INTRODUCTION TO CORROSION AND CORROSION INHIBITORS SECTION ...shodhganga.inflibnet.ac.in/bitstream/10603/72595/3/chapter 1.pdf · Chapter I Introduction to corrosion and corrosion

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

Corrosion is the destructive attack of a material by reaction with its environment.

The serious consequences of the corrosion process have become a problem of

worldwide significance. In addition to our everyday encounters with this form of

degradation, corrosion causes plant shutdown, waste of valuable resources, loss or

contamination of product, reduction in efficiency, costly maintenance and expensive

overdesign. It also jeopardizes safety and inhibits technological progress. The

multidisciplinary aspect of corrosion problems combined with the distributed

responsibilities associated with such problems only increase the complexity of the

subject. Corrosion control is achieved by recognizing and understanding corrosion

mechanisms by using corrosion-resistant materials. Mild steel is widely used in many

industries because of economically cost-effective and easy fabrication, but it is prone to

undergo corrosion in aggressive environmental conditions. The environment may be a

liquid, gas or mixture of solid and liquid.

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 damages to infrastructure, waterways, ports, railroads,

hazardous materials storage, drinking water, sewer systems, 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, 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 arise in the pipe lines due to aggressiveness of the liquid

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


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

cooling and heating systems in many operations. However, all kinds of water passes

through these lines contain very high chloride concentration and considerable amount of

sulfate and other ions of different metals. For this reason, the injection of corrosion

inhibitors through different sites of pipes is very important.

Mainly corrosion can have two forms, uniform and localized. Uniform corrosion

accounts for the greatest tonnage of metal consumed. Yet the localized forms of

corrosion are difficult to predict and control. Some forms of localized corrosion are

galvanic, crevice, pitting and erosion-corrosion. For uniform corrosion, the corrosive

environment must have the same access to all parts of the metal surface and the metal

itself must be metallurgically and compositionally uniform. Atmospheric corrosion is

probably the most prevalent example of uniform corrosion at a visually apparent rate

[1]. Of course since there are different types of atmospheres all over the world, the rate

of atmospheric corrosion will differ for each type.

Fig. 1.1: Oil pipeline corrosion.


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Fig. 1.2: Gas pipeline corrosion.

Fig. 1.3: Inner view of corroded pipeline.


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Pickling is a metal surface treatment used to remove impurities such as stains,

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

(Fig. 1.4). 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 [2].

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 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 [3]. 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 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 no 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, to 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.


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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 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 [4].

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 it negatively affects metal in oil production, refinery,


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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 affect 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 get adsorb 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 problem. However, most

inhibitor’s evaluations are generally based on the test results under stagnant or low flow

rate (<1 m s-1) conditions [5].

Fig. 1.6: Corrosion in boilers.

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. In

some days we are going to face acute shortage of these materials. An impending

metal crisis does not seem anywhere to be a remote possibility but it will be a

reality. There is bound to be metal crisis and we are getting the signals. To

preserve these valuable resources, we need to understand how these resources are


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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 prevention mechanism 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 played a very important role in these

disasters. Applying the knowledge of corrosion prevention 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

by corrosion products and become unfit for consumption. Corrosion prevention is

integral part of the system 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 [6] corrosion occurs because of the creation of a large

number of micro electrochemical cells or local cells (Fig. 1.7) at heterogeneities

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

surface. 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 difference can be measured when a metal is electrically connected to a standard

electrode.

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

which case the voltage is expressed as either positive or negative. The difference in this

potential makes the 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 are usually developed due to the following reasons [7]:


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Stress from welding or other works.

Compositional differences at the metal surface.

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

Fig.1.7: Model for local cell theory of corrosion.

1.1.2.2. Wagner and Trauds’ theory

Wagner and Traud [8] 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 might have formed when the metal is molten and these impurities are passed

onto 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. 1.8):

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 (oxidized

from Fe0 to Fe2+ state) 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.

Ionic conductor

Cathodic site Anodic site


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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. 1.8: 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)

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)


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the metal.







222 HeH (1.3)


follows:

2

2 2 OHFeOHFe (1.4)



hydroxide.




OHOFeOHFe 2323 32 (1.6)


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the metal.







222 HeH (1.3)


follows:

2

2 2 OHFeOHFe (1.4)



hydroxide.




OHOFeOHFe 2323 32 (1.6)


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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 for corrosion, but the following

classification is adapted [9-11]:

1.1.3.1. Uniform or general corrosion

Uniform or general corrosion, as the name implies, results in a fairly uniform

penetration (or thinning) over the entire exposed metal surface. The general attack

results from local corrosion cell action, that is, multiple anodes and cathodes are

operating on the metal surface at any given time. The location of the anodic and

cathodic areas continues to move about on the surface, resulting in uniform corrosion.

In this case the exposed metal/alloy surface area is entirely corroded in an environment

such as a liquid electrolyte (chemical solution, liquid -metal), gaseous electrolyte (air,

CO2, SO2 etc.,) or a hybrid electrolyte (solid, water, biological organisms, etc.).

Some types of general 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 sodium chloride environment.

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

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

Stray-current corrosion on pipelines near railroad.


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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. 1.9). It occurs when

dissimilar metals are electrically coupled in a common solution, the more negative

(more active) metal will be the anode of the galvanic corrosion cell and its corrosion

rate will increase. The more positive (more noble) metal will be the cathode and its

corrosion rate will decrease and it is detectable by the presence of a buildup of corrosion

at the joint between the dissimilar metals. 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. 1.9: Galvanic corrosion of mild steel pipe connected to copper connector.

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: It is similar to pitting corrosion in a stagnant electrolyte after its

initiation. This form of corrosion initiates due to changes in local chemistry such as

depletion of oxygen in the crevice, increase in pH with increasing hydrogen ion

concentration and increase of chloride ions. Oxygen depletion implies that, cathodic

reaction for oxygen reduction cannot be sustained within the crevice area and

consequently metal dissolution occurs. Crevice corrosion may take place on any metal


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

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 corrosion need not be metallic.

Wood, plastic, rubber, glass, concrete, asbestos, wax and living organisms have been

reported to cause crevice corrosion. It is frequently more intense in chloride

environments. The mechanism of crevice corrosion is electrochemical in nature and it is

illustrated in Fig. 1.10. It requires a prolong time to start the metal oxidation process,

but it may get accelerated afterwards.

Fig. 1.10: Crevice corrosion of mild steel connectors.

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

termed as “under film” corrosion. 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. Their use should be avoided unless absence of an

adverse effect has been proven by field experience.

Pitting corrosion: Pitting Corrosion is "self-nucleating" crevice corrosion, starting

at occluded cells (Fig. 1.11). 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, 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


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may start in the crevice or the pits. Large surface areas will become cathodic and pits or

crevices will become anodic and corrode. Metal dissolution will thus be concentrated in

small areas and will proceed at much higher rates than with uniform corrosion. Large

crevices are less likely to corrode because water movement causes mixing and

replenishes oxygen, hydrogen ions, bicarbonate or hydrogen sulfide.

Fig. 1.11: Pitting corrosion in mild steel pipes.

Intergranular corrosion: Intergranular corrosion is a localized form of corrosion.

It 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.

1.1.3.4. Stress corrosion cracking (SCC)

Structural parts subjected to a combination of a tensile stress and a corrosive

environment may prematurely fail at a stress below the yield strength (Fig. 1.12). This

phenomenon is known as environmentally induced cracking (EIC) which is divided into

the following categories:

Stress-corrosion cracking (SCC)

Hydrogen-induced cracking (HIC) and

Corrosion-fatigue cracking (CFC).


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Fig. 1.12: Stress corrosion cracking of mild steel.

1.1.3.5. Erosion corrosion

The term “erosion” applies to deterioration due to mechanical force. When the

factors contributing to erosion accelerate the rate of corrosion of a metal, the attack is

called “erosion corrosion”. 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. Surfaces which have undergone erosion corrosion are generally fairly

clean, unlike the surfaces from many other forms of corrosion.

1.1.4. Rate of corrosion

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

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

corrosion stops. The slowest step among these 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 increase of corrosion

rate. 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 [12].


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1.1.5. Factors influencing the corrosion rate

Primary factors influencing the corrosion rate are the conditions of the metal

surface and the secondary factors are the nature of the environment [13].

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 has more reactive and is

more susceptible for corrosion and metal with high electrode potential is less reactive

and less susceptible for corrosion. For example, metals like K, Na, Mg, Zn etc., have

low electrode potential and undergo corrosion very easily where as noble metals like

Ag, Au, and Pt have higher electrode potential, their corrosion rates are negligible. But

there are few exceptions 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 [14]. If we know the electrode potentials of metals in some electrolyte, we may

predict whether the 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)

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

ΔG = -nFE (1.9)

where, ΔG 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.

In corrosion processes where hydrogen evolution is the cathodic reaction,

hydrogen over potential (resistance to hydrogen evolution) is another important factor

effecting the corrosion. Each metal in a given environment has characteristic hydrogen

overpotential. Thus a metal with low hydrogen overpotential in a given environment

undergoes facile corrosion. In the case of alloys, other entities added in micro-quantities

will alter the hydrogen overpotential and thereby affect the corrosion process.


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1.1.5.2. Surface state of the metal

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 formed 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 and Ti develop such a layer on their

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

form such 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 has an

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

layer formed on metals like Zn, Fe, Mg, etc,). Highly polished and smooth surface

resists corrosion while a rough surface containing various types of imperfections such

as clearage steps, dislocations, point defects etc., is liable to severe attack.

1.1.5.3. Protective film

Certain metals like Al have a tendency to form a protective film on the surface

which acts as physical barrier (passivation) between the metal and the medium, thereby

curtailing the corrosion [15].

Secondary factors

1.1.5.4. pH of the medium

In general, rate of corrosion is higher in acid 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 where as low pH serve corrosion takes place. But for metals like Al,

corrosion rate is high even at high pH. For metals like zinc, iron, magnesium etc.,

hydrogen evolution is thermodynamically favored cathodic reaction. Corrosion of each

metal in acidic medium is therefore highly pH dependent. Decrease in pH facilitates the

rate of hydrogen evolution and hence increases the corrosion rate of metals. 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


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varying pH conditions of the medium, a corroding surface may exhibit activity,

immunity or passivity [14].

