1

CHAPTER 1

INTRODUCTION

1.1 Corrosion

Corrosion has been an important problem for centuries, but with the

rapid multiplying uses of metal, the increasing occurrence of corrosive

environment and the depletion of supplies of ores, it has became much more

serious in recent years. Except noble metals (like Gold and Platinum) all

metals exist in nature in combined forms as their carbonates, hydroxy

carbonates, oxides, sulphides, chlorides and silicates. Because of their higher

energy, pure metals are less stable and they have a natural tendency to form

more stable metal compounds that is corrosion. Corrosion is defined as the

destruction or deterioration of a material by chemical, electrochemical or

metallurgical interaction between the environment and material. The

interaction between the material and their environment sometimes give

beneficial effects such as passivation, pickling of steel, storage of electrical

energy in dry cells, etc., but most of the time it leads to destructive results.

1.1.1 Cost of the corrosion

Corrosion is a naturally occurring phenomenon commonly defined

as the deterioration of a metal or its properties due to the reaction with its

environment. Like other natural hazards such as earthquake or severe weather

disturbances, corrosion can cause dangerous and expensive damage to

everything from automobiles, home appliances and drinking water system,

pipelines, bridges and public buildings. Corrosion is probably the greatest

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consumer of the metal known to the man. The tonnage of metals like steel,

copper, aluminium, lead, zinc and tin lost through corrosion is extremely high.

The cost of corrosion is enormous considering many costly processes involved

in the manufacture of metals. Business India news on 24 April, 2010 states

that in India the annual cost of corrosion is Rs 2.0 lakh crore. World annual

cost of corrosion is $2.2 trillion is over 3 % of world’s GDP. The annual

corrosion cost of key sectors such as infrastructure has been put at Rs 22,600

crore, utility services Rs 47,100 crore, production and manufacturing Rs

17,650 crore and defense and nuclear waste Rs 20,000 crore. Correspondingly,

cost of mitigation has been estimated at one per cent of the savings. The figure

cannot be off the mark as US and Japan mark their losses around four percent

of Gross Domestic Product (GDP) (Miksic et al 2004, Natesan et al 2006 and

Natesan et al 2008).

Apart from its direct costs, corrosion poses serious threat to our

natural resources. As a result of rapid industrialization of many countries, the

competition for and the price of the metal resources are in the increase.

Although corrosion is inevitable, its cost can be considerably reduced. It is

thus quite justifiable that crores of rupees are being spent on research to find

out means of preventing corrosion.

1.1.2 Theories of corrosion

1.1.2.1 Acid theory

The acid theory suggests that the presence of acid such as carbonic

acid is essential for corrosion. According to this theory rusting of iron is due to

the combined action of oxygen, carbon dioxide and moisture, converting metal

into soluble ferrous bicarbonate which is further oxidized to basic ferric

carbonate and finally hydrated ferric oxide. The reactions are given in the

equations (1.1) - (1.3). This theory is supported by the fact that rust analysis

3

generally shows the presence of ferrous and ferric carbonates along with ferric

oxide and retardation of rusting in presence of lime or NaOH to the water

which iron is immersed.

Fe + O + 2CO� +H�O → Fe�HCO� � (1.1)

2Fe�HCO� � +H�O + O → Fe�OH CO� + 2CO� + 2H�O (1.2)

2Fe�OH CO� + 2H�O → 2Fe�OH � + 2CO� (1.3)

1.1.2.2 Electrochemical theory

The electrochemical or local cell theory considers that corrosion is

due to the existence of separate anodic and cathodic parts, between which

current flows through a conducting solution. This type of corrosion occurs

where a conducting liquid is in contact with metal or alloy. Immersion

corrosion, underground corrosion and atmospheric corrosion are fall under this

category.

Electrochemical reaction involves transfer of electrons between

anode and cathode through an electrolyte. The anodic reaction is dissolution of

metal as corresponding metallic ions with the liberation of free electrons. The

cathodic reaction consumes electrons either by evolution of hydrogen or by

absorption of oxygen, depending upon the pH of the environment. The anodic

and cathodic reactions are as follows,

At anodic part, oxidation (or) dissolution of metal occurs

M → M�� + 2e� (1.4)

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At cathodic part reduction reaction occurs, which depends on nature

of the corrosive environment. If the corrosive environment is acidic, hydrogen

evolution occurs at cathodic part.

2H� +2e� → H� ↑ (1.5)

If the corrosive environment is slightly alkaline or neutral,

absorption of oxygen occurs at cathodic part.

1 2⁄ O� +H�O +2e� → 2OH� (1.6)

The metal ion and hydroxide ions react to give corresponding metal

hydroxide.

M�� + 2OH� → M�OH � (1.7)

1.1.2.3 Homogeneous theory

It is not necessary that the anodic and cathodic reactions should take

place at spatially separated areas explained by Wagner and Traud (1938), but

shift randomly over the corroding metal in respect of time and space.

According to this, the entire surface supports anodic and cathodic reaction and

is considered as homogeneous theory of corrosion, while local cell theory may

be regarded as heterogeneous theory of corrosion.

1.1.3 Classification of corrosion

Corrosion has been classified in many different ways. In one way it

is classified into low-temperature and high-temperature corrosion. In another

way it is classified into wet corrosion and dry corrosion. The convenient way

of classification is in which it manifests itself. In this way, it is classified into

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eight different forms. Each form can be identified by mere visual observation.

In most cases the naked eye is sufficient, but sometimes magnification is

required. The identification of the different forms of corrosion is very much

helpful in understanding the intensity of problem and finding the solutions.

The eight forms are given below,

� Uniform or General attack

� Galvanic or Two metal corrosion

� Crevice corrosion

� Pitting corrosion

� Intergranular corrosion

� Selective leaching or Parting

� Erosion corrosion

� Stress corrosion

The eight forms of corrosion are discussed in terms of their

characteristics, mechanisms and preventive measures as follows,

1.1.3.1 Uniform or General attack

Uniform or General attack is the most common form of corrosion. It

is characterized by a chemical or electrochemical reaction in which corrosion

takes place uniformly over the entire exposed surface area of the material. This

type of corrosion is not too great concern from the technical standpoint

because life of equipment can be accurately estimated using simple tests like

immersion of specimen in the fluid of our interest. This form of corrosion can

be prevented or reduced by proper selection of materials, protective coatings,

use of inhibitors and applying cathodic protection to the affected equipment.

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1.1.3.2 Galvanic or Two-metal corrosion

Two dissimilar metals or alloys are exposed to an electrolyte

(corrosive or conductive) a potential difference is developed between these

two. If these two are electrically coupled or placed in contact, this potential

difference causes an electron flow from the more active metal to less active

metal. The more active metal undergoes corrosion (anodic) and the less active

metal is protected from the corrosion (cathodic). Because of the electric

current and two dissimilar metals are involved, this corrosion is called as

Galvanic or Two-metal corrosion.

Galvanic corrosion has several applications e.g.:- Dry cells,

cathodic protection etc., (Fontana 1987). Galvanic corrosion can be

minimized by

1. Coupling of dissimilar metals which are close together in

galvanic series.

2. Use anode area as large as possible.

3. Electrically insulate dissimilar metals from each other.

4. Apply protective coatings on the metal surface.

5. Add inhibitors to reduce the aggressiveness of the environment.

6. Proper designing of metal article.

7. Electrically connect a third metal that is anodic to both metals in

the galvanic contact.

1.1.3.3 Crevice corrosion

Crevice corrosion is an intensive localized corrosion occurs within

crevices and other shielded area on metal surfaces exposed to a corrosive

environment. Crevice corrosion is more likely to occur in holes, gasket

surfaces, lap joints, surface deposits and crevices under bolt and rivet heads

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that retain solutions and take longer time to dry out. This form of corrosion is

sometimes called as deposit or gasket corrosion. The rate of crevice corrosion

is based on some important factors such as lack of oxygen, changes in acidity,

inadequate anodic treatment and decrease or depletion of an inhibitor. Crevice

corrosion can be prevented by the use of high resistance alloy, use of welded

joints instead of riveted or bolted joints, maintaining clean surfaces, use of

solid non-absorbent gaskets and frequent removal of accumulated deposits and

designing containment vessels to avoid stagnant areas and ensure complete

drainage.

1.1.3.4 Pitting corrosion

Pitting corrosion is extremely localized attack that results from

inhomogeneities in metal due to inclusions, coring and distorted zones which

set up difference of potential at localized spots to cause deep isolated hole or

pits. These holes may be smaller or larger in diameter, but in most cases they

are relatively small and because of their smaller size and are often covered by

corrosion products, they are not easily identified. It is very much difficult to

measure quantitatively and compare the extent of pitting because of the

varying depths and number of pits that may occur on identical conditions.

Pitting is an autocatalytic process. This process is self-stimulating and

self-propagating. This corrosion processes usually requires an initiation period.

The period ranges from months to years depending on both the specific metal

and the corrosive. Depth of pitting is some time expressed as pitting factor,

which is the ratio of the depth of deepest pit to the average penetration as

calculated from weight loss.

This form of corrosion can be prevented by removing deposits of

solids from exposed metal surface, by selection of material, by using corrosion

8

inhibitors suitable for the environment and by preventing formation of oxygen

concentration cell (Bonora et al 1974).

1.1.3.5 Intergranular Corrosion

Intergranular corrosion is a localized attack at and adjacent to the

grain boundaries with relatively little corrosion of the grains. In this corrosion

alloy disintegrates and loss its strength. Intergranular corrosion is a

non-uniform corrosion. It can be caused by impurities at the grain boundaries,

enrichment of one of the alloying elements or depletion of one of these

elements in the grain boundaries. The well-known example is the failure of

18-8 stainless steel due to IGC (Sastri 1998). When 18-8 stainless steel is

heated to 950 - 1450°F, depletion of chromium takes place by metallurgical

reaction with carbon, this chromium carbide being insoluble and precipitates

out of the solid solution along the grain boundaries. When carbon content is

higher than 0.02% then the resultant structure is susceptible to intergranular

attack. Stainless steel may be protected from intergranular corrosion by the

following measures: (1) subjecting the sensitized material to a

high-temperature heat treatment in which all chromium carbide particles are

re-dissolved,(2) lowering the carbon content below 0.02 % so that carbide

formation is minimal, and (3) alloying the stainless steel with another metal

such as niobium or titanium, which has a greater tendency to form carbides

than does chromium so that the chromium remains in solid solution.

1.1.3.6 Selective leaching or Parting

Selective leaching is generally found in solid solution alloys. It

occurs when one element or constituent is completely removed by corrosion

processes. Selective leaching is also called as dealloying and parting. The most

common examples are dezincification of brass and graphitization.

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Dezincification of brass is the selective leaching of zinc from yellow brass

leaving porous mass of copper which may be red colour or copper colour.

Graphitization is the selective leaching of iron from grey cast iron leaving

weak, porous and inert graphite. It is a slow process and occurs in relatively

mixed corrosive environments such as soil or water. Selective leaching also

occur with other alloy systems in which aluminium, cobalt, chromium and

other elements are vulnerable to preferential removal. Selective leaching

reduces the mechanical properties of the alloy. The colour and appearance of

the material is also changed. Selective leaching can be minimized by reducing

the aggressiveness of environment, cathodic protection and using less

susceptible alloys.

1.1.3.7 Erosion corrosion

Erosion corrosion usually occurs due to the combined action of

chemical attack and mechanical abrasion or wear as a consequence of fluid

motion. This type of corrosion is associated with systems where the corrosive

fluid having high velocities. Rate of erosion corrosion depends upon the nature

of surface film, corrosion environment, velocity, presence and size of air

bubbles, chemical compositions, suspended solid, corrosion resistance and

metallurgical properties of metals and alloys. This type of attack can be

observed in piping system such as bends, elbows, and tees, valves, pumps,

blowers, centrifugals, impellers, heaters and condensers etc. All types of

equipments exposed to moving fluids are susceptible to erosion corrosion.

Among the available alloys, Ti has been found to possess the

greatest resistance to erosion corrosion and 70: 30 brass is known to have poor

resistance to erosion. Erosion corrosion can be prevented by selection of

materials with better resistance, proper design, change of aggressive

environment, use of surface coating and cathodic protection.

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1.1.3.8 Stress corrosion

Stress corrosion sometimes termed as stress corrosion cracking,

results from the combined action of an applied tensile strength and a corrosive

environment. In this corrosion small cracks are formed first and then propagate

in a direction perpendicular to the stress. The main cause of stress corrosion

cracking depends upon metallurgical factors such as chemical composition of

alloys, preferential orientation of grains, composition and distribution of

precipitates. The source of stress can be applied, residual, thermal, welding or

corrosion products in constructed regions. As stress corrosion crack penetrates

the metal, the cross sectional area decreases and the cracking failure occurs

due to mechanical action. Long-term stress corrosion cracking tests are

necessary to assess the cracking tendencies of alloys. The mechanisms of

stress corrosion can be broadly classified as dissolution based and cleavage

based. Corrosion plays an important role in the initiation of cracks. Stress

corrosion can be minimized by lowering the magnitude of stress, eliminating

the critical environment species, changing of alloy composition, applying

cathodic protection, adding inhibitors and coatings (Lu et al 2005).

1.2 Potential – pH diagrams (E-pH)

Thermodynamic data can be used to map out the occurrence of

corrosion, passivity and nobility of a metal and also it provides a means for

predicting the equilibrium state of a system of specific component.

M.Pourbaix developed the thermodynamic interpretation of corrosion in a

systematic way and presented his results in the form of potential–pH diagrams

otherwise known as Pourbaix diagrams. Pourbaix diagrams are convenient

way of summarizing much thermodynamic data and providing the useful

means of summarizing the thermodynamic behavior of metal and associated

species in given environmental conditions. Pourbaix diagrams are also very

much useful in determining the stable chemical species for metals in contact

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with aqueous solution. Figure 1.1 is a simplified Pourbaix diagram for iron

(Fontana 1987).

Figure 1.1 Potential – pH diagram for iron-water system at 25°°°°C

Potential–pH diagrams are also available for corrosion of metals in

atmosphere. The metal ion concentration in water film is fluctuating and is

generally high. Apart from metal- water system one must consider the

presence of SO42-

, Cl – and NO3– which may be dissolved in the film or

moisture and under certain conditions which may form solid phase complexes

with the corroding metal. This diagram clearly distinguishes the regions of

immunity, corrosion and passivity. But these E / pH diagrams have some

inherent limitations which are

• It does not deal with the effect of impurities or alloying element

present in the metal.

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• In most cases, the measured electrode potential varies with time.

So extreme care is necessary in applying Pourbaix diagrams in

such cases.

• These diagrams give no information on rates of reaction.

• They represent only equilibrium conditions in certain specific

environments and do not take into account the factors such as

velocity and temperature variation during the course of the

process.

• The effect of pH can be varied by the presence of unintentional

or otherwise, of anions such as Cl –, NO3– and complexing ions

which are likely to form.

• Passivity is also depending on solubility, adhesion, cohesion, ion

conductivity and crystal formation of passive layer.

1.3 Kinetics of electrochemical corrosion reaction

Consider an electrochemical reaction, where the reaction takes

place at the metal – solution interface and the rate of reaction is proportional to

current-potential dependence. Over voltage (polarization) η is the potential

change, E - Er, from the equilibrium half-cell electrode potential Er, caused by

a net surface reaction rate for the half-cell reaction was introduced by Nernst

and Caspari. The dependence of η on the current-density for a hydrogen

evolution reaction has been shown to be η = a + b log i by Tafel. Butler gave

a kinetic treatment of reversible electrode in which the concepts of the partial

anodic and cathodic currents related to η through an exponential equation.

