers studies the inhibitory action of 4-phenylthiazole

derivatives on the corrosion of 304L stainless steel in HCl solution [110]. The inhibition

mechanism of 1, 3-diketone malonates on the corrosion of mild steel in aqueous

hydrochloric acid solution has been investigated by Lubanski et al [111]. Experimental and

theoretical investigations of semicarbazones and thiosemicarbazones as organic corrosion

inhibitors were studied by Khaled Goulart et al [112]. Hamdy and coworkers assessed the

inhibition of mild steel corrosion in hydrochloric acid solution by some triazole derivatives

using potentiodynamic polarization and EIS methods [113]. Influence of the 1-methyl-3-

pyridin-2-yl-thiourea on the corrosion inhibition of mild steel in 0.5 M sulphuric acid

solution was studied by Hosseini et al [114].

Jawich et al. investigated the effect of newly synthesized heptadecyl-tailed mono-

and bis-imidazolines on the inhibition of mild steel corrosion in a carbon dioxide-saturated

saline medium [115]. Thermodynamic and electrochemical investigations of 2-[(4-

phenoxy-phenylimino) methyl]-phenol on the corrosion of mild steel in 1 M HCl was

reported by Hulya Keles [116]. The corrosion inhibition effect formazan of benzaldehyde

on Mild Steel in 1 M and 2 M HCl media has been studied by Ananda [117]. Larabi et al.

tested some hydrazide derivatives as corrosion inhibitors for mild steel in 1M HCl solution

[118]. Corrosion inhibition potential of Sodium diethyldithiocarbamate on cold rolled steel

in 0.5 M hydrochloric acid solution has been studied by Li et al [119]. Liu and coworkers

studied the corrosion inhibition and adsorption behavior of 2-((dehydroabietylamine)

methyl)-6-methoxy phenol on mild steel surface in seawater [120]. The inhibitor effect of

tryptamine on the corrosion of mild steel in 0.5 M hydrochloric acid was evaluated by

Pongsak et al [121]. Experimental and theoretical investigation on the on the corrosion

inhibition of mild steel by 3-amino-1, 2, 4-triazole-5-thiol has been studied by Basak et al

[122].

Hossein et al. explained the structure-inhibition relationship study of the

benzimidazole, aniline and their derivatives on iron corrosion [123]. Musa et al. compare

the corrosion inhibition of mild steel in sulphuric acid by 4,4-dimethyloxazolidine-2-thione

using different electrochemical techniques [124]. Synthesis, characterization and corrosion

inhibition efficiency of 2-(6-methylpyridin-2-yl)-1Himidazo [4, 5-f][1,10] phenanthroline

on mild steel in sulphuric acid solution has been studied by Obi-Egbedi and coworkers


38

[125]. The influence of 4-amino-5-phenyl-4H-1, 2,4-trizole-3-thiol on the corrosion of

mild steel has been studied by Musa et al [126]. Outirite and coworkers assessed the ac

impedance, X-ray photoelectron spectroscopy and density functional theory studies of 3,5-

bis(n-pyridyl)-1,2,4-oxadiazoles as efficient corrosion inhibitors for carbon steel surface in

hydrochloric acid solution [127].

Xuehui et al. explored the use of 2,3,5-triphenyl-2H-tetrazolium chloride and 2,4,6-

tri(2-pyridyl)-striazine as corrosion inhibitors on mild steel in hydrochloric acid solution

[128]. Adsorption and inhibitive performance studies of benzimidazole derivatives on mild

steel corrosion in acid medium has been reported by Popova et al [129]. Popova

investigated the temperature effects on mild steel corrosion in acid media in presence of

azoles namely indole, benzimidazole, benzotriazole, benzothiazole [130]. Inhibition effects

of some Schiff’s bases on the corrosion of mild steel in hydrochloric acid solution have

been investigated by Prabhu et al [131]. Quartarone et al. discussed the use of indole-5-

carboxylic acid on the corrosion of mild steel deaerated 0.5 M sulfuric acid solutions using

weight-loss and different electrochemical techniques [132]. Some triazole derivatives have

been synthesized and evaluated as corrosion inhibitors for mild steel in natural aqueous

environment by Ramesh and coworkers [133]. Sathiyanarayanan et al. investigated the

corrosion inhibition effect of some tetramines for mild steel in 1M hydrochloric acid

medium [134].