1.1.5.5. 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 10 °C rise in temperature. In an open vessel, allowing dissolved

oxygen to escape, corrosion rate increases with increase in 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 as the temperature is

raised 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 10 °C rise in temperature [16]. 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. 1.13) 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 [17].


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Fig. 1.13: Effect of temperature on the corrosion rate of low carbon steel in tap water.

1.1.5.6. 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, and then corrosion

diminishes near 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 tends 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 of the common rust is comprises of hydrated

ferric oxide. Frequently, a black layer of magnetic hydrous ferrous ferrite (Fe3O4. nH2O)

forms between Fe2O3 and FeO. Hence, the rust film normally consists of three layers of

iron oxide in different states of oxidation. Although increase in oxygen concentration

initially accelerates the corrosion of iron, but, beyond a critical concentration, the

corrosion rate drops down again to a low value. If we inject more and more oxygen into

water, under some specific conditions (in water of high purity) and at high temperature

that may result in the formation of a passive protective dense film composed of metal

oxides on the metal surface, and corrosion would decrease [18]. The injection of oxygen

into water is one of the corrosion control methods at power stations.

At constant O2

concentration


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Cohen [19] reported that the corrosion rate in the presence of oxygen is 65 times

higher the rate in the absence of oxygen. Whitman [20] stated that the corrosion rate

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

the protective films on the metal surfaces. Frese [21] showed that, iron tends to become

passive with high oxygen. Fig. 1.14 shows the effect of oxygen concentration on the

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

Fig. 1.14: 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

An inhibitor is a chemical substance or combination of substances which when

added in very low concentrations in a corrosive environment effectively prevents or

reduces corrosion without significant reaction with the components of the environment.

Concentrations of corrosion inhibitors can vary 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 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 inhibitor.

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 variable. Corrosion inhibitors are used in oil and gas exploration and

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

treatment and product additive industries [22]. In the oil extraction, processing and

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

of defense. A great number of scientific studies have been devoted to the subject of

corrosion inhibitors [23-36].

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 recirculation systems, oil

production, oil refining, and acid pickling of steel components. One of the more

recognizable applications for inhibitors is in antifreeze for automobile radiators. The

application of inhibitors must be viewed with caution by the user because inhibitors

may afford excellent protection for one metal in a specific system but can aggravate

corrosion for other metals in the same system. Inhibitors can be organic or inorganic

compounds and they are usually dissolved in aqueous environments. Some of the most


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effective inorganic inhibitors are chromates, nitrites, silicates, carbonates, phosphates

and arsenates. The organic inhibitors include amines, heterocyclic nitrogen compounds,

sulfur compounds such as thioethers, thioalcohols, thioamides, thiourea and hydrazine.

Chromates and zinc salts are used increasingly less due to their toxicity and are

nowadays largely replaced by organic inhibitors. 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 inhibition efficiency but also safety of use,

economic constraints and compatibility with other chemicals in the system, and

environmental concerns [27].

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

cooling system must satisfy the following criteria [28]:

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 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.


22

The protective mechanism of cathodic inhibitors is generally based on the reaction with

the products of a cathodic electrochemical reaction (OH−) [29 - 31]. For example, Zn2+

react 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, CO and CP in organic molecules of inhibitors 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 metallic atoms, then 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, preferably 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 [32-34]. 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

action of inhibitors in acid solutions depends on adsorption onto the metal surface in

oxide-free solutions. The adsorbed inhibitors then acts to retard the cathodic and/or

anodic electrochemical corrosion processes. 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 change with experimental conditions. Thus,

the action of an inhibitor depends on its concentration, the pH of the acid, the nature of

the anion 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 [35, 36]:


23

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.

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 contains 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 of 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 a 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 is 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,


24

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

to what extent the secondary inhibition is effective than the primary inhibition. For

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

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.

Blocking of reaction sites: The blocking decreases the number of metal atoms at

which corrosion reactions can occur. During this, mechanisms of the reactions are

not affected, and the Tafel slopes of the polarization curves remain unaffected.

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 Fe(OH)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 pyridinium salts.


25

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.

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 [37-42].

1.2.3.1. Passivating inhibitors

Passivating inhibitors cause a large anodic shift of the corrosion potential 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


26

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 reduce corrosion by slowing the reduction reaction rate of the

electrochemical corrosion cell. This is done by blocking the cathodic sites by

precipitation. 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 which are referred to as

cathodic poisons reduce the hydrogen reduction reaction rate and thus lower the overall

corrosion rate. Other cathodic inhibitors remove reducible species from the

environment. Removal of oxygen from the corrosive environment will significantly

decrease the corrosion rate. This can be done through (a) the use of oxygen scavengers

such as sodium sulfite and hydrazine which react with the oxygen and remove it from

the solution (b) vacuum de-aeration or (c) boiling to lower the dissolved oxygen

concentrations.

1.2.3.4. Anodic inhibitors

Anodic inhibitors such as chromates, phosphates, tungstates and other ions of

transition elements with high oxygen content are those that stifle the corrosion reaction

occurring at the anode by forming a sparingly soluble compound with a newly produced

metal ion. They are adsorbed on the metal surface forming a protective film or barrier,

thereby reducing the corrosion rate. Anodic inhibitors build a thin protective film along

the anode and increasing their potential and thus slow down the corrosion reaction.

Chromates, nitrates, tungstate and molybdates are some examples of anodic inhibitors.


27

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 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 their concentrations

are 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, and zinc-phosphates.

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.


28

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 environmental conditions [43].

1.2.4. Adsorption

Adsorption is a surface phenomenon exhibited by solids which consists of

adhesion in an extremely thin layer of the molecules of gases, liquids and dissolved

substances with which they are in contact. 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. There are two types of adsorptions depending on the nature of

forces involved [41, 44]. (a) Chemisorption: In this type of adsorption, a single layer of

molecules, atoms or ions is attached to the surface by chemical bonds and is essentially

irreversible. (b) Physical adsorption: In this type, the attachment is by the weaker van

der Waal’s forces, whose energy levels approximate to those of condensation. The

factors that influence the adsorption of inhibitor ions on metal surfaces are [45]:

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


29

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 electronegativity and

follow 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 Kuster. It 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:

1. The surface of the adsorbent is uniform.

2. Adsorbed molecules do not interact.

3. All adsorptions 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 molecule 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.


30

Different adsorption isotherms were found to describe the adsorption of inhibitors

on metal surface such as Langmuir [46 – 48], Temkin [49–51], Frumkin [52 – 54],

Flory– Huggins [55 – 57], Dhar–Flory–Huggins and Bockris–Swinkels [58], and

Freundlich [58, 59].

1.2.5. Polarization

Polarization [44] 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 the corrosion current despite 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. 1.15a, where the

corrosion current is largely determined by the anode [59]. Fig.1.15b shows the cathodic

polarization where the cathode alone undergoes polarization. Here the cathodic

polarization curve is much steeper and the corrosion current is under cathodic control. If

both electrodes undergo polarization, it is said to be under mixed control as shown in

Fig.1.15c.

Fig. 1.15: Evans polarization diagrams: (a) anodic control (b) cathodic control (c) mixed

control.

Icor Icor Icor

(a) (b) (c)

Ec

Ecor

Ea

Ec

Ecor

Ea

Ec

Ecor

Ea


31

1.2.6. Electrochemical impedance

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 [44]. 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.

It gives mechanistic information. 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 [60]. The selection of an

effective and appropriate inhibitor for the corrosion of metals has been widely carried

out based on empirical approach [61-62]. Recently, quantum chemical calculations have

been performed enormously to complement the experimental evidence. Quantum


32

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.


33

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 involves transfer of electrons

from a substance to an oxidizing agent. The term antioxidant was used to refer

specifically to a chemical that prevents 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 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 get

adsorbed 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 [63].

Antioxidants from different origins have high bioavailability, and therefore high

protective efficiency against free radicals [64]. Free radicals and singlet oxygen

scavengers (antioxidants) have metal and alloy corrosion inhibition character, which

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

accepting - donating hydrogen or electron behaviors [65]. 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 [66].


34

Synthetic and natural antioxidants are free radical scavengers containing some

N- heterocyclic compounds and their derivatives 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

demonstrate 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 [67]. Christ Tamborski et al. [68] studied

antioxidant and anticorrosion properties of new aromatic phosphine compounds. The

antioxidant and anticorrosion properties of the some natural products have been

investigated by Shanab and coworkers [69].


35

SECTION - IV: CORROSION INHIBITION STUDIES ON MILD STEEL:

A REVIEW

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

studied using different organic moieties. The use of organic inhibitors for preventing

corrosion is a promising alternative solution. Organic compounds containing functional

electronegative groups, pi-electron in triple or conjugated double bonds and presence of

aromatic rings in their structure are the major adsorption centers and usually are good

inhibitors [72]. The adsorption characteristics of organic molecules are also affected by

sizes, electron density at the donor atoms and orbital character of donating electrons

[70, 71]. The choice of effective inhibitors is based on their mechanism of action and

their electron donating capability. The inhibiting ability of an inhibitor depends upon

the 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 the ability to accept or donate electrons in order to get adsorbed on metallic

surfaces by electrostatic interaction between the unshared electron pair of corrosion

inhibitor and metal.

Abboud et al. [76] synthesized and studied the inhibition of mild steel corrosion in

1 M HCl by 2, 2'-bis(benzimidazole) using various corrosion monitoring techniques and

found that the inhibition efficiency value increases with the inhibitor concentration and

reaches a maximum of 97.8 % at 10-4 M. The effect of N, N'-bis (salicylaldehyde)-1,3-

diaminopropane and its corresponding cobalt complex on the corrosion behaviour of

steel in 1 M sulphuric acid solution was investigated by Abdel-Gaber et al [77].

Thermodynamic characterization of metal dissolution and inhibitor adsorption processes

on mild steel using 2, 5-bis (n-thienyl)-1,3,4-thiadiazoles in hydrochloric acid system

was investigated by Bentiss et al. [78] and they discussed about thermodynamic

functions of dissolution and adsorption processes from experimental polarisation. 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 [79]. All the data

revealed that AMPA acts as inhibitor in acidic environment. Furthermore, polarization

data showed that the compound behaves as a mixed-type inhibitor. Aminopyrimidine

derivatives as inhibitors for corrosion of 1018 carbon steel in nitric acid solution was

reported by Abdallah et al [80]. It was found that the aminopyrimidine derivatives


36

provide a good protection to steel against pitting corrosion in chloride containing

solutions.