1.3.1 Activation controlled corrosion reaction

Activation polarization is generally caused by slow electrode

reaction. The reaction at the electrode requires activation energy in order to

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proceed. The most important example for activation controlled corrosion

reaction is that of hydrogen ion reduction at cathode. The relationship between

current and potential for a corroding system in which anodic reaction is metal

dissolution reaction and cathodic reaction is hydrogen evolution reaction can

be derived by the application of electrochemical kinetic theory.

For the metal dissolution reaction,

� → ��� + 2�� (1.8)

(1.9)

Where Ear is the reversible potential of anodic dissolution reaction, iaº is

exchange current density for anodic reaction and α a βa are the transfer

coefficients of metal dissolution and deposition reaction.

Similarly for cathodic reaction

2H� +2e� → H� ↑ (1.10)

(1.11)

Where is Ecr is the reversible potential of cathodic dissolution reaction, Icº is

the exchange current density for cathodic reaction and α c βc are transfer

coefficients of reduction reaction.

Normally the corrosion potential (Ecorr) will be far away from the

equilibrium potential of reversible reaction. Hence the contribution from the

deposition reaction of metal dissolution reaction and the anodic partial reaction

−

β−−

−α

= )EE(RT

Fexp)EE(

RT

Fexpii r

aar

aao

aa

−

β−−

−α

= )EE(RT

Fexp)EE(

RT

Fexpii r

ccr

cco

cc

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of reduction is negligible. Therefore the net current of the mixed electrode

system is

i = ia - ic (1.12)

(1.13)

At corrosion potential E = Ecorr i = 0

i.e.

for anodic reaction and

i.e for cathodic reaction

Substituting the terms Ear and Ec

r in terms of Ecorr

(1.14)

The above equation can be rewritten in terms of Tafel slopes ba and bc as

(1.15)

since E - Ecorr = η

(1.16)

−

β−−

−α

= )EE(RT

Fexp)EE(

RT

Fexpi r

aar

aao

a

−

α= )EE(

RT

Fexpii r

acorrao

acorr

−

β−= )EE(

RT

Fexpii r

ccorrco

ccorr

−

β−−

−α

= )EE(RT

Fexp)EE(

RT

Fexpii corr

ccorr

acorr

−−−

−=

c

corr

a

corrcorr

b

)EE(3.2exp

b

)EE(3.2expii

η−−

η=

ca

corrb

3.2exp

b

3.2expii

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The above expression forms the basis of measuring corrosion rate

by electrochemical method.

1.3.2 Diffusion controlled reaction

Concentration polarization or diffusion over potential is the

potential difference of a cathode in absence and presence of external current.

The corrosion process in neutral media consists of metal dissolution reaction

as anodic and oxygen reduction as cathodic reaction. In such cases,

(1.17)

(1.18)

Where D is diffusion coefficient, δ is diffusion layer thickness and Cb is concentration of reduction species.

i = ia – ic

(1.19)

id = limiting diffusion current density

At corrosion potential E = Ecorr, i = 0, therefore icorr = id

It follows that the id is the most significant parameter in the

corrosion reaction in which the cathodic reaction is diffusion controlled and

any factor that increases id will increase the corrosion rate.

−

α= )EE(

RT

Fexpii corr

acorra

δη

== bdc

FDCii

dcorr

a

corra iEERT

Fii −

−= )(exp.α

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1.4 Methods used for studying corrosion rates

1.4.1 Weight loss method

Weight loss methods are commonly used to measure corrosion

rates. A test specimen is polished, cleaned, degreased and weighed. This

specimen is exposed to the corroding medium for a known time and removed.

This treated specimen is cleaned to remove the corrosion products and

weighed. From the weight difference corrosion rate is calculated. To avoid or

minimize the error it is necessary to carry out a control test, irrespective of the

method used to remove corrosion products. The amount of metal dissolved by

the reagent used for cleaning corrosion product must be determined.

1.4.2 Electrochemical methods

Electrochemical methods which can be used for the determination

of corrosion rates have been published by several authors (Chandrasekaran et

al 2005, Gunasekaran and Chauhan 2004, Abdel-Gaber et al 2006 and

Hougaarad et al 1983). The majority of corrosion behavior are due to

electrochemical process ie., they are governed by the law of electrochemical

kinetics. Therefore it is very important to study the electrochemical

characteristics of metals during corrosion tests in order to understand

mechanism and the rate of the most corrosion process.

Tafel polarization method: This method involves the

measurement of over potentials for various current densities and a plot of η

versus log i is made. The slope gives Tafel constants [ba and bc] and the

intercept corresponds to icorr. Disadvantages of this method are

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1. Polarization of several hundred milli volts from open circuit

potential perturbs the system significantly and alters the surface

conditions.

2. This method is applicable to conducting media and for systems

controlled by activation controlled reactions.

3. In actual practice, the linear region does not exceed a decade

due to interference from concentration polarization.

Linear polarization resistance (LPR) method: Stern and Geary

have shown that when the over potential (η) is less than ±20 mV, there is a

linear relationship between current and potential. Measuring (dη / di) η → 0,

the corrosion current can be obtained from

(1.20)

The above equation is valid only for activation-controlled reactions.

Improvements and the simultaneous determination of Tafel slopes and

corrosion current at corrosion potential have been suggested by many authors

(Reeve et al 1973). The graphical method of linear polarization curve analysis

has been obtained by Oldham and Mansfeld (Oldham et al 1973). Mansfeld

developed a computer programme (CORFIT) of quantitative determination of

icorr which requires simultaneous determination of both Rp and B. However

this method requires knowledge of ba and bc.

The main advantage of Linear Polarization method is that it gives

instantaneous corrosion rate and is extremely helpful in determining the effect

of process change with time on corrosion rate. Based on this method,

corrosion rate measuring meters have been developed. This method is useful

Rp

K

di

d

bb

bbi

ca

ca

corr =×+

×=

η)(303.2

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for periodic monitoring of corrosion process and the small potential scan does

not perturb the system, very low corrosion rates with high accuracy can be

followed. The main limitation is that this method requires a conducting liquid

as a medium.

1.4.3 Coulostatic method (Sato et al 1978)

This method is suited especially for the measurement of corrosion

rate of metals in high resistive media. The polarization resistance (Rp) is

measured from the η-t transient of the electrode on discharging a charged

capacitor (c) through the cell. The electrode potential decays as

ηt=ηt = 0 exp (-t/ Cdl Rp) (1.21)

Where ηt is over potential at any time ‘t’, ηt=0 is over potential immediately

after charging the double layer of the electrode and Cdl is differential capacity

of the double layer.

The plot of log ηt versus time (t) is a straight line and the slope

gives 1 / 2.303 Cdl Rp and the intercept is ηt = 0.

1.4.4 Electrochemical impedance spectroscopic method (EIS)

AC impedance measurements are useful for identification of

phenomena governing the electrochemical systems. EIS technique provides

data on both electrode capacitance and charge transfer resistance, thereby

providing valuable mechanistic information. For this reason, EIS become a

powerful tool in the study of corrosion, semiconductors, batteries,

electroplating, and electro-organic synthesis (Sha Cheng et al 2007, Mansfeld

et al 1982, Lorenz et al 1981, Issaacs et al 1982, Glarum et al 1982, McCann et

19

al 1982, Sluyters et al 1961, Sluyters et al 1962, Glarum et al 1981 and

Franceschetti et al 1982). EIS offers three main advantages over

DC techniques. Very small excitation amplitude of 5–10 mV is applied for the

EIS measurements. This small amplitude causes only minimal perturbation of

electrochemical test system and reduces errors caused by the measurement

technique.

Measurement Accuracy: The measurements are based on the

consideration that an electrochemical cell representing the conducting system

is analogous to an electronic or equivalent circuit consists of an array of

resistors and capacitors. It does not involve any potential scan so that

measurements can be made even in low conducting solutions where DC

techniques are subjected to serious potential–control errors. The measurements

are carried out by studying the response of electrochemical system to an

excitation over a wide range of frequency, may be extended from 10 –1 MHz

depending on the nature of process being studied. In A.C theory E = IZ where

E and I are waveform amplitudes for potential and current, Z is the impedance.

The expression for impedance is

Ztotal = Z′real + Z′′imaginary (1.22)

i.e. impedance can be expressed as a complex numbers, where resistance is the

real component and combined capacitance and inductance is the imaginary

component.

The A.C impedance data can be presented in many forms and each

mode of presentation has its own advantages and disadvantages.

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Nyquist plot: In this plot, Z′′ is plotted against Z′ at each

excitation frequency and is also known as Cole – Cole plot. From this plot Rs,

Rct and Cdl can be calculated.

The advantages of Nyquist plot are:

1. The plot format make it easy to see the effects of ohmic

resistance

2. The shape of the curve does not change when Rs changes, it is

possible to compare the resistance of two separate experiments

3. It emphasizes the circuit components which are in series such as

ohmic resistance

The disadvantages are:

1. Frequency does not appear explicitly

2. Although the Rs and Rp are already known, but the electrode

capacitance can be calculated only after frequency information

is known.

Bode plot: In this plot, logZ is plotted against frequency. The

main advantage of Bode plot over Nyquist plot is that the frequency appears as

one of the axes and it is easy to understand how the impedance depends on

frequency. From this plot, capacitance value can be calculated using the

following formula,

Z = 1 / Cdl, at ω = 1 (1.23)

Bode plot avoids the longer measurement times associated with low

frequency Rp determinations, furthermore, allows more effective extrapolation

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data from higher frequency range. In some electrochemical process, there is

more than one rate – determining step. From this plot it is easy to identify the

frequency break points associated with each limiting step. The main

disadvantage is that the shape of the curve changes if the circuit values

changes.

Randels plot: The Randels plot is useful in determining whether

Warburg impedance is a significant component of the equivalent circuit model

(reaction mechanism). For a completely reversible system under pure diffusion

control, the Warburg impedance Zω is

Warburg Impedance (1.24)

Thus the linearity of the Randels plot can be used as a test of

diffusion control, the Warburg impedance coefficient can also be calculated

from the slope.

Faradaic Distortion: On superimposing a sinusoidal alternating

voltage i.e. Em sinωt to the electrode at corrosion potential, harmonic current

components are produced due to the non-linear relationship between current

and potential. Measurements of fundamental (i1), second harmonic (i2) and

third harmonic (i3) current components are made for getting icorr, ba and bc

from the following relationships,

(1.25)

(1.26)

→ω

=ω S2S

Z

2231

21

corr

ii,i248

ii

−=

+=

1

2

corr

1

i

i4

i

i

Em6.4

1

ba

1

22

(1.27)

The advantage of this method is that the measurement of corrosion

current is possible at corrosion potential without the usage of anodic and

cathodic Tafel slopes.

Electrochemical Noise (Dawson et al 1983): Impedance and

polarization analysis are ideal techniques for use along with general corrosion

tests. However, electrochemical noise, i.e. the spontaneous fluctuations in

potential and current, offer some possibilities of assessing localized corrosion.

The corrosion potential noise can be measured either between two identical

corroding electrodes or between a corroding electrode and a low noise

reference electrode (Hiladky et al 1982). Electrochemical noise is random

process comprising initiating events controlled by a poisson’s distribution

(stochastic) coupled to kinetic parameters (deterministic). The noise data can

be presented auto correction functions and spectral density plots, the time

record being transformed to the frequency domain by the Fast Fourier

Transform (FFT) or the Maximum Entropy Method (M.E.M) (Childers et al

1978 and Haykin 1979). This technique is useful in monitoring a number of

practical situations (Dawson et al 1983). Noise analysis and monitoring have

been successfully applied to corrosion involving film breakdown process (e.g)

pitting of stainless steel and aluminium. However general corrosion systems

have not been investigated to the same extent (Al-Zanki et al 1986).

1.5 Corrosion prevention

There are many corrosion prevention methods available and some

of the important corrosion control methods are described as follows:

+=

1

2

corr

1

i

i4

i

i

Em6.4

1

bc

1

23

Proper designing: Proper design should permit least contact

between the structure and corroding agent. It is desirable that the design allows

for adequate cleaning and flushing of critical parts of the equipment. Crevices,

recesses, pockets and sharp corners should be avoided, because they favor the

formation of stagnant areas and accumulation of solids.

Using pure metal: Impurities in a metal cause heterogeneity, which

decreases corrosion resistance of the metal. The corrosion resistance of a metal

can be improved by increasing the purity of the metal.

Using metal alloys: It is common to increase both strength and

corrosion resistance by the use of suitable alloying elements. For example

small amounts of phosphorus and copper improve the resistance of structural

steel to atmospheric corrosion. About 10 % aluminium renders iron extremely

resistant to high temperature oxidation. Intergranular corrosion in stainless

steels can be avoided either by reducing the carbon percentage or by the

addition of titanium or columbium.

Cathodic protection: In general, cathodic protection is an approach

where the metal surface to be protected is made into cathode of a corrosion

cell. Since corrosion and the material loss are occur only at the anode, this

approach protects the metal. There are two types of cathodic protection

method one is sacrificial anodic method and another one is impressed current

technique. In the sacrificial anodic protection method highly active metal or

more anodic metal is connected to the metal surface which is to be protected

from the corrosion. In this cell the more active metal is acting as an anode and

it is corroded thereby it protects the metal surface. In the impressed current

technique, an impressed current is applied in opposite direction to nullify the

corrosion current, and convert the corroding metal from anode to cathode.

24

Modifying the environment: The corrosive nature of environment

reduced thereby corrosion is controlled. This can be done by the removal of

harmful constituents or by the addition of specific substances, which is

neutralize the effect of corrosive constituents of the environments.

Application of protective coatings: Protecting the surface of an

article by applying protective coating is the oldest and most common method

to control corrosion. Protective surface coating includes metallic coatings,

organic coatings, anodizing and ceramic coatings.

Use of inhibitors: This will be discussed in detail in the following

chapter.

1.6 Inhibitors

A corrosion inhibitor is a substance which when added in small

quantities to a corrosive environment, effectively decreases the corrosion of

the metal. In a sense, a corrosion inhibitor can be considered as a retarding

catalyst. Corrosion inhibitors are added to many systems including cleaning

pads, cooling systems, various refinery units, chemical operations, steam

generators, ballast tank, oil and gas production units. Inhibitors function by

� Adsorption as a film on to the surface of the corroding medium.

� Inducing the formation of a thick product.

� Changing the characteristics of environment either by producing

protecting precipitates or by inactivating an aggressive

constituent.

So that it prevent or arrest corrosion processes. It can also interface

with cathodic, anodic or both reactions.

25

1.6.1 Classification of corrosion inhibitors

Inhibitors can be classified on the basis of environments and

reaction mechanism involved during the inhibition.

1.6.1.1 Based on environment

a) Acid inhibitors

This may be classified into

� Inorganic inhibitors

� Organic inhibitors

Inorganic inhibitors: The protection of metal in corrosion medium

with inorganic inhibitors is due to the reduction of electropositive ions and

deposition on the metal surface and lowering of over voltage of the main

cathodic depolarization reaction (Tamashov et al 1967). Based on the

electrode potential ions like Cu, Hg, Ir, Pt, Rh should improve the resistance of

corrosion of titanium and the most effective is platinum due to its low

hydrogen over voltage.

The influence of halide ions on inhibitive effect of Congo red on in

strong acid solutions has been found to be effective inhibitors (Oguzie 2004).

Recently it is shown that the addition of heavy metal ions such as Pb2+, Tl+,

Mn2+, Cd2+ is found to inhibit corrosion of iron in acids. This effect is

attributed due to lower potential deposition of metal ions leading to complete

coverage of added metal on iron surface.

Zinc ions and Nickel ions were also found to reduce the corrosion

rate of Fe and Ti (Shedriks et al 1972). Even 0.2 ppm nickel ions render

passivity of Ti in 3.5% NaCl. This effect of nickel ions may be due to

lowering over voltage of hydrogen on Ti.

