CAP2

1.0 INTRODUCTION.

1.1 CORROSION.

Vast range of constructional materials such as; metals and alloys, plastics, rubber,

ceramics, composites, wood, etc, serves as the backbone for modern day technological

processes. Since metals are involved in most cases, mechanical, physical and chemical

properties must be considered, and in this connection it should be observed that whereas

mechanical and physical properties can be expressed in terms of constants, the chemical

properties of a given metal are dependent entirely on the precise environmental

conditions prevailing during service (Shreir, 1994). The interaction of a metal or alloy (or

a non-metallic material) with its environment is clearly of vital importance in the

performance of materials of construction. While a metal or alloy may be selected largely

on the basis of its mechanical or physical properties, the fact is that there are very few

applications where the effect of the interaction of a metal with its environment can be

completely ignored, although the importance of this interaction will be of varying

significance depending on the situation (Shreir, 1994).

Corrosion (Latin Corrode – to gnaw away), is the gradual deterioration of a material

caused by the chemical (or) electrochemical reaction with its environment. Corrosion has

been defined as the undesirable deterioration of a metal or alloy, i.e. an interaction of the

metal with its environment that adversely affects those properties of the metal that are to

be preserved. This definition- referred to as the deterioration definition - is also

applicable to non-metallic materials such as glass, concrete, etc. and embodies the

concept that corrosion is always deleterious (Uhlig, 1948).

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The word Corrosion stands for material or metal deterioration or surface damage in an

aggressive environment. Corrosion is a chemical or electrochemical oxidation process, in

which the metal transfers electrons to the environment and undergoes a valence change

from zero to a positive value (Perez, 2004). The environment may be a liquid, gas or

hybrid soil-liquid. These environments are called electrolytes since they have their own

conductivity for electron transfer. Though the materials undergo deterioration in very

many ways, the term “corrosion” is presently restricted to surface degradation due to the

chemical attack. Corrosion occurs when protective mechanisms have been overlooked,

broken down, or have been exhausted, leaving the metal vulnerable to attack.

The corrosion of metals can be regarded in some ways as reverse extractive metallurgy.

Most metals exist in nature in the combined state, for example, as oxides, sulphides,

carbonates, or silicates. In these combined states the energies of the metals are lower. In

the metallic state the energies of metals are higher, and thus there is a spontaneous

tendency for metals to react chemically to form compounds. For example, iron oxides

exist commonly in nature and are reduced by thermal energy to iron, which is in a higher

energy state. There is, therefore, a tendency for the metallic iron to spontaneously return

to iron oxide by corroding (rusting) so that it can exist in a lower energy state.

The scope of the term ‘corrosion’ is continually being extended; Fontana and Staehle

(1990) have stated that corrosion will include the reaction of metals, glasses, ionic solids,

polymeric solids and composites with environments that embrace liquid metals, gases,

non-aqueous electrolytes and other non-aqueous solutions .

Vermilyea,(1962) defined corrosion as a process in which atoms or molecules are

removed one at a time, this includes evaporation of a metal into vacuum, since atomically

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it is similar to other corrosion processes. Evans, (1960) considers that corrosion may be

regarded as a branch of chemical thermodynamics or kinetics, as the outcome of electron

affinities of metals and non-metals, as short-circuited electrochemical cells, or as the

demolition of the crystal structure of a metal. These considerations lead to the conclusion

that there is probably a need for two definitions of corrosion, which depend upon the

approach adopted (Uhlig, 1971; Shreir, 1994):

1. Definition of corrosion in the context of Corrosion Science: the reaction of a solid with

its environment.

2. Definition of corrosion in the context of Corrosion Engineering: the reaction of an

engineering/constructional material with its environment with a consequent deterioration

in properties of the material.

1.2 Effect of corrosion

The effects of corrosion are many and varied. Effects on the safe, reliable and efficient

operation of equipments or structures are often more serious than the simple loss of a

mass of metal. Failures of various types and the consequent expensive replacements may

occur, even though the amount of metal destroyed is quite small. The first area of

significance of corrosion studies is the economical aspect including the objective of

reducing material losses resulting from the corrosion of pipes, tanks, metal components

of machines, ships, bridges, marine structures etc. The second is the safety of operating

equipments from resulting catastrophic consequences due to corrosion. Examples are

metallic containers for radioactive materials, turbine blades and rotors, pressure vessels,

boilers, bridge cables, aeroplane components, automotive steering mechanisms. The third

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dimension of corrosion studies is conservation of materials as applied primarily to metal

resources.

It is very well known that the supply of metals is limited and their wastage include

corresponding losses of energy and water reserves associated with the production and

fabrication of metal structures. The human efforts required for the design and repair of

corroded equipments can well be utilized for other socially useful purposes.

Currently, research in corrosion is primarily motivated by the economic factors. Losses

incurred by industry, military and municipalities amount to many billions of dollars

annually. Loss due to corrosion is approximately estimated to be 3-5% of the Gross

National Product (GNP) (Jones, 1996). 15% or more could be saved by the application of

existing technologies to mitigate corrosion. The existing technologies include the

methodologies like proper design, selection of materials, coatings and linings, cathodic

protection and inhibitors (Shreir, 1994).

Economic losses due to corrosion are classified as direct losses and indirect losses. The

direct losses are the cost of replacing the corroded structures and machines or their

components such as condenser tubes, mufflers, pipelines and metal roofing including

labour cost. Direct losses also include an extra cost of using corrosion resistance metals

and alloys instead of carbon steel where the latter does have adequate mechanical

properties but not sufficient corrosion resistance. The cost of galvanizing, nickel plating

of steel, use of corrosion inhibitors, or dehumidification of storage rooms for metal

equipments also accounts for direct losses.

Indirect losses, which are more difficult to assess, may either be economical or social.

These may include contamination of the product, loss of valuable product from a

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corroded container, damage of adjacent equipment, decreased safety and production (e.g.

sudden failure of equipment may cause fire, explosion or release of toxic products) and

change of appearance.

Obviously, indirect losses are a substantial part of the economic overhead imposed due to

corrosion, although it is difficult to arrive at an accurate estimate of total losses of an

industry. The indirect losses are in fact very difficult to assess in the event of loss of

health or life through explosions, unpredictable failure of chemical equipment or

wreckage of aeroplanes, trains or automobiles through sudden failure by corrosion of

critical parts. To sum up, it can be stated that corrosion is a potent force which incurs

economic loss, depletes resources and causes irreversible and untimely failures of

industrial plants, equipments and their components.

1.3. Corrosion and its mechanism

As a first approach to the principles which govern the behaviour of metals in specific

environments it is preferable for simplicity to disregard the detailed structure of the metal

and to consider corrosion as a heterogeneous chemical reaction which occurs at a

metal/non-metal interface and involves the metal itself as one of the reactants. Therefore,

corrosion can be expressed by electrochemical reaction comprising of both anodic and

cathodic reactions.

For mild steel, the anodic reaction involves the dissolution of the metal according to

equation (1.1)

Fe Fe2+ + 2e (1.1)

At the cathode, the electrons released are utilized in the reduction of species depending

on the nature of the environment. This could involve hydrogen reduction according to

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equation (1.2) in acidic environments, reduction of oxygen in near neutral or alkaline

environments according to equation (1.3), or reduction of sulphate ions in the absence of

oxygen following equation (1.4).

2H+ + 2e H2 (1.2)

O2 + 2H2O + 4e 4OH- (1.3)

4H2 + SO42- S2- + 4H2O (1.4)

The driving force for the corrosion is the thermodynamic instability of the metal in its

surrounding environment. The electron potential of a metal is a measure of its tendency

to corrode. The metals corrode faster when dissolution potential is negative. In the

absence of any surface active compound, the oxidation of iron is the primary anodic

reaction. The possibility for the oxidation of iron in aqueous solution can best be

demonstrated by various oxide formation reactions along with their electrode potential

values (E°).

Fe + 2OH- FeO + H2O + 2e- E° = -0.422V (1.5)

3Fe + 8OH- Fe3O4 + 4H2O + 8e- E° = -0.473V (1.6)

2Fe + 6OH- Fe2O3 + 3H2O + 6e- E° = -0.431V (1.7)

Fe + H2O FeO + 2H+ +2e- E° = -0.450V (1.8)

3Fe + 4H2O Fe3O4 + 8H+ + 8e- E° = -0.421V (1.9)

2Fe + 3H2O Fe2O3 + 6H+ + 6e- E° = -0.463V (1.10)

From the values of oxide formation potentials, it is seen that iron oxidizes readily in

water and the occurrence of the above reactions depends on the potentials established on

the electrode surface. In general the chemical stability of a metal exposed to a solution is

a function of the potential of the metal with respect to the solution, and the pH of the

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solution. The regions where the various species are stable can be plotted on a map of

potential against pH, know as an E-pH or Pourbaix diagram (Pourbaix, 1974).

There are three regions in the Pourbaix diagram:

1. When the metal is in the thermodynamically stable state, the metal is immune from

corrosion.

2. When the stable species is a soluble corrosion product, the metal is said to be active,

and will corrode relatively rapidly.

3. When the stable species is an insoluble corrosion product, then the metal is said to be

passive, and will corrode relatively slowly.

The oxide formation potentials are pH dependent and the relationship between the pH

and the potential is demonstrated by Pourbaix diagram.

Fig.1.1 illustrates the Pourbaix diagram for iron in water at 250C. It shows all possible

equilibra between various anions and cations of iron (or its compound) and water

together with their nature and activities. For example, parallel lines of curve (x)

represents the equilibrium between Fe(OH)3 and Fe2+ (aq) at various activities of the latter

ranging from 100 to 10-6. The arbitrary value of 10-6 has been selected and used to

classify various regions as show in Fig.1.1.

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Fig.1.1. The Pourbaix diagram for Fe-H2O system at 250C.

(Source: www.corrosion-doctors.org)

From the diagram, three different regions of interest exists, and are discussed as follows;

Immunity zone, where the metal is in equilibrium with Fe2+(aq) or FeO2H- (aq) at an

activity less than 10-6.

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Corrosion zone, in which the activity of Fe2+, Fe3+(aq) or FeO2-(aq) is greater than 10-6

Passivation zone: in this region Fe(OH)2, Fe3O4 or Fe(OH)3 are in equilibrium with metal

ions at an activity less than 10-6. However, due to the lack of kinetic information, the

Pourbaix diagram can not be used to predict the rate of metallic corrosion.

1.4. Forms of corrosion.

Corrosion can affect the metal in a variety of ways which depend on its nature and the

precise environmental conditions prevailing, leading to the various forms of corrosion

identified below;

1.4.1. Pitting Corrosion.

This form of corrosion is extremely localized and shows itself as holes on a metal

surface. The initial formation of pits is difficult to detect due to the small size, but it

requires a prolong time for visual detection. Pitting corrosion may occur due to

breakdown of a protective film (passive oxide film or organic coating). This form of

corrosion can be found on aluminum and its alloys and automobile chromium-plated

bumpers or body coated (painted) parts due to film breakdown at isolated surface sites.

Pits vary in shape, but are very small surface holes due to the extremely localized anodic

reaction sites (Schreir, 1979).

The appearance of pits on a metal surface is not very appealing, but they can be harmless

if perforation does not occur. The initiation of pits occurs at localized sites on a metal

surface defects, which may be due to coating failure, mechanical discontinuities or

microstructural phase heterogeneities such as secondary phases (Vereecken, 1994.).

Besides the prolong time needed for pit formation or its growth, it is assumed that many

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anodic and cathodic reactions take place at localized sites. Both rates of anodic and

cathodic reactions are slow; however, the reactions continue inward in the direction of

gravity in most cases. This suggests that the bottoms of pits are rich in metal ions due to

the large number of anodic reactions.