Ganesha Achary et al. [81] 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 results indicated that the

corrosion inhibition efficiency and extent of surface coverage were increased with

increase in inhibitor concentration and decreased with increase in temperature and acid

concentration. Bouklah et al [82] studied corrosion inhibition behavior and

thermodynamic properties of 2, 5-bis (4-methoxyphenyl)-1,3,4-oxadiazole as a

corrosion inhibitor for mild steel in normal sulfuric acid medium. Results obtained

revealed that 4-MOX showed excellent performance as corrosion inhibitor for mild steel

in sulfuric acid media and its efficiency attains 96.19 % for 8 × 10−4 M at 333 K. The

role of some new thiosemicarbazide derivatives namely, 1-ethyl-4(2,4-dinitrophenyl)

thiosemicarbazide (I), 1,4-diphenylthiosemicarbazide (II), 1-ethyl-4-

phenylthiosemicarbazide (III) as corrosion inhibitors for carbon steel in 2 M HCl was

investigated by Badr et al [83]. Results obtained revealed that the inhibition efficiency

(% IE) follows the sequence: I > II > III.

Yuanyuan et al. [84] reported that 5, 10, 15, 20-tetraphenylporphyrin and 5, 10,

15, 20-tetra-(4-chlorophenyl) porphyrin are good inhibitors for iron corrosion in H2SO4

solutions. The maximum protection efficiency for the porphyrin-covered iron electrodes

is more than 80 %. The new isoxazolidines were synthesized and their influence on

corrosion inhibition of mild steel in 1M and 5 M HCl at 50–70 °C has been studied by

Ali et al [85]. All compounds have shown very good corrosion inhibition efficiency (IE

%) in acidic solution in the range of 100 – 400 ppm. Abboud et al. [86] studied 2, 3-

quinoxalinedione as novel corrosion inhibitor for mild steel in 1M HCl, and found good

inhibiting properties for steel corrosion in acidic bath with efficiencies of around 88 %.

A series of new thiazole derivatives have been synthesized and investigated as corrosion

inhibitors for carbon steel in 2 M HCl solutions by Al-Sarawya et al [87]. The results

showed that the inhibition efficiency of the investigated compounds depend on

concentration and the nature of the inhibitor. The presence of substituted donating group

(–OCH3) plays an important role in the inhibition efficiency of these investigated

compounds. The inhibition action of N-(furfuryl)-N′-phenyl thiourea on the corrosion of


37

mild steel in hydrochloric acid medium was reported by Divakara Shetty et al [88].

Good inhibition efficiency (> 93%) has been evidenced at 28 °C and 50 °C and

inhibition is governed by chemisorption mechanism.

Amin et al. [89] evaluated the inhibiting effects of the newly synthesized (GlyD1),

2-(4-(dimethylamino)benzylamino)acetic acid hydrochloride on mild steel corrosion in

4.0 M H2SO4 solutions at different temperatures (278–338 K). The inhibition

performance of GlyD1 was much better than those of N, N-bis (2-aminoethyl)glycine

(GlyD2) and Gly itself. Results obtained from the different corrosion evaluation

techniques were in good agreement with each other. New macrocyclic polyether

compounds containing a 1, 3, 4-thiadiazole moiety have been prepared to study the

corrosion inhibitive effect of mild steel in normal hydrochloric acid solutions [90]. The

results of these investigations have shown enhancement in inhibition efficiencies with

the extent of the polyethylene glycol unit that forms a cavity. Molecular modeling has

been conducted in an attempt to correlate the corrosion inhibition properties with the

calculated quantum chemical parameters. 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 [91]. It was found that this

compound acts as a good inhibitor for mild steel in 1 M HCl even at very low

concentration (1 ppm).

The effect of formation of covalent bonds between carbon and iron atoms on the

adsorption of p-toluene and p-hydroxymethylbenzene on iron for its protection was

studied by Shimura et al [92]. The inhibitive effect of indole-3-acetic acid on the

corrosion of mild steel in 0.5 M HCl medium was reported by Avci et al [93].

According to the obtained results from all measurements, inhibition efficiency was

about 77 % with 1.7 ×10-3 M inhibitor present and increased to about 93 % at the 1 ×10-

2 M inhibitor concentration. A new corrosion inhibitor namely N-(piperidinomethyl)-3-

[(pyridylidene)amino]isatin (PPI) has been synthesized and its influence on corrosion

inhibition of low carbon steel in 1N hydrochloric acid solution was studied by Quraishi

et al [94]. The PPI showed more than 94 % inhibition efficiency at an optimum

concentration of 300 ppm. The inhibition efficiency of inhibitor was found to vary with

inhibitor concentration, solution temperature, immersion time and acid concentration.

Experimental and theoretical investigations of three newly synthesized organic

compounds on mild steel corrosion in sulfuric acid medium was reported by Bahrami


38

and co-workers [95]. It was found that the investigated compounds exhibit a good

inhibition effect especially at 8–10 ppm range concentration, which makes them

commercially important.

Bhrara et al. [96] evaluated the inhibiting effects of butyl triphenyl

phosphonium bromide (BuTPPB) on the corrosion of medium carbon steel in 0.5 M

sulphuric acid solution. At 298 K, inhibition efficiency was found to be 94.5 % for

10−7 M BuTPPB which increased to about 99 % for 10−2 M concentration. 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 (A), 2-{[(2)-1-

(4-methylphenyl)methylidene]amino}-1-benznethiol (B) and 2-[(2-sulfanylphen-

yl)ethanimidoyl)]phenol (C) on the corrosion of mild steel in 15 % hydrochloric acid

solution has been reported by Behpour and co-workers [97]. The results of the

investigation showed that the compounds A and B with mean efficiency of 99 % at

200 mg/L concentration for mild steel corrosion in hydrochloric acid, and they act as

mixed inhibitors.

The corrosion inhibition properties of tris(benzimidazole-2-ylmethyl)amine

(TBMA) by different techniques were analyzed and the data showed that, this

compound can retard anodic reaction [98]. DFT results clearly showed that TBMA

posses corrosion inhibition properties by having a delocalization region in the

benzimidazole ring that gives up their π-electrons through its HOMO orbital to the

metal LUMO orbital to form a adsorption layer over the metallic surface. Yuce et al.

evaluated the adsorption and corrosion inhibition effect of 2-thiohydantoin (2-THD) on

mild steel in 0.1 M HCl solution [99]. The results showed that 2-THD acts as a mixed

type inhibitor by suppressing simultaneously the cathodic and anodic processes via

physical adsorption on the MS surface followed by Langmuir adsorption isotherm. New

long alkyl side chain acetamide, isoxazolidine and isoxazoline derivatives were

synthesized and their corrosion inhibitive property was studied by Yildirım and Cetin

[100]. The best inhibition was generally obtained at 50 ppm inhibitor concentration in

the acidic medium. All the tested inhibitors except two showed promising inhibition

efficiencies in oil medium also. Kinetics of mild steel corrosion in aqueous acetic acid

solutions has been studied by Singh and Mukherjee [101]. The results obtained by

weight loss method and polarization technique are in good agreement with each other.

The surface studies support the conclusions drawn from the weight loss method.


39

Kenya and co-workers studied gallic acid (GA) as corrosion inhibitor for carbon

steel (CS) in chemical decontamination formulation at 4.25 mM concentration. [102]. A

formulation containing Citric acid (CA) (1.4 mM), EDTA/NTA (1.4 mM), Ascorbic

acid (AA) (1.0–2.0 mM) and GA (4.25 mM) was found to be more efficient in

dissolving hematite and providing 31% corrosion inhibition (passivation) to the CS.

Doner et al. reported the corrosion inhibitive behaviour of 2-amino-5-mercapto-1,3,4-

thiadiazole (2A5MT) and 2-mercaptothiazoline (2MT) on mild steel using experimental

and theoretical studies [103]. It was shown that both 2A5MT and 2MT act as good

corrosion inhibitors for mild steel protection. The effect of the presence of extra NH2

group and N atom in 2A5MT on the ability to act as corrosion inhibitor was investigated

by theoretical calculations. Xianghong et al. [104] studied the inhibition effect of

Tween-20 as a nonionic surfactant on the corrosion of cold rolled steel in 1.0 – 8.0 M

hydrochloric acid. The results showed that Tween-20 is a good inhibitor in 1.0 M HCl,

and the inhibition efficiency increases with the inhibitor concentration, while decreases

with increasing the hydrochloric acid concentration and temperature.

The corrosion inhibition effect of 3-[(2-hydroxy-benzylidene)-amino]-2-thioxo-

thiazolidin-4-one (HBTT) on mild steel in the 0.5 M H2SO4 medium has been

investigated by Ali Doner and co-workers [105] using the conventional

potentiodynamic polarization studies, linear polarization studies (LPR), electrochemical

impedance spectroscopy studies (EIS). SEM was used for surface characterization. The

results showed that HBTT posses excellent inhibition effect towards MS corrosion. The

corrosion inhibition of iron in HCl, HClO4, H2SO4 and H3PO4 solutions (1M for each)

by cefatrexyl has been studied by Morad [106]. The results obtained at 30 °C revealed

that cefatrexyl acts as a weak inhibitor in HCl solution but shows excellent inhibition

performance in the remaining acids. The corrosion inhibition mechanism of some amino

acids namely cysteine (CYS), serine (SER), amino butyric acid (ABU) alanine (ALA)

and valine (VAL), threonine (THR), phenylalanine (PHE), tryptophan (TRP) and

tyrosine (TYR) were studied on the mild steel using experimentally and theoretically by

Eddy [107]. From this study, the expected trends for the variation of the inhibition

efficiencies of the amino acids is CYS > SER > ABU, THR > ALA >VAL and TRP >

TYR > PHE, respectively.