26

Organic inhibitors: Usually the corrosion of metals and alloys in

acid solution is very severe and this kind of attack can be inhibited by a large

number of organic substances. In general, nitrogen, oxygen and sulphur

containing compounds with a hydrocarbon part attached to the polar group are

used as inhibitors (Weihua Li et al 2007, popova et al 2006). Many nitro and

nitroso compounds have been tried as corrosion inhibitors. Certain azo

compounds such as cango red dye, methyl orange and methyl red have been

found to be effective inhibitors. Many types of quinine such as anthroquinone

and its derivatives act as oxidative passivators. Organic corrosion inhibitors

can be anodic, cathodic or mixed type depending on its size, carbon chain

length, atomicity, conjugation and nature of bonding atoms.

b) Neutral inhibitors

Inhibitors, which are effective in acidic solutions, do not function

effectively in neutral solutions. In neutral solutions, interaction of inhibitors

with oxide covered metal surfaces and prevention of oxygen reduction

reactions at cathodic sites takes place. Such inhibitors protect the surface

layers from aggressiveness. The first step involves displacement of pre

adsorbed water molecules by inhibitors followed by chemical or

electrochemical reactions at the surface. Hence inhibitors, which block

cathodic reduction of oxygen by restricting diffusion of oxygen to the surface

of metal, are cathodic inhibitors and those which prevent anodic dissolution by

the formation of thin passivating film on metal surface are called anodic

inhibitors.

c) Alkaline inhibitors

The metals, which form amphoteric oxides, are prone to corrosion

in basic solution. Inhibitors in basic solution mainly involve metals like Al,

Zn, Cu, Fe, etc. Many naturally occurring organic compounds are often used

27

as inhibitors for metals in basic solutions e.g. tannin, gelatins, saponin,

agar-agar etc. Compounds such as thiourea, substituted phenols, naphthol,

β-diketone etc. have been used as effective inhibitors in basic solutions due to

the formation of metal complexes. Lysine was also found as effective

inhibitors in 4.0 N sodium hydroxide solutions (Jeyaraj et al 2003).

d) Vapour phase inhibitors

Metals in closed space such as in parcels, during storage undergo

corrosion. This type of corrosion can be prevented by using certain substances

called vapor phase inhibitors (VPI), which are specific in nature. The vapors of

inhibitors form an extremely thin film over certain metal surfaces, especially

those of iron and steel, thereby rendering them passive. The inhibitors are

mostly crystalline solids whose vapor pressure in controlled by the structure of

crystal lattice and character of the atomic bond in the molecule. The protective

vapors expand within the enclosed space until the equilibrium determined by

their partial pressure of the vapor is reached. At higher values of vapor

pressure protected the space reaches saturation stage (Trabanelli 1987). This

method is easy to apply as paper coated VPI for temporary protection. The life

time of the inhibitor is very long.

1.6.1.2 Based on electrode process

a) Anodic inhibitors

An anodic inhibitor increases the anode polarization and hence

moves corrosion potential to the positive direction. A number of inorganic

inhibitors such as orthophosphates, silicates etc fall under anodic type. Even

though anodic inhibitors are widely used, a few of them have some

undesirable property. If such inhibitors are used in very low concentration they

28

cause stimulation of corrosion such as pitting and hence anodic inhibitors are

denoted as dangerous.

b) Cathodic inhibitors

These inhibitors shift the corrosion potential in negative direction.

Here the cations migrate towards cathode surfaces where they are precipitated

chemically or electrochemically and thus block these surfaces e.g. Action of

As3+ and Sb3+ on dissolution of Fe in acids.

c) Mixed inhibitors

These inhibitors retard both anodic and cathodic processes. The

potential is smaller and the direction is determined by the relative size of the

anodic and cathodic sites. Such inhibitors will have the advantage over other

inhibitors as they control both partial corrosion reactions and hence they are

very safe to apply.

1.6.2 Theories of inhibition

a) Adsorption theory

According to adsorption theory, inhibitors are adsorbed on the

metal surface forming a protective layer. The mode of adsorption leads to its

classification as physical and chemical adsorption.

Physical adsorption or Physisorption: The electrostatic attraction

between ions or dipole of the inhibiting species and electrically charged

surface of the metal gives physical adsorption. The charge on the metal surface

depends on free corrosion potential Ecorr and potential with respect to zero

charge (PZC).

29

The potential of metal measured against a reference electrode is

known as zero-charge potential, which is carried out under the condition that

metal is in zero charge. At zero-charge potential, electrodes adsorb substances

dissolved in the electrolyte. At PZC, the net charge on electrode is zero. At

potential more positive and negative than PZC, the electrode is positively and

negatively charged respectively.

The concept of PZC can also be explained by means of synergistic

effect of iron in H2SO4 by means of chloride ion (Rozenfeld 1981).

Chemical adsorption or Chemisorption: It is due to the

interaction between the metal surface and an inhibitor molecule. The adsorbed

molecule is in contact with the surface of the metal. In this, a co-ordinate bond

is involved in the electron transfer from metal inhibitors to metal (Fathy

Bayoumi et al 2005 and Hackerman et al 1962).

Chemisorption is slower compared to electrostatic adsorption

process and it has higher activation energy. It depends upon temperature and

inhibitor efficiency. Chemisorption is specific to certain metals electron

transfer from inhibitor to metal is facilitated by the presence of unshared lone

pair of electrons, π electrons due to multiple bonds and aromaticity.

The strength of the adsorption bond depends upon

i. The electron density of donar atom of functional group

ii. Polarizability of the group

30

b) Film theory

This theory revealed that effective protection of metals by inhibitors

is due to the formation on the metal surface a layer of insoluble or slightly

soluble corrosion products.

c) Hydrogen over voltage theory

This theory postulates that inhibitors, which are adsorbed on the

metal retard either anodic or cathodic or in some cases both the reactions.

This leads to rapid polarization of anodic or cathodic sites, and thus overall

corrosion rate is retarded.

d) Quantum chemical approach

Direct interaction between surface metal atoms and outermost

electrons of organic molecule sometimes leads to chemisorption phenomenon

and thereby cause inhibition. Chemisorption of organic inhibitors may be

taken as linear combination of participating wave function of inhibitor

molecule as well as of surface metal atoms. The binding energy of metal

inhibitor adduct may thus be correlated to the energy difference between

lowest free molecular orbital (LFMO) and highest occupied molecular orbital

(HOMO) of metal atoms and inhibitor molecule respectively.

1.6.3 Effect of inhibitors on corrosion processes

Electrochemical studies have shown that inhibitors in acid solution

may affect corrosion reactions of metal in the following ways.

a) Formation of diffusion layer

The adsorbed inhibitors may form a surface film, which acts as a

physical barrier to restrict the diffusion of ions, or molecules to or from metal

31

surface and so retard the corrosion reactions. This effect occurs particularly

when the inhibitor species are large molecules e.g. proteins such as gelatins,

agar-agar etc. Similar effect also occurs when the inhibitor can undergo

reaction to form a multi molecular surface film e.g. acetylene compounds and

sulphoxides.

b) Blocking of reaction sites

The interaction of absorbed inhibitors with surface metal atoms may

prevent these metal atoms from participating in either anodic or cathodic

reactions of corrosion. The simple blocking effect decreases number of surface

metal atoms at which these reactions can occur, and hence rates of these

reactions are proportional to the extent of adsorption. The mechanism of

reactions is not affected and Tafel slope of polarization curve remains

unchanged. Behavior of this type has been observed for iron in sulphuric acid

solutions containing 2,6-dimethyl quinoline, β-naphthaquinoline or aliphatic

sulphides.

c) Precipitation on the electrode reaction

The electrode reactions of corrosion involve the formation of

adsorbed intermediate species with surface metal atoms e.g. adsorbed

hydrogen atoms in the hydrogen evolution reaction; adsorbed (FeOH) in the

hydrogen evolution reaction, adsorbed (FeOH) in the anodic dissolution of

iron. The presence of adsorbed inhibitors will interface with formation of these

adsorbed intermediates but electrode process may then proceed via alternative

paths through intermediates containing inhibitor. In this process inhibitor

species act in a catalytic manner and remain unchanged, since participation by

inhibitor is generally characterized by a change in Tafel slope observed for the

process.

32

1.7 Adsorption isotherm

In order to gain more information about the mode of adsorption of

inhibitors on metal surface, the experimental data have been tested with

several adsorption isotherms. The fractional surface coverage values (θ) to

different concentrations of inhibitors from capacitance measurements as

described elsewhere to explain the best isotherm to determine adsorption

process. It gives relationship between coverage of an interface with adsorbed

species and concentration of species in solution. Various adsorption isotherms

have formulated and are given in Table1.1 (Damaskin et al 1971).

Table1.1 Adsorption isotherms

S.No. Isotherm Equation

1 Henry βc = θ

2 Freundlich βcn = θ

3 Langmuir βc = θ / 1-θ

4 HFL βc = (θ /1-θ)e(2- θ) / (1-θ)2

5 Frumkin βc = θ /1-θ e (-2aθ)

6 Temkin βc = e(a-θ) / 1-e [-a(1-θ)]

7 Parsons βc = θ / 1-θ e (2-θ)/(1-θ) e (-2aθ)

8 Blomgren-Bockris βc = θ / 1-θ e (pθ3/2 – q θ 3)

9 BDM Log c + log (θ / 1- θ ) = c + βθ3/2

where β = e , a = Interaction parameter a > 0, attraction and a < 0, repulsion

Most of organic inhibitors obey Langmuir (El-Awady et al. 1992),

Temkin, Frumkin and Flory–Huggins isotherm. The adsorption isotherm

relationships of Frumkin are represented in the following equation:

��� � �����– ���� = ��� + 2!" (1.28)

33

Where a is the lateral interaction term describing molecular

interaction in adsorbed layer and heterogeneity of the surface, K is binding

constant (equilibrium constant) of adsorption reactions, C is inhibitor

concentration in bulk of the solution. If this relation gives a straight line, then

Frumkin isotherm is applicable.

El-Awady et al. kinetic model and Flory–Huggins isotherm are

represented according to the following equations:

��� � ����� = ��ℎ

′ + $���� (1.29)

%&'�( = ���) + )����1 − " (1.30)

Where Y is the number of inhibitor molecules occupying one active site,

X the number of adsorbed water molecules replaced by one molecule of

organic inhibitor, K and K' are the binding constants. The above expressions

include equilibrium constant of adsorption process K that is related to standard

free energy of adsorption (∆Gads) by

= �++.+ �)-�−∆/012

3 45⁄ (1.31)

The value of 55.5 is the concentration of water in solution in mole l−1.

For El-Awady et al. model a plot of log [θ/(1−θ)] versus logC give

a straight lines of slops (X) and intercept (K'). In Flory–Huggins isotherm, a

plot of log θ/C versus log [1−θ] gave straight lines of slope (X) and intercepts

(log XK). Obey the Flory–Huggins isotherm. The values of equilibrium

constant, change in free energy ∆Gads and number of active site are calculated

from e slope and intercept values.

34

1.8 Literature review of inhibitors for phosphoric acid

The inhibition effect of benzyltrimethylammonium iodide (BTAI)

on the corrosion of cold rolled steel (CRS) in phosphoric acid produced by

dihydrate wet method process (7.0 M H3PO4) solution was investigated for the

first time by weight loss, potentiodynamic polarization, electrochemical

impedance spectroscopy (EIS) and scanning electron microscopy (SEM)

methods. The results show that BTAI is a good inhibitor, and the adsorption of

BTAI obeys Langmuir adsorption isotherm. Polarization curves show that

BTAI behaves as a mixed-type inhibitor (Xianghong Li et al 2011).

The inhibition of the corrosion of mild steel in hydrochloric acid

solutions by 4-amino-5-phenyl-4H-1, 2, 4-trizole-3-thiol (APTT) inhibitor was

studied using weight loss technique. Basic kinetic parameters of the corrosion

inhibition process were obtained by reaction kinetic equations. Rustles show

that the inhibition increases with increasing of inhibitor concentration

(Ahemed Y. Musa et al 2010). The synergistic inhibition effect of rare earth

cerium (IV) ion and sodium oleate (SO) on the corrosion of cold rolled steel

(CRS) in 3.0 M phosphoric acid has been investigated by weight loss,

potentiodynamic polarization, electrochemical impedance spectroscopy (EIS)

and scanning electron microscope (SEM) methods. The results revealed that

sodium oleate has a moderate inhibitive effect and Ce4+ has a poor effect.

However, incorporation of Ce4+ with SO improves the inhibition performance

significantly and exhibits synergistic inhibition effect. SO acts as a cathodic

inhibitor, while SO/Ce4+ mixture acts as a mixed-type inhibitor (Xianghong Li

et al 2010). The effect of benzimidazole (BI), 2-methyl benzimidazole (2MBI)

and 2-aminobenzimidazole (2ABI) on the corrosion of mild steel was

evaluated in 1 M phosphoric acid at various concentrations using

electrochemical techniques (Electrochemical Impedance Spectroscopy (EIS)

and DC polarization). Inhibition of imidazole derivatives was evaluated at

35

concentrations between 5 × 10−2–10−4 M. It was observed that inhibition

efficiency increased with increasing inhibitor concentration (A. Ghanbari et al

2010).

The effect of 2-phenyl-1-hydrazine carboxamide on the corrosion of

mild steel in phosphoric acid has been studied by weight loss and

electrochemical techniques and results revealed that this compound act as

mixed type inhibitor and increasing the acid concentration increased the metal

corrosion but did not affect the inhibition efficiency (Dadgarinezhad et al

2009). The corrosion inhibition performance of acetylacetonate complexes of

zinc (II), manganese (II), cobalt (II) and copper (II) on the mild steel substrate

in 1M phosphoric acid was studied by DC polarization. It could be seen that

the above complexes decreased corrosion rate of mild steel in phosphoric acid

media due to the adsorption of complexes on metal surface and acted as

cathodic inhibitors. Presence of chloride ions in the electrolyte enhanced

inhibition performance of acetylacetonate complexes due to synergism

(Ghanbari et al 2009). The synergistic inhibition effect of red tetrazolium (RT)

and uracil (Ur) on the corrosion of cold rolled steel (CRS) in 1to10 M

phosphoric acid solution was studied by weight loss and potentiodynamic

polarization methods. The results revealed that RT had a moderate inhibitive

effect and acted as a mixed-type inhibitor in phosphoric acid. For the Ur, it

showed a poor inhibition effect and acted as a cathodic inhibitor (Xianghong

Li et al 2009).

The inhibitive action of mangrove tannins extracted from mangrove

barks and phosphoric acid on pre-rusted steel in 3.5% NaCl solution was

evaluated and the inhibitive efficiency was compared with that of mimosa

tannins. From the electrochemical studies, the inhibition efficiency of

solutions containing 3.0 g/l tannins depended upon the concentration of

36

phosphoric acid added and the pH of the solution. At pH 0.5 and pH 2.0

inhibitions were found to be great with mangrove and mimosa tannins alone,

while at pH 5.5 the addition of phosphoric acid alone gave the highest

inhibition effect (Afidah A. Rahim et al 2008). The influence of chloride ions

on inhibitive performance of cetyl trimethyl ammonium bromide (CTAB) in

1.0 - 4.0M of phosphoric acid for cold rolled steel has been studied using

weight loss and Tafel polarization techniques. The results revealed that a

synergistic effect has been observed for CTAB with NaCl at each acid

concentration (Xueming Li et al 2008).

The corrosion behaviour of graphite and stainless steels in

phosphoric acid solution (40%) was studied by the use of different

electrochemical methods and scanning vibrating electrode technique (SVET).

It was found that the current density measured by polarization curves

increased with the presence of chloride and sulphate ions in the acid solution

inspite of the tested material. A generalized corrosion was occurred on

graphite whereas a localized corrosion was observed for stainless steels. These

results showed that the graphite as component material in some of the

equipments of the phosphoric acid industry (Iken et al 2007).