1.4.2. Crevice Corrosion.

Crevice corrosion is similar to pitting corrosion after its initiation stage in a stagnant

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

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

concentration [H+], and increase of chlorine Cl- ions. Oxygen depletion implies that

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

consequently, metal dissolution occurs. The problem of crevice corrosion can be

eliminated or reduced using proper sealants and protective coatings. The mechanism of

crevice corrosion is electrochemical in nature. It requires a prolong time to start the metal

oxidation process, but it may be accelerated afterwards (Fontana, 1986).

1.4.3. Stress-Corrosion Cracking

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

may prematurely fail at a stress below the yield strength. This phenomenon is known as

environmentally induced cracking (EIC), which is divided into the following categories:

stress-corrosion cracking (SCC), hydrogen-induced cracking (HIC) and corrosion-fatigue

cracking (CFC) (Vereecken, 1994). These three categories can develop under the

influence of an applied potential. It is known that SCC occurs under slow strain-rate

(SSR) ) only if the tensile strain rate or the applied potential is within a narrow and yet,

critical range; otherwise metals and alloys would appear to be immune to SCC either due

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to film repair at low strain rates or mechanical failure at high strain rates. The primary

characteristic of HIC is the brittle mechanical fracture caused by diffusion of atomic

hydrogen into the material because hydrogen is very small and has the capability of

migrating through the crystal lattice (Vereecken, 1994). The detrimental effect of atomic

hydrogen diffusion on mechanical properties is a continuous decrease in ductility (as well

as in strength). This particular mechanism is also known in the literature as hydrogen

embrittlement (HE) and it is a form of irreversible hydrogen damage. If hydrogen atoms

within the lattice defects, such as voids, react to form molecular hydrogen, and these

molecules form babbles, then the metallurgical damage is called blistering.

1.4.4. Underground Corrosion.

Factors affecting the corrosivity of a given soil include porosity, electrical conductivity,

dissolved salts, moisture, acidity and alkalinity. In poor aerated soils, an observed

corrosion is in the form of pitting. Localized corrosion of this kind is obviously more

deleterious to a pipeline than a higher overall corrosion rate occurring more uniformly

(Mikhailovskii, et al., 1997).

1.5. Factors influencing corrosion

Corrosion rate and extent depend mainly on the environment and nature of the metal.

1.5.1. Nature of the Environment

The nature of the environment affects the rate of corrosion due to various reasons. Such

factors like pH of the solution, temperature, velocity and concentration of solution play a

vital role.

(a) Concentration of Solution

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Corrosion rate is not linear with concentration even at a wide range. In equipments such

as distillation column, reactors and evaporators, concentration changes continuously,

making prediction of corrosion rate rather difficult.

(b) pH of the solution

Metals such as iron are soluble in acid. Rate of corrosion in the middle pH range is

controlled by the rate of transport of an oxidizer (usually dissolve O2) to metal surface.

Noble metals such as gold, platinum are not affected by pH changes. Oxidizing that

accelerate the corrosion of some materials may also retard corrosion of others through

deformation of their oxides on the surface.

(c) Velocity

An increase in the velocity of relative movement between a corrosive solution and

metallic surface frequently tends to accelerate corrosion. This type of corrosion occurs

frequently in smaller diameter tubes or pipes through which corrosive liquid may be

circulated at high velocities (e.g. heat exchangers and evaporator tubes) (Schreir, 1979).

(d) Temperature

The rate of corrosion increases with an increase in temperature. Suitability of a metal for

high temperature service is dependent upon the properties inherent in the metals

composition and the condition of their application. A large number of metal failures

occur due to the development of excessive thermal stress at high temperatures.

1.5.2. Nature of the metal

The extent of corrosion depends upon the position of the metal in the galvanic series.

When two metals are in electrical contact in the presence of electrolyte the metal higher

up in the galvanic series becomes anodic and suffers corrosion. Greater the difference

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between the two metals in the galvanic series, faster will be the corrosion of the anodic

metal. Further, the reduction in the over voltage also plays an important role in

accelerating corrosion. The relative areas of the anode and cathode also contribute to the

corrosion.

Corrosion rate is also influenced by the orientation of the crystal at the metal surface and

by the size of the grains of the metal / alloys. The presence of impurities in the metal also

influences the rate of corrosion (Roberge, 1984).

The solubility of the corrosion product formed is an important factor in electrochemical

corrosion. If the corrosion product is soluble, corrosion rate will be higher. If the

corrosion leads to the formation of insoluble product, then a protective film may be found

over the metal/alloy which may suppress corrosion (Roberge). The nature of the oxide

film formed influences the rate of corrosion. If the oxide film formed is porous, the

diffusion of oxygen can easily occur and consequently corrosion rate is enhanced. On the

other hand, aluminium and heavy metals form impervious oxide film which protects the

metal from further oxidation. Only fission or a crack in this film can trigger corrosion.

1.5.3. Biological effects.

Macro and microscopic organisms influence corrosion in two principal forms.

(i.) By creating obstructions on the surface of the metal, thereby producing differential

aeration cells, or

(ii). By adsorbing hydrogen from the metal surface and thus eliminating the hydrogen as

a resistance factor in the corrosion cell. Most sulphate reducing bacteria operate in this

form, creating sulphuric acid in the vicinity of the cathodic areas of the metal,

consequently accelerating corrosion (Brasunas, 1984).

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1.6. Corrosion Monitoring

Corrosion measurement involves the application of various techniques to determine the

corrosiveness of the environment and the rate of metal loss. Corrosion measurement is

usually a quantitative method of evaluating the performance and effectiveness of

corrosion control and prevention techniques. After evaluation is completed, the result

obtained is used to improve and enhance the techniques.

The methods for corrosion measurement can be broadly divided into four categories.

They are [Roberge, 1984]

1. Weight loss methods, where the corrosion is directly measured by the loss in weight of

a sample. This is appropriate for all systems but may require a long exposure time if the

corrosion rate is low (Roberge, 1984).

2. Electrochemical corrosion rate methods such as linear polarization, ac impedance and

electrochemical noise. These require a conducting solution and so are not appropriate for

gases or for non-aqueous systems, such as almost dry hydrocarbons (Wiston, 2000).

3. Electrical resistance measurement, where the loss of metal is followed by the increase

in electrical resistance as the metal films. This is appropriate for all systems.

4. Fluid analysis, for example the measurement of the increase in dissolved iron levels as

a fluid passes through a pipeline (Wiston, 2000).

It is important that the monitoring is representative, so location of corrosion probes

should reflect the worst expected corrosion, for example at high flow rates or at areas

where water may separate.

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Similar methods can be used for testing in the laboratory. Here it is important to

reproduce the actual exposure conditions as far as possible.

1.7. Corrosion Prevention

The corrosion process can be retarded by using different methods as the following:

i- Methods based on modification of the environment:

a- By de-aeration or adjusting the pH of the environment.

b- By de-humidification of air.

c- By addition of inhibitors.

ii- Methods based on modification of the metal:

a- By addition of alloying elements.

b- By heat treatment

iii- Methods based on protective coating:

a- Coating by a reaction product (chemical or electrochemical treatment of metal

surface).

b- Organic coating (paints, resins).

c- Inorganic coating (enamels, cement).

d- Metal coating.

e- Temporary protective.

1.7.1.1. Metal Coatings

Coating a substrate metal with a metallic material may be carried out to provide

decorative effect or prevent corrosion. In both cases the corrosion behaviour of the

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substrate/coating system must be considered as a whole. Ideally, a protective coating will

provide a complete barrier and exclude the corrosion environment from the substrate

metal which it is desired to protect; however, no coating is ever perfect. In reality all

coatings contain discontinuities which will lead to contact between the substrate and

corrosion environment. These discontinuities may occur during application or

subsequently due to mechanical damage. Consideration of the corrosion behaviour of the

substrate, the coating and the bimetallic system produced is therefore required.

Metallic coatings can be classified as either anodic or cathodic to the substrate depending

on their position in the electrochemical series and the interaction between the two metals

in the specific environment.

Anodic coatings: Anodic coating is used to describe a coating material, which has a more

negative electrochemical potential than the substrate. In this situation, it is possible for

the coating to sacrificially protect the substrate corroding preferentially at any location

where the two metals are in contact within an electrolyte. Corrosion of the more anodic

metal provides electrons to support the cathodic reaction at the cathodic material hence,

the anodic reaction for the steel, i.e. dissolution, will be reduced or may even be

completely stopped. This type of corrosion protection is referred to as cathodic protection

or sacrificial protection. Zinc coatings on steel (galvanizing) are examples of anodic

coatings. At breaks or pores in the coating, or at cut edges, the coating corrodes

preferentially and provides sacrificial protection (galvanic protection) to the underlying

steel substrate.

Cathodic coatings: They behave in the opposite way to anodic coatings in that the coating

is more electropositive than the substrate. Nickel or copper coatings on steel are examples

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of cathodic coatings. These coatings can provide excellent corrosion protection.

However, an outstanding limitation of such coatings is that they must provide a complete

barrier to the substrate from the environment. If the substrate is exposed to the corrosion

environment through breaks, pores or cut edges, the underlying steel will become the

anode and corrosion will be dramatically accelerated because the anodes is small while

the cathode area is much larger, severe pitting in the substrate may result causing the

coating to flake away from the substrate.

1.7.1.2. Materials Selection and design against corrosion

Most corrosion resistant materials are protected by a passive corrosion product film.

Common corrosion resistant materials are:

• Stainless steel: these are iron-based alloys with the passive film being formed by

chromium. Several classes of a stainless steel are available: ferritic steels are basically

iron-chromium alloys, with about 13% Cr; austenitic steels contain nickel to stabilize the

austenitic structure, a common composition is 18% Cr, 8% Ni; martensitic steels can be

hardened by precipitation of iron carbide in the same way as carbon steels.

• Aluminium and its alloys are protected by a film of alumina, Al2O3, which is very

insoluble and protective. Many of the alloys are, however, susceptible to pitting

associated with copper or iron containing precipitates.

• Nickel base alloys are similar to stainless steels, but tend to be more corrosion resistant

because nickel is a somewhat more noble metal (it is also much more expensive).

• Titanium is protected by a passive film of TiO2. This is one of the most resistant alloys

to localized corrosion, and it is also relatively light but strong. It is relatively expensive.

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Design features can have an important influence on corrosion behaviour. Favorable

designs should encompass the following considerations:

The ideal design will have smooth surfaces, with no crevices or regions where liquid will

accumulate, or from which liquid cannot be drained.

It will not normally use mixtures of different metals (in order to avoid galvanic

corrosion), and

It will avoid highly stressed regions (to prevent stress corrosion cracking).

1.7.1.3. Environment Modification and Inhibitors.

The simplest modification to the environment is the removal of oxygen. In neutral

solution this reduces the rate of corrosion of iron and steel to a very low value. Increasing

the pH to above about 10 will also reduce the corrosion rate of iron and steel, as the iron

will passivate. If these methods are not applicable, then chemicals may be added to the

environment to interfere with the corrosion process, usually by forming a film of some

kind. These chemicals called corrosion inhibitors are substances which, when added in

small quantities to a normally corrosive environment, reduces the corrosion rate of the

metal, without significantly changing the concentration of corrosive species.

Broadly, there are only two ways in which inhibitors can exert their action:

• By forming an adsorbed layer on the metal surface, or,

• By forming a three-dimensional film, either a passive film or a precipitated film.

1.7.2. TYPES OF INHIBITORS.

Numerous classes of inhibitors exist. However, to narrow this down, classification shall be done

by considering certain criteria.

1.7.2.1. Classification based on electrode process

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1.7.2.1.1. Anodic inhibitors.