Divakara Shetty and coworkers analyze the inhibiting effect of N-(furfuryl)-N′-

phenyl thiourea on the corrosion of mild steel in hydrochloric acid medium [135]. The

corrosion inhibitive behaviour of 5-((E)-4-phenylbuta-1,3-dienylideneamino)- 1,3,4-

thiadiazole-2-thiol Schiff base on mild steel in hydrochloric acid solution was reported by

Solmaz [136]. Adsorption and corrosion inhibition effect of 2-((5-mercapto-1,3,4-

thiadiazol-2-ylimino)methyl)phenol Schiff base on mild steel has been investigated by

Solmaz and coworkers [137]. The effect of Schiff base furoin thiosemicarbazones on the

corrosion of mild steel in hydrochloric acid solution has been described by Stanly Jacob

[138]. Inhibitive and adsorption behaviour of carboxymethyl cellulose on mild steel

corrosion in sulphuric acid solution has been studied by Solomon et al [139]. Yongming et

al. reports the experimental and molecular dynamics studies of 2-amino-5-phenyl-1,3,4-

thiadiazole for mild steel corrosion in 0.5 M H2SO4 and 1.0 M HCl solutions [140].

Polarization, EIS and molecular dynamics simulation studies of 2-amino-5-(n-pyridyl)-

1,3,4-thiadiazole for mild steel corrosion in 0.5 M H2SO4 medium has been investigated by


39

Yongming and coworkers [141]. Zhihua et al. analyzed the corrosion inhibition effect of

some oxo-triazole derivatives on mild steel in acidic solution [142]. Synthesis and evaluate

the new long alkyl side chain acetamide, isoxazolidine and isoxazoline derivatives as

corrosion inhibitors for mild steel in hydrochloric acid solutions has been studied by

Yıldırım and Etin [143]. Inhibition effect of 1-ethyl-3-methylimidazolium dicyanamide

against steel corrosion has been investigated by Tuken et al [144]. Zhang and coworkers

explored the inhibitive performance and theoretical study of 2-(4-pyridyl)-benzimidazole

for mild steel corrosion in hydrochloric acid medium [145]. Synthesis and electrochemical

behaviors of the novel TTF derivatives containing thiazole groups has been investigated by

Zhao et al [146].

1.4.2. Plant extracts as green corrosion inhibitors

Plant extracts are an incredibly rich source of natural chemical compounds that can

be extracted by simple procedures with low cost and are biodegradable in nature. The

actual inhibitors in the plant extracts are usually alkaloids and other organic nitrogen

bases, as well as carbohydrates, proteins and their acid hydrolysis products. Alkaloids have

an ability to coordinate the transition metals or their alloys via d-orbitals of metal and

empty p-orbitals of hetero atoms in the inhibitor molecules. A number of natural

compounds have been used as corrosion inhibitors for metals and their alloys in acidic,

alkaline and neutral solutions. The exploration of natural products as inexpensive eco-

friendly corrosion inhibitors is an essential field of study. They are environmentally

friendly, ecologically acceptable, low-cost, readily available and renewable sources of

materials. The extracts from their leaves, barks, seeds, fruits and roots comprise of

mixtures of organic compounds containing nitrogen, sulphur and oxygen atoms, and some

have been reported to function as effective inhibitors of metal corrosion in different

aggressive environments.

Inhibitive action of chamomile, halfabar, black cumin and kidney bean on the

corrosion of steel in 1M sulphuric acid has been investigated by Gaber et al [147].

Inhibitive performance of lupine extracts on mild steel in 1 M sulphuric and 2 M

hydrochloric acids was studied by Gaber and coworkers [148]. Nabey et al. explored the

possible use of olive leaf extracts on the corrosion of mild steel in brine solution [149].