Corrosion inhibition of mild steel in 0.67M phosphoric acid by

phenacyldimethyl sulfonium bromide and six of its p-substituted derivatives

was studied by different chemical, electrochemical and scanning electron

microscopy techniques. Potentiodynamic polarization curves indicated that the

compounds acted as mixed-type inhibitors and steel dissolution was controlled

by charge-transfer mechanism (Arab et al 2006). Corrosion inhibition of

triazole derivatives (n-PAT) on mild steel in phosphoric acid solution has been

investigated by weight loss and polarization methods. The results indicated

that these compounds acted as mixed-type inhibitors retarding the anodic and

37

cathodic corrosion reactions with emphasis on the former and do not change

the mechanism of either hydrogen evolution reaction or mild steel dissolution

(Wang Lin 2006). The synergistic inhibition effect of 4-(2-pyridylazo)

resorcinol (PAR) and chloride ion on the corrosion of cold rolled steel in

1M phosphoric acid was studied by weight loss and potentiodynamic

polarization method. Results revealed that the single PAR is not an effective

inhibitor for steel corrosion in phosphoric acid, but in the presence of chloride

ion PAR act as a good inhibitor due to the synergism (Libin Tang et al 2006).

Synergistic effect of isopropylamine with Cl – and SO42- on the

inhibition of corrosion of mild steel in phosphoric acid medium was found to

be 92.03% and 97.50 % inhibition efficiency respectively (Chandrasekaran et

al 2005).The effect of some quaternary N-heterocyclic compounds on the

corrosion of mild steel in solutions of phosphoric acid has been investigated.

The results revealed that the studied compounds are good inhibitors for mild

steel corrosion in phosphoric acid solutions. The inhibition efficiency of the

studied inhibitors increased with inhibitor concentration, but decreased with

acid concentration upto critical concentration above which it started to

increase (Noor 2005). The inhibition effect of isopropylmine (PA) on

corrosion of mild steel in phosphoric acid solution was investigated by

polarization technique at different temperature (Chandrasekaran et al 2005).

The effects of single OP and the mixture of various concentrations of OP and

0.1 M NaCl on the corrosion of cold-rolled steel in 1.0-3.5 M phosphoric acid

have been investigated by weight loss method and polarization method. This

study revealed that the cold-rolled steel in phosphoric acid has been more

efficiently inhibited by OP in the presence of NaCl than single OP. The

polarization curves showed that OP acts as cathodic inhibitor, while the

complex of OP and NaCl acts as mixed-type inhibitor which mainly inhibits

the cathodic corrosion of the steel (Li et al 2005). The corrosion rates of steel

38

in concentrated phosphoric acid (1.0 to 11.0M) were determined by the weight

loss method at three temperatures 298, 308 and 323K. Results showed that the

corrosion rate increased with both acid concentration and temperature

(Benabdellah et al 2005).

The effects of single o-phenanthroline and the mixture of various

concentrations of NaCl and 0.0002M o-phenanthroline on the corrosion of

cold rolled steel in 1.0 to 4.0M phosphoric acid have been investigated by

weight loss and polarization methods. The study revealed that the cold rolled

steel in phosphoric acid has been found to be more efficiently inhibited by

o-phenanthroline in the presence of NaCl than single o-phenanthroline at a

relatively higher concentration of NaCl and there was a synergistic effect

between o-phenanthroline and NaCl. (Mu et al 2004). The role of a rust

converter with tannic and phosphoric acids for painted and unpainted steel

with different corrosion and salts contaminating the rust were evaluated

(Ocampo et al 2004). Anodising of commercial pure titanium in phosphoric

acid solution at different concentrations (0.5 to 4M) has also been investigated

by galvanostatic and potentiodynamic polarization techniques. The samples

exhibited a different anodic behaviour and development of gel-like layer

during the formation of thin anodic film on titanium in phosphoric acid

solution by potentiodynamic conditions (Cydzik 2004).

The corrosion under heat transfer of 904L stainless steel in

phosphoric acid with silica (SiO2) as inhibitor was examined by

electrochemical and spectroscopic techniques (Bellaouchou et al 2003). The

dielectric properties of anodic film formed on tantalum in dilute phosphoric

acid solution at 20 and 85°C have been investigated by electrochemical

impedance spectroscopy. The slightly higher dielectric constant obtained at

85°C is due to reduced amount of incorporated phosphorus species in the outer

39

layer of anodic films formed at this temperature, together with a reduced

thickness of the outer layer, compared with those of anodic films formed at

20°C (Lu et al 2003). The influence of an aqueous phosphoric acid solution on

the corrosion product of mild steel samples has been carried out using

Mossbauer spectroscopy. It has been observed that the transformation of rust

by phosphoric acid depends strongly on the concentration of the phosphoric

acid. The formation of acid ferric phosphate with 8M concentration of the

phosphoric acid was observed (Nigam et al 1990). Polyaniline (PANI)

coatings were electro synthesized on steel samples (13% and 4.44%Cr) using

sulphuric and phosphoric acids as supporting electrolytes. Protective

properties of PANI coatings in the supporting electrolytes were investigated

(Kraljic et al 2003).

The non-toxic inhibitors are of considerable interest in

investigations into the replacement of hazardous classical molecules. The

action of four amino acids containing sulphur on the corrosion of mild steel in

phosphoric acid solution with and without Cl-, F- and Fe3+ ions has been

studied by both the polarization resistance method and electrochemical

impedance spectroscopy (EIS). Both cysteine and N-acetylcysteine (ACC)

showed higher inhibition efficiency than methionine and cystine (Morad et al

2002). The effect of corrosion in the presence of copper (II) and nitrate ions in

3M phosphoric acid solution at 98oC on the corrosion-electrochemical

properties of 12Kh18N1OT steel was studied. Sodium nitrate and copper

phosphate were used as corrosion inhibitor (Filimonov et al 2002). The

inhibiting action of 2-mercaptobenzimidazole on the corrosion of zinc in

phosphoric acid solution has been investigated by weight loss and polarization

methods. The studies revealed that the inhibitor is effective for the inhibition

of zinc in phosphoric acid solution and retards the anodic and cathodic

corrosion reactions (Wang et al 2002).

40

The electrochemical polarization behavior of Monel (400) in

different compositions of binary and ternary solution mixtures of concentrated

phosphoric, sulphuric, formic and acetic acids has been studied by

potentiostatic polarization technique at 25°C (Singh et al 2001). The

polarization and weight loss studies showed that the 2-mercaptopyrimidine

was effective for the inhibition of low carbon steel over a wide concentration

range of aqueous phosphoric acid solution (Wang 2001). The inhibition

efficiency of 2-chloroethyl phosphoric acid in controlling corrosion of carbon

steel immersed in an aqueous phosphoric acid solution containing 60ppm Cl-,

in the absence and presence of inhibitor has been evaluated. The results

revealed that in the presence of chloride it showed synergistic effect (Amairaj

et al 2001). The inhibition on the corrosion under heat transfer of the stainless

alloy 904L by benzotriazole (BTA) in phosphoric acid composed by 40%

H3PO4+4% H2SO4+300 ppm Cl− has been studied by electrochemical and

spectroscopic techniques. Results obtained by polarization measurements

showed that BTA affects both anodic and cathodic processes. With the

increase in concentration of BTA the corrosion rate decreased and the

inhibition efficiency increased (Bellaouchou et al 2001). The inhibiting action

of 2-mercapton benzimidazole on the corrosion of mild steel in phosphoric

acid solution was investigated by weight loss and polarization methods (Wang

2001a). Weight loss studies showed that both 2-mercaptothiazoline and a cetyl

pyridinium chloride are effective for the inhibition of low carbon steel over a

wide concentration range of aqueous phosphoric acid solution (Wang et al

2001b). The effect of nitrate ions and possible products of their reduction

(ammonium, hydrazine and hydroxylammonium ions) on electrochemical

characteristics of stainless steel were studied in phosphoric acid (Filimonov et

al 2001). Steady-state and electrochemical impedance spectroscopy

measurements have been made on anodic layers on 1050 and 2024T3

aluminium alloys prepared from solutions of phosphoric acid, boric acid and

41

sodium tetraborate, before and after impregnation treatment with zinc

(Dasquet et al 2001).

The erosion-corrosion of an austenitic alloy in a phosphoric acid

media containing sulphide ions has been studied (Bellaouchou et al 2000).

Corrosion resistant low-alloy steel, corrosion resistant iron-titanium alloys

have also been developed which may find use in the hardware of phosphoric

acid making plants and other equipment (Mukherjee et al 2000). The corrosion

inhibition of steel in phosphoric acid by thiosemicarbazide derivatives was

studied by different chemical and electrochemical techniques (Ameer et al

2000). The corrosion resistance of niobium, tantalum and Nb-20, 40, 60 and

80 wt% Ta alloys in 20, 40, 60 and 80 wt% phosphoric acid solutions at

boiling point 150°C and 200°C was evaluated (Robin et al 2000). The

corrosion inhibition of steel in phosphoric acid by thiosemicarbazide

derivatives was studied using different chemical and electrochemical

techniques. Protection efficiency up to 99% was obtained with small amount

(10-4M) of cinnamaldehyde thiosemi-carbazone (CTSCN) (Khamis et al 2000).

Corrosion behavior of Monel in phosphoric acid was studied (Jeyaprabha et al

2000). Weight loss and polarization studies showed that 2-mercapto

benzoxazole was effective for the inhibition of low-carbon steel over a wide

concentration range of aqueous phosphoric acid solutions. The inhibitor

retards the anodic and cathodic corrosion reaction with emphasis on the

former. (Wang et al 2000).

The corrosion of tantalum was investigated in sub- and supercritical

oxidizing solutions of hydrochloric, sulfuric and phosphoric acid between

360°C and 500°C (Friedrich et al 1999). The influence of propargyl alcohol

(PA) on the corrosion behavior of mild steel in phosphoric acid was

investigated. PA inhibits the cathodic corrosion reaction at 40ºC, but

42

accelerated the anodic one without changing the mechanism of both reactions.

Inhibition of mild steel in phosphoric acid by PA is attributed to adsorption of

PA onto steel surface via triple-bond carbon atoms (Morad 1999). Effect of

mineral compounds in phosphoric acid polluted by sulfide ions on corrosion of

nickel was observed (Guenbour et al 1999). The electrochemical behaviour of

316 stainless steel in phosphoric acid solution containing nitrate, dichromate,

tungstate and molybdate anions as inhibitors are presented and discussed

(El-dahan 1999). The inhibiting properties of some organic phosphonium and

ammonium compounds were studied with respect to the corrosion of zinc in

1M H3PO4 solutions (Morad 1999a).

The anodic films formed at 1 mA/cm2 on tantalum in concentrated

phosphoric acid (85%) and sulphuric acid (95%) solutions at 25°C have been

examined directly in the transmission electron microscope employing ultra

micro tomed sections (Shimizu et al 1998). The corrosion rates of AISI 304L

and 316L stainless steels prepared by powder metallurgy (P\M) have been

studied by continuous electrochemical methods in different concentrations of

phosphoric acid solutions at room temperature (298K). For comparison

purposes, a simultaneous study was carried out on similar composition of cast

AISI 304L and AISI 316L stainless steels specially prepared for this study.

The sintered AISI 304L and 316L stainless steels had the highest corrosion

rates, these being much higher in phosphoric acid (Otero et al 1998).

It could be seen from the literature survey that the most of the

inhibitors showed the effective inhibition effect in phosphoric acid medium

and most of the organic inhibitors obeyed Langmuir or Temkin adsorption

isotherm.

43

1.9 Literature review of inhibitors for hydrochloric acid

Corrosion inhibition efficiency of 1-phenyl-3-methyl-5-pyrazolone

(PMP) as corrosion inhibitor on mild steel in acid solution was investigated by

means of weight loss, potentiodynamic polarization curve, electrochemical

impedance spectroscopy (EIS), Raman spectrum and Quantum chemical

method. Weight loss measurements gave an inhibition efficiency of about 32%

in the presence of 1×10-5 M PMP, which increased to about 93% at PMP

concentration of 1×10-3 M. EIS results revealed that PMP took effects

excellently as a corrosion inhibitor for mild steel in 1 M hydrochloric acid

media, and its efficiency attains more than 97.2% at 1×10-3 M at 298 K

(Kun cao et al 2012).

The corrosion inhibition properties of ceftadizime (CZD) for mild

steel corrosion in HCl solution were analyzed by electrochemical impedance

spectroscopy (EIS), potentiodynamic polarization and gravimetric methods.

The results reveal that CZD acts as mixed type inhibitor and it follows

physisorption and chemisorption (Ashish Kumar Singh et al 2011). The

corrosion inhibition properties of disulfiram (DSR) for mild steel in HCl

solution were analyzed by electrochemical impedance spectroscopy (EIS),

potentiodynamic polarization, atomic force microscopy, scanning electron

microscopy and gravimetric methods. Physical adsorption is proposed for the

inhibition and the process followed the Langmuir adsorption isotherm and

kinetic/thermodynamic model of El-Awady et al (Ashish Kumar Singh and

M.A.Quraishi 2011). The inhibition effect of triazolyl blue tetrazolium

bromide (TBTB) on the corrosion of cold rolled steel (CRS) in 1.0 M HCl

solution was investigated for the first time by weight loss, potentiodynamic

polarization curves, and electrochemical impedance spectroscopy (EIS)

methods. The results show that TBTB is a very good inhibitor, and is more

efficiency in 1.0 M HCl. The adsorption of TBTB on CRS surface obeys

44

Langmuir adsorption isotherm. Polarization curves reveal that TBTB acts as a

mixed-type inhibitor in hydrochloric acids (Xianghong Li et al 2011). The

corrosion inhibition and adsorption processes of 1,2-diaminoanthraquinone

(DAQ) on mild steel in HCl was studied at different temperatures (303–333 K)

by means of weight loss measurement and UV-visible spectrophotometric

methods. The results indicate that the studied compound exhibits good

performance as inhibitor for mild steel corrosion in 1 M HCl (N.O.Obei-gbedi

et al 2011). Corrosion inhibition of mild steel in 1M HCl by cefadroxil has

been studied using electrochemical impedance spectroscopy (EIS),

potentiodynamic polarization and weight loss methods. The inhibitor showed

more than 96% inhibition efficiency at optimum concentration of

11.0×10-4 mol l-1 (Sudhish K. Shukla et al 2011).

The inhibition efficiency of imidazole derivatives against mild steel

corrosion in hydrochloric acid were evaluated by weight loss, potentiodynamic

polarization, linear polarization and electrochemical impedance spectroscopy.

The weight loss results showed that these are excellent corrosion inhibitors,

electrochemical polarizations data revealed that the mixed mode of inhibition

and the results of impedance spectroscopy have shown that the change in the

impedance parameters, charge transfer resistance and double layer capacitance

(Niketan S. Patel et al 2010). The inhibition efficiency of DCI (Dicycloimine

hydrochloride) has been evaluated by conventional weight loss method and

electrochemical polarization studies. The results revealed that DCI acts as an

effective inhibitor (around 90% of IE) in hydrochloric acid medium

(Rajalakshmi et al 2010). The corrosion behavior of carbon steel in

1M hydrochloric acid solution in the absence and in the presence of three

compounds of ethoxylated fatty amide was studied by weight loss and

galvanostatic polarization techniques. The inhibition efficiency was found to

45

be increased with increasing inhibitor concentrations, number of ethylene

oxide unit and with decreasing temperature (Zaafarany et al 2010).