They form or facilitate the formation of passivating films that inhibits the anodic metal

dissolution reaction. Anodic inhibitors are usually used in near-neutral solutions where sparingly

soluble corrosion products, such as oxides, hydroxides, or salt are formed.

Addition of these inhibitors causes the corrosion potential to shift in a cathodic direction (more

positive potential). Anodic inhibitors are often called passivating inhibitors when they cause large

shifts in the corrosion potential. Passivation may be oxidizing, or nonoxidizing. The oxidizing

anions such as chromate, nitrate, etc do not require the presence of oxygen, while the

nonoxidizing anions like tungstate and phosphate, (Shibli and Saji, 2005) do not require the

presence of oxygen.

When the concentration of an anodic inhibitor is not sufficient, corrosion may be accelerated,

rather than inhibited, hence they are called dangerous inhibitors. The critical concentration above

which inhibitors are effective depends on the nature and concentration of the aggressive ions.

1.7.2.1.2. Cathodic inhibitors.

Cathodic inhibitors either slow the cathodic reaction itself, or they selectively precipitate on

cathodic areas to increase circuit resistance and restrict diffusion of reducible species towards the

cathode (Ref.). Here, cations migrate towards the cathode surfaces where they are precipitated

chemically or electrochemically and thus block these surfaces. Action of As3+ and Sb3+ on

dissolution of Fe in acids is an example.

Some cathodic inhibitors make the discharge of hydrogen gas more difficult, and they are said to

increase the hydrogen overvoltage. Other cathodic inhibitors utilize the increase in alkalinity at

cathodic sites to precipitate insoluble compounds on the metal surface. The cathodic reaction,

hydrogen ion and/or oxygen reduction causes the environment immediately adjacent to the

cathodes to become alkaline; therefore, ions such as calcium or zinc may be precipitated as oxides

to form a protective layer on the metal (Speller, 1951).

1.7.2.1.3. Mixed inhibitors.

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About 80% of inhibitors that cannot be designated specially as anodic or cathodic and are known

as mixed inhibitors. Such type of inhibitors retards both the anodic and cathodic processes. The

shift in 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 in that they

control both the cathodic and anodic corrosion reactions and hence are very safe to apply.

1.7.2.1.4. Vapour phase inhibitors.

Temporary protection against atmospheric corrosion, particularly in close environments can be

achieved using vapour-phase inhibitors (VPI). Substances having low significant pressure of

vapour with inhibiting properties are effective. The vapour-phase inhibitors are used by

impregnating wrapping paper or by placing them loosely inside a closed container slow

vaporization of the inhibitor protects against air and moisture. Vapour phase inhibitors (VPI), are

also called volatile corrosion inhibitors (VCI).

The VCIs contain anticorrosive chemicals, which extend their corrosion inhibiting properties to a

metal surface by volatilization within an enclosed space. In spite of the fact that VCIs have been

used for a long time for inhibiting atmospheric corrosion, the action mechanism of these

compounds is not completely clear. It is believed that the major parameters responsible for their

efficiency are vapor pressures and the interactions with the metal surface. In general, when

mechanisms are discussed as to how VCIs function to prevent corrosion, very simplified

explanations are often presented. These explanations contain the assumption that the outer surface

of a metal is composed of a well-behaved metal oxide of fairly uniform thickness (Da-quan

Zhang et al., 2006). The VCI then attaches itself to oxide through weak chemical bonding and

forms an adsorbed monolayer to shield this interface from penetration by corrosive agents such as

water, erosive ions (Cl- or SO42-), etc. The adsorbed monolayer may change the rate of

electrochemical reactions like the dissolution of metal or the reduction of oxygen.

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Desirably, it is necessary for a VPI to provide inhibition rapidly and to have also a lasting effect.

Hence, the compound should have a high volatility to saturate all of the accessible vapour space

as quickly as possible, but at the same time, it should not be too volatile, because it would be lost

rapidly through any leaks in the package or container in which it is used. The optimum vapour

pressure of a VPI should then be just sufficient to maintain an inhibiting concentration on all

exposed metal surface.

1.7.2.1.5. .Inorganic inhibitors (Wiston, 2000),

The common inorganic inhibitors used are crystalline salts, for instance, sodium

chromate and molybdate. The only active groups of these compounds that functions to

reduce corrosion are the negative anions they carry.

1.7.2.1.6. Organic inhibitors

Organic corrosion inhibitor can be anodic, cathodic and mixed type depending on its size, carbon

chain length, aromaticity, conjugation and nature of bonding atoms.

The effectiveness of organic inhibitors is related to the extent to which they adsorb and cover the

metal surface. Adsorption depends on the structure of the inhibitor, surface charge of the metal

and the type of electrolyte. Mixed inhibitors protect the metal in three possible ways viz.,

physical adsorption, chemisoption and film formation. Physical (or electrolyte) adsorption is a

result of electrostatic attraction between the inhibitor and the metal surface. When the metal

surface is positively charged, adsorption of negatively charged (anionic) inhibitors is facilitated.

Positively charged molecules acting in combination with a negatively charged intermediate can

inhibit a positively charged metal. Anions, such as halide ions, in solution adsorb on the

positively charged metal surface, and organic cations subsequently adsorb on the dipole.

Corrosion of iron in sulphuric acid containing chloride ions is inhibited by quaternary ammonium

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cations through this synergistic effect. Physically adsorb inhibitors interact rapidly, but they are

also easily removed from the surface. Increase in temperature generally facilitates desorption of

physically adsorbed inhibitor molecules.

Most effective inhibitors are those that chemically adsorb (chemisorbs), a process that involves

charge sharing or charge transfer between the inhibitor molecules and metal surface.

Chemisorptions take place more slowly than physical adsorption. As temperature increases,

adsorption and inhibition also increase. Chemisorption is specific and is not completely

reversible. Adsorbed inhibitor molecules may undergo surface reactions, producing polymeric

films. Corrosion protection increases markedly as the films grow from nearly two-dimensional

adsorbed layers to three-dimensional films up to several hundred angstroms thick and inhibition

is effective only when the films are adherent, and insoluble, preventing the excess of the solution

to the metal.

1.7.2.1.7 ‘Green’ Corrosion Inhibitors.

Inorganic compounds including chromates, dichromates, phosphates, arsenates, phosphates,

nitrates, nitrites, and sulphides of alkali metals have been in use widely as corrosion inhibitors.

However, due to strict environmental regulations, their continued use has faced relentless

condemnation. Consequently, organic compounds, which function as adsorption inhibitors are

now replacing the inorganic corrosion inhibitors.

In line with this, large numbers of organic compounds, principally those containing

heteroatoms like oxygen, nitrogen or sulphur groups in a conjugated system are now

known to serve as corrosion inhibitors for different metals in various aggressive media.

22

1.8. Objective of Study

It is obvious that plant extracts show corrosion inhibition potential in different

aggressive media. Plant materials are the richest source of heterocyclic compounds and

high molecular aromatic contents, and adsorption of these compounds play a key role in

corrosion inhibition of metals (Zucchi and Omar, 1986; Khamis, et al., 2005; Oguzie,

2008b).

Recent development in the area of corrosion control has been on the screening of various

plant extracts for anticorrosive properties. But there does not seem to be any approach for

the detection of active ingredients responsible for the corrosion inhibition effect.

Pertinently, study of the corrosion inhibition of mild steel in acid medium in presence of

green inhibitors is undertaken in this study.

The present work is geared towards the determination of anticorrosion effect of the

following plants which have been chosen based on the literature survey.

a) Moringa oliferia

b) Mimosa pudica

c) Dacryodes edulis

d) Euphorbia hirta.

This study includes,

a) Weight loss measurement at different temperatures and calculation of various

kinetic parameters.

b) Potentiostatic polarization at room temperature and calculation of corrosion

current density and corrosion potential.

23

c) Impedance measurements at room temperature and calculation of series

resistance, charge transfer resistance and double layer capacitance.

d) Surface characterization by scanning electron microscope (SEM) to get the

quantitative analysis of the absorbed layers of inhibition.

e) Quantum chemical calculations in order to get useful quantitative and semi-quantitative

information.

CHAPTER TWO: LITERATURE REVIEW.

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 (Oguzie, 2005a). Many nitro and nitroso compounds have been tried as corrosion

inhibitors. Certain azo compounds such as methyl orange and methyl red have been found to be

effective inhibitors (ref). The inhibitive capabilities of some organic dyes namely; Safranine-O,

Thymol blue and Fluorescein-Na on the electrochemical corrosion of mild steel in sulphuric acid

solution using the gasometric technique have been studied, (Oguzie and Ebenso, 2005). The

inhibitory properties of indigo dye during corrosion of mild steel in aerated sulphuric acid

solutions at 30–50 °C have been assessed (Oguzie, et al., 2004). Also, (Oguzie, et al., 2005),

investigated the inhibitory mechanism of methylene blue dye on the corrosion of mild steel in 2

M H2SO4 solutions. (Oguzie, 2005b), again investigated the inhibitory mechanism of methylene

blue dye on the corrosion of mild steel in hydrochloric acid solution using the gravimetric

method. Tang and co-workers (2003) researched on the corrosion inhibiting effect of neutral red

on the corrosion of cold rolled steel in 1.0 M hydrochloric acid (HCl) using weight loss method

and potentiodynamic polarization technique.

24

Congo red dye has been assessed for its inhibitive properties on mild steel corrosion in sulphuric

acid solution by Oguzie, (Oguzie, 2004).

The corrosion behaviour of AA2024-T3 pre-treated with bis-[triethoxysilylpropyl]tetrasulphide

have been studied by Cabral and co-workers (Cabral et al., 2005). Imidazolines and amidic

precursors were evaluated as corrosion inhibitors in an aqueous solution of 1.0 M HCl (Olivares-

Xometl et al., 2006). The adsorption behavior of dodecylamine on a copper–nickel alloy in 0.2M

NaCl solutions was studied using electrochemical methods and atomic force microscopy (Qu et

al., 2005). Inhibitive action of a non-ionic surfactant of tween-40 on the corrosion of cold rolled

steel in 0.5–7.0 M sulphuric acid was studied by Xianghong and Guannan, (Xianghong and

Guannan, 2005). Abdallah et al (2006) has studied the effect of some aminopyrimidine

derivatives on the corrosion of 1018 carbon steel in 0.05 M HNO3 solution (Abdallah et al.,

2006). Inhibiting action of 2,2I-Dithiobis(3-cyano-4,6-dimethylpyridine) on the corrosion of mild

steel in 1–5 M H2SO4 solutions at 35–50 0C was investigated by Morad and Kamal, (2006).

Corrosion inhibition of steel in hydrochloric acid by decylamides of α-amino acids derivatives

was studied using gravimetric and electrochemical techniques (Olivares et al., 2006). Benzyl

triphenyl phosphonium bromide has been evaluated as a corrosion inhibitor for mild steel in

aerated 0.5 M sulfuric acid solution (Bhrara and Singh, 2006). Abdallah (2003) studied the

inhibitive effect of some substituted pyrazolones on the corrosion of 304stainless steel in

sulphuric acid. The inhibition effect of halide ions such as iodide, bromide and chloride ions on

the corrosion of iron in 0.5 mol L-1 H2SO4 and the adsorption behaviour of these ions on the

electrode surface have been studied (Venkatachari, et al., 2006). The influence of pH and

concentration of potassium ethyl xanthate (KEtX) on the spontaneous dissolution of copper in

acidic chloride solutions containing oxygen has been studied ( Scendo, 2005). Studies on the

corrosion inhibition of iron in 0.5 M H2SO4 solutions by alkyl quaternary ammonium halides

inhibitors have been carried out (Lin Niu, et al., 2005). Reports on the corrosion inhibition of

mild steel in 0.5 M H2SO4 in the temperature range 30–60 °C using sodium naphthalene

25

disulphonic acid (NDSA) as an inhibitor has been documented (Bayoumi and Ghanem, 2005).