The effect of the extract of punica granatum and their main constituents, ellagic acid (EA)

and tannic acid (TA), as mild steel corrosion inhibitors in 2 M HCl and 1 M H2SO4


40

solutions was investigated by Behpour et al [150]. Raja and coworkers assessed the

corrosion inhibition behaviour of the black pepper extract on mild steel in 1 M H2SO4

[151]. Corrosion inhibition of mild steel by the aqueous extracts of fruit peels in 1M HCl

has been studied by Rocha et al [152]. A comprehensive study on crude methanolic extract

of Artemisia pallens (Asteraceae) and its active component as effective corrosion

inhibitors of mild steel in acid solution has been reported by Garai et al [153]. The

corrosion inhibition and adsorption characteristics of uncaria gambir extract on mild steel

in 1M HCl medium has been investigated by Hussin and Kassim [154].

Ibrahim et al. studied the anticorrosive effect of thyme leaves extracts on mild steel

in HCl solution [155]. The influence of bamboo leaf extracts on the corrosion inhibition of

mild steel in HCl and H2SO4 solutions has been reported by Li et al [156]. Adsorption and

corrosion inhibitive behaviour of osmanthus fragran leaves extracts on carbon steel in

hydrochloric acid has been studied by Li and coworkers [157]. Corrosion inhibition effect

of justicia gendarussa extract on mild steel in 1 M HCl medium has been investigated by

Satapathy et al [158]. Inhibitory action of aqueous coffee ground extracts on the corrosion

of carbon steel in HCl solution has been explored by Torres et al [159]. Uwah and

coworkers studied the adsorption characteristics and inhibitive action of ethanol extracts

from nauclea latifolia on the corrosion of mild steel in H2SO4 solutions [160]. The effect of

apricot juice as corrosion inhibitor of mild steel in phosphoric acid has been reported by

Yaro et al [161].

Oguzie evaluate the inhibitive effect of some plant extracts on the acid corrosion of

mild steel [162]. A comprehensive study on crude methanolic extracts of artemisia pallens

and its active component as effective corrosion inhibitors of mild steel in acid solution has

been reported by Garai et al [163]. Ostovari and coworkers studied the corrosion inhibition

of mild steel in 1 M HCl solution by henna extracts [164]. Inhibitory action of Phyllanthus

amarus leaves and seed extracts on the corrosion of mild steel in HCl and H2SO4 solutions

has been studied by Okafor et al [165].

Corrosion inhibition performance of caffeic acid on mild steel in 0.1 M H2SO4 has

been investigated by de Souza et al [166]. Deng and coworkers studied the mild steel

corrosion inhibition by Ginkgo leaves extracts in hydrochloric acid and sulphuric acid

solutions [167]. Corrosion inhibition of C38 steel in 1 M hydrochloric acid medium by

alkaloids extract from the plant Oxandra asbeckii has been investigated [168]. Raja and


41

coworkers studied the Neolamarckia cadamba alkaloids as eco-friendly corrosion

inhibitors for mild steel in 1 M HCl solution [169]. Tan et al. study the correlation of

phenolic compositions and corrosion inhibition properties of Rhizophora apiculata bark

extracts [170]. The effect of two oleo-gum resin exudate from ferula assa-foetida and

dorema ammoniacum on mild steel corrosion in acidic medium has been evaluated by

Behpour et al [171]. The effect of salvia officinalis leaves extracts on the corrosion of 304

stainless steel in hydrochloric acid has been evaluated by Soltani et al [172]. The corrosion

inhibition effect of polyalthia longifolia for mild steel in HCl solution has been

investigated by Vasudha et al [173]. Inhibition performance of aqueous extracts of coffee

senna on the corrosion of mild steel in acidic environments has been evaluated by Akalezi

et al [174]. Adsorption and corrosion inhibition property of gnetum africana leaves

extracts on carbon steel has been assessed by Nanna et al [175].

Recently, a review on green inhibitors for corrosion of metals has been reported by

Kesavan and coworkers [176]. The nature of inhibition performance of dodonaea viscosa

(L.) leaves extracts on mild steel surface has been investigated by Leelavathi et al [177].