Khaled [108] synthesized N-(5,6-diphenyl-4,5-dihydro-[1,2,4]triazin- 3-yl)-

guanidine (NTG) and studied its inhibitive performance towards the corrosion of mild


40

steel in 1 M hydrochloric acid and 0.5 M sulphuric acid. These studies have shown that

NTG is a very good inhibitor in acid media and the inhibition efficiency goes up to 99

% and 96 % in 1M HCl and 0.5M H2SO4, respectively. Adsorption of 2-amino-5-

mercapto-1,3,4-thiadiazole (2A5MT) on mild steel (MS) surface in 0.5 M HCl solution

and its corrosion inhibition effect was studied by Solmaz and co-workers [109] in both

short and long immersion times (over 120 h). The obtained results indicate that the

stability of 2A5MT is good in the typical temperature range of working conditions of

pickling. Fouda et al. suggested that, some furfural hydrazone derivatives act as

corrosion inhibitors on C-steel in H3PO4 solution [110]. These hydrazone derivatives

are fairly efficient inhibitors for C-steel dissolution in 3M H3PO4. The oxo-triazole

derivative (DTP) was synthesized and its inhibitory action on the corrosion of mild steel

in sulphuric acid was investigated by Zhihua Tao et al. [111] using weight loss,

potentiodynamic polarization, EIS and SEM. The results revealed that DTP is an

excellent inhibitor and the inhibition efficiencies obtained from weight loss and

electrochemical experiment were in good agreement. Shuwei et al [112] used

molecular dynamics and density functional theory to study the relationship between

structure and inhibitory behaviour of imidazoline derivatives like 3-ethylamino-2-

undecyl imidazoline (EUI) and chloride-3-ethylamino-3-(2,3-two hydroxyl) propyl-2-

undecyl imidazoline sodium phosphate(CEPIP) for Q235 steel in CO2 saturated solution

at 298 K, and calculated by weight loss and electrochemical techniques. The results

indicated that the two imidazoline derivatives could both adsorb on the Fe surface

firmly through the imidazoline ring and heteroatoms, and hence the two inhibitors both

have excellent corrosion inhibition performance.

Corrosion monitoring of mild steel in sulphuric acid solutions in the presence of

some thiazole derivatives like 2-amino-4-(p-tolyl)thiazole (APT), 2-methoxy-1,3-

thiazole (MTT) and thiazole-4-carboxaldehyde (TCA) at iron (110) surface dissolved in

aqueous solution were studied via molecular dynamics (MD) simulations by Khaled and

Amin [113]. Chemical and electrochemical measurements are agreeing with

computational study and APT is the most effective inhibitor among the tested thiazoles.

EIS and polarization studies to evaluate the inhibitors effect of 3H-phenothiazin-3-one,

7-dimethylamin on mild steel corrosion in 1 M HCl solution was reported by Sorkhabi

et al [114]. It was found that it acts as a strong inhibitor for mild steel in 1 M HCl even

at very low concentration (1 ppm). Tris-hydroxymethyl-(2-hydroxybenzylidenamino)-


41

methane (THHM) was synthesized and its anticorrosive property for cold rolled steel in

hydrochloric acid has been studied by Qing et al [115]. Ali et al. [116] studied the bis-

isoxazolidines obtained by the cycloaddition reaction of 1-pyrroline 1-oxide with 2,7-

di(10-undecenyloxy)naphthalene and 1,4-di(10-undecenyloxy)benzene as new class of

corrosion inhibitors for mild steel in acidic media. All the three inhibitors molecules at

60 °C and 400 ppm concentration achieved inhibition efficiencies in the ranges 97–98

% and 92–96 % as determined by gravimetric method for corrosion of mild steel in 1 M

HCl and 0.5 M H2SO4, respectively.

The effect of two triazole derivatives namely 1-[2-(4-nitro-phenyl)-

5[1,2,4]triazol-1-ylmethyl-[1,3,4]oxadiazol-3-yl]-enthanone (NTOE) and 1-(4-methoxy-

phenyl)-2-(5-[1,2,4]triazol-1-ylmethyl-4H[1,2,4]triazol-3-ylsulfanyl)-ethanone (MTTE)

against the corrosion of mild steel in 1 M hydrochloric acid was studied by Zhanga et al

[117]. The inhibition ability of the two follows the order NOTE > MTTE, and the

inhibition deficiencies determined by polarization, EIS, and weight loss methods are in

good agreement with each other. Gokhan Gece [118] reported the application of

quantum chemical methods in corrosion inhibitor studies. It begins with a concise

summary of the most used quantum chemical parameters and methods and then

summarizes the results of research articles in corrosion science over the past 20 years.

Quantum chemical study of the inhibition of the corrosion of mild steel in H2SO4 by

some antibiotics was studied by Nabuk et al [119]. Some Schiff base compounds

containing disulfide bond were analyzed as corrosion inhibitors for mild steel in HCl

[120] by Behpour et al. Quantum chemical calculations were further applied to reveal

the adsorption structure and explain the experimental results. The effect of organic

compounds on the electrochemical behaviour of steel in acidic media, a review has been

reported by Abd El–Maksoud [121]. Natural product as corrosion inhibitor for metals in

corrosive media - a review was reported by Raja and Sethuraman [122].

The relationship between the quantum chemical parameters for three triazole

compounds and their inhibition ability was studied using electrochemical measurements

(potentiodynamic polarization and EIS), molecular dynamic method and quantum

chemical calculations were done by Musa et al [123]. The quantum chemical

calculations showed that the triazole ring is the active site in these inhibitors and they

can adsorb on Fe surface by donating electrons to d-orbitals of Fe. Adsorption isotherms

and corrosion inhibitive property of some oxadiazol-triazole (TOMP) derivatives in


42

acidic solution was analyzed by Zhihua et al [124]. Results obtained revealed that

TOMP is an effective corrosion inhibitor for mild steel in sulphuric acid and its

efficiency attains more than 97.6% at 298 K. Solmaz [125] studied the inhibition effect

of a Schiff base 5-((E)-4-phenylbuta-1,3-dienylideneamino)-1,3,4-thiadiazole-2-thiol

(PDTT) on mild steel corrosion in 0.5 M HCl for both short and long immersion time.

The Schiff base has shown remarkable inhibition on the corrosion of mild steel in 0.5 M

HCl solution. The high inhibition efficiency was attributed to the blocking of active

sites by adsorption of inhibitor molecules on the steel surface.

Four Gemini surfactants were synthesized and evaluated as corrosion inhibitors

for carbon steel in 0.5 M HCl solution by Negm et al [126]. The results showed that

inhibition efficiencies increased on increasing the inhibitor doses, hydrophobic chain

length and reached a maximum at 500 ppm by weight for stearyl derivative. Corrosion

inhibition of mild steel in 0.5 M H2SO4 and 1.0 M HCl by 2-amino-5-phenyl-1,3,4-

thiadiazole (APT) has been studied using potentiodynamic polarization and

electrochemical impedance spectroscopy (EIS) measurements by Yongming Tang et al.

[127]. The results showed that inhibition efficiency increases with the increase of APT

concentration in both acids, and higher inhibition efficiency is obtained in 0.5 M H2SO4.

Obot et al. [128] evaluated ketoconazole (KCZ) as corrosion inhibitor for mild steel in

aerated 0.1 M H2SO4 by gravimetric method. Quantum chemical approach was further

used to calculate some electronic properties of the molecule in order to ascertain

correlation between the inhibitive effect and molecular structure of ketoconazole.

The behavior of Schiff base, N, N′-bis(salicylidene)-1,2-ethylenediamine (salen),

its reduced form (N, N′-bis(2-hydroxybenzyl)-1,2-ethylenediamine) and a mixture of its

preceding molecules, ethylenediamine and salicylaldehyde as corrosion inhibitors on

carbon steel in 1 mol L−1 HCl solution was studied by da Silva et al [129]. Experimental

results showed that the reduced salen showed highest efficiency among the inhibitors

studied. The results obtained in the presence of salen were similar to those obtained in

the presence of the salicylaldehyde and ethylenediamine mixture, showing that in acid

medium salen molecule undergoes hydrolysis, regenerating its precursor molecules.

Potentiodynamic polarization, electrochemical impedance spectroscopy (EIS),

weight loss measurements and atomic force microscopy techniques were used to

investigate the inhibitory effect of diethylcarbamazine (DECM) on the corrosion of mild


43

steel in HCl solution was studied by Singh et al [130]. The inhibitor showed >90 %

inhibition efficiency at 5.01 × 10−4 M. Three compounds of N-alkyl-sodium

phthalamates were synthesized and tested as corrosion inhibitors for carbon steel in

0.5 M aqueous hydrochloric acid by Flores et al [131]. Tests showed that inhibition

efficiencies were related to aliphatic chain length and dependent on concentration. N-1-

n-tetradecyl-sodium phthalamate displayed moderate efficiency against uniform

corrosion, which is 42–86% at 25 °C and 25–60 % at 40 °C. Some 4-phenylthiazole

derivatives were tested as corrosion inhibitors for 304L stainless steel in 3 M HCl using

weight loss and galvanostatic techniques by Fouda et al [132]. The results showed that

inhibition efficiency increases with increasing concentration of 4-phenylthiazole

derivatives.

Four 1,3-diketone malonates compounds were synthesized and tested as corrosion

inhibitor by Fragoza-Mar et al. [133] on mild steel in 1 M aqueous hydrochloric acid.

Both tautomers displayed high corrosion inhibition efficiency (75–96 %) at 100 mg L−1,

which increased with temperature (25–55 °C) and dependent on diketo population.

Goulart et al. [134] studied the inhibition ability of four thiosemicarbazones and two

semicarbazones towards carbon steel corrosion in 1 M HCl. Results were evaluated by

molecular modeling, potentiodynamic polarization and electrochemical impedance

spectroscopy (EIS) at different inhibitor concentrations. Both experimental and

theoretical data showed that thiosemicarbazones are better corrosion inhibitors than the

semicarbazones. A comparative study of 5-amino-1,2,4-triazole (5-ATA), 5-amino-3-

mercapto-1,2,4-triazole (5-AMT), 5-amino-3-methylthio-1,2,4-triazole (5-AMeTT) and

1-amino-3-methylthio-1,2,4-triazole (1-AMeTT) as inhibitors for mild steel corrosion in

0.1 M HCl solution at 20 ºC was carried out by Hassan et al [135]. The studies have

shown that 5-AMT was the most efficient inhibitor reaching the values of inhibition

efficiency up to 96 % at the concentration of 10-3 M.