The inhibiting efficiency of non-toxic imidazole derivatives on copper corrosion in sulphuric acid

have been investigated (Stupnisˇek-Lisac, et al., 2002). 1, 12-bis (1, 2, 4-triazolyl) dodecane

(dTC12) have been reported to be an excellent corrosion inhibitor for carbon steel in deaerated 1

M HCl solution (Ait Chikh, et al., 2005). The effect of 2,5-bis(4-pyridyl)-1,3,4-thiadiazole (4-

PTH) on the corrosion of mild steel in acidic media (1 M HCl, 0.5 M H 2SO4, 1M HClO4) has

been investigated (Lebrini, et al., 2006). Eight diazoles namely, Benzimidazole, 2-

Methylbenzimidazole, 2-Hydroxymethylbenzimidazole, 2-Aminobenzimidazole, 2-

Mercaptobenzimidazole, 5(6)-Nitrobenzimidazole, 5(6)-Carboxybenzimidazole and 2-

Benzimidazolylacetonitrile, have been investigated as corrosion inhibitors of mild steel in 1 M

hydrochloric acid (Popova, et al., 2004). (Bentiss, et al., 2006) evaluated the inhibitive effect of

the new pyridazine derivative, namely 1,4-bis(2-pyridyl)-5H-pyridazino[4,5-b]indole (PPI)

against mild steel corrosion in 1 M HCl solutions. Adsorption and corrosion inhibitive properties

of three different organic molecules: 2- naphthalenesulfonic acid, 2,7-naphthalenedisulfonic acid

and 2-naphthol-3,6-disulfonic acid were investigated on Armco-iron electrode cathodically

polarized, in 0.5mol dm-3 H2SO4 solution (Vracar and Drazie, 2002). (Frignani, et al., 2005),

investigated the influence exerted by phenylthiourea (PTU) towards the corrosion of Fe79B16Si5

metallic glass in deaerated 0.1 N H2SO4. The effect of propargylic alcohol on the corrosion

inhibition of low alloy steel in sulphuric acid has been studied (Gojic, 2001). Several new

isoxazolidines namely, 1-methylamino-3-pentadecanol, 2-tert-butyl-5-dodecylisoxazolidine, 2-

isopropyl-5-dodecylisoxazolidine, 2-methyl-5-dodecylisoxazolidine, 2-methyl-5-

hexylisoxazolidine, having varying degree of steric environment and hydrophobic chain length,

were tested for corrosion inhibition of mild steel in 1M and 5M HCl at 50–70 °C (Ali, et al.,

2005). (Harek and Larabi, 2004) studied the Corrosion Inhibition of Mild Steel in 1 mol dm–3 HCl

by Oxalic N-Phenylhydrazide N'-Phenylthiosemicarbazide. Also, N-Phenyl oxalic dihydrazide

(PODH) and oxalic N-phenylhydrazide N'-phenylthiosemicarbazide (OPHPT), were tested as

26

inhibitors for the corrosion of mild steel in molar HCl (Larabi, et al., 2005). The inhibition

effect of some PEGs on carbon steel corrosion at 25oC in 0.5 N HCl as corroding solution was

evaluated (Ashassi-Sorkhabi, et al., 2006). 2-amino-5-mercapto-1,3,4-thiadiazole and 2-methyl-

5-mercapto-1,3,4-thiadiazole have been reported as copper corrosion inhibitors in neutral chloride

environments (Orlin and Annick, 2004). The performance of poly(diphenylamine) as corrosion

inhibitor for iron in 0.5 M H2SO4 has also been evaluated (Jeyaprabha, et al., 2005). Zinc

corrosion in sulphuric acid containing different m-substituted aniline-N-salicylidenes has been

studied with respect to inhibitor and acid concentration, period of exposure and temperature

(Talati, et al., 2005). Five heterocyclic compounds, having a five atom ring, fused with the

benzene ring (indole, benzimidazole, benzotriazole, benzothiazole and benzothiadiazole) were

investigated as corrosion inhibitors of mild steel in 1 N HCl (Popova and Christov, 2006) using

impedance and polarisation resistance methods. (Emregul, et al., 2006) has studied the effect of

three Schiff base compounds namely, (E)-2-(1-(2-(2-hydroxyethylamino)ethylimino)

ethyl)phenol, 2,2'-(1E,1'E)-1,1'-(2,2'-azanediylbis(ethane-2,1-diyl)bis(azan-1-yl-1-ylidene))

bis(ethan-1-yl-1-ylidene)diphenol and 2,2'-((2E,12E)-3,6,9,12-tetraazatetradeca-2,12-diene-2,13-

diyl)diphenol on the corrosion behavior of steel in 2 M HCl at 298 K by weight loss

measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS)

methods. The effectiveness of the cationic surfactant; 1-dodecyl-4-methoxy pyridinium bromide

as corrosion inhibitor for mild steel in 2M HCl solution has been reported (Migahed, 2005). The

effects of methylthiourea (MTU), phenylthiourea (PTU) and thiobenzamide (TBA) on the

corrosion of mild steel in 0.1 M H2SO4 solution at temperatures of 30 ◦C, 40 ◦C and 50 ◦C have

been studied (Dehri and Ozcan, 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 , N'-(4-dimethylamino-benzylidene)-hydrazine

carbodithioic acid and N'-(3-nitro-benzylidene)-hydrazine carbodithioic have been studied

(Khaled, 2006) using chemical and electrochemical (potentiodynamic and electrochemical

27

impedance spectroscopy, EIS) measurements. (Mounim, et al., 2005) reported on the inhibiting

effects of some oxadiazole derivatives on the corrosion of mild steel in perchloric acid solution.

The inhibitor 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 mol dm−3 HCl

medium on mild steel has been investigated at 303 K (Emregul, et al., 2005). The anti-corrosive

properties of 2-aminomethylbenzimidazole (AMB) and bis(benzimidazol-2-ylethyl)sulfide

(BBES) has been analyzed using electrochemical techniques like polarization curves and

electrochemical impedance spectroscopy (EIS) (Cruz, et al., 2005). The results showed that AMB

behaved as a cathodic inhibitor, while BBES functioned as mixture inhibitor. The effect of some

mercapto-triazole derivatives on the corrosion and hydrogen permeation of mild steel in 1.0 M

HCl has been investigated (Hui-Long, et al., 2004). (Giacomelli, et al., 2004) studied the inhibitor

effect of succinic acid on the corrosion resistance of mild steel in sulfuric acid solutions using

potentiodynamic polarization, electrochemical impedance spectroscopy, weight loss and optical

microscopic analysis. Three macrocyclic compounds, namely 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 (MTAT), 3,4,9,10-tetramethyl-

7,12-dithia-1,2,5,6,8,11-hexaazacyclododecane-2,4,7,8,10,12-hexaene (MTAD), 3,4,9,10-

tetramethyl- 7,12-dioxa-1,2,5,6,8,11-hexaazacyclododecane-2,4,7,8,10,12-hexaene(MOAD)were

synthesized and studied for their corrosion inhibitive effect in 5N HCl by weight loss and

potentiodynamic polarization studies (Quraishi and Rawat, 2002). (Azhar, et al., 2001) studied

the inhibitive effect of [2,5-bis(n-pyridyl)-1,3,4-thiadiazoles]on mild steel in 1 M HCl and 0.5 M

H2SO4. Studies on the influence of [(2-pyridin-4-ylethyl)thio]acetic acid (P1) and pyridine (P2)

on the corrosion inhibition of steel in sulphuric acid solution has been reported (Bouklah, 2005).

The effect of benzimidazole derivatives on mild steel corrosion in 1 M HCl at five different

temperatures has been studied (Popova, et al., 2003). The influence of l-ascorbic acid (AA) on

mild steel corrosion in pH = 2–6 solutions has been investigated using electrochemical and

weight loss techniques (Ferreira, et al., 2004). Electrochemical measurements have been

28

performed to investigate the effectiveness of cationic surfactants of the N-alkyl quaternary

ammonium salt type, namely, myristyltrimethylammonium chloride (MTACl),

cetyldimethylbenzylammonium chloride (CDBACl), and trioctylmethylammonium chloride

(TOMACl), as corrosion inhibitors for type X4Cr13 ferritic stainless steel in 2MH2SO4 solution

(Fuchs-Godec, 2006). (Khaled, 2003) studied the inhibitive action of some benzimidazole

derivatives namely 2-aminobenzimidazole (AB), 2-(2-pyridyl)benzimidazole (PB), 2-

aminomethylbenzimidazole (MB), 2-hydroxybenzimidazole (HB) and benzimidazole (B), against

the corrosion of iron (99.9999%) in solutions of hydrochloric acid using potentiodynamic

polarization and electrochemical impedance spectroscopy (EIS). 1,2,4-triazole, 3-amino-1,2,4-

triazole, benzotriazole and 2-mercaptobenzothiazole were evaluated as corrosion inhibitors for

protection of the 2024 aluminium alloy in neutral chloride solutions (Zheludkevich, et al., 2005).

The inhibition effect of three amino acids against steel corrosion in HCl solutions has been

investigated by potentiodynamic polarization method (Ashassi-Sorkhabi, et al., 2004).

Tryptophan has been tested as a copper corrosion inhibitor in 0.5 M aerated sulfuric acid in the

20–50 °C temperature range (Moretti and Guidi, 2002). (Larabi, et al., 2006), investigated the

inhibition of corrosion of copper in hydrochloric acid by 2-mercapto-1-methylimidazole by dc

polarization, ac impedance and weight loss techniques. The co-inhibition characteristic of sodium

tungstate has been evaluated along with potassium iodate on mild steel corrosion (Shibli and Saji,

2005). The inhibitive effect of thiourea (TU) on the corrosion behaviors of bulk nanocrystallized

and coarse grain industrial pure iron (BNIPI & CGIPI) were investigated in 1 mol L -1 HCl at

room temperature by means of electrochemical impedance spectroscopy (EIS) and

potentiodynamic polarization curve (Shen, et al., 2006). Reports on the study of the conditions

and mechanism of copper corrosion inhibition by linear sodium heptanoate (Rocca, et al., 2001),

with a formula of CH3(CH2)5COONa has been documented. The inhibition effect of some amino

acids towards the corrosion of aluminum in 1 M HCl + 1 M H2SO4 solution was investigated

(Ashassi-Sorkhabi, et al., 2005), using weight loss measurement, linear polarization and SEM

29

techniques. The corrosion inhibition of Cu–Ni alloys was investigated in aqueous chloride

solutions using amino acids (Badawy, et al., 2006). The effect of cysteine (cys) on the anodic

dissolution of copper in sulfuric acid media has been studied at room temperature using

electrochemical methods (Matos, et al., 2004).