The inhibitory action of phyllanthus amarus extracts on the corrosion of mild steel in

seawater has been studied by Sribharathy et al [178]. Mohana and Shivakumar studied the

corrosion inhibition performance of ziziphus mauritiana leaves extracts on mild steel in

different acidic environments [179]. Chen et al. introduce ginkgo biloba leaves extracts as

new corrosion inhibitor for mild steel [180]. Hussein and coworkers investigated the

corrosion inhibition of carbon steel in 1M HCl solution using sesbania sesban extracts

[181]. A review on phytochemicals as green corrosion inhibitors in various corrosive

environments has been reported by Buchweishaija [182]. The inhibitive effect of cuminum

cyminum plant extracts on the corrosion of mild steel in an aqueous solution of seawater

has been evaluated by Sribharathy et al [183].

Electrochemical studies of mild steel corrosion inhibition in aqueous solution by

uncaria gambir extracts has been studied by Hussin and Kassim [184]. Patel et al. studied

the corrosion inhibition behaviour of mild steel in the presence of various plant extracts in

0.5 M sulphuric acid solution [185]. Inhibition of steel corrosion by chamomile extracts

has been studied by Hammouti et al [186]. A new eco-friendly green corrosion inhibitor,

eupatorium odoratus for mild steel corrosion in sulphuric acid has been introduced by

Onuegbu et al [187]. Sangeetha et al. studied the banana peel extracts as potent corrosion

inhibitors for carbon steel in sea water [188]. Evaluation of nicotiana leaves extracts as


42

corrosion inhibitor for mild steel in acidic and neutral environments has been reported by

Abd-El-Khalek et al [189]. Alpina galinga extracts as green corrosion inhibitor for mild

steel in acid media has been studied by Sankar et al [190]. Eddy et al. studied the joint

effect of halides and ethanol extract of lasianthera africana on the corrosion inhibition of

mild steel in sulphuric acid medium [191].

The effect of environmentally benign fruit extracts of shahjan (Moringa Oleifera)

on the corrosion of mild steel in hydrochloric acid solution was studied by Singh et al

[192]. Nasrin et al. studied the inhibitive property of extracts of salvia officinalis leaves on

the corrosion of 304 stainless steel in hydrochloric acid solution [193]. Corrosion

inhibition and adsorption behavior of extracts from piper guineensis on mild steel

corrosion in acid media has been studied by Ikpi et al [194]. Effect of water soluble rosin

on the corrosion inhibition of carbon steel has been investigated by Ayman et al [195].

Inhibitive action of clematis gouriana extracts on the corrosion of mild steel in acid

medium was studied by Gopiraman et al [196]. Inhibition effect of environmentally benign

karanj (pongamia pinnata) seed extracts on the corrosion of mild steel in hydrochloric acid

solution has been studied by Ambrish Singh et al [197]. Corrosion inhibition of C38 steel

in 1 M hydrochloric acid medium by alkaloids extracts of oxandra asbeckii plant by

Lebrini et al [198]. Anti-corrosive effectiveness of strychnos nux-vomica and calotropis

procera extracts as eco-friendly corrosion inhibitor for mild steel in 1 M sulfuric acid

medium was studied by Raja and Sethuraman [199, 200]. Eddy and Ebenso [201]

investigated the adsorption and inhibitive properties of ethanol extracts of musa sapientum

peels as a green corrosion inhibitor for mild steel in H2SO4 medium. Corrosion inhibition

mechanism of mild steel by certain plant extract in dilute HCl has been investigated by

Chauhan et al [202].


43

SECTION - V: SCOPE OF THE PRESENT WORK

Corrosion is the destructive phenomenon which affects almost all metals. Although

iron was not the first metal used by man, it has certainly been the most used, and must

have been one of the first in which serious corrosion problems were encountered. From the

literature survey it is clear that, there is a great need for more research in the field of

corrosion of metals in general mild carbon steel in particular for its practical importance.