The effect of 1-methyl-3-pyridin-2-yl-thiourea on the corrosion resistance of mild

steel in H2SO4 solution was investigated by Hosseini et al [136]. The results showed

that inhibition efficiency increases with the increase of inhibitor concentration. Jawich

et al. [137] for the first time, compared the corrosion inhibition properties of

diethylenetetramine-derived imidazoline (I), tetraethylenepentamine-derived

imidazoline (II) and bis-imidazoline (III), all having heptadecyl pendants as

hydrophobes. At the concentration of 100 ppm at 40 °C, imidazolines I, II, and III


44

imparted inhibition efficiencies of 84 %, 95 % and 96 % respectively. Imidazolines

cover most of the steel surface before their critical molar concentrations are reached.

The corrosion inhibition of mild steel in 1M HCl solution by a Schiff base compound

named 2-[(4-phenoxy-phenylimino)methyl]-phenol (APS) was investigated by Keles

[138] at different temperatures (25–55 °C) using electrochemical measurements. Time

dependency of mild steel in 1 M HCl solution in the absence and presence of APS was

also studied. N-Phenyl oxalic dihydrazide (PODH) and oxalic N-phenylhydrazide N′-

phenylthiosemicarbazide (OPHPT) were synthesized and tested as inhibitors for the

corrosion of mild steel in 1M HCl by Larabi et al [139]. Inhibition efficiencies up to 92

% for OPHPT and 79 % for PODH were obtained.

Sodium diethyldithiocarbamate (DDTC) as corrosion inhibitor for cold rolled steel

(CRS) in 0.5 M HCl solution was investigated by Lei Li et al. [140] using Tafel

polarization and electrochemical impedance spectroscopy (EIS). All the data indicate

that DDTC can inhibit the corrosion of CRS in HCl solution. Polarization data showed

that DDTC acts as a good inhibitor though it can accelerate the anodic reaction to some

extent. 2-((Dehydroabietylamine) methyl)-6-methoxyphenol (DMP) was investigated by

as a corrosion inhibitor for mild steel in synthetic seawater medium by weight loss

measurements, electrochemical tests, SEM, XPS, quantum chemical calculations and

molecular dynamics Liu et al [141] (MD). The results showed that DMP inhibits the

corrosion of mild steel in synthetic seawater, and inhibition efficiency increases with the

increase in concentration of the inhibitor. The inhibitory effect of tryptamine on the

corrosion of mild steel in 0.5 M hydrochloric acid at 30 °C was investigated by

Lowmunkhong et al. [142] using linear polarization, potentiodynamic polarization and

electrochemical impedance spectroscopy (EIS) techniques. When the concentration of

tryptamine is 500 ppm the inhibition efficiency calculated by these techniques is around

97 %. The inhibition effect of 3-amino-1,2,4-triazole-5-thiol (3ATA5T) was

investigated in 0.5 M HCl on carbon steel (CS) by Mert et al. [143] at various

concentrations and temperatures. The results showed the correlation between

experimental (inhibition efficiencies, ΔGads, Ea) and quantum chemical parameters

(dipole moment, EHOMO, ELUMO). The high inhibition efficiency was described in terms

of strong adsorption of protonated inhibitor molecules on the metal surface which forms

a protective film.


45

Safarizadeh and Khosravan [144] investigated the interaction energies of

benzimidazole, aniline and nine of their derivatives on the surface of iron in

hydrochloric acid medium through cluster model using quantum chemical calculations

at DFT level. The interaction energy is then used as a descriptor in QSIR method for

corrosion behavior modeling of the inhibitors in solutions at different concentrations.

The findings are indicative of the effectiveness of the new descriptor. The corrosion

protection of mild steel in 2.5 M H2SO4 solution by 4,4-dimethyloxazolidine-2-thione

(DMT) was studied by Musa et al. [145] at different temperatures by measuring

changes in open circuit potential (OCP), potentiodynamic polarization and

electrochemical impedance spectroscopy (EIS). Results showed that DMT inhibited

mild steel corrosion in 2.5 M H2SO4 solution and inhibition efficiencies increased with

the concentration of the inhibitor, but decreased proportionally with temperature.

The corrosion inhibition of mild steel in 2.5 M H2SO4 solution by 4-amino-5-

phenyl-4H-1, 2, 4-trizole-3-thiol (APTT) was studied at different temperatures, utilising

open circuit potential, potentiodynamic and impedance measurements by Musa et al

[146]. The results indicate that APTT performed as an excellent mixed-type inhibitor

for mild steel corrosion in 2.5 M H2SO4 solution. The corrosion inhibition properties of

a new class of oxadiazole derivatives, namely 3,5-bis(n-pyridyl)1,2,4-oxadiazoles (n-

DPOX) for C38 carbon steel corrosion in 1 M HCl medium were analysed by Outirite et

al [147]. An adequate structural model of the interface was used and values of the

corresponding parameters were calculated and discussed. The inhibition efficiency,

increase with the increase in inhibitor concentration and attained a maximum value of

97 % at 12 ×10-4 M of 3-DPOX. At all concentrations, IE(%) follows the ranking order:

3-DPOX > 2DPOX> 4-DPOX. Popova et al. [148] investigated eight diazoles as

corrosion inhibitors for mild steel in 1 M hydrochloric acid using gravimetric and

polarization techniques. It was found that the inhibition efficiency increased with the

increase of organic substrate concentration and the adsorption followed the Frumkin

isotherm.

The inhibitory properties of four azoles (indole, benzimidazole, benzotriazole,

benzothiazole) were investigated in case of mild steel corrosion in 1 M HCl by Popova

[149], the effect of temperature was followed. Impedance spectroscopy, polarization

resistance, gravimetric and polarization curves methods were applied. The corrosion

inhibition effect of 4-{[(1Z)-(2-chloroquinolin-3-yl)methylene]amino} phenol (CAP),


46

N-[(1Z)-(2-chloroquinolin-yl)methylene]-N-(4-methoxyphenyl)amine (CMPA) and N-

[(1E)-(2-chloroquinolin-3-yl)methylene]-N-(4-nitrophenyl)amine (CNPA) on mild steel

in 1 M HCl has been investigated by Prabhu et al. [150] using mass loss, polarization

and electrochemical impedance techniques at 300 K.

Inhibition of mild steel corrosion in de-aerated 0.5 M sulfuric acid solutions

containing various concentrations of indole-5-carboxylic acid was studied in the

temperature range of 25 - 55ºC using weight loss, potentiodynamic polarization and

spectrophotometric tests has been carried out by Quartarone and co-workers [151].

Selected triazole derivatives have been synthesized and evaluated as corrosion inhibitors

for mild steel in natural aqueous environment by weight loss, potentiodynamic

polarization and ac impedance methods by Ramesh and Rajeswari [152]. All the

condensed products showed good inhibition efficiency. The effect of changing

functional groups of some triazole derivatives on their inhibition efficiency was also

reported using weight loss and potentiodynamic techniques. 3-Salicylalidene amino-

1,2,4-triazole phosphonate (SATP) was found to be the best corrosion inhibitor compare

to the other compounds. Sathiyanarayanan et al. [153] attempted to study the inhibitive

effect of triethylene tetramine (TETA) and hexamethylene tetramine (HMTA) for mild

steel in 1 M hydrochloric acid in the concentration range of 10−6 to 10−2 M by weight

loss, DC polarization and AC impedance spectroscopy methods. Results indicate that

the addition of tetramines to the acid reduce the rate of metal attack.

The inhibitory effect of 2-((5-mercapto-1,3,4-thiadiazol-2-

ylimino)methyl)phenol Schiff base (MTMP) on mild steel corrosion in 0.5 M HCl

solution was studied by Solmaz et al [154]. It was shown that, the MTMP Schiff base

has remarkable inhibition efficiency on the corrosion of mild steel in 0.5 M HCl

solution. The inhibition efficiency depends on the concentration of inhibitor and reaches

97 % at 1 mM MTMP. Jacob and Parameswaran [155] investigated heterocyclic Schiff

base furoin thiosemicarbazone for its corrosion inhibition towards mild steel in 1 M HCl

solution using weight loss, Tafel polarization and electrochemical impedance

spectroscopy techniques. Furoin thiosemicarbazone revealed good corrosion inhibition

efficiency even at low concentrations towards mild steel in HCl medium. Chemical

methods were used to assess the inhibitive and adsorption behaviour of carboxymethyl

cellulose (CMC) by Solomon et al. [156] for mild steel in H2SO4 solution at 30–60 °C.

Results obtained show that CMC acts as inhibitor for mild steel in H2SO4.


47

Two thiadiazole derivatives namely 2-amino-5-(3-pyridyl)-1,3,4-thiadiazole (3-

APTD) and 2-amino-5-(4-pyridyl)-1,3,4-thiadiazole (4-APTD) were investigated as

steel corrosion inhibitors in 0.5 M H2SO4 by Tang et al. [157] using polarization and

electrochemical impedance spectroscopy. The experimental results showed that the

inhibition efficiency of 4-APTD is higher than that of 3-APTD, and the molecular

dynamics (MD) simulations showed that the adsorption of 4-APTD on iron surface has

the higher binding energy than that of 3-APTD. Tuken et al. [158] studied the inhibition

efficiency of 1-ethyl-3-methylimidazolium dicyanamide (EMID) against mild steel

corrosion in acidic environment. Surface analysis showed that the assembled inhibitor

film could protect the metal successfully during 120 h exposure in 0.1 M H2SO4. The

inhibitor was fixed successfully within polypyrrole film and applied to steel surface as

highly protective coating.

Zhang et al. [159] investigated the corrosion inhibition performance of 2-(4-

pyridyl)-benzimidazole (PBI) against corrosion of mild steel in 1 M HCl using weight

loss and electrochemical measurements. The theoretical results from DFT and MD

simulations revealed that adsorption of PBI depends on the formation of coordinative

bonds between PBI molecule and iron surface, and the binding energy between PBI

molecule and iron surface is the highest among the three studied compounds. Five new

tetrathiafulvalene (TTF) derivatives bearing two or four thiazole groups were prepared

by Zhao et al [160]. The derivatives were characterized by 1H NMR, IR, MS spectra,

elemental analyses and X-ray analysis. The preliminary electrochemical properties of

the derivatives were investigated by cyclic voltammetry (CV) and two one-electron

quasi-reversible waves with redox potentials were observed. The synergistic inhibition

effect of rare earth cerium(IV) ion and 3,4-dihydroxybenzaldehye (DHBA) on the

corrosion of cold rolled steel (CRS) in H2SO4 solution was first investigated by Li et al.