Electrochemical investigation of ascorbic acid adsorption on low-carbon steel in 0.50 M Na2SO4

solutions have been carried out (Goncalves and Mello, 2001). It was observed that the incidence

of monochromatic light may affect the electrochemical behavior of the electrode in this medium

as well as the interaction between the electrode and ascorbic acid. The action of light was

attributed to the complex interaction of several photosensitive phenomena. The effect of some

thiophene derivatives on the electrochemical behavior of AISI 316 austenitic stainless steel in

acidic solutions containing chloride ions has been investigated (Galal,et al., 2005). Evidence of

the formation of the surface layer was confirmed from surface reflectance infrared spectroscopy

measurements. The elemental composition of the surface as indicated from energy dispersive X-

ray analysis and X-ray photoelectron spectroscopy (XPS) proved that the application of the

inhibitor increases the concentration of Cr at the surface of the specimen. The efficiency of 2-

amino-5-mercapto-1,3,4-thiadiazole and 2-methyl-5-mercapto-1,3,4-thiadiazole as copper

corrosion inhibitors in neutral chloride environments has been investigated (Blajiev and Hubin,

2004). The study was carried out by means of a density-functional quantum-chemical approach

and by impedance spectroscopy. Obtained estimates of the total dc path resistance showed that

the studied thiadiazole derivatives were excellent copper corrosion inhibitors. The molecular

behavior of some aniline derivatives as corrosion inhibitors of iron and copper in acidic media

(HCl and H2SO4) were investigated quantum-electrochemically by some recently developed

models based on density functional theory and cluster/ polarized continuum approaches. The

adsorption of aniline molecules on iron seemed to have chemical properties, while that on copper

has a more physical nature. Moreover, in both cases, adsorptions were almost parallel with the

corroding surfaces and took place through an sp3 nitrogen lone pair (Lashgari, et al., 2007).

30

Fundamental study of corrosion inhibition behavior of selected organic anions on low-carbon

steel in saturated Ca(OH)2 solution containing chlorides have been carried out. The compounds

studied include eight amino acids, two carboxylic acids, ascorbic acid and a mimosa tannin

extract (Martinez, et al., 2007). All the inhibitors suppressed the reduction of O2, a process that is

known to be strongly catalyzed on passive iron. The highest degree of inhibition was observed for

ascorbic acid. Corrosion Inhibition of Aluminum Alloy 2024-T3 by Aqueous Vanadium Species

has been studied (Ralston, et al., 2008). Potentiodynamic polarization measurements were carried

out on Al alloy 2024-T3 in 50 mM NaCl solutions in which pH and vanadate concentration were

systematically varied. Results showed that inhibition by vanadates occurred mainly in alkaline

solutions where tetrahedrally coordinated vanadates, metavanadate and pyrovanadate, were

abundant. Inhibition was not observed in solutions where octahedrally coordinated decavanadates

predominated. Effects of 1,5-naphthalenediol (ND) on corrosion inhibition of aluminum have

been investigated in 0.50 M NaCl using potentiodynamic polarization, potentiostatic current-time,

air-exposed potential, and electrochemical impedance spectroscopy measurements. The study was

also complemented by the quartz crystal analysis (QCA), cyclic voltammetery, scanning electron

microscopy (SEM), energy dispersive X-ray analysis (EDX), and Fourier transform infrared

(FTIR) spectroscopy. The addition of ND to the electrolyte decreased both general and pitting

corrosion of aluminum in 0.50 M NaCl to a great extent due to its adsorption on the aluminum

electrode surface (Sherif and Park, 2005). The adsorption of ND passivated or blocked the flawed

areas of the aluminum oxide film, and pits formed on the aluminum metal surface from the severe

chloride attack. (Gasparac, et al., 2000a) investigated the efficiency of imidazole derivatives for

corrosion inhibition of copper in 0.5 M hydrochloric acid. Corrosion inhibition was studied using

impedance spectroscopy. Imidazole and its derivatives 4-methylimidazole, 4-methyl-5-

hydroxymethylimidazole, 1-phenyl-4-methylimidazole, 1-(p-tolyl)-4-methylimidazole were

investigated. This study showed that 1-(p-tolyl)-4-methylimidazole is the best inhibitor in this

series and that it acts as mixed inhibitor. Similar studies using potentiodynamic methods have

31

shown that 1-(p-tolyl)-4- methylimidazole has the best inhibitory efficiency (Gasparac, et al.,

2000b). The corrosion inhibition of copper in aerated 0.5 M H2SO4 solution in presence of two

classes of heterocyclic compounds, namely phenylazo-pyrazolones (PAP) or hydroxyl quinoline

and bromobenzyl-carboxy-1,2,3 triazole (BCT) derivatives has been studied. The inhibition effect

was attributed to the adsorption of the dye molecules, the precipitation of Cu chelates and/or

formation of p-complexes at the electrode surface (Elmorsi and Hassanein, 1999). The properties

of organic coatings containing Polyaniline (PANI) and PANI in combination with other anti-

corrosive pigments have been studied (Kalendova, et al., 2008). The adhesion, barrier, and

anticorrosion properties of the coatings containing PANI and selected chemically active pigments

were studied as well as the combination of PANI with zinc dust. The comparison of the results of

corrosion tests completed in the atmosphere of SO2 and of NaCl revealed that the PANI +

Zn3(PO4)2·2H2O combination increased the anti-corrosion efficiency of organic-coatings.

3-mercaptopropyltrimethoxysilane (MPS) has been used as a copper corrosion inhibitor in 0.100

mol L-1 KCl solution. The inhibition was studied as a function of the MPS pretreatment

concentration in ethanol. From the polarization resistance, the inhibition efficiency improved with

increase in MPS concentration during the pretreatment. Polarization studies suggested that MPS

is an anodic as well as a cathodic inhibitor, in the presence of dissolved oxygen (Tremont, et al.,

2000). Abdel Rehim and co-workers studied the influence of the concentration of adenine (AD),

as a safe inhibitor, on the corrosion of low carbon steel (LCS) in aerated 4.0 M H 2SO4 solutions

(Abdel Rehim, et al., 2008). The investigations involved weight loss, potentiodynamic

polarization, impedance and electrochemical frequency modulation (EFM) methods. Results

obtained revealed that together with iodide ion, AD is an effective corrosion inhibitor for LCS

corrosion in H2SO4 solutions. Potentiodynamic polarization studies showed that AD alone and the

mixture of AD and iodide ions acted as mixed-type inhibitors for the corrosion of LCS in 4.0 M

H2SO4 solution. Variation of carbon steel corrosion rate with flow conditions in the presence of

an inhibitive formulation (Fatty amines associated with phosphonocarboxylic acid salts) used for

32

the treatment of water in cooling circuits was assessed (Srisuwan, et al., 2008). Two distinct

surface areas were visualized on the metal surface and the ratio between the two zones was

dependent on the flow conditions. A non-monotonic variation of the corrosion current densities

was also observed and explained by the variation of the ratio between the two layers, which each

have different intrinsic protective properties. The inhibitive effect of four oleo chemicals

(namely; 2-Pentadecyl-1,3-imidazoline (PDI), 2-Undecyl-1,3-imidazoline (UDI), 2-Heptadecyl-

1,3-imidazoline (HDI), 2-Nonyl-1,3-imidazoline (NI)), regarded as green inhibitors, has been

studied for the corrosion protection of mild steel in 0.5 M H2SO4. The methods employed were

weight loss, potentiodynamic polarization and electrochemical impedance techniques. The

inhibition efficiency increased with increase in inhibitor concentration, immersion time and

decreased with increase in solution temperature (Rafiquee, et al, 2009). The corrosive behaviour

of mild steel in 1M HCl solutions containing selected imidazolines of fatty acids with C7-C17 was

investigated using weight-loss method, potentiodynamic polarization technique and scanning

electron microscopy (Quaraishi, et al., 2008). The results obtained revealed that all the studied

imidazolines are effective in reducing corrosion of mild steel in HCl media. Potentiodynamic

polarization data showed that the compounds studied are mixed type inhibitors in the acid

solution. (Eddy, et al., 2009) has studied the inhibition efficiency of some antibiotics against mild

steel corrosion using weight loss and quantum chemical techniques. 3,5-Diamino-1,2,4-triazole

Schiff base derivatives and their inhibition efficiency, based on the effect of changing functional

groups, has been reported to establish a relationship between inhibitor efficiency and molecular

structure using weight loss method, electrochemical and Fourier transform infrared spectral

techniques. It was also found that the molecules containing more electron donating groups have

higher inhibition efficiency than the corresponding compounds with low electron donating groups

(Gopi, et al., 2010). The inhibiting action of 4-amino-antipyrine (AAP) and its schiff bases 4-

[(benzylidene)-amino]-antipyrine (BAAP), 4-[(4-hydroxy benzylidene)-amino]-antipyrine

(SAAP) and 4-[(4-methoxy benzylidene)-amino]-antipyrine (AAAP) which are derived from 4-

33

amino-antipyrine with benzaldehyde, salicylaldehyde and anisaldehyde, toward the corrosion

behaviour of mild steel in 1 M HCl solution has also been studied (Govindaraju, et al., 2009)

using weight loss, potentiodynamic polarization, electrochemical impedance and FT-IR

spectroscopic techniques. Although AAP was found to retard the corrosion rate of mild steel, the

synthesized schiff base compounds were also seen to retard the corrosion rate very effectively.

Ssanbhag and co-workers has investigated the corrosion inhibition characteristics of N′-[(1E)-(4-

hydroxy phenyl) methylene] isonicotinohydrazide (HIH) and N′-[(1E)-(4-hydroxyl-3-methoxy

phenyl) methylene] isonicotinohydrazide (HMIH) on mild steel corrosion in 1 M hydrochloric

acid by weight loss, potentiodynamic polarization and impedance techniques. The inhibition

efficiency was found to increase with increase in inhibitor concentration but decreased with

increase in temperature (Shanbhag, et al., 2007). Tramadol[2-[(dimethylamino)methyl]-1-(3-

methoxyphenyl)cyclohexanol hydrate], a drug, has been tested as a corrosion inhibitor for mild

steel in 0.5 M HCl and 0.25 M H2SO4 separately at 300, 310 and 320 K using mass loss and

galvanostatic polarization techniques (Prabha, et al., 2006). The protection efficiency increased

with increase in inhibitor concentration and decreased with increase in temperature in both the

acid solutions. Galvanostatic polarization studies showed that the inhibitor is of mixed type with a

slight predominance of cathodic character. (Granero, et al., 2009) carried out a study of the

behaviour of a mixture of amines and amides, commercially known as Dodigen 213-N (D-

213 N), as a corrosion inhibitor for ASTM 1010 mild steel in 10% w/w HCl solution. The weight

loss and electrochemical techniques used were corrosion potential measurement, anodic and

cathodic polarization curves, and electrochemical impedance spectroscopy (EIS). The corrosion

potential values shifted to slightly more positive values, thus indicating mixed inhibitor

behaviour. The anodic and cathodic polarization curves showed D-213 N as an effective

corrosion inhibitor, since both the anodic and the cathodic reactions were polarized in comparison

with those obtained without inhibitor. The effect of betanin (2,6-pyridinedicarboxylic acid, 4-(2-

(2-carboxy-5-(beta-D-glucopyr-anosyloxy)-2,3-dihydro-6-hydroxy-1H-indol-1-yl)ethenyl)-2,3-

34

dihydro-(S-(R*,R*))) on the corrosion inhibition of mild steel has been investigated in 1 M HCl

solution (Ashassi-Sorkhabi and Es’haghi, 2008). Weight loss method, potentiodynamic

polarization, and electrochemical impedance spectroscopy techniques were applied to study the

mild steel corrosion behaviour in the absence and presence of different concentrations of betanin

under the influence of various experimental conditions. Polarization curves showed that betanin

behaves mainly as a mixed-type inhibitor. The inhibiting behaviour of Nile blue and Indigo

Carmine organic dyes on mild steel corrosion has been evaluated in 1 M HCl solution, separately,

by weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy

techniques. Polarization curves indicate that the inhibition of the both inhibitors is of a mixed

anodic–cathodic nature, and Langmuir isotherm was found to be an accurate isotherm describing

the adsorption behaviour (Abdeli and Ahmadi, 2009). (Eddy, et al., 2009) investigated the

corrosion inhibition of ampicillin (AMP) and its synergistic combination with halides (KI, KCl

and KBr) for the corrosion of mild steel in H2SO4 using gravimetric, gasometric, thermometric

and infrared (IR) methods. The adsorption of AMP on the mild steel surface was found to obey

the Langmuir adsorption isotherm model and combination of AMP with the halides (KI, KBr and