The influence of ethylenediamine tetra-acetic acid (EDTA) on the

corrosion of mild steel in 1M hydrochloric acid was investigated by

potentiodynamic polarization and electrochemical impedance spectroscopy

(EIS). The efficiency of EDTA was found to be higher than thiourea for mild

steel in 1M hydrochloric acid medium. (Ahmed Y. Musa 2009). The inhibition

of corrosion of mild steel in hydrochloric acid solutions by 2-benzoylpyridine

(2BP) and pyridoxolhydrochloride (PXO) at 303K, 313K and 323K has been

investigated by weight loss and hydrogen evolution techniques. 2BP exhibited

better inhibition efficiency (78.99%) than PXO (71.93%) (A.O.James et al

2009). The corrosion behavior of 304 SS in 1N hydrochloric acid solution

containing different concentrations of N-benzyl - N′ - phenyl thiourea (BPTU),

at different temperatures was investigated by potentiostatic polarization

technique. The results obtained show that BPTU is an efficient anodic

inhibitor with efficiency of greater than 93% and the inhibition was governed

by physisorption mechanism (Herle.Ramadev et al 2009). The bixin in acidic

media was tested for corrosion inhibition of Ti in 0.1N hydrochloric acid

solution over the temperature range 30o to 40oC by electrochemical methods. It

revealed that bixin act as a corrosion inhibitor in halide medium and protects

the metals from corrosion with great efficiency (Jinendra Singh Chauhan et al

2009). It has been stated that the inhibition action of carmine and fast green

dyes on corrosion of mild steel in 0.5M hydrochloric acid was investigated by

mass loss, polarization and electrochemical impedance (EIS) methods at

300K. The inhibition efficiency of fast green (98 %) is higher than that of

carmine (92 %) and found to be maximum in 1 × 10-3M solution (R.A. Prabhu

et al 2009). 5-allyl-4-phenyl-4H-[1,2,4] triazole-3-thiol of phenyl hydrazides

of fatty acids from neem, rice bran and karanja oils have been synthesized and

46

evaluated as corrosion inhibitors for mild steel in hydrochloric acid solution by

weight loss method. The results revealed that the inhibition efficiency of these

compounds was found to vary with concentration, solution temperature, and

immersion time (Toliwal et al 2009).

Effect of sodium lauryl sulfate, a surfactant on corrosion of mild

steel in hydrochloric acid was studied by weight loss, electrochemical

polarization and metallurgical research microscopy. Results obtained reveal

that SLS is good inhibitor and shows very good corrosion inhibition efficiency

(Atulkumar et al 2008). Inhibitive and absorption properties of sparfloxacin

for the corrosion of mild steel in hydrochloric acid have been investigated

using gravimetric, gasometric and thermometric methods. Inhibition efficiency

of sparfloxacin was increased with increase in concentration of inhibitor

(N.O.Eddy et al 2008). The inhibition effect of N-benzyl - N-phenyl thiourea

on the corrosion of mild steel in 0.01 and 0.5N hydrochloric acid medium has

been investigated by potentostatic polarization studies and revealed that PBTU

is an effective anodic inhibitor and showed above 94% inhibition efficiency

(Divakara shetty et al 2008). The interaction of 1, 2, 3 benztriazole (BTAH) on

austenitic stainless steel in hydrochloric acid medium was studied and results

were compared with reported results. The inhibition efficiency of BTAH was

89.1% (Satpati et al 2008). Effect of inhibitor mixtures (TVE-3A, TVE-3B and

TVE-3C) containing formaldehyde in concentration with phenol or cresol on

corrosion behaviour of N80 steel in 15% hydrochloric acid solution was

investigated. TVE-3B has shown the maximum efficiency of 68% at ambient

temperature, whereas maximum inhibition efficiency shown by TVE-3A and

TVE-3C was found to be 62.2% and 65.7% respectively (Kumar et al 2008).

The inhibition effect of 4-[(E)-(phenylimino)methyl]phenol (PIP),

4-[(E)-(4-flourophenylimino)methyl]phenol(FIP), 4-[(E)-(4-chloro phenyl

47

imino) methyl]phenol(CIP), 4-[(E)-(4-bromophenylimino)methyl]phenol(BIP)

and 4-[(E)-(4-nitrophenylimino)methyl]phenol(NIP) on the corrosion of mild

steel in hydrochloric acid was investigated. Results reveal that all the imines

are efficient inhibitors for corrosion of mild steel in hydrochloric acid. The

order of inhibition efficiency is PIP > FIP > CIP > BIP > NIP (A.V.Shanbhag

et al 2007). The inhibition efficieny of methoxy phenol (MPH) and noyl

phenol (NPH) on corrosion of N80 steel in 15% hydrochloric acid has been

studied and found that the maximum inhibition efficiency is about 83 and

78% at 75mM inhibitor concentration respectively (Vishwanatham et al 2007).

Four gemini surfactants namely N-trimethyl butane-diyl-1,2-ethane-bis-

ammonium bromide (BEAB), N-hexane-diyl-1,2- ethane-bis-ammonium

bromide (HEAB), N-dodecane-diyl-1,2- ethane-bis-ammonium bromide

(DDEAB) and N-hexadecane-diyl-1,2- ethane-bis-ammonium bromide

(HDEAB) were synthesized and their influence on the inhibition of corrosion

on mild steel in 1N hydrochloric acid was investigated. Results revealed that

all the compounds were mixed type inhibitors and found to inhibit the

corrosion of mild steel by blocking the active sites of the metal (Sharma et al

2007).

The effects of 2-hydroxy-1-naphthaldehyde glycine (HNG) and

2-hydroxy-1-naphthaldehyde (HN) on the corrosion of mild steel in

hydrochloric acid have been studied. Weight loss measurements reveal that

HNG exhibits higher inhibition efficiency than HN (B.I.Ita et al 2006). Five

heterocyclic compounds having a five atom ring fused with the benzene ring

(indole, benzimidazole, benzotriazole, benzothiazole and benzothiadiazole)

were investigated as corrosion inhibitors for mild steel in 1N hydrochloric acid

by impedance and polarization resistance methods. Four of these compounds

exhibit inhibition properties, while one of them, benzothiadiazole, stimulates

the corrosion process (Popova et al 2006). The effect of three Schiff base

48

compounds namely, (E)-2-(1-(2-(2-hydroxyethylamino) ethylimino)

ethyl)phenol (I), 2,2΄-(1E,10E)-1,10-(2,2΄-azanediylbis (ethane-2,1-diyl) bis

(azan-1-yl-1-ylidene) bis (ethan-1-yl-1-ylidene) diphenol (II) and 2,2΄-

(2E,12E)-3,6,9,12-tetraazaetra deca-2,12-diene-2,13- diyl) diphenol (III) on

the corrosion behavior of steel in 2M hydrochloric acid solution has been

investigated at 298 K . Results proved that the compound III to be the best

inhibitor with a mean efficiency of 93% at 10 ×-2 M additive concentration

(Kaan C Emregul et al 2006).

The inhibition effect of N-cyclohexyl-N’-phenyl thiourea (CPTU)

on the corrosion of mild steel in 0.01 and 0.1 N hydrochloric acid has been

investigated by potentiostatic polarization technique. Results obtained reveal

that CPTU is an efficient anodic inhibitor for mild steel in hydrochloric acid

(S.Divakara Shetty et al 2005). Electrochemical studies of the inhibition and

the effect of a series of semicarbazides and thiosemicarbazides on the

corrosion of iron in 1M hydrochloric acid solution were performed and the

results were found to be 4-ethyl-3-thiosemicarbazide (ETSC) as the better

effective inhibitor than 4-allyl-3-thiosemicarbazide (ATSC) (Nahle .A et al

2005). The inhibitive action of 5-membered heterocycles (Thiophene and

Furan) on the corrosion of brass in 1M hydrochloric acid indicated that the

inhibiting effect grows with rise in the temperature 293K to 333K. The

maximum inhibition efficiency was found to be 98% for thiopene and 95% for

Furan at 1×10-1

M concentration (Adeyemi .O.O et al 2005). The corrosion

inhibitive effect of poly (p-Anisidine) on iron in 1M hydrochloric acid was

studied by electrochemical techniques has shown a remarkable performance of

inhibition efficiency when compared with monomer (Manivel et al 2005).

Studies have been conducted on the inhibitive effect of triethylene tetramine

(TETA) and hexamethylene tetramine (HMTA) for mild steel in 1M HCl in

the concentration range of 10-6 to 10-2 M by weight loss, D.C polarization

49

method and A.C impedance spectroscopy. Results indicated that the inhibition

efficiency of the hexamethylene tetramine was less when compared to that of

triethylene tetramine (Sathiyanarayanan et al 2005). The efficiency of

benzylidene-pyrimidin-2-yl-amine (A), (4-methyl-benzylidene)-pyrimidine-2-

yl-amine (B) and (4-chloro-benzylidene)-pyrimidine-2-yl-amine as corrosion

inhibitors for mild steel in 1M hydrochloric acid have been determined by

weight loss measurement and electrochemical polarization method. The results

showed that these inhibitors have a good corrosion inhibition efficiency even

at very low concentrations and the inhibition efficiency followed the order

C > B > A (Ashassi-Sorkhabi et al 2005). Electrochemical measurements

were performed to investigate the effectiveness of the cationic surfactant;

1-dodecyl-4-methoxy pyridinium bromide as corrosion inhibitor for mild steel

in 2M hydrochloric acid solution. The inhibition efficiency (93.3%) was found

to increase as the concentration of the surfactant increased until critical micelle

concentration (300 ppm) was reached (Migahed 2005). The inhibition effect of

N, N′-bis(salicylidene)-2-hydroxy-1,3-propanediamine (LOH) and N, N′-bis(2-

hydroxyacetophenylidene)-2-hydroxy-1,3-propanediamine (LACOH) in

2 M hydrochloric acid medium on mild steel with known compound has been

investigated at 303K (Emregul et al 2005).

Compounds such as 2,5-Bis (4-dimethylaminophenyl)-1,3,4-

oxadiazole (DAPO) and 2,5-bis(4-dimethylaminophenyl)-1,3,4-thiadiazole

(DAPT) have been synthesized and their inhibiting action on the corrosion of

mild steel in 1M hydrochloric acid at 30°C has been investigated by various

corrosion monitoring techniques. DAPO was found to be more efficient in

1M hydrochloric acid medium and potentiostatic polarization studies proved

that both are behaving as mixed-type inhibitors in 1M hydrochloric acid

(Bentiss et al 2004). New and effective aldimine types of corrosion inhibitors

namely N-methylidene octylamine (MOA), N-ethylidene octylamine (EOA)

50

and N-propylidene octylamine (POA) have been synthesized. Their inhibition

efficiency was investigated for the corrosion of mild steel in 1M hydrochloric

acid solution and it was found that the decrease in corrosion rate was in the

order POA > EOA > MOA (Subramania et al 2004). The effect of 3-amino-1,

2, 4-triazole (ATR) as a corrosion inhibitor for mild steel in acid media has

been investigated and reported that this compound inhibits mild steel corrosion

by affecting both cathodic and anodic reactions (Garcia-Ochoa et al 2004).

The effect of isomers of 3-pyridyl-substituted 1, 2, 4- and 1, 3, 4-thiadiazoles

(3-PTHD and 3-PTH) on the corrosion of mild steel in acid medium has been

investigated. A comparison of the results showed that 3-PTHD was the best

inhibitor in acid medium. It was found to behave better and polarization curves

indicated that 3-PTH and 3-PTHD are mixed type inhibitors in 1M HCl

medium (Bentiss et al 2004).

Three different water-soluble surfactants based on maleic

anhydride-oleic acid adduct (MO) were synthesized, such as triethanol

ammonium salt of MO adduct (TEASMO), triethanolamine ester of MO

adduct (TEAEMO) and polyoxyalkylated MO adduct (POAMO-23). The data

showed that TEASMO exhibited minimum inhibitive efficiency (65%) and on

the other hand, the maximum inhibitive efficiency was noticed (95%) with

POAMO-23 (Osman et al 2003). The inhibition efficiency of benzimidazole

derivatives were examined by corrosion monitoring techniques. Results

revealed that all the five diazoles studied have well pronounced inhibiting

properties in corrosion of mild steel in hydrochloric acid (Popova et al 2003).

The inhibition effect on the corrosion of mild steel in 1M HCl by

3,5-di(m-tolyl)-4-amino-1,2,4-triazole (m-DTAT) and 3,5-di(m-tolyl)-4H-

1,2,4-triazole (m-DTHT) has been investigated at 30ºC by electrochemical and

weight loss measurements. Polarization curves revealed that m-DTHT is a

51

mixed type inhibitor whereas m-DTAT is a cathodic type inhibitor. Inhibition

efficiency up to 95% for m-DTAT and 91% for m-DTHT were obtained

(El Mehdi et al 2003). 5-Bis (n-methoxyphenyl)-1, 3, 4-oxadiazoles is used as

corrosion inhibitors in acidic medium. Correlation between inhibition

efficiency and chemical structure had been studied by weight loss

measurement and electrochemical methods. The results showed that these

inhibitors showed a good corrosion inhibition effect even at very low

concentrations (Bentiss et al 2002).

The corrosion inhibition characteristics of 2-amino thiophenol

(ATP) and 2-cyanomethyl benzothiazole (CNMBT) on two types of steel in

1M hydrochloric acid medium were investigated at different temperatures

(25, 30, 35, 40 and 50°C). Results were correlated to the chemical structure of

the inhibitors. The inhibition efficiency of CNMBT was found to be higher

than that of ATP (Abd El-Rehim et al 2001). The influence of some organic

acid hydrazides namely salicylic acid hydrazide (SAH), anthranilic acid

hydrazide (AAH), benzoic acid hydrazide (BAH) and cinnamic acid hydrazide

(CAH) on the corrosion inhibition of mild steel in the presence of 1N HCl was

studied. The potentiodynamic polarization studies indicated that all the

hydrazides except SAH are mixed type inhibitors (Quraishi et al 2001c). The

effects of benzoin (BN), benzil (BL), benzoin-(4-phenyl thiosemi- carbazone)

(BN4PTSC) and benzil-(4-phenyl thiosemicarbazone) (BL4PTSC) on the

corrosion of mild steel in hydrochloric acid have been studied. The results

revealed that BN exhibited higher inhibition efficiency than BN4PTSC, BL

and BL4PTSC and chemical adsorption mechanism has been proposed for the

action of inhibitors (Ita et al 2001). The inhibiting effects of quinoline,

8-hydroxyquinoline, benzo(f)quinoline, quinoline-2-thiol, triphenylbenzyl, and

tetrabenzyl phosphonium chloride on the corrosion of mild steel (0.26 wt-%C)

52

in de-aerated 3M hydrochloric acid solution have also been studied (Abdel-Aal

et al 2001).

The inhibition effect of 3, 6-bis (2-methoxyphenyl)-1, 2-dihydro-1,

2, 4, 5-tetrazine (2-MDHT) on the corrosion of mild steel in acidic media has

been investigated. 2-MDHT is able to reduce the corrosion of steel more

effectively in 1M hydrochloric acid. Potentiodynamic polarization studies

showed that 2-MDHT is acting as a mixed-type inhibitor in 1M hydrochloric

acid (Elkadi et al 2000). The corrosion inhibition behavior of some substituted

dithiobiurets namely 1,5-diphenyl- 2, 4-dithiobiuret (DPDTB), 1-tolyl-5-

phenyl-2,4-dithio biuret (TPDTB),1- anisidy - l - 5 - phenyl - 2, 4 -

dithiobiuret (APDTB),1-chorophenyl-5-diphenyl-2,4-dithiobiuret (CPDTB)

were studied in 1 to 5 M hydrochloric acid on mild steel. Among the

compounds studied APDTB exhibited the best performance giving more than

98% inhibition efficiency (IE) in hydrochloric acid solutions. DPDTB and

CPDTB were found to reduce hydrogen permeation through mild steel in

hydrochloric acid solutions (Quraishi et al 1999). 2-undecane-5-mercapto-1-

oxa-3, 4-diazole (UMOD), 2-heptadecene-5-mercapto-1-oxa-3, 4-diazole

(HMOD) and 2-decene-5-mercapto-1-oxa-3, 4-diazole (DMOD) were synthesized

in the laboratory and their influence on the inhibition of corrosion of mild steel

in 1N hydrochloric acid was investigated by weight-loss and potentiodynamic

polarization techniques (Ajmal et al 2000). Hexylamine and dodecylamine

were investigated as inhibitors on mild steel corrosion in hydrochloric acid

corrosion (Bastidas et al 2000). The comparative study of corrosion inhibition

of triazole derivatives indicated that the efficiency of the 4-aminotriazole was

greater than that of the 4H-triazole (Bentiss et al 2000).