[161] using weight loss and potentiodynamic polarization methods. The effect of

inhibitor concentration, temperature, immersion time and acid concentration on

synergism were discussed in detail. The results revealed that DHBA has moderate

inhibitive effect and its adsorption obeys Temkin adsorption isotherm. The corrosion

inhibition of mild steel in 1 M HCl solution by streptomycin has been studied by Shukla

et al. [162] using Tafel polarization, electrochemical impedance spectroscopy (EIS) and

weight loss measurements. The inhibitor showed 88.5 % inhibition efficiency at

optimum concentration of 500 ppm.


48

Sodium tungstate (Na2WO4) has been investigated as corrosion inhibitor of cold

rolled steel (CRS) in per acetic acid (PAA) solution by weight loss, potentiodynamic

polarization and electrochemical impedance spectroscopy (EIS) by Qu et al [163]. All

the data obtained from the experiments indicate that Na2WO4 is capable of inhibiting

the corrosion of CRS in PAA solution. Ostovari et al. [164] investigated the corrosion

inhibitory action of henna extract (Lawsonia inermis) and its main constituents

(lawsone, gallic acid, α-d-Glucose and tannic acid) on corrosion of mild steel in 1 M

HCl solution through electrochemical techniques and surface analysis. Maximum

inhibition efficiency (92.06 %) was obtained at 1.2 g/l henna extract. Inhibition

efficiency increases in the order: lawsone > henna extract > gallic acid > α-d-

Glucose > tannic acid. The corrosion inhibition of 2-hydrazino-4,7-

dimethylbenzothiazole on low carbon steel in industrial water has been investigated by

Badiea and Mohana [165] at different temperatures and fluid velocities at different

concentrations of the inhibitor using mass loss, potentiodynamic polarization and

electrochemical impedance spectroscopy measurements.

Different electrochemical methods have been employed by da Trindade and

Gonçalves [166] in order to confirm the ability of caffeine (1,3,7-trimethylxanthine) to

adsorb to low-carbon steel in ethanol solutions and thereby inhibit the corrosion

process. The presence of the adsorbed organic compound on low-carbon steel electrode

surface was confirmed by comparing the voltammograms, Tafel plots and EIS curves of

the electrode in the absence and presence of caffeine. The inhibition performance of

3,5-bis(4-methoxyphenyl)-4-amino-1,2,4-triazole (4-MAT) on mild steel in normal

hydrochloric acid medium at 30 °C was tested by Bentiss et al. [167] using weight loss,

potentiodynamic polarization and electrochemical impedance spectroscopy (EIS)

techniques. 4-MAT inhibits the acidic corrosion even at very low concentration,

reaching a value of inhibition efficiency up to 98 % at the concentration of 3 × 10−4 M.

Influence of hydrodynamic conditions on the corrosion of St52-3 type steel rotating disc

electrode (RDE) in 3.5 % NaCl and its corrosion inhibition using K2HPO4 have been

studied by Ashassi-Sorkhabi and E. Asghari [168].

The corrosion inhibition behavior of benzotriazole (BTZ), Na3PO4 (SP) and their

mixture on carbon steel in 20 wt.% (0.628 mol l−1) tetra-n-butylammonium bromide

aerated aqueous solution was investigated by Liu et al. [169] using weight-loss test,

potentiodynamic polarization measurement, electrochemical impedance spectroscopy


49

and scanning electron microscope/energy dispersive X-ray techniques. The results

revealed that inhibitor mixtures have shown synergistic effects at lower concentration of

inhibitors. At 2 g l−1 BTA and SP showed optimum enhanced inhibition compared with

their individual effects. Shukla [170] and co-workers studied the corrosion inhibition of

mild steel in 1 M HCl solution by cefotaxime sodium by Tafel polarization,

electrochemical impedance spectroscopy (EIS) and weight loss measurements. The

inhibitor showed 95.8 % inhibition efficiency at the optimum concentration of 300 ppm.

Mahdavian and Attar [171] studied the corrosion inhibition of some metal

acetylacetonate complexes including Co(acac)2, Cu(acac)2, Mn(acac)2 and Zn(acac)2

using electrochemical impedance spectroscopy (EIS) in 3.5 % NaCl for mild steel. The

results were compared to zinc potassium chromate (ZPC) solution in 3.5 % NaCl.

Corrosion inhibition of these metal complexes followed the order:

ZPC > Co(acac)2 > Zn(acac)2 > Mn(acac)2 while Cu(acac)2 displayed corrosion catalytic

activity. Liua et al. [172] studied the inhibitory behaviour of 2-undecyl-1-aminoethyl

imidazoline (AEI-11) and 2-undecyl-1-aminoethyl-1-hydroxyethyl quaternary

imidazoline (AQI-11) on CO2 corrosion of N80 mild steel in single liquid phase and

liquid/particle two-phase flow using weight loss, linear polarization, potentiodynamic

polarization, EIS methods and SEM observations. In both phases, AQI-11 exhibited

better inhibition ability than AEI-11 due to the polycentric adsorption sites on its

structure.

Zhang and Hua [173] investigated the acid corrosion inhibition process of mild

steel in 1 M HCl by 1-butyl-3-methylimidazolium chloride (BMIC) and 1-butyl-3-

methylimidazolium hydrogen sulfate ([BMIM]HSO4) using electrochemical impedance,

potentiodynamic polarization and weight loss measurements. For both inhibitors, the

inhibition efficiency increased with increase in the concentration of inhibitor, and

effectiveness of the two inhibitors are in the order [BMIM]HSO4 > BMIC. The

inhibition behaviour of 2-undecyl-1-ethylamino-1-methylbenzyl quaternary imidazoline

(2UMQI) and KI on mild steel in 1 M H2SO4 solutions was investigated by Okafor and

Zheng [174] at 25 °C using electrochemical methods. Inhibition efficiency of 2UMQI

was enhanced by the addition of iodide ions. In the presence of KI, the potential of

unpolarization, was observed to increase with KI concentration. The synergistic effect

of iodide ions and benzisothiozole-3-piperizine hydrochloride (BITP) on the corrosion

inhibition of mild steel in 0.5 M H2SO4 solution has been studied by Pavithra et al.


50

[175] using both chemical and electrochemical methods. The BITP showed maximum

inhibition efficiency of 93 % in HCl media whereas its inhibition efficiency in sulphuric

acid is about 85 %. Hegazy et al. [176] synthesized, characterized and tested novel

cationic gemini surfactants namely: bis(p-(N,N,N-octyldimethylammonium

bromide)benzylidene)benzene-1,4-diamine (I), bis(p-(N,N,N-decyldimethylammonium

bromide)benzylidene)benzene-1,4-diamine (II), and bis(p-(N,N,N-

dodecyldimethylammonium bromide)benzylidene)benzene-1,4-diamine (III) as

corrosion inhibitors for carbon steel in 1 M HCl solution. The obtained results showed

that, the synthesized inhibitors are excellent inhibitors for carbon steel in 1 M HCl

solution.

Two series of cationic Schiff base surfactants namely: 2-(benzylideneamino)-3-

(2-oxo-2-alkoxyethyl)-1,3-benzothiazol-3-ium bromide and 2-[(4-

methoxybenzylidene)amino]-3-(2-oxo-2-alkoxyethyl)-1,3-benzothiazol-3-ium bromide

were prepared by Negm et al. [177] and their structures were confirmed using elemental

analyses, FTIR, and 1H NMR spectra. The surface activity of the synthesized Schiff

bases showed their tendency towards adsorption at the interfaces. Nam and Kim [178]

examined the effect of niobium (Nb) addition on the electrochemical properties of low

alloy steel using electrochemical techniques in 10 wt.% sulfuric acid solution as well as

surface analysis techniques. These results suggest that the interaction of elements

improves the corrosion resistance of low alloy steel by the formation of Nb, C, S, P, and

Fe products on the surface. Ionic liquids with chemical 1,3-dioctadecylimidazolium

bromide and N-octadecylpyridinium bromide were synthesized by Likhanova et al.

[179] using conventional and microwave-assisted reactions, respectively. Ionic liquids

were tested as corrosion inhibitors and polarization curves displayed corrosion

protection efficiency between 82 % & 88 % at 100 ppm for mild steel in 1 M aqueous

solution of sulfuric acid.

Adsorption of four derivatives of piperidinylmethylindoline-2-one on mild steel

surface in 1 M HCl solution and its corrosion inhibition properties have been studied by

Ahamad et al. [180] using a series of techniques such as polarization, electrochemical

impedance spectroscopy (EIS), weight loss and quantum chemical calculation methods.

Soltani et al. [181] studied the inhibition effect of four double Schiff bases on the

corrosion of mild steel in 2 M HCl by polarization, electrochemical impedance

spectroscopy (EIS) and weight loss measurements. Quantum chemical calculations have


51

been performed. Several quantum chemical indices were calculated and correlated the

corresponding inhibition efficiencies. A cationic gemini-surfactant, namely 1,4-bis (1-

chlorobenzyl-benzimidazolyl)-butane (CBB) was synthesized and its inhibitory effect

on the corrosion of mild steel in 0.5 M HCl solution was investigated by Wang et al.

[182] using weight loss and electrochemical techniques. The results showed that CBB

acts as an excellent corrosion inhibitor in 0.5 M HCl by suppressing simultaneously

cathodic and anodic processes via chemical adsorption on the surface of steel, which

followed the Langmuir adsorption isotherm. Obot et al. [183] synthesized 2,3-

diphenylbenzoquinoxaline (2,3DPQ) and assessed its inhibitory action on the corrosion

of mild steel in 0.5 M H2SO4 by weight loss method at 30 °C. The results of the

investigation showed that this compound has excellent inhibitory properties for steel

corrosion in sulphuric acid. Quantum chemical calculations were employed to give

further insight into the mechanism of inhibition action of 2,3-DPQ. Singh and Quraishi

[184] studied the adsorption and inhibition effect of cefazolin on mild steel in 1 M HCl

between 308 & 338 K using weight loss, EIS, potentiodynamic polarization and atomic

force microscopy techniques. The results showed that inhibition efficiency increases

with increase in inhibitor concentration.