KCl) enhanced the inhibition efficiency and adsorption behaviour of the inhibitor indicating

synergism. The corrosion inhibition properties of these compounds; 2-Pentadecyl-1,3-imidazoline

(PDI), 2-Undecyl-1,3-imidazoline (UDI), 2-Heptadecyl-1,3-imidazoline (HDI), 2-Nonyl-1,3-

imidazoline (NI) on aluminium in 1 M HCl and 0.5 M H2SO4 were investigated (Quraishi, et al.,

2007) by weight loss, potentiodynamic polarization, electrochemical impedance and scanning

electron microscopic techniques. Weight loss studies showed that the inhibition efficiency

increased with increase in the concentration of the inhibitor and was found to be inversely related

to time and temperature while it showed no significant change with increase in acid

concentration. The adsorption of these compounds on aluminium surface followed the Langmuir

adsorption isotherm and potentiodynamic polarization data showed that the compounds studied

were mixed type inhibitors. The inhibition performance of Basic yellow 13 dye on mild-steel

35

corrosion in hydrochloric acid solution was studied at 25 °C using weight loss and

electrochemical techniques (Ashassi-Sorkabi, et al., 2009). Inhibition efficiency increased with

increase of basic yellow 13 concentrations. ΔGads was calculated and its negative value indicated

spontaneous adsorption of the Basic yellow 13 molecules on the mild-steel surface and strong

interaction between inhibitor molecules and metal surface. The corrosion inhibition of mild steel

in 0.5 M sulfuric acid by two newly synthesized Schiff bases namely, 4-X-2-{[2-(2-pyridin-2-yl-

ethylsulfanyl) ethylimino] methyl}-phenol [X = –NO2 (S 1) and –OMe (S 2)] has been investigated

in the temperature range 298–318 K using Tafel polarization and electrochemical impedance

spectroscopy (EIS) methods ( Hosseini, et al., 2009). The Temkin isotherm was found to provide

an accurate description of the adsorption behavior of the Schiff bases.

Two triazole derivatives, 3,4-dichloro-acetophenone-O-1′-(1′,3′,4′-triazolyl)-methaneoxime (4-

DTM) and 2,5-dichloro-acetophenone-O-1′-(1′,3′,4′-triazolyl)-methaneoxime (5-DTM) have been

tested for inhibition effects for mild steel corrosion in 1 M HCl solutions by weight loss

measurements, electrochemical tests and scanning electronic microscopy (SEM). The

potentiodynamic polarization experiment revealed that the triazole derivatives are inhibitors of

mixed-type and electrochemical impedance spectroscopy (EIS) confirmed that changes in the

impedance parameters (R ct and C dl) are due to surface adsorption (Li, et al., 2007).

(Singh and Quraishi, 2010) studied the inhibition effect of all the three Mannich bases against the

corrosion of mild steel in 1 M HCl solution by weight loss, electrochemical impedance

spectroscopy (EIS), potentiodynamic polarization, and atomic force microscopy techniques. The

adsorption of Mannich bases obeyed Langmuir adsorption isotherm. Polarization curves indicated

a mixed nature of inhibition. All the Mannich bases were observed to be adsorbed physically at

lower concentration, whereas chemisorption was favoured at higher concentration.

The corrosion inhibition of C38 steel in molar HCl by N,N-bis[2-(3,5-dimethyl-1H-pyrazol-1-

yl)ethyl]buthylamine and 5-{N,N-bis[2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl] amino} pentanol

has been investigated at 308 K using electrochemical and weight loss measurements (Zerga, et

36

al., 2010). Measurements showed that these compounds acted as good inhibitors without

changing the mechanism of the corrosion process. Moreover, the inhibiting efficiency increased

with the increase in concentration of the studied inhibitors.

(Gopi, et al., 2007) studied the corrosion inhibition of mild steel by means of newly synthesized

triazole phosphonates 3-Vanilidene amino 1,2,4-triazole phosphonate (VATP), 3-Anisalidene

amino 1,2,4-triazole phosphonate (AATP) and 3-paranitro benzylidene amino 1,2,4-triazole

phosphonate (PBATP) along with cetyl trimethyl ammonium bromide (CTAB) in natural aqueous

environment using weight loss measurement, potentiodynamic polarisation and ac impedance

measurement. Addition of molybdate was noticed to increase the inhibition efficiency of triazole

in a synergistic manner. Results from experimental observation indicated VATP as a better

corrosion inhibitor for mild steel in aqueous solution. Additionally the formulation consisting of

VATP, sodium molybdate and CTAB offered good corrosion inhibition efficiency.

The effect of seven amphiphilic compounds, on steel API 5L X52 corrosion in sulphuric acid 1 M

solution was studied by (Pérez-Navarrete, et al., 2010) using potentiodynamic polarization

curves, and electrochemical impedance spectroscopy (EIS). Potentiodynamic polarization curves

indicated that these compounds acted mainly as cathodic type inhibitors.

Even with the broad spectrum of organic compounds available for use, the final choice of an

appropriate inhibitor for a particular application is restricted by several factors, ranging from

increased environmental awareness and the need to promote environmentally friendly processes,

to the vast variety of possible corrosion systems available.

Accordingly, the need exists for the development of new class of corrosion inhibitions that are of

low toxicity and good efficiency.

2.2. Natural products as corrosion inhibitors.

The investigation of natural products which are of plant origin as inexpensive and eco-friendly

corrosion inhibitors has gained interest recently. Plant extracts have become important as an

environmentally acceptable, readily available and renewable source for wide range of inhibitors.

37

They are the rich sources of ingredients which have very high inhibition efficiency. In addition to

being environmentally friendly and ecologically acceptable, plant products are of low-cost and

renewable sources of materials (Sharma, et al., 2008, Khamis, et al., 2005, Quraishi, 2004,

Zucchi and Omar, 1985). Extracts from their leaves, barks, seeds, fruits and roots comprise of

organic compounds containing nitrogen, sulphur and oxygen atoms and some have been reported

to function as effective inhibitors of metal corrosion in different aggressive environments.

The inhibition effect of Zenthoxylum alatum plant extract on the corrosion of mild steel in 5%

and 15% aqueous hydrochloric acid solution has been investigated (Chauhan and Gunasekaran,

2007) by weight loss and electrochemical impedance spectroscopy (EIS). Plant extract was able

to reduce the corrosion of steel more effectively in 5% HCl than in 15% HCl. The adsorption of

this plant extract on the mild steel surface was observed to obey the Langmuir adsorption

isotherm.

The inhibitive effect of the extract of khillah (Ammi visnaga) seeds, on the corrosion of SX 316

steel in HCl solution was determined (El-Etre, 2006) using weight loss measurements as well as

potentiostatic technique. It was found that the presence of the extract reduces markedly the

corrosion rate of steel in the acid solution. Also noticed was the increase in inhibition efficiency

as the extract concentration was increased.

(Oguzie, 2005a) investigated the efficacy of Telfaria occidentalis extract as a corrosion inhibitor

for mild steel in 2M HCl and 1M H2SO4 solutions. Telfaria occidentalis extract was found to

inhibit mild steel corrosion in 2M HCl and 1M H2SO4 solutions. Inhibition efficiency increased

with extract concentration but decreased with rise in temperature.

The inhibition effect of Zenthoxylum alatum plant extract on the corrosion of mild steel in 20, 50

and 88% aqueous orthophosphoric acid was investigated (Gunasekaran and Chauhan, 2004) by

weight loss and electrochemical impedance spectroscopy (EIS). Observation showed that plant

extract was able to reduce the corrosion of steel more effectively in 88% phosphoric acid than in

20% phosphoric acid.

38

The aqueous extract of the leaves of henna (lawsonia) was tested as corrosion inhibitor of C steel,

nickel and zinc in acidic, neutral and alkaline solutions, using the polarization technique (El-Etre,

et al., 2005). It was found that the extract acts as a good corrosion inhibitor via mixed inhibition

mechanism for the three tested electrodes in all tested media.

Comparison of steel anticorrosive protection formulated with natural tannins extracted from

acacia and from pine bark have been carried out (Matamalaa, et al., 2000). The results indicated

that pine tannins present better reactivity than acacia tannins, with better corrosion inhibition and

better adherence to metallic substrates.

Inhibition of aluminium corrosion in 2M sodium hydroxide solution in the presence and absence

of 0.5M NaCl using damsissa (Ambrosia maritime, L.) extract has been studied (Abdel-Gaber, et

al., 2008) employing different chemical and electrochemical techniques. Chemical gasometric

technique showed that addition of chloride ions or damsissa extract to sodium hydroxide solution

decreased the volume of the hydrogen gas evolved while potentiodynamic results manifested that

chloride ion retard the anodic dissolution of aluminium, below the pitting potential, in sodium

hydroxide solution. Damsissa extract, in presence or absence of chloride ion, influenced both the

anodic dissolution of aluminium and the generated hydrogen gas at the cathode indicating that the

extract behaved as mixed-type inhibitor.

Ocimum basilicum extract has been established to inhibit aluminium corrosion in the acidic and

alkaline environments (Oguzie, et al., 2006a). Inhibition efficiency was found to increase with

extract concentration but decreased with rise in temperature, suggesting physical adsorption of

the organic matter on the metal surface.

(Oguzie, 2006a) also studied the corrosion inhibition of mild steel in 2M HCl and 1M H2SO4 by

leaf extracts of Occimum viridis via the gasometric technique.

The inhibitive behaviour on steel of flavanoid monomers that constitute mangrove tannins

namely catechin, epicatechin, epigallocatechin and epicatechingallate has been investigated

(Rahim, et al., 2006) in an aerated HCl solution via electrochemical methods. The monomers

39

were found to be mainly cathodic inhibitors and the inhibition efficiency was dependent on

concentration.

Wild strain of Bacillus mycoides has been investigated for possible corrosion inhibition potential

(Eimutis, et al., 2006). Aluminium, zinc and mild steel were subjected to influence of wild strain

Bacillus mycoides for 2 years under laboratory conditions at controlled temperature and humidity.

Results obtained from the electrochemical impedance measurements performed for both biotic

and abiotic samples indicated microbially influenced corrosion inhibition for aluminium,

corrosion acceleration for zinc and indifference for steel.

The inhibitive action of the acid extracts of seeds, leaves and bark from the Ficus virens plant

towards hydrochloric and sulfuric acid corrosion of aluminium has been tested using mass loss

and thermometric techniques. It was found that the extract acts as a good corrosion inhibitor for

aluminium corrosion in all concentration of hydrochloric and sulfuric acid solution. It was also

found that the Ficus virens extract provides a good protection against pitting corrosion in chloride

ion containing solution (Jain, et al., 2006).

(Oguzie, et al., 2007) studied the effectiveness of Gongronema latifolium extract as an

environmentally friendly corrosion inhibitor for aluminium in strong acid (2M HCl) and alkaline

(2M KOH) environments. The results showed that the extract was well adsorbed on the metal

surface and significantly repressed aluminium corrosion in both environments.

The effectiveness of guar gum as an inhibitor for carbon steel corrosion in sulphuric acid

environments has been studied (Abdallah, 2004). Rosmarinus officinalis L. has been reported to

inhibit the corrosion of Al-Mg in chloride solution (Kliskic, et al., 2000).