Sulphamethoxazole was tested as a corrosion inhibitor for mild steel

in 1.0M HCl solution. The results showed that sulphamethoxazole was found

53

to be an effective inhibitor for mild steel in this medium. The protection

efficiency increases with increasing inhibitor concentration (5×10-5 to 1×10-3

M) but decreases with increasing temperature (30 to 60°C) (Foad El Sherbini,

1999a). The effect of addition of 2[5-(2-pyridyl)-1, 2, 4-triazol-3-yl] phenol

(PPT) on mild steel dissolution in 1M hydrochloric acid was studied. The

results showed that PPT revealed a good corrosion inhibition effect and

potentiodynamic polarization studies indicated that PPT is a mixed-type

inhibitor (Bentiss et al 1999a). It has been observed from weight loss and

hydrogen gas evolution measurements that 4-phenylsemicarbazide (4PSC) and

semicarbazide (SC) actually have very significant effects on the corrosion of

mild steel in hydrochloric acid. 4PSC and SC tend to inhibit the corrosion of

mild steel in hydrochloric acid to a remarkable extent, with 4PSC exhibiting a

maximum inhibition efficiency (82%) than that of SC (66%) (Ita et al 1999).

Four nitrogen substituted thiobisformamidines – phenylthiobis-formamidines

(PTBF), tolyl thiobis formamidines (TTBF), anisidyl thiobisformamidines

(ATBF), and 4-chlorophenyl thiobisformamidines (CPTBF) were synthesized

and their corrosion inhibiting behavior for mild steel in 1M, 3M, and 5M

hydrochloric acid was studied (Ajmal et al 1999). The effect of urea (U),

thiourea (TU), acetamide (A), thioacetamide (TA), semicarbazide (SC),

thiosemicarbazide (TSC), methoxybenzaldehyde thiosemicarbazone (MBTSC),

2-acetylpyridine- (4-phenyl) thiosemicarbazone (2AP4PTSC), 2-acetylpyridine-(4-

methyl)thiosemicarbazone (2AP4MTSC), benzoin thiosemicarbazone

(BZOTSC) and benzil thiosemicarbazone (BZITSC) were investigated for

mild steel corrosion in hydrochloric acid The results (at 308ºC and 408ºC)

indicated that the order of efficiency of the thiocompounds in solution and the

extent of their tendency to adsorb on mild steel surfaces are as follows: TSC >

TU > TA, whereas for the thiosemicarbazone derivatives, the order is

BZOTSC > BZITSC > MBTSC > 2AP4MTSC < 2AP4PTSC. Physical

54

adsorption mechanism has been proposed for all the inhibitors except MBTSC,

BZITSC and BZOTSC, which are chemically adsorbed (Ebenso et al 1999).

The inhibition efficiency of the formulation consisting of

1-hydroxyethane-1, 1-diphosphonic acid (HEDP), molybdate and Zn2+ in

controlling the corrosion of mild steel in neutral, aqueous environment

containing 60ppm Cl- has been evaluated (Apparao et al 1998). The inhibiting

effect of cationic surfactant N, N, N-dimethyl 4-methyl benzyl dodecyl

ammonium chloride on mild steel in hydrochloric acid solutions was

investigated (El-Dahan et al 1998).

The effect of α-pyridoin and 2, 2'-pyridiI on the corrosion behaviour

of mild steel in hydrochloric acid solution has been studied. Weight loss and

hydrogen evolution measurements revealed that α-pyridoin gives a better

inhibition effect than 2,2'-pyridil (Ita et al 1997). The inhibiting action of

linear and cyclic thiocarbamides on mild steel corrosion in lM hydrochloric

acid is examined. The thiocarbamides studied are adsorbed through the S-atom

which is the adsorption centre, forming a donor-acceptor bond between the

unpaired electrons of the S-atom and the positive active centre of the metal

surface (Stoyanova et al 1997). The influence of N-heterocyclics viz.

imidazole (IA), benzimidazole (BIA) and 2-methyl imidazole (MIA) on the

corrosion and hydrogen permeation through mild steel in 1N hydrochloric acid

has been investigated (Muralidharan et al 1997).

This literature survey gives the clear idea about various inhibitors

used for mild steel corrosion in HCl environment. Almost all inhibitors shows

better inhibition efficiency in the room temperature, very few only inhibits

effectively in the higher temperature.

55

1.10 Literature review of inhibitors for sulphuric acid

The inhibitive effect of 4-Aminoantipyrine (4-AAP) on the

corrosion of mild steel in 0.5 M H2SO4 solution at 303 – 323 K was studied by

weight loss measurement as well as computational techniques. Results

obtained showed that 4-aminoantipyrine (4-AAP) is a good inhibitor for the

corrosion of mild steel in sulphuric acid solution. The inhibition efficiency

increased with increase in concentration of 4-AAP and also synergistically

increased in the presence of KI and KSCN, but decreased with temperature

(Acha U. Ezeoke et al 2012).

The inhibition effects of 2-amino-5-mercapto-1,3,4-thiadiazole

(2A5MT) and 2-mercaptothiazoline (2MT) on mild steel corrosion in

1 M H2SO4 were studied with potentiodynamic polarization, linear

polarization resistance and electrochemical impedance spectroscopy

techniques. It was shown that both 2A5MT and 2MT act as good corrosion

inhibitors for mild steel protection (Ali Doner et al 2011).

The corrosion and corrosion inhibition effect of carboxymethyl

cellulose (CMC) for mild steel in sulphuric acid medium was investigated

using weight loss and hydrogen evolution techniques at 30–60 0C. The effect

of addition of halide ions (Cl-, Br-, and I-) was also studied. It was found that

CMC functions as an inhibitor for acid induced corrosion for mild steel.

Inhibition efficiency increases with increase in immersion time but decreases

with increase in temperature (S.A.Umoren et al 2010). Chemical methods

were used to assess the inhibitive and adsorption behaviour of carboxymethyl

cellulose (CMC) for mild steel in H2SO4 solution at 30–60 °C. Results

obtained show that CMC act as inhibitor for mild steel in H2SO4. The

inhibition efficiency was found to increase with increase in CMC

concentration but decreased with rise in temperature (M.M.Solomon et al

56

2010). Ketoconazole (KCZ) has been evaluated as a corrosion inhibitor for

mild steel in aerated 0.1 M H2SO4 by gravimetric method. The effect of KCZ

on the corrosion rate was determined at various temperatures and

concentrations. The inhibition efficiency increases with increase in inhibitor

concentration but decrease with rise in temperature (I.B.Obot et al 2010).

The corrosion inhibition of stainless steel with different

concentrations of 1-methyl-3-pyridine-2-YI-thiourea (MPT) in sulphuric acid

was investigated by potentiostatic polarization measurements. MPT acts as

efficient inhibitor in sulphuric acid (S.M.A Hosseini et al 2009). The corrosion

inhibition of mild steel in sulphuric acid in the presence of methocarbamol was

studied using thermometric and gasometric (hydrogen evolution) methods.

The study revealed that the methocarbamol lowered the corrosion rate of mild

steel in sulphuric acid medium and showed the maximum inhibition efficiency

of methocarbamol under increased level of concentration with decreased level

of temperature. The phenomenon of physical adsorption is proposed from the

thermodynamic parameters (Ebenso et al 2009). The inhibition effect of cetyl

trimethylammonium bromide (CTAB) on acid corrosion of mild steel in

sulphuric acid at different temperatures has been investigated. It showed the

maximum inhibition efficiency 92% at room temperature at 10-3 mol/lit, and

any increase in temperature decreased the inhibition efficiency (Mukta

Sharama et al 2009). Inhibitive and adsorption properties of penicillin G for

the corrosion of mild steel was investigated using gasometric and

thermometric methods. Penicillin- G was found to inhibit the corrosion of mild

steel in sulphuric acid. Inhibition efficiencies of penicillin- G increased as the

concentration of penicillin-G increases but decreased with increase in

temperature. (Eddy et al 2009). The inhibitor effect of naturally occurring

biological molecule caffeic acid on the corrosion of mild steel in 0.1M H2SO4

was investigated by weight loss, potentiodynamic polarization,

57

electrochemical impedance and Raman spectroscopy. The different techniques

confirmed the adsorption of caffeic acid onto the mild steel surface and

consequently effects inhibition of the corrosion process. Caffeic acid acts by

decreasing the available cathodic reaction area and modifying the activation

energy of the anodic reaction (De Souza et al 2009).

The corrosion inhibition of mild steel in 1 M sulphuric acid by

polyvinyl pyrrolidone (PVP) and the synergistic effect of iodide ions were

investigated using weight loss and hydrogen evolution methods in the

temperature range of 30-60 0C. The corrosion rates of mild steel decreased

with the increasing concentration of PVP, while the inhibition efficiency

increased. The inhibition efficiency of PVP decreased with rise in temperature

(S.A.Umoren et al 2008). The inhibitory effect of diethanolamine (DEA) on

corrosion of mild steel in 0.5 M sulphuric acid was investigated by various

corrosion monitoring techniques. The inhibition efficiency varied in the range

of 88.7% to 55.3 % for a concentration range of 10-3M to 10-7M at 303 K

respectively (Ramananda Singh et al 2008). The corrosion inhibition of mild

steel in 2M sulphuric acid using Alizarin yellow GG (AYGG) (an azo dye) in

the presence of iodide ions was studied at 30 – 60oC by using weight loss and

hydrogen evolution methods. It has been observed that the inhibition

efficiency increased with increase in concentration of AYGG and decreased

with increase in temperature. The inhibition efficiency of AYGG

synergistically increased on addition of KI (Ebenso et al 2008).

The corrosion inhibition of mild steel in one normal sulphuric acid

solution by polyethylene glycol methyl ether (PEGME) has been studied using

electrochemical polarization. Polyethylene glycol methyl ether is a very

effective corrosion inhibitor for mild steel in sulphuric acid medium

(A.K.Dubey et al 2007). Inhibitive action of cetyl pyridinium bromide (CPB)

58

on the mild steel corrosion in sulphuric acid was investigated by

electrochemical polarization techniques. The results revealed that this inhibitor

inhibit more effectively the mild steel corrosion in sulphuric acid (A.K.Dubey

et al 2007). Three new Schiff bases, N,N΄-ethylen-bis (salicylidenimine) [S1],

N,N΄ -isopropylien-bis (salicylidenimine) [S2], and N-acetylacetone imine,

N-(2-hydroxybenzophenone imine) ortho-phenylen [S3] have been

investigated as corrosion inhibitors for mild steel in 0.5M sulphuric acid by

Tafel polarization and electrochemical impedance spectroscopy (EIS). The

three Schiff bases functioned as good inhibitors reaching inhibition

efficiencies of 97–98% at 300 ppm concentration and S2 showed better

efficiency than the other two Schiff bases (Hosseini et al 2007). The efficiency

of hexamethylenetetramine (HMTA) as corrosion inhibitor for steel in de-

aerated acid solution has been determined by electrochemical studies. It was

found that the HMTA acts a good corrosion inhibitor and also inhibits the

corrosion by controlling both the anodic and cathodic reactions in 0.1M

sulphuric acid (Bayol et al 2007).

The corrosion rates in the presence of 2,5-bis(4-methoxyphenyl)-

1,3,4-oxadiazole (4-MOX) as a steel corrosion inhibitor in 0.5M sulfuric acid

were measured by weight loss method over the range of temperatures from

303 to 343K. Results revealed that 4-MOX performed excellently as a

corrosion inhibitor for mild steel in sulfuric acid media and its efficiency

attained more than 96.19 % at 8 ×10-4

M at 333K (Bouklah et al 2006). 2,2΄-

Dithiobis(3-cyano-4,6-dimethyl pyridine) was found to be an efficient

inhibitor for the corrosion of mild steel in 1 to 5M sulfuric acid solutions at

35 to 50ºC. The inhibition efficiency is slightly increased or remained

approximately unchanged irrespective of the acid concentration or the solution

temperature. The inhibition action is attributed to chemisorption of the

heterocyclic compound on the steel surface by blocking its active sites (Morad

59

et al 2006). Inhibition of mild steel corrosion in de-aerated 0.5M sulfuric acid

solution containing various concentrations of indole-5-carboxylic acid was

studied over the temperature range from 25 to 55ºC. It gave the highest

inhibition efficiency of 92% at 4×10-3M of indole-5-carboxylic acid and

behaved as mixed-type inhibitor (Quartarone et al 2006). The inhibiting action

of hexadecylpyridinium bromide (HDPB) on mild steel in 0.5M sulfuric acid

solution in the temperature range of 30 to 60ºCwas studied using

potentiodynamic technique and weight loss measurement. Results showed that

the inhibition efficiency increased with increase in temperature and maximum

range of inhibition efficiency of HDPB was obtained around 97% at 1×10-3 M

at 333K (Mahmoud M. Saleh 2006).

Benzyl triphenyl phosphonium bromide (BTPPB) has been

evaluated as a corrosion inhibitor for mild steel in aerated 0.5M sulfuric acid

solution by galvanostatic polarization and potentiostatic polarization methods.

The results of mass loss and potentiodynamic polarization measurements on

the corrosion inhibition of mild steel in 0.5 M sulphuric acid in the

temperature range of 30-60 0C using sodium naphthalene disulphonic acid

(NDSA) as an inhibitor reveals that inhibition efficiency increased with the

increase in concentration of NDSA. The adsorption of inhibitor followed

Flory-Huggins adsorption isotherm (Fathy M.Bayoumi et al 2005). The effect

of formazones as a new class of corrosion inhibitor on mild steel in sulphuric

acid has been investigated by various techniques. The results revealed that the

compounds gave a maximum inhibition efficiency of 98-99% at 0.5 to 10 M

concentrations of inhibitor (Selvaraj et al 2005). The inhibitive capabilities of

some organic dyes namely; safranine-O (SO), thymol blue (TB) and

fluorescein-Na (F-Na) on the electrochemical corrosion of mild steel in

sulphuric acid solution was rapidly assessed using the gasometric technique.

The results indicated that all the compounds act as inhibitor in the acidic

60

medium and inhibition efficiency increased with concentration for SO and TB

but decreased with concentration for F-Na (Ebenso et al 2005). The influence

of some substituted dianils on corrosion of mild steel in 1N sulphuric acid has

been studied. Methylene blue dye (MB) was investigated as a corrosion

inhibitor for mild steel in 2 M sulphuric acid solution in the presence of halide

additives namely KCl, KBr and KI. It was found that in the presence of halide

additives the inhibition efficiency increased. (Oguzie et al 2005). Nasser

(2005) results revealed that the some aliphatic sulphides inhibit the corrosion

of mild steel in 1M sulphuric acid in the order: Ethyl butyl sulphide > Allyl

butyl sulphide > Ethyl propylsulphide > Diethyl sulphide > Ethyl methyl

sulphide.