The corrosion inhibition efficiencies of two crown type polyethers, namely

dibenzo-bis-imino crown ether (C-1) and dibenzo-diaza crown ether (C-2) which are

macrocyclic Schiff base and its reduced form (macrocyclic amine), respectively, for the

steel in 1 M H2SO4 have been investigated by Hasanov et al. [185] using Tafel

extrapolation and linear polarization methods. The obtained results of these calculations

for the compounds were found to be consistent with the experimental findings. Obot et

al. [186] tested acenaphtho [1,2-b] quinoxaline (AQ) as a novel corrosion inhibitor for

mild steel in 0.5 M H2SO4 solution using chemical technique at 30 °C. AQ acts as an

effective inhibitor for mild steel in acidic medium. Inhibition efficiency increased with

increasing concentration of AQ. A zero-order kinetics relationship with respect to mild

steel was obtained with and without AQ from the kinetic treatment of the data. DFT

study gave further insight into the mechanism of inhibition action of AQ. The inhibitory

effect of novel nonionic surfactants on the corrosion of carbon steel (CS) in 1 M HCl

was studied by Hegazy and Zaky [187] at different temperatures (20–60 °C) by weight

loss, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization

methods. The CS surface morphology was investigated by SEM. The obtained results


52

showed that the prepared non-ionic surfactants are excellent inhibitors in 1 M HCl, and

the inhibition efficiency increases with the increase in inhibitor concentration and

temperature.

The inhibition ability of 4,4-dimethyloxazolidine-2-thione (DMT) for mild steel

corrosion in 1 M HCl solution at 30 °C was studied by Musa et al. [188] using

potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) technique,

and scanning electron microscopy (SEM). Results showed that DMT performed good

inhibiting effect for the corrosion of mild steel in 1 M HCl solution and inhibition

efficiency is higher than 82 % at 4 × 10−3 M of DMT. Acetyl thiourea chitosan polymer

(ATUCS) has been synthesized and evaluated as corrosion inhibitor by Fekry and

Mohamed [189]. The electrochemical behavior of mild steel in naturally aerated 0.5 M

H2SO4 acid containing different concentrations of ATUCS has been studied by

potentiodynamic polarization, electrochemical impedance spectroscopy (EIS)

measurements and surface examination via scanning electron microscope (SEM)

technique. Electrochemical impedance spectroscopy measurements under open-circuit

conditions confirmed well polarization results. ATUCS has shown very good inhibition

efficiency in 0.5 M sulphuric acid solution and reaches to 94.5 % efficiency for

0.76 mM concentration. Negm et al. [190] examined the effects of two biodegradable

corrosion inhibitors derived from vanillin and aminophenol (Meta: VPAP and Para:

VOAP) on the corrosion of carbon steel in 1 M HCl solution. The geometry of the

inhibitors showed that VPAP has slightly higher corrosion inhibition efficiency than

VOAP. The inhibitors showed good biodegradability in the environment that is within

28 days they were completely degraded.

Nataraja et al. [191] investigated the inhibition potential of ziprasidone for the

corrosion of steel in 1 M HCl and 0.5 M H2SO4 using weight loss, polarization,

electrochemical impedance spectroscopy and quantum chemical methods. Ziprasidone

is composed of benzisothiozole-3-piperizine (BITP) and an indole moiety. The results

showed that nearly 10 times lower concentration of ziprasidone showed the same

efficiency that was rendered by BITP. This is related to the planarity of ziprasidone

molecule, potential adsorption sites and extensive distribution of LUMO orbitals on

indole moiety which cause larger back donation. Inhibitive performance of some

synthesized thiophenol derivatives on the corrosion behavior of mild steel in 0.1 M HCl

solution was investigated by Kosari et al. [192] using electrochemical techniques,


53

quantum chemical calculations and optical microscopy. Inhibitor molecules directly get

adsorbed at surface on the basis of donor–acceptor interactions between π-electrons of

benzene, sulfur and nitrogen atoms and the vacant d-orbitals of iron atoms. The

inhibitive effect of a newly synthesized imidazoline phosphate against Q235 steel and

its adsorption behavior were investigated by Zhang et al. [193] in 1 M HCl solution

using weight-loss, potentiodynamic polarization and electrochemical impedance

spectroscopy (EIS) techniques. The quantum chemistry calculations exhibited that the

imidazoline ring and heteroatoms (N, O, and P) were the active sites of inhibitor.

The inhibition effect of three pyrazine derivatives such as 2-methylpyrazine

(MP), 2-aminopyrazine (AP) and 2-amino-5-bromopyrazine (ABP) on the corrosion of

cold rolled steel (CRS) in 1 M H2SO4 solution was studied by Li et al. [194] using

weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy

(EIS) methods. The results showed that all pyrazine compounds are good inhibitors, and

the inhibition efficiency follows the order: ABP > AP > MP. The corrosion inhibition

effect of 2-[4-(methylthio) phenyl] acetohydrazide (HYD), 2-{[4-(methylthio) phenyl]

acetyl} hydrazinecarbothioamide (TAD) and 5-[4-(methylthio) benzyl]-4H-1,2,4-

triazole-3-thiol (TRD) on steel in 1 M HCl was investigated by Nataraja et al. [195]

using mass loss and electrochemical methods. The efficiency stands in the order

TRD > TAD > HYD, and is confirmed by the Quantum chemical studies. Singh et al.

[196] studied the inhibitory effect of two Schiff bases on the corrosion of the mild steel

in 1 M HCl using electrochemical impedance spectroscopy and Tafel polarization

measurements. It is suggested that the effects of Schiff bases depend on concentrations

and the molecular structures. Popova [197] and co-worker investigated seven

quaternary ammonium bromides of different heterocyclic compounds as corrosion

inhibitors of mild steel in 1 M HCl using gravimetric and polarization techniques. For

comparison, gravimetric experiments were carried out in 1 M H2SO4 as well. The

inhibition efficiency was found to depend on both concentration and molecular structure

of the inhibitor.

The influences of a benzimidazole derivative, namely 1,8-bis (1-chlorobenzyl-

benzimidazolyl)-octane (CBO) on the corrosion behaviour of mild steel in different

concentrations HCl solutions were studied by Wang et al. [198] using weight loss,

potentiodynamic polarization, electrochemical impedance spectroscopy (EIS)

measurements and SEM observations. The results showed that CBO acted as an


54

excellent and a mixed-type inhibitor via strongly chemical adsorption onto mild steel

surface to suppress simultaneously both anodic and cathodic processes according to the

Langmuir adsorption isotherm. The inhibition effect of triazolyl blue tetrazolium

bromide (TBTB) on the corrosion of cold rolled steel (CRS) in 1 M HCl and 0.5 M

H2SO4 solution was investigated for the first time by Li et al. [199] using weight loss,

potentiodynamic polarization and electrochemical impedance spectroscopy methods.

The results showed that TBTB is a very good inhibitor, and is more efficient in 1 M

HCl than in 0.5 M H2SO4. Obot et al. [200] studied the anti-corrosive effect of xanthone

(XAN) on the corrosion of mild steel in 0.5 M H2SO4 by gravimetric and UV–visible

spectrophotometric methods between 303 & 333 K. Results obtained revealed that XAN

performed excellently as a corrosion inhibitor for mild steel in sulphuric acid. Quantum

chemical calculations have been performed using DFT, and several quantum chemical

indices were calculated and correlated with the inhibitory effect.

The corrosion inhibition properties of ceftadizime (CZD) on mild steel corrosion

in HCl solution were analyzed by Singh et al. [201] using electrochemical impedance

spectroscopy, potentiodynamic polarization and gravimetric methods. The experimental

data showed a frequency distribution and therefore a modeling element with frequency

dispersion behaviour and a constant phase element (CPE) have been used. 2-

Aminothiazole (AT) was polymerized by Solmaz [202] using electrochemical technique

on mild steel electrode from 0.01 M monomer containing 0.3 M ammonium oxalate

solution. The results obtained indicated that, the polymer film adherent to the steel

surface. The polymer film gives good corrosion protection against the attack of

corrosive environment. The inhibitory action of 2-mercapto benzimidazole (2MBI) on

mild steel in 1 M hydrochloric acid has been investigated by Benabdellah et al. [203] at

308 K using weight loss measurements and electrochemical techniques. Inhibition

efficiency increases with 2MBI concentration to attain 98 % at 10−3 M.

Palomar-Pardave et al. [204] investigated the corrosion inhibition efficiency of

2-amino 5-alkyl 1,3,4 thiadiazole compounds with different alkyl chain lengths, namely,

2 ethyl, 3 n propyl, 5 n penthyl, 7 hepthyl, 11 undecyl and 13 tridecyl in the system,

steel/1 M H2SO4. The results showed that the inhibition mechanism involves blockage

of steel surface by inhibitor molecules by Langmuir-type adsorption process and the

alkyl chain length plays an important role in the inhibition efficiency of the synthesized

inhibitors. The inhibitory effect of three novel nonionic surfactants, 2-((alkylimino)


55

methyl)phenyl bis(53- hydroxy-3,6,9,12,15,18,21,24,27,30,33,36,39,42,45,48,51

heptadecaoxatripentacontyl) phosphate (I–III), on the corrosion of carbon steel in

0.5 H2SO4 was studied by Hegazy et al. [205] using polarization, EIS and weight loss

measurements. It was found that all the inhibitors were effective and their inhibition

efficiency was significantly increased with increasing both concentration and

temperature. The adsorption behaviour of cefapirin (CFP) on mild steel in 1 M HCl

solution was studied by Singh et al. [206] using potentiodynamic polarization,

electrochemical impedance spectroscopy and weight loss techniques. The hydrophobic

character of MS increased with increasing concentration of CFP. The results showed

that CFP performed excellently as corrosion inhibitor for MS.

The inhibition performance of four benzylidine malononitriles (BMNs) on mild

steel in 1 M HCl was studied by Yadava et al. [207] using weight loss, potentiodynamic

polarization and electrochemical impedance spectroscopy techniques. The results of

quantum chemical calculations were correlated with experimental inhibition

efficiencies. The inhibition performance of cationic gemini-surfactant (CBB) and its co-

adsorption behavior with halides on mild steel in 0.25 M H2SO4 solution was studied by

Wang et al. [208] using weight loss and electrochemical techniques. Results showed

that the compound could effectively inhibit the mild steel corrosion and acted as a

mixed-type inhibitor by suppressing simultaneously anodic and cathodic reactions.