The effect of extracts of Chamomile (Chamaemelum mixtum L.), Halfabar (Cymbopogon

proximus), Black cumin (Nigella sativa L.), and Kidney bean (Phaseolus vulgaris L.) plants on

the corrosion of steel in aqueous 1M sulphuric acid has been investigated using electrochemical

impedance spectroscopy (EIS) and potentiodynamic polarization techniques. Potentiodynamic

40

polarization curves indicated that the plant extracts behave as mixed-type inhibitors (Abdel-

Gaber, et al., 2006).

The protective effect and adsorption behaviour of Azadirachta indica extract in controlling mild

steel corrosion in 1M H2SO4 and 2M HCl has been assessed (Oguzie, 2006b). Azadirachta indica

extract effectively inhibited steel corrosion in the acid media studied by virtue of adsorption. The

inhibitor adsorption characteristics were approximated by Langmuir isotherm. (Sharma, et al.,

2009) also investigated the effect of this same plant on the corrosion of zinc in sulphuric acid

solutions.

Seed extract of Garcinia kola (Oguzie, et al., 2006b) was investigated as a corrosion inhibitor for

mild steel in 2M HCl and 1M H2SO4 solutions using the gasometric technique. Results indicated

that the extract inhibited the metal corrosion in the acidic environments and inhibition efficiency

increased with concentration. Temperature studies revealed a decrease in efficiency with rise in

temperature while corrosion activation energies increased in the presence of the extract. (Okafor,

et al., 2007) also investigated ethanol extract of Gracinia kola for inhibition of mild steel

corrosion in H2SO4 solutions.

Effect of berberine abstracted from Coptis chinensis and its inhibition efficiency on corrosion of

mild steel in 1 M H2SO4 was investigated (Li, et al., 2005).The weight loss results showed that

berberine is an excellent corrosion inhibitor for mild steel immersed in1 M

H2SO4.Potentiodynamic curves suggested that berberine suppressed both cathodic and anodic

processes.

Two different aerobic bacteria, viz. Pseudomonas alcaligenes and Pseudomonas cichorii have

been reported (Chongdar, et al., 2005) to influence the corrosion of mild steel electrodes

incubated in phosphate-buffered basal salt solution (BSS). In the medium containing P. cichorii,

significant reduction in the corrosion rate was observed due to the surface reaction leading to the

formation of corrosion inhibiting bacterial biofilm.

41

The inhibitive action of leaf extracts of Sansevieria trifasciata on aluminium corrosion in 2 M

HCl and 2 M KOH solutions has also been studied (Oguzie, 2007) using the gasometric

technique. The results indicate that the extract functioned as a good inhibitor in both

environments via a mechanism of physical adsorption. The adsorption characteristics of the

inhibitor were approximated by Freundlich isotherm.

Four siderophores isolated from bacteria have been studied as possible corrosion inhibitors for

iron in deaerated 1 N HCl. The compounds were aerobactin, enterobactin, parabactin, and

rhodotorulic acid. Polarization measurements showed all four compounds as good inhibitors

(McCafferty and McArdle, 1995).

(Okakor, et al., 2008) studied the inhibitive action of leaves (LV), seeds (SD) and a combination

of leaves and seeds (LVSD) extracts of Phyllanthus amarus on mild steel corrosion in HCl and

H2SO4 solutions using weight loss and gasometric techniques. The results indicate that the

extracts functioned as a good inhibitor in both environments and temperature studies revealed an

increase in inhibition efficiency with rise in temperature while activation energies decreased in

the presence of the extract. A mechanism of chemical adsorption of the plants components on the

surface of the metal was proposed for the inhibition behaviour.

The inhibiting action of the calyx extract of Hibiscus sabdariffa on mild steel corrosion in 2 M

HCl and 1 M H2SO4 solutions has been assessed using the gasometric technique. The results

demonstrated that Hibiscus sabdariffa extract suppressed the corrosion reaction in both acid

media. The inhibition mechanisms, estimated from the temperature dependence of inhibition

efficiency as well from kinetic and activation parameters showed that the extract functioned via

mixed inhibition mechanism (Oguzie, 2008).

The influence of natural honey (chestnut and acacia) and natural honey with black radish juice, on

corrosion of tin in aqueous and sodium chloride solutions was studied (Radojcic, et al., 2008)

using weight loss and polarization techniques. Results showed that the inhibition efficiency of

acacia honey was lower than that of chestnut honey, while the addition of black radish juice

42

increased the inhibition efficiency of both honey varieties. The process of inhibition was

attributed to the formation of multilayer adsorbed film on the tin surface. The adsorption of

natural honey and honey with black radish on tin was found to follow the Langmuir adsorption

isotherm.

(Oguzie, 2008b) investigated the Corrosion inhibition of selected plants on mild steel corrosion in

2 M HCl and 1 M H2SO4 using a gasometric technique. The studied plants materials include leaf

extracts Occimum viridis, Telferia occidentalis, Azadirachta indica and Hibiscus sabdariffa as

well as extracts from the seeds of Garcinia kola. The results indicated that all the extracts

inhibited the corrosion process in both acid media by virtue of adsorption and inhibition

efficiency improved with concentration of the extracts studied.

The corrosion inhibitive effect of the extract of black pepper on mild steel in 1 M H 2SO4 media

was evaluated by conventional weight loss studies, electrochemical studies viz., Tafel

polarization, ac impedance and scanning electron microscope (SEM) studies (Raja and

Sethuraman, 2008). Results from weight loss study revealed that black pepper extract acts as a

good inhibitor even at high temperatures. The inhibition is through adsorption which is found to

follow Temkin adsorption isotherm. Tafel polarization revealed the mixed mode inhibition of

black pepper extract.

Inhibition of the corrosion of mild steel by ethanol extract of Musa species peel has been studied

(Eddy, et al., 2009) using hydrogen evolution and thermometric methods of monitoring

corrosion. The result of the study revealed that different concentration of ethanol extract of Musa

species peel inhibit mild steel corrosion. Inhibition efficiency of the extract was found to vary

with concentration, temperature, period of immersion, pH and electrode potentials.

A study of rosemary oil as a green corrosion inhibitor for steel in 2 M H3PO4 has been carried out

(Bendahou, et al., 2006).The oil was found to be a good inhibitor for steel corrosion, but, its

efficiency decreased with temperature. Polarisation measurements showed that rosemary oil acted

essentially as a cathodic inhibitor.

43

The inhibitive effect of Jasminum nudiflorum Lindl leaves extract on the corrosion of cold rolled

steel (CRS) in 1.0 M hydrochloric acid (HCl) was investigated (Li, et al., 2010) by weight loss,

potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) methods. The

results showed the extract as a very good inhibitor, and the inhibition efficiency increases with

concentration. The adsorption of the extract obeyed Langmuir adsorption isotherm.

The corrosion behaviour of mild steel and aluminium exposed to H2SO4 solution and their

inhibition in H2SO4 containing Gum Arabic (GA) used as inhibitor has been assessed using

weight loss and thermometric techniques. Corrosion rate increased both in the absence and

presence of inhibitor with increase in temperature. Corrosion rate was also recorded to decrease

in the presence of inhibitor compared to the free acid solution. He proposed the phenomenon of

chemical adsorption for mild steel corrosion, while physical adsorption mechanism was proposed

for aluminium corrosion (Umoren, 2008).

2.3. Ethnobotanical details of the plants.

The botanical details of the plant and the major constituents present in the plants which were

screened for corrosion inhibition studies are discussed as follows.

2.3.1. Moringa oleifera.

Moringa oleifera Lam. belonging to the single genus family Moringaceae is a small fast-growing

ornamental tree originally belonging to India. Root, bark, pods and leaves of this tree are used in

traditional medicine for the treatment of human diseases, the leaves are enriched in vitamins A

and C. Pods and young leaves of the plant are primarily used for vegetative purpose (Mughal, et

al., 1999). M. oleifera seed oil is also high in tocopherols (Tsaknis, et al., 1999). The seeds of this

plant are also employed for water purification (Okuda, et al., 2001). Anti-cancer (Guevara et al.,

1999), anti-inflammatory and hepatoprotective (Kurma and Mishra, 1998a, 1998b) activities in

various tissues have also been reported. Detailed information on the nutritional value and

44

chemical composition of M. oleifera seed oil is available (Ching and Mohamed, 2001; Lalas and

Tsakins, 2002; Ryynanen, et al., 2004). Number of phytochemicals from various parts of

Moringa oleifera have also been isolated (Bennett et al., 2003; Faizi et al., 1994a, 1994b; Francis,

et al., 2004; Lakshminarayana, et al, 2005) except the saponins which are commonly found in

trees (Wina, et al, 2005). An immunoenhancing polysaccharide (Mondal, et al, 2004) and

niaziminin, having structural requirement to inhibit tumorpromoter induced Epstein Barr virus

activation have been reported from the leaves (Murakami, et al., 1998).

2.3.2. Mimosa pudica.

Mimosa pudica Linn belonging to the family Mimisoideae is commonly known as sensitive plant.

The plant has very sensitive leaves which fold on touching and has reddish roots. The plant is

widely distributed through tropical and subtropical parts of India, common in waste place where

the climate is moist and warm. The useful parts of this plant are roots, leaves and flower heads.

The whole plant is used medicinally in ayurvedic folk medicine and its photochemical studies

revealed the presence of minosine, orientin, isoorientin, β- sterol, D-pinitol, norepinepherine,

crocetin, tannins and turgorins. Mimosa pudica is an important plant which is used for various

ailments in ayurvedic system of medicine (Nadkarani, 1982, 2001; Chopra, et al., 1996; Rastogi,

et al., 2005).

In previous studies, it was found that the decoction of Mimosa pudica leaves has anticonvulsant

properties (NgoBum, et al., 2002). Both the ethanolic and aqueous extracts of the leaves of

Mimosa pudica possess hyperglycemic and antidepressant activities in mice and rats respectively

(Amalraj and Ignacimuthu, 2002; Molina, et al., 1999).

2.3.3. Dacryodes edulis.

Dacryodes edulis is a dioecious shade loving species of non-flooded forests in the humid tropical

zone (Adebayo-Tayo and Ajibesin, 2008; Hutchison and Daiziel, 1958). Dacryodes edulis is a

versatile plant in African ethnomedicine, as its various parts are employed to treat several

diseases. The bark of the plant has long been used to cicatrize wound in Gabon. It is also used as

45

gargle and mouth-wash to treat tonsillitis. The leaf decoction is prepared to produce vapour that

treats fever and headache, while the stems are employed as chewing sticks for oral hygiene

(Ajibesin, et al., 2008). Β-caryophyllene has been reported to occur as the main constituent of the

leaf essential oil of D. edulis (Onocha, et al., 1999). Ajibesin (2005) had identified phenolics such

as ethylgallate and quercitrin in the plant leaves.

In a study conducted in Cameroon, the extracts of 42 medicinal plants used for anaemia, diabetes,

AIDS, malaria and obesity were investigated for phytochemical substances and antioxidant

properties. The leaves of Dacryodes edulis elicited very high antioxidant effect when analyzed

against three assay methods: Folin (Folin Ciocalteu Reagent), FRAP (Ferric Reducing

Antioxidant Power) and DPPH (1, 1-diphenyl-2-picryhydrazyl), ranking second behind

Alchornea cordifolia (Agbor, et al., 2007). This antioxidant property was attributed to the

presence of flavonoids in the plant extract.