Mild steel is widely used in many industries because of economically cost-effective and

easy fabrication. However, it is prone to undergo corrosion in aggressive environmental

conditions during process called acid cleaning or pickling. Therefore, more attention was

made on mild carbon steel in the present study.

Addition of corrosion inhibitors is one of the requirements to protect metals and

alloys against attack in many industrial environments. Hence, the development of new

corrosion inhibitors based on organic/inorganic compounds containing nitrogen, oxygen,

sulphur and phosphorous atoms is of growing interest in the field of chemical industries to

solve the corrosion problems and reduced the economic cost of equipments. The thesis

presents the application of organic compounds and plants extracts as corrosion inhibitors.

The present work was designed to understand the inhibition mechanism of corrosion

inhibitors on mild steel in acid and industrial water media. The behavior of mild steel in

the presence of inhibitive formulation was investigated by steady-state electrochemical

measurements. The mechanism of metal corrosion is priority of our research by evaluating

the adsorption thermodynamic parameters.

The thesis involves the investigation of corrosion and corrosion inhibition of mild

steel in acid medium using some of the inhibitors such as 4-(4-bromophenyl)-N'-(2,4-

dimethoxybenzylidene) thiazole- 2 -carbohydrazide (BDTC), 4-(4-bromophenyl)-N'- (4-

methoxybenzylidene) thiazole -2- carbohydrazide (BMTC), 4- (4-bromophenyl) - N' - (4

hydroxybenzylidene) thiazole - 2 carbohydrazide (BHTC), 4-(((4-((5-Mercapto-1,3,4-

oxadiazol-2-yl)methyl)-5-methylthiazol-2-yl)imino)methyl)benzene-1,2-diol (MOMMBD)

and 4-(((4-((5-Mercapto-1,3,4-oxadiazol-2-yl)methyl)-5-methylthiazol-2yl)imino)methyl)-

2,6-dimethoxyphenol (MOMMDP), N-(1-(5-fluoro-2-(methylthio) pyrimidin-4-yl)

piperidin-4-yl)-2,4,6-trimethyl benzene sulfonamide (FMPPTS) N-(1-(5-fluoro-2-

(methylthio)pyrimidin-4-yl)piperidin-4-yl)-3,4 dimethoxy benzene sulfonamide

(FMPPDS). N-(1-(5-fluoro-2-(methylthio)pyrimidin-4-yl)piperidin-4-yl)-3-


44

methoxybenzenesulfonamide (FMPPMS), N-(1-(5-fluoro-2-(methylthio) pyrimidin-4-yl)

piperidin-4-yl) -2,5-dimethoxybenzene sulfonamide (FMPPDBS), N-(1-(5-fluoro-2-

(methylthio)pyrimidin-4-yl)piperidin-4-yl)-4-nitrobenzene sulfonamide (FMPPNBS), N-

(1-(5-fluoro-2-(methylthio) pyrimidin-4-yl) piperidin-4-yl)-3 methoxy benzene

sulfonamide (FMPPMBS). The corrosion inhibition character of Pterolobium hexapetalum

(PH), Celosia argentea (CA), Achyranthes aspera (AA), Plumeria rubra (PR) was studied

in industrial water medium. The behavior and mechanisms of the inhibitors have been

investigated in the temperature range of 303-333 K at different concentrations of inhibitors

using mass loss, potentiodynamic polarization and electrochemical impedance

spectroscopy (EIS) techniques. Scanning Electron Microscopy (SEM) and FT-IR spectral

studies were also used to analyze the surface adsorbed film.

The frame work of the investigation is based on the following objectives:

1. To identify the efficient corrosion inhibitors for mild steel.

2. To optimize the temperature of the environment and concentration of the

inhibitor.

3. To study the adsorption thermodynamic parameters for the corrosion inhibition of

mild steel surface.

4. Establishing the behavior of corrosion inhibitors on mild steel surface.


45

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INTRODUCTION TO CORROSION AND CORROSION INHIBITORSshodhganga.inflibnet.ac.in/bitstream/10603/66586/3/chapter 1.pdf · Chapter I Introduction to Corrosion and Corrosion Inhibitors