Mendes et al. [209] used quantum mechanical calculations based on the density

functional theory under periodic boundary conditions to construct a model for the

inhibition performance of imidazole on iron surface. It is shown that, at high coverage,

imidazole molecules form C–C intermolecular bonds creating a protective film, which

constitutes a hydrophobic medium and, consequently, prevents adsorption of water

molecules. 2-Thiohydantoin (2-THD) was investigated as a corrosion inhibitor of mild

steel in 0.1 M HCl solution by Yuce et al. [210] using potentiodynamic polarization,

electrochemical impedance spectroscopy, and linear polarization resistance

measurements. The results showed that 2-THD acts as a mixed type inhibitor in 0.1 M

HCl by suppressing simultaneously cathodic and anodic processes via physical

adsorption on the MS surface followed by the Langmuir adsorption isotherm.

Corrosion of carbon steel pipes and tanks by concentrated sulfuric acid: a review

was reported by Pimenta and Marques [211]. The inhibition effect of 3-(4-((Z)-indolin-

3-ylideneamino)phenylimino)indolin-2-one Schiff base (PDBI) on mild steel corrosion


56

in 1 M HCl solution was studied by Singh [212] using potentiodynamic polarization

curves, weight loss, electrochemical impedance spectroscopy and linear polarization

resistance. PDBI has remarkable inhibition efficiency on the corrosion of mild steel in 1

M HCl solution. John and Joseph [213] studied the interaction and corrosion properties

of three different 1,2,4-triazole precursors on mild steel in 1 M hydrochloric acid by

polarization, EIS, adsorption, surface studies and computational calculations at 300 K.

Electrochemical and theoretical studies agree fairly well with each other. The results

showed that 4-(4-(dimethylamino)benzylideneamino))-4H-1,2,4-triazole-3,5-

diyl)dimethanol (DBATD) is a better inhibitor than (4-(benzylideneamino)-4H-1,2,4-

triazole-3,5-diyl) dimethanol (BATD) and 4-amino-4H, 3,5-di(methoxy)-1,2,4,triazole

(ATD). Four environmentally friendly corrosion inhibitors were derived from vanillin

by Negm et al. [214] and evaluated gravimetrically and electrochemically as corrosion

inhibitors for carbon steel in 1 M HCl. The inhibition efficiencies of these inhibitors

depend on their concentration and the chemical structures. Corrosion inhibition of four

diquaternary amines such as N,N-pentane-2,4-diylidenedipyridin-4-amine (NDSI), N,N-

(3-benzylidenepentane-2,4-diylidene)dipyridin-4-amine (NBDSI), N,N-[3-(4-

methoxybenzylidene)pentane-2,4-diylidene]dipyridin-4-amine (NMDSI) and N,N-[3-

(4-chlorobenzylidene)pentane-2,4-diylidene]dipyridin-4-amine (NCDSI) on carbon

steel was investigated by Negm et al. [215] using gravimetric measurements,

polarization and electrochemical impedance spectroscopy (EIS).

Moretti et al. [216] tested the use of 2-butyl-hexahydropyrrolo[1,2-b ][1,2]

oxazole (BPOX) as corrosion inhibitor of mild steel with a polished or a pre-corroded

(for 2 h) surface in 0.5 M aerated hydrochloric acid in the 30–60 °C temperature range.

The inhibition effect of new heterocyclic compounds, namely 2-aryl-benzothiazin-3-one

(P1) and 3-aryl-benzothiazin-2-one (P2) on mild steel corrosion in 1 M HCl was

investigated by Ghailane et al. [217] using electrochemical measurements. It is also

found that the inhibition of P1 is greater than P2. The obtained results showed that the

experimental and theoretical studies agree well and confirm that P1 is the better

inhibitor. Corrosion inhibition of carbon steel in normal hydrochloric acid solution at

30 °C by new 4-amino-1,2,4-triazole derivative, namely 3,5-bis(2-thienylmethyl)-4-

amino-1,2,4-triazole (2-TMAT) has been studied by Tourabi et al. [218] using

electrochemical impedance spectroscopy (EIS) and polarization techniques.


57

Experimental results have shown that 2-TMAT showed good corrosion inhibition and

the inhibition efficiency increased with increase in inhibitor concentration.

The inhibition behavior of imidazole (IM) and 2-phenyl-2-imidazoline (2-PI) for

AA5052 was investigated by He et al. [219] using weight loss and electrochemical

method, contact angle measurements and scanning electron microscopy. The results

showed that IM and 2-PI can inhibit the corrosion of AA5052 and the inhibition

efficiency of 2-PI is higher. Cao et al. [220] studied the adsorption behavior and

inhibition mechanism of 2-aminomethyl benzimidazole (ABI), bis(2-

benzimidazolylmethyl) amine (BBIA) and tri-(2-benzimidazolylmethyl) amine (TBIA)

on the surface of mild steel by quantum chemical calculations and molecular dynamics

(MD) simulations. It was found that the three molecules showed similar ability to

donate electrons while the difference in inhibition performance should mainly be

attributed to the difference in accepting electrons. The application of 2-amino-4-methyl-

thiazole (2A4MT) as corrosion inhibitor for mild steel protection was investigated by

Yuce et al. [221] in 0.5 M HCl solution. Electrochemical impedance spectroscopy and

potentiodynamic measurements were used at various concentrations and temperatures.

The high corrosion inhibition efficiency of 2A4MT was associated with its strong

adsorption as a barrier film on the MS surface. Kosari et al. [222] examined the

inhibitive performance of two pyridine derivatives namely pyridine-2-thiol (P2T) and 2-

pyridyl disulfide (2PD) for mild steel corrosion under stagnant condition and

hydrodynamic flow in HCl solution at 25 °C. P2T and 2PD reveal 98 % efficiency in

200 mg/L concentration at stagnant condition.


58

SECTION - V: SCOPE OF THE PRESENT WORK

Corrosion is a destructive phenomenon which affects almost all metals.

Although iron was not the first metal used by man, it is certainly 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 great need for more research in the

field of corrosion of metals in general and mild carbon steel in particular due to its

practical importance. Mild steel is widely used in many industries because of its

economically cost-effectiveness and easy fabrication. However, it is subject to corrosion

under aggressive environmental conditions especially during acid cleaning or pickling.

Therefore in the present study greater attention was bestowed on mild carbon steel.

Addition of corrosion inhibitors is one of the requirements to protect metals

and alloys against attack of corrosion in many industrial environments. Hence,

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 to reduce the

economic cost of equipments. The thesis aims at the application of different organic

compounds as corrosion inhibitors. The present work was designed to understand the

inhibition mechanism of corrosion inhibitors on mild steel in acid medium. The

behavior of mild steel in the presence of inhibitive formulation was investigated by

steady-state electrochemical measurements. The mechanism of metal corrosion is the

prime priority of the research which will be achieved by evaluating the adsorption

thermodynamic parameters.

The present work involves the investigation of corrosion and corrosion inhibition

of mild steel in acid medium using some of the inhibitors such as 2-methyl-2-{4-[5-(6-

methyl pyridin-2-yl)-[1,3,4]oxadiazol-2-yl]-phenyl}-propionitrile (6-MMOPP), 2-

methyl-6-(5-pyridin-4-yl-[1,3,4]oxadiazol-2-yl)-pyridine (5-MPOP), 2-[5-(4-bromo-

phenyl) [1,3,4]oxadiazol-2-yl]-6-methyl-pyridine (4-BPOMP), 6-methyl-4-morpholin-

4-yl-2-oxo-1,2,3,4-tetrahydro-pyrimidine-5-carboxylic acid ethyl ester (P1), 6-methyl-

4-morpholin-4-yl-2-thioxo-1,2,3,4-tetrahydro-pyrimidine-5-carboxylic acid ethyl ester

(P2), 6-methyl-4-morpholin-4-yl-2-oxo-1,2,3,4-tetrahydro-pyrimidine-5-carboxylic acid

hydrazide (P3), 6-methyl-4-morpholin-4-yl-2-thioxo-1,2,3,4-tetrahydro-pyrimidine-5-

carboxylic acid hydrazide (P4), 6-bromo-2-(4-fluoro-phenyl)-1H-benzoimidazole


59

(BFB), 2-[3-(6-bromo-1H-benzoimidazol-2-yl)-phenyl]-2-methyl-propionitrile (BPMP),

6-bromo-2-(3,4-dimethoxy-phenyl)-1H-benzoimidazole (BDB), 8-bromo-5-

morpholino-3-(4-propylphenyl)-[1,2,4]triazolo[4,3-c]pyrimidine (5a), 8-bromo-3-(2-

fluoro-3-methoxyphenyl)-5-morpholino-[1,2,4]triazolo[4,3 c]pyrimidine (5b), 8-bromo-

3-(2-fluoro-4,5-dimethoxy-phenyl)-5-morpholin-4-yl-[1,2,4]triazolo[4,3-c]pyrimidine

(5c), (3-methoxy-phenoxy)-acetic acid hydrazide (3-MAH), 4-amino-5-(3-methoxy-

phenoxymethyl)-4H-[1,2,4]triazole-3-thiol (4-AMTT) and 3-(3-methoxy-

phenoxymethyl)-6-phenyl-7H-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazine (3-MPTT). 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) EDAX 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. Proposed to synthesize novel nitrogen containing heterocyclic compounds.

2. Proposed to characterize newly synthesized compounds using spectroscopic

methods such as elemental analysis, FT-IR, 1H-NMR, 13C-NMR and LC-MS.

3. Proposed to carry out the antioxidant assay for newly synthesized compounds.

4. Proposed to establish the synthesized compounds as corrosion inhibitors for

carbon steel in acid medium.

5. Proposed to find the optimum conditions of temperature of the environment and

concentration of inhibitors.

6. Proposed to evaluate the adsorption thermodynamic parameters.

7. Proposed to establish the behavior of corrosion inhibitors on carbon steel.

8. Proposed to identify the protective layer that formed on the metal surface.


60

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