The effect of thiourea (TU), methylthiourea (MTU) and

phenylthiourea (PTU) on the corrosion behaviour of mild steel in 0.1 M

solution of sulfuric acid has been investigated in relation to the concentration

of thioamides (–CS–NH2). Their inhibition efficiencies increased in the order

of phenylthiourea > methylthiourea > thiourea (Ozcan et al 2004). An anionic

surfactant [p-myristyloxycarbonyl methoxy-p-sodiumcarboxylate-azobenzene]

was prepared. The inhibition efficiency of this surfactant has been studied by

both chemical and electrochemical techniques at 25ºC. The inhibition

efficiency reached to an extent of 95.82% at 10−4M of the inhibitor (Migahed

et al 2004). Gravimetric method was used to study the inhibitory properties of

indigo dye during corrosion of mild steel in aerated sulphuric acid solutions in

the range 30°–50°C. The effect of halide salts KCl, KBr and KI was also

investigated. (Oguzie et al 2004). The addition of halide salts synergistically

increased the inhibition efficiency of indigo in the order KI >KBr >KCl. The

inhibitive action is probably based on the adsorption ability of the polar N or O

atom in the inhibitor which is bonding to the metal surface by chemical

adsorption mechanism (E.E. Oguzie et al 2004).

61

Macrocyclic compounds constitute a potential class of inhibitors.

The influence of some substituted dianils on corrosion of mild steel in 1N HCl

and 1N H2SO4 has been studied using weight loss and electrochemical

techniques. All the five compounds studied, 1,4-dicinnamyledene

aminophenylene (DCAP) showed the best performance. The inhibition

efficiency values of aromatic dianils showed that at a common concentration

of 500 ppm follow the order DCAP > DBAP > DDAP > DVAP > DSAP

(Quraishi et al 2003). The inhibition effects of sodium

dodecylbenzenesulphonate (SDBS) and hexamethyl enetetramine (HA) on the

corrosion of mild steel in sulphuric acid solution have been studied (Hoesseini

et al 2003a). The inhibition effects of 3,5 bis (2-pyridil) 4-amino 1,2,4 triazole

(NBTA) and 1-10 phenantrolin (PHEN) on corrosion of mild steel in acid

solution was studied (Arshadi et al 2002). A macrocyclic compound namely

2,3,9,10-tetraphenyl-6,13-dithia-1,4,5,7,8,11,12,14-octaaza-cyclotetradeca

-1,3,6,8, 10,13-hexaene (PTAT) was also synthesized to study the corrosion

inhibitive effect on pickling of mild steel in 20% sulphuric acid at 95°C

(Quraishi et al 2001a).

The corrosion and inhibition behaviors of mild steel in aerated

sulphuric acid in the presence of propargyl alcohol (PA) and potassium iodide

were investigated by electrochemical methods. The experimental results

suggested that the presence of iodide ions in the solutions stabilized the

adsorption of PA molecules on the metal surfaces and improved the inhibition

efficiency of PA (Feng et al 1999). Effectiveness of quaternary ammonium salt

used as corrosion inhibitor on mild steel in 5% sulphuric acid solution at

temperatures between 30° and 60°C (Dahan et al 1999). The inhibition of mild

steel corrosion in 0.5M sulphuric acid with simple amines (cyclohexylamine,

pyridine, triethylamine) was investigated by DC polarization, electrochemical

impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS).

62

The results indicated that a strong dependence of the inhibition performance

on the nature of the metal surface, in addition to the structural effects of

amines (Li et al 1997).

Inhibitor survey for the mild steel corrosion reveals that all the

inhibitors shows very good inhibition efficiency for the mild steel in H2SO4

environment.

1.11 Literature review of synthesized inhibitors for acid medium

The corrosion rates in the presence of new synthesized pyridazine

derivatives (P1, P2, P3 and P4) as a steel corrosion inhibitors in

1 M hydrochloric acid, were measured by the weight loss method, in the range

of temperatures from 303 to 353 K. Results obtained revealed that the

inhibition efficiency of these compounds decreases markedly with increasing

temperature and its value reaches 48.5% at 353 K at 10-3M (B.Zerga et al

2012). The effect of adding new pyridazine derivatives, ethyl[4-(2-chloro

benzyl)-3-methyl-6-oxopyridazin-1(6H)-yl]acetate (P1), ethyl [4-(2-

chlorobenzyl) -3-methyl-6-thioxopyridazin-1(6H)-yl] acetate (P2), 5-(2-

chlorobenzyl)-2-(2-hydroxyethyl)-6-methylpyridazin-3(2H)-one (P3) and 5-

(2-chlorobenzyl)-2-(2-hydroxyethyl)-6-methylpyridazine-3(2H)-thione(P4), on

the electrochemical behaviour of steel in molar hydrochloric acid was

investigated by using weight loss method, potentiodynamic polarization and

electrochemical impedance spectroscopy (EIS) measurements. Results

reported in this study show that the addition of these compounds inhibits the

corrosion of steel and the extent of inhibition depends upon the type and

concentration of the pyridazine compounds ( B.Zerga et al 2012).

The inhibition effect of synthesized 1-(6-ethoxy-6-oxohexyl)

pyridazinium bromide(I), 1-benzylpyridazinium bromide (II), 1-phenethyl

63

pyridazinium bromide (III) and 1-(4-phenoxybutyl)pyridazin-1-ium bromide

(IV) on the corrosion of mild steel in hydrochloric acid was investigated by

gravimetric method. The inhibition efficiency was increased with increase in

concentration of all four inhibitors and also synthesized inhibitors acts as a

mixed type inhibitor (M.Messali et al 2011). The inhibition effect of a new

synthesized organic inhibitor, namely 1-(3-Nitrobenzylidene)Thiosemi

carbazide (A) on the corrosion of mild steel in 0.5 M sulphuric acid have been

investigated at room temperature using weight loss, electrochemical

impedance spectroscopy (EIS) and Tafel polarization measurements. The

inhibition efficiencies obtained from all methods employed are in good

agreement with each other. The obtained results show that compound (A) is a

very good inhibitor with efficiency of 98% at 100 ppm additive concentration

in acid solution (Athareh DADGARINEZHAD et al 2011). Schiff bases

derived from condensation reaction of Acrolein with 2-aminophenol (SB1),

cinnamaldehyde with 2-aminophenol (SB2) and cinnamaldehyde with

phenylene diamine (SB3) were prepared. These schiff bases identified by

UV-Vis, IR, CHN and H1NMR.The study also included the use of these schiff

bases as inhibitors for corrosion of carbon steel in acidic media 0.5 N HCl.

The rate corrosion was measured by electrochemical and weight loss methods

and it was found that their results are in agreement between them. The results

indicated that these schiff bases inhibited the corrosion efficiently

(Mohammed Qasim Mohammed et al 2011). The inhibition effect of triazolyl

blue tetrazolium bromide (TBTB) on the corrosion of cold rolled steel (CRS)

in 1.0 M HCl and 0.5 M H2SO4 solution was investigated by weight loss,

potentiodynamic polarization curves and electrochemical impedance

spectroscopy (EIS) methods. The results show that TBTB is a very good

inhibitor (Xianghong Li et al 2011). 1, 3-Bis-(morpholin-4-yl-phenyl-methyl)-

thiourea (MBT) was synthesized and their influence on the inhibition of

corrosion on mild steel in various hydrochloric acid concentrations has been

64

investigated by weight loss, potentiodynamic polarization, electrochemical

impedance (EI), Tafel polarization, and scanning electron microscope (SEM)

and FT-IR methods. The result of weight loss study shows that the corrosion

inhibition efficiency (IE) is directly proportional to the concentration of the

inhibitor and inversely proportional to the temperature (Devaraj Karthik et al

2011).

Acenaphtho [1,2-b] quinoxaline (AQ) was tested 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

(I.B. Obot et al 2010). The corrosion inhibition of mild steel in 0.5 M sulfuric

acid solutions by some new synthesized organic compounds namely (E)-2-

acetyl-3-(butyl amino)-N-phenyl buten-2-thioamide (compound A), (E)-3-(4-

(dimethyl amino) phenyl amino)-2-acetyl-N-phenyl buten-2-thioamide

(compound B) and (E)-3-(2,3-dimethyl phenyl amino)-2-acetyl-N-phenyl

buten-2-thioamide (compound C) was investigated using weight loss and

potentiostatic polarization techniques. These measurements reveal that the

inhibition efficiency obtained by these compounds increased by increasing

their concentration. The inhibition efficiency follows the order A > B > C

(S.M.A. Hosseini et al 2010). The influence of inhibitor concentration and

temperature on the corrosion behaviour of steel in molar HCl solution has been

investigated by weight loss method. Results obtained show that the inhibitory

effect of 2-phenylthieno (3, 2-b) quinoxaline (P4) increases with increasing P4

concentration to attain the highest value (95%) at 5×10-4M (El Ouali et al

2010). Newly synthesized compounds such as benzoic-triazole derivative 3,

5-dimethylbenzoicacid [1, 2, 4] triazol-1-ylmethyl ester (DBT) were also

investigated. The results revealed that DBT was an excellent inhibitor and

exhibited mixed-type character (Zhihua Tao et al 2010). Sulphaniilic acid and

65

sulphanilamide Schiff bases have been synthesized and evaluated as inhibitors

for mild steel in sulphuric acid by weight loss and electrochemical methods.

The inhibition efficiency increases with increase in concentration of inhibitor

and decreases with temperature (S.Chitra et al 2010). Four heterocyclic

compounds, namely 4- phenyl-5-acetyl/carbethoxy-3-methyl-6-hydroxy-6-

methyl-4,5,6,7-tetrahydro-2,1-benzoisoxazole and benzopyrazole (BIS1, BP1

and BIS2, BP2) were synthesized and their influence on the inhibition of

corrosion of mild steel in sulphuric acid was investigated by means of weight

loss, potentiodynamic polarization, electrochemical impedance and scanning

electron microscopy. All the four inhibitors exhibit excellent inhibition

efficiency towards corrosion of mild steel in sulphuric acid (K. Parameswari et

al 2010).

The inhibition effect of Bis (benzimidazol-2-yl) disulphide

(BIMDS) on corrosion behavior of mild steel (MS) in 1.0 M HCl and

0.5 M H2SO4 was studied using different techniques. These studies have

shown that studied compound is a good inhibitor for MS in 1.0 M HCl and

0.5 M H2SO4 solutions. Inhibitor showed better performance in 0.5 M H2SO4

solutions than 1.0 M HCl (Ishtiaque Ahamad et al 2009). 3-{[8-

trifluoromethyl)quinoline-4-yl]thio}-N’-(2,3,4-trihydroxybenzylidene)propano

hydrazide (TQTHBH) was synthesized, characterized and tested as a corrosion

inhibitor for mild steel in hydrochloric acid and sulphuric acid solutions using

weight loss, electrochemical impedance and potentiodynamic polarization

methods. The newly synthesized TQTHBH acts as good corrosion inhibitor for

mild steel in acid medium (V. Ramesh Saliyan et al 2009).

A new corrosion inhibitor N-(3,4-dihydroxybenzylidene)-3-{[8-

(trifluoromethyl) quinolin-4-yl]thio} propane hydrazide (DHBTPH) was

synthesized, characterized and tested as a corrosion inhibitor for mild steel in

66

sulphuric acid (0.5M, 1M) solutions using weight-loss method,

electrochemical methods. The results showed that DHBTPH is a very good

inhibitor for mild steel in acidic medium and the inhibition efficiency was

found to be in the decreasing order 0.5M H2SO4 > 1M H2SO4 (Ramesh Saliyan

et al 2008). The cycloaddition reactions of the cyclic nitrones 1-pyrroline 1-

oxide and 3,4,5,6-tetrahydropyridine 1-oxide with alkenes, 11-phenoxy-1-

undecene and 11-p-methoxyphenoxy-1-undecene, afforded cycloaddition

products (bicyclic isoxazolidines) in excellent yields. One of the cycloadducts

on reaction with propargyl chloride and ring opening with zinc in acetic acid

afforded quaternary ammonium salt and aminoalcohol, respectively. All the

new inhibitor molecules in the presence of 400 ppm at 60 °C achieved

inhibition efficiencies, determined by gravimetric method, in the range

99–99.6% and 85–99% for mild steel in 1 M HCl and 0.5 M H2SO4 (S.A. Ali

et al 2008). The newly synthesized 2-(Alkylsulfanyl)-N-(pyridin-2-yl)

acetamide derivatives were tested as a corrosion inhibitor on steel in acidic

mediums. All tested inhibitors showed promising inhibition efficiencies

(A. Yıldırım et al 2008). Quinolin-5-yl methylene-3- { [ 8- ( tri fluoro methyl )

quinolin-4-yl]thio}propanohydrazide (QMQTPH) was synthesized,

characterized and tested as a corrosion inhibitor for mild steel in 1 M and 2 M

HCl solution using potentiodynamic polarization and electrochemical

impedance spectroscopy (EIS). The results showed that QMQTPH is an

excellent inhibitor for mild steel in acid medium (V. Ramesh Saliyan et al

2008).

The effect of three Schiff base compounds with increasing number

of coordination sites, namely, 2-{(E)-[(2-hydroxyethyl)imino]methyl} phenol

(I), 2-[(E)-({2-[(2-hydroxyethyl)amino]ethyl}imino)methyl]phenol (II) and

2,2′-{iminobis[ethane-2,1-diylnitrilo(E)methylidene]}diphenol (III) have been

investigated at 298 K by weight loss measurements, potentiodynamic

67

polarization and electrochemical impedance spectroscopy (EIS) methods. The

inhibition efficiencies obtained from all methods employed are in good

agreement. Results show compound III to be the best inhibitor with a mean

efficiency of 93% at 10−2 M additive concentration. Studies showed all three

compounds to act as mixed type inhibitors (Canan Kustu et al 2007). The

inhibition effect of synthesized 4-[(E)-(Phenylimino) methyl] phenol (PIP),

4-[(E)-(4-fluorophenylimino) methyl] phenol (FIP), 4-[(E)-(4-chloro

phenylimino) methyl] phenol (CIP), 4-[(E)-(-4-bromophenylimino) methyl]

phenol (BIP) and 4-[(E)-(-4-nitro phenylimino) methyl ]phenol (NIP) on the

corrosion of mild steel in hydrochloric acid was studied by weight loss and

electrochemical techniques (Shanbhag et al 2007). Three triazole derivatives

(4 – chloro – acetophenone –O - 1′- (1′,3′,4′-triazolyl) – metheneoxime

(CATM), 4 – methoxyl – acetophenone - O- 1′- (1′,3′,4′-triazolyl) -

metheneoxime (MATM) and 4 – fluoro – acetophenone - O- 1′- (1′,3′,4′-

triazolyl) - metheneoxime (FATM)) have been synthesized as new inhibitors

for the corrosion of mild steel in acid media. The inhibition efficiencies of

these inhibitors were evaluated by means of weight loss and electrochemical

techniques such as electrochemical impedance spectroscopy (EIS) and

polarization curves. The inhibition efficiency of all three inhibitors increased

with increase in concentration of inhibitors (Weihua Li et al 2007). The

inhibition effect of a new bipyrazole derivative namely N-benzyl-N,N-bis[(3,5-

dimethyl-1H-pyrazol-1-yl)methyl]amine (BBPA) on the corrosion of steel in

1 M HCl is studied at 308 K. Weight-loss measurements, potentiodynamic

polarization, linear polarization and impedance spectroscopy (EIS) methods

were used. Results show that BBPA is a good inhibitor and inhibition

efficiency reaches 87% at 5×10− 4 M (K. Tebbji et al 2007). This study

examined the use of 4H-triazole derivatives namely 3,5-diphenyl-4H-1,2,4-

triazole (DHT), 3,5-bis(4-pyridyl)-4H-1,2,4-triazole (4-PHT) and 3,5-bis(4-

methyltiophenyl)-4H-1,2,4-triazole (4-MTHT) as inhibitor for corrosion and

68

dissolution protection of mild steel in normal hydrochloric acid solution. The

experimental results revealed that 4-MTHT was the best effective inhibitor

(99.6%) at 5×10-4

M and the inhibition efficiency was found to be in the order:

4-MTHT > 4-PHT > DHT (Bentiss et al 2007).

The inhibiting action of newly synthesized 2,2′-Dithiobis(3-cyano-

4,6-dimethylpyridine (PyS)2 on the corrosion of mild steel in 1–5 M H2SO4

solutions at 35–50 °C has been investigated by polarization resistance (Rp),

polarization curves and electrochemical impedance spectroscopy (EIS). (PyS)2

showed excellent performance and its efficiency did not affect either by

increasing the acid concentration or rise of temperature (M.S. Morad et al

2006). Some S-containing newly synthesized thio compounds have been tested

as inhibitors for the corrosion of steel in 1 M HCl solution. Weight loss

measurements, potentiodynamic polarization and impedance spectroscopy

(EIS) methods are used. The inhibiting action increases with the concentration

of the compounds tested (M. Elayyachy et al 2006). The inhibiting action of a

Schiff base 4-[(4-hydroxy-3-hydroxymethyl-benzylidene)-amino]-1,5-

dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (phv), derived from 4-amino-

1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (phz) and 4-hydroxy-3-

methoxy-benzaldehyde (vn), towards the corrosion behavior of steel in

2 M HCl solution has been studied using weight loss, polarization and

electrochemical impedance spectroscopy (EIS) techniques. The inhibitor

efficiencies calculated from all the applied methods were in agreement and

were found to be in the order: phv > phz > vn (Kaan C. Emregül et al 2006).

Selected oxa-diazoles of fatty acids; namely 2-hepta decene-5-mercapto-l-oxa-

3,4-diazole (HMOD); 2-undecane-5-mercapto-l-oxa-3,4-diazole (UMOD); and

2-decene-5-mercapto-l-oxa-3,4-diazole (DMOD), were synthesized. The

inhibition efficiency of these compounds was found to vary with

concentration, immersion time and temperature. All these compounds showed

69

good inhibition efficiency and act as mixed type inhibitors (Quarishi et al

2006). Imidazolines and amidic precursors were synthesized with good yields

through an optimized process. These compounds were evaluated as corrosion

inhibitors in an aqueous solution of 1M hydrochloric acid by gravimetric and

polarization techniques. The results indicated that the inhibition efficiency of

these compounds depended on the molecular structure and concentration of

inhibitor (Olivares-Xometl et al 2006). The corrosion inhibition of mild steel

in 0.5 M hydrochloric acid solutions by some new hydrazine carbodithioic

acid derivatives namely N΄-furan-2-yl-methylene-hydrazine carbodithioic acid

(A), N΄-(4-dimethylamino-benzylidene)-hydrazine carbodithioic acid (B) and

N΄-(3-nitro-benzylidene)-hydrazine carbodithioic (C) were also studied. The

measurements showed that the inhibition efficiency obtained by these

compounds increased by increasing their concentration. The inhibition

efficiency was found to follow the order: C > B > A (K.F. Khaled et al 2006)

A potential class of heterocyclic inhibitors namely 2-amino -1,3,4-

thiadiazole(AT), 5-methyl-2-amino-1,3,4- thiadiazole(MAT), 5-ethyl -2-

amino-1,3,4- thiadiazole (EAT)and 5-propyl -2-amino-1,3,4-thiadiazole

(PAT) were synthesized and their influence on inhibition of mild steel

corrosion in 1N hydrochloric acid was investigated by potentiostatic

polarization technique. It could be seen from the result that all the thiadazoles

exhibited good inhibition efficiency and act as mixed type inhibitor (Quraishi

et al 2005). The influence of some new bipyrazole derivatives synthesized in

the laboratory on the corrosion inhibition of mild steel in 1 M hydrochloric

acid solution was studied. It was found that ethyl 4-[bis (3, 5-dimethyl-1H-

pyrazol-1-yl) methyl)] amino benzoate (bipyrazol-4) was the best inhibitor

(Elayyachy et al 2005). Inhibitory effect of some new synthesized bipyrazole

compounds, namely, N,N-bis[(3,5-dimethyl-1H-pyrazol-1-yl)methyl]-N-(4-

methylphenyl)amine(P1) and methyl-1-[((methylphenyl) {[3-(methoxycarbony

70

l)-5-methyl-1H-pyrazol-1-yl] methyl} amino)methyl]-5-methyl-1H-pyrazole-

3-carboxylate (P2) on corrosion of pure iron in 1 M HCl solution has been

studied using chemical technique as weight loss and electrochemical

techniques as potentiodynamic polarization, linear polarization and

impedance. The inhibition efficiencies obtained from gravimetric, cathodic

Tafel plots, linear polarization resistance and EIS methods are in good

agreement. The results obtained reveal that these compounds are efficient

inhibitors (A. Chetouani 2005). A triazole-based cationic gemini surfactant,

3,5-bis(methylene octadecyl dimethylammonium chloride)-1,2,4-triazole (18-

triazole-18) has been synthesized, and its effect on corrosion inhibition of A3

steel in 1 M HCl has been studied using the weight-loss method. The result

showed that 18-triazole-18 acted as an excellent inhibitor in 1 M HCl (Ling-

Guang Qiu et al 2005). The influence of diethyl pyrazine-2,3-dicarboxylate

(P1) on the corrosion of steel in 0.5 M H2SO4 solution has been studied by

weight loss measurements, potentiodynamic polarization and linear

polarization resistance (Rp) and impedance spectroscopy (EIS) methods. The

inhibiting action increases with the concentration of pyrazine compound to

attain 82% at 10−2 M (M. Bouklah et al 2005). The efficiency of benzylidene-

pyrimidin-2-yl-amine (A), (4-methyl-benzylidene)-pyrimidine-2-yl-amine (B)

and (4-chloro-benzylidene)-pyrimidine-2-yl-amine, as corrosion inhibitors for

mild steel in 1 M HCl have been determined by weight loss measurements and

electrochemical polarization method. The results showed that these inhibitors

revealed a good corrosion inhibition even at very low concentrations

(H. Ashassi-Sorkhabi et al 2005).

The effect of some mercapto-triazole derivatives synthesized in the

laboratory containing different hetero atoms and substituents in the organic

structures on the corrosion and hydrogen permeation of mild steel in 1M HCl

was investigated. It has been reported that all the mercapto-triazole derivatives

71

perform excellently as corrosion inhibitors for mild steel in 1M HCl and

behaved as mixed-type inhibitors (Hui-Long Wang et al 2004).

The inhibition of the corrosion of mild steel in 1 M HCl by 3,5-

di(m-tolyl)-4-amino-1,2,4-triazole (m-DTAT) and 3,5-di(m-tolyl)-4H-1,2,4-

triazole (m-DTHT) has been investigated at 30 °C using electrochemical and

weight loss measurements. Polarization curves reveal that m-DTHT is a mixed

type inhibitor whereas m-DTAT is a cathodic type inhibitor. Inhibition

efficiencies up to 95% for m-DTAT and 91% for m-DTHT can be obtained

(B El Mehdi et al 2003). A new corrosion inhibitor namely 4-amino-3-butyl-5-

mercapto-1,2,4-triazole (ABMT) has been synthesized and its inhibitive

performance towards the corrosion of mild steel in 1 N sulphuric acid (H2SO4)

investigated by weight loss and potentiodynamic polarization techniques.

Potentiodynamic polarization measurements clearly reveal that the

investigated inhibitor is of mixed type (M.A Quraishi et al 2003). A mercapto-

triazole compound, namely 4-salicylideneamino-3-phenyl-5-mercapto-1,2,4-

triazole (SAPMT), was synthesized and its inhibition effect on the corrosion of

mild steel in 1.0 M hydrochloric acid (HCl) solution was investigated by

weight loss and electrochemical techniques. Results obtained revealed that

SAPMT performed excellently as a corrosion inhibitor for mild steel in HCl

solution (Hui-Long Wang et al 2003). Three heterocyclic compounds namely

3-anilino-5-imino-4-phenyl-1, 2,4-thiadiazoline (AIPT), 3-anilino-5-imino-4-

tolyl-1, 2,4-thiadiazoline (AITT), and 3-anilino-5-imino-4-chlorophenyl-1,

2,4-thiadiazoline (AICT) were synthesized and their corrosion inhibition effect

on mild steel in 1M hydrochloric acid was investigated (Quraishi et al 2003b).

Compounds such as 2,3,9,10-tetramethyl-6,13-dithia-1,4,5,7,8,11,12,14-

octaaza-cyclotetradeca-1,3,6,8,10,13-hexaene (MTAH) were synthesized to

study the corrosion inhibitive effect on pickling of mild steel (MS) in 20%

sulphuric acid at 95°C. MTAH exhibited inhibition efficiency of 95% and the

72

efficiency further increased in the presence of KI due to synergistic effect

(Quraishi et al 2003).

A new corrosion inhibitor namely 4-amino-3-butyl-5-mercapto-1, 2,

4-triazole (ABMT) has been synthesized and its inhibitive performance

towards the corrosion of mild steel in 1N sulphuric acid was investigated by

weight loss and potentiodynamic polarization techniques. The inhibition

efficiency of ABMT was 99% at 1000 ppm and the inhibitor is of mixed type.

(Quraishi et al 2002).

A new corrosion inhibitor namely 4-salicylideneamino-3-

hydrazino-5-mercapto-1, 2, 4-triazole (SAHMT) has been synthesized and its

influence on corrosion inhibition of oil-well tubular steel (N-80) and mild steel

in 15% hydrochloric acid solution under boiling condition has been studied by

weight loss method. Potentiodynamic polarization measurements clearly

revealed that the investigated inhibitor is of mixed type and inhibits the

corrosion of both the steels by blocking the active site of the metal (Quraishi et

al 2001). Three different long chain fatty acid oxadiazoles namely 2-undecane-

5-mercapto-1-oxa-3, 4-diazole (UMOD), 2-heptadecene-5-mercapto-1-oxa-3,

4-diazole (HMOD) and 2-decene-5-mercapto-1-oxa-3, 4-diazole (DMOD)

were synthesized in the laboratory and were evaluated as corrosion inhibitor

for mild steel in 15% hydrochloric acid at 105±2°C by weight loss method.

UMOD was found to be the best corrosion inhibitor. It exhibited 94%

inhibition efficiency for N-80 steel and 72% inhibition efficiency for mild

steel (Quraishi et al 2001b). New class of corrosion inhibitor such as namely

2,5-bis (4-dimethylaminophenyl)-1,3,4-thiadiazole (DAPT) was synthesized

and its inhibiting action on the corrosion of mild steel in 1M hydrochloric acid

at 30°C was investigated (Bentiss et al 2001).

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The inhibition effect of 3, 6-bis (2-methoxyphenyl)-1, 2-dihydro-

1,2,4,5-tetrazine (2-MDHT) on the corrosion of mild steel in acidic media has

been investigated by weight loss and various electrochemical techniques.

Results obtained reveal that this organic compound is a very good inhibitor. 2-

MDHT is able to reduce the corrosion of steel more effectively in 1 M HCl

than in 0.5 M H2SO4 (L. Elkadi et al 2000). The influence of 3-methyl-2,6-

diphenyl piperidin-4-one (MDPO) and 2-phenyl decahydroquinoline-4-one

(PDQO) synthesized in the laboratory and corrosion inhibition of mild steel in

sulphuric acid has been studied using weight loss and various electrochemical

AC and DC monitoring techniques. Both the compounds inhibit excellently

the corrosion of mild steel in sulphuric acid (S.Muralidharan et al 2000).

A new corrosion inhibitor, namely, 3,5-bis (2-thienyl)-4-amino-

1,2,4-triazoles (2-TAT) has been synthesized and its inhibiting action on the

corrosion of mild steel in acid baths (1 M HCl and 0.5 M H2SO4) has been

investigated by various corrosion monitoring techniques, such as corrosion

weight loss tests and electrochemical impedance spectroscopy. 2-TAT is able

to reduce the steel corrosion more effectively in 1 M HCl than in 0.5 M H2SO4

(F. Bentiss et al 1999).

A new class of corrosion inhibitors, namely,3,5-bis(n-pyridyl)-4-

amino-1,2,4-triazoles has been synthesized and its inhibiting action on the

corrosion of mild steel in 1M hydrochloric acid has been studied (Mernari et al

1998). Attempts were made to develop effective corrosion inhibitors of

synthesized four macrocyclic compounds by condensing o-ethylene diamine

and o-phenylene diamine with ethylacetoacetate and succinic acid. Their

inhibiting action was evaluated on corrosion of mild steel in hydrochloric acid

by weight loss and potentiodynamic polarization methods (Rawat et al 1998).

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This literature survey shows that numerous synthesized organic

compounds were used as corrosion inhibitor in the acid environment. Very few

mannich bases was synthesized and used as the corrosion inhibitor for mild

steel in the acid environment.

1.12 Scope of the investigation

Mild steel is one of the well-known materials used in chemical and

allied industries for handling of acids, alkali and salt solutions due to its low

cost and easy availability for fabrication of reaction vessels, tanks and pipes

etc. But its susceptibility to corrosion in acid medium is the major obstacles

for its use on larger scale (S. Bilgic et al 2001).

Mineral acids are mainly used in chemical industry particularly in

pickling process to remove oxide scale from metal surface at elevated

temperature (H. P. Sachin, et al 2009). Almost all the industries find it difficult

to control corrosion of mild steel in mineral acid environment.

The use of organic inhibitors is one of the practical methods used to

control dissolution of metal in acid medium. The corrosion inhibition of these

compounds is attributed to their molecular structure. Adsorption of these

compounds over metallic surface is featured by its planarity and lone pair of

electrons present in hetero atom. The inhibition of corrosion process is mainly

decided by the formation of donor-acceptor surface complex between free or π

electrons of an inhibitor and vacant d orbital of metal atom (J.L Mora-

Mendoza et al 2002).

A survey of literature reveals that the selection of inhibitor is mostly

based upon the type and number of hetero atoms like N, O and S present in

organic compounds. The type and amount of hetero atoms present and the

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length of compound decides the inhibition efficiency. Many of the available

organic compounds were used as corrosion inhibitors for mild steel corrosion

(K.Parameshwari et al 2010). Some of the newly synthesized organic

compounds were used as corrosion inhibitor for mild steel corrosion (Athareh

dadgarinezhak et al 2011 and Nada F. Atta et al 2010) and also newly

synthesized mannich base was used as corrosion inhibitor for mild steel

corrosion (Duan Xiao-yun et al 2008).

The literature survey reveals that the most of available and

synthesized organic compounds inhibit mild steel corrosion at room

temperature only. It is very much essential to find out suitable inhibitor to

inhibit mild steel corrosion at elevated temperature.

In the present investigation three mannich bases are proposed to be

synthesized by using cyclohexylamine, formaldehyde and urea/acetamide/

acetone. Synthesis of mannich base has increase the length of parent

compound and hence hetero atoms present in the structure. The following

synthesized mannich bases are going to be tested as corrosion inhibitor to find

out the inhibition efficiency of inhibitors on mild steel corrosion in acid

medium at elevated temperature.

� Aminocyclohexane-N’-methylurea.

� Aminocyclohexane-N’-methylacetamide.

� Aminocyclohexane-N’-methylacetone.

The main objectives of the investigations are

1. Characterization of the synthesized mannich bases using FT-IR

and NMR techniques.

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2. To evaluate the inhibition efficiency of synthesized mannich

bases on mild steel corrosion in 1 N HCl and 1 N H2SO4

medium.

3. To study the adsorption isotherms of synthesized mannich bases

on mild steel corrosion in 1 N HCl and 1 N H2SO4 medium.

4. To study the activation and adsorption parameters of these

inhibitors in 1 N HCl and 1 N H2SO4 environment.

5. To study the morphology of the inhibited mild steel surface.

It is proposed to carry out the characterization of synthesized

mannich bases for structural elucidation. It is planned to carry out a study on

synthesized mannich bases for mild steel corrosion in 1 N HCl and 1 N H2SO4

environments by weight loss, potentiodynamic polarization and AC impedance

methods to evaluate the effectiveness of inhibitors. It is also planned to carry

out the surface examination studies by FT-IR spectral analysis and scanning

electron micrograph (SEM).