2.3.4. Euphorbia hirta.

Euphorbia hirta is well known and used by the major tribes in Nigeria. In Nigeria, the name in

Yoruba is Emi-ile. The plant E. hirta is commonly called asthma plant because of its alleged

efficacy in East and West Africa in the treatment of asthma and various respiratory ailments

(Odugbemi, 2008; Brown, 1995; Oliver, 1959). The plant is used as a diuretic, febrifuge,

galactogogue, purgative and vermifuge. It is reported as medication for intestinal amoebic

dysentery (Ogbulie et al., 2007; Stuart, 1979). Traditionally, the plant is squeezed in water and

the extract taken orally as a remedy for asthma. The phytochemicals in E. hirta include volatile

oil, alkaloids, tannins, saponins and steroids (Hashemi et al., 2008). The analgesic, antipyretic

and anti-inflammatory properties of the plant have been reported (Lanhers et al., 1991). The

ethanolic extracts of the plant have been found to demonstrate antibacterial activity and

toxicological potential (Ogbulie et al., 2007). In vitro antifungal and antibacterial properties as

well as ulcer-protective effect and hepatoprotective activity of the plant extract in rats have also

been demonstrated (Somchit et al., 2001; Rao et al., 2003; Edwin et al., 2007). Bioassay-guided

46

fractionation of the methanol extracts of Euphorbia hirta Linn (Euphorbiaceae) aerial parts led to

the isolation of flavonol glycosides; afzelin, quercitrin, and myricitrin (Liu, et al., 2007).

2.3.5 PIPER GUINENSIS (AFRICAN BUSH PEPPER)

Piper guinensis (African bush pepper) or commonly called uziza in Igbo are climbing glabrous

creeper cultivated in various part of India, Malaya Island, Nigerian and other West African

countries. There is considerable local use of this species as a condiment and it is widely found in

almost all Nigerian market. The roots, fruits and leaves of this plant are widely used in the

treatment of asthma, bronchitis, fever and pain in the abdomen, as stimulant and in haemorrhoidal

infection. The fresh fruits of piper guinensis are often eaten raw for their spicy taste. The fruits

are also dried and then pounded and sieved; this powder is added to tea or coffee or used for

seasoning vegetables (Ebenso et al 2008). Piper guinensis is a plant among the candidate with

enormous potential for use as a bio insecticide, it is a member of the piperraceae family. It is

used in small quantities for flavour in foods and medicinal purposes, the excess post harvest is

usually wasted, since new stock come to meet the previous season stock.

2.3.6 MISTLETOE (Viscum album)

Natural tannins are complex mixtures extracted from plant materials that are either being

investigated or are in use as corrosion inhibitors in aqueous solutions (Oguzie et al,

2004).Tannins acts as corrosion inhibitors on bare steel in acid or neutral solution due to their

ability to form chelate compounds with ferrous and ferric ions that are released in the course of

steel corrosion process(Roberge,1994).

47

Viscum album growing on a populous species has the following scientific classification: Kingdom: Plantae,

Division: magnoliophyta , Class: magnoliopsida, Order: santalales, Family: santalsceae, Genus: Viscum, Specie: V. album

It is a hemi-parasitic shrub which grows on the stem of other trees. It has stems 30-100cm

(12-39in)long with dichotomous branching. The leaves are in opposite pairs, strapped shaped

entirely leather textured, 2.8cm (0.79-3.1in) long and 0.8-2.5cm broad.

Yellowish-green in color. The specie is deciduous and the flowers are inconspicuous

yellowish-green in 2-3mm diameter. The fruit is a white/yellow berry containing one (very

rarely several) seed embedded in the very sticky, glutinous fruit pulp (Morgenstern, 1996).

V. album has an added appearance; a yellowish ball hanging high up on the tree, visible only

when the host plant have lost its leaves. V. album is ever green and sustains its greenish

yellow leaves throughout the winter. It’s growing habit is distinctly round; it’s twigs branch

frequently and its elongated, oral leaves always grow in opposite pairs. The tiny

inconspicuous yellowish flowers appear in May. The berries are distinctly sticky, hence, the

latin name Viscum album-(white sticky stuff) and easily cling to branches and soon send out a

sucker rootlet that penetrate the back of the host three and tap its sap for nutrient and water.

Though, V. Album is a parasite and as such dependent on the host plant for nutrient and

water. It does not rely on host for CO2. or kill the host plant and thus is not really harmful to

the host. While birds feed on the berries without apparent harm, they are toxic to humans

(Morgenstern, 1996)

The toxic lectin viscumin has been isolated from v. album. Viscumin is a cyto-toxic protein

that binds to galactose residues ofcell surface glycoproteins and may be internalized by

48

endocytosis. Viscum strongly inhibits protein synthesis by inactivating the 60 S-ribosomal

sub units.

V. album is chiefly used to lower blood pressure and heart rate, ease anxiety and promote

sleep. In low doses it can also relieve panic attacks and headaces and also improves the

ability to concentrate. There is no doubt that certain constituents of the plants, especially the

viscotoxins exhibits anti-cancer activities (Morgenstern, 1996) .

They are antispasmodic, cardiac, cytostatic, diuretic, hypotensive, narcotic, nervine,

stimulant, tonic and vasodilator. V. album has a reputation for curing epilepsy and other

convulsive nervous activities that causes the spams, but larger dosages can produce the

problem. V. album has also been employed in checking internal hemorrhages, in treating high

blood pressure and in treating cancer of the stomach, lungs and ovaries (Morgenstern, 1996).

Externally, the plant has been used to treat arthritis, rheumantism, chilblains, leg ulcer and

varicose veins. A homeopathic remedy is made from equal quantities of the berries and

leaves. It is difficult to make because of the viscidity of the sap. (Morgenstern, 1996)

Oguzie et al(2004a) worked on the influence of halide ion on the inhibition effect of Congo

red dye on the corrosion of mild steel in sulphuric acid solution and he formed out that Congo

red dye inhibits mild steel corrosion in sulphuric acid solution. The inhibition efficiency

increased with concentration but decreased with rise in temperature. The adsorption process

followed Flory Huggins isotherm. Also Oguzie et al , (2004b) worked on the corrosion

inhibition and adsorption behavior of bismark brown dye on aluminium in sodium hydroxide

solution and they found out that Bismark Brown dye inhibited aluminium corrosion in 0.1M

NaOH solution, the inhibition efficiency increasing with concentration. Furthermore, Oguzie

et al, worked on inhibition of mild steel corrosion in acid in media by aqueous extracts from

Garcinia Kola seed and they discovered that the extracts from seeds of Garcinia Kola inhibits

mild steel corrosion in 2 M HCl and 1M H2SO4 solutions by physical adsorption onto the

49

metal surface. Also inhibition efficiency increased with increase in concentration but

decreased with rise in temperature.

Finally, Oguzie et al also studied the inhibitive effect of occinum vividis extract on the acid

corrosion of mild steel and he discovered that O. Vividis inhibited mild steel in 2 M HCl and

1 M H2SO4 at the temperature studies. Inhibition efficiency increased with an increase in

extract concentration and synergistically increased in the presence of halide ions. Temperature

studies recorded a decrease in efficiency with rise in temperature

50

CHAPTER THREE

3.0 EXPERIMENTAL

3.1 Material Preparation

3.1.1 Metal Specimen

This study was carried out on mild steel metal procured from the Materials and

Metallurgical Engineering Department of Federal University of Technology, Owerri. The

mild steel has the following compositions: C; 0.05%, Mn; 0.60%, P; 0.36%, Si; 0.03%. It

was pressed–cut mechanically using the guillotine machine into coupons of uniform

dimensions 3cm x 3cm and a hole drilled at the centre of the coupon with radius 2.5mm.

Its thickness was 1mm. The coupons were used without further polishing. However, they

were degreased in absolute ethanol, dried in acetone and stored in moisture–free

desiccator containing silica gel prior to use.

3.1.1 REAGENTS

All reagents used were of BDH analytical grades purchased and used without further

purification. They included acetone, ethanol, conc. HCl, conc. H2SO4 and potassium

iodide. The blank corrodents were 1M HCl and 0.5M H2SO4 and were prepared from

their stock solutions. Potassium iodide used for corrosion experiment has molarity of

0.0005M. All gravimetric measurements were carried out using a FAJA weighting

balance of model FA 2004A and weighing capacity 0.0001-200g.

3.1.6 Extraction Process

51

Each leaf (Dacryodis edulis, Viscuum ablum, Euphobeace hirtha linn, Piper guinesis

Moringa olifera and Mimosa pudica) extract solutions were prepared by boiling a

weighed amount of the ground leaves (20g) in 800ml of the acid using a reflux condenser

and 1000ml round-bottomed flask for three hours. This was carried out at low heating

temperature. The experiments were carried out separately for each of the acids (1M HCl

and 0.5M H2SO4). The solutions were allowed to cool to room temperature, filtered and

then stored in an air-tight glass container. The stock concentrations were determined by

carefully drying the residues and weighing to obtain the amount of the extracts infused in

the solutions.

GRAVIMETRIC MEASUREMENTS:

Blank Experiment;

250ml volumetric flask was filled with the stock acid solutions and then transferred to a

measuring cylinder and 200ml was measured and poured into a beaker and covered. Two

mild steel coupons labeled A and B for the two acids (two for each acid) were weighed

and suspended in the acid solution with the aid of a wooden cross bar and a polyester

rope at room temperature and also at 400C, 500C and 600C. The set up was allowed to

stand for six hours after which the coupons were retrieved, scrubbed with bristle brush

under running water, dried with acetone and warm air and then weighted. The final

weights of the coupons were less than the initial indicating that the mild coupons have

undergone corrosion leading to the dissolution of some parts.

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Gravimetric Measurements in the Presence of these Extracts

A micro syringe was used to aspirate the calculated volumes of extract stock solutions

into 250ml volumetric flasks and topped with the stock acid under investigation. The

contents were shaken vigorously to facilitate better dissolution of the extracts in the acid

solution. The 200ml of the mixture was transferred into a beaker and then covered. Two

mild steel coupons for each extract concentration were weighed and same procedure

carried out as in blank experiment.

The total surface area of mild steel coupon is given as

. . . . . . . . . . . . . . . . . . 3.1.9a

Where total surface area of immersed coupon

Length of coupon

width of coupon

thickness of coupon

radius of hole drilled on coupon.

Corrosion rates of mild steel in the different test solutions were determined using the

formular;

. . . . . . . . . . . . . . . 3.1.9b

Where corrosion rate in mg/dm2/day

53

weight loss (mg).

time of retrieval in days

total surface area of mild steel coupon in dm2

From the value of corrosion rates in the presence and absence of the inhibitor, the

inhibition efficiencies were calculated using;

. . . . . . . . . . . . . . 3.1.9c

50mg/l exhibited lowest inhibition efficiency in both acid solutions while 800 mg/l and

1000 mg/l showed highest inhibition efficiency in 1M HCl and 0.5M H2SO4 respectively.

3.2.0 Experiment with a Halide (KI)

The effect of halide additives was studied by introducing 5.0mM of the halide ion

(potassium iodide) with each plant extracts to enhance the inhibiting performance of the

extract components. Ion-pair interactions between the organic cations and the halide ion

will result in increased surface coverage. Protonated or molecular species in the extract

components will be responsible for synergistic increase in inhibition efficiency.

3.3.0 Effect of Temperature on Corrosion and Corrosion Inhibition

This was carried out using the extract concentrations that inhibited least and the ones that

showed highest inhibition efficiency. Corrosion rates, inhibition efficiency and activation

energy were then calculated.

2.3. Electrochemical measurements

Metal samples for electrochemical experiments were machined into cylindrical

specimens and fixed in polytetrafluoroethylene (PTFE) rods by epoxy resin in such a way

54

that only one circular surface of area 0.64 cm2 was left uncovered. The round surface was

necessary in order to avoid edge effects. The exposed surface was ground with silicon

carbide abrasive paper (from grade #200 to #1000), degreased in acetone, rinsed with

distilled water and dried in warm air. Electrochemical experiments were conducted in a

Model K0047 corrosion cell using a VERSASTAT 400 Complete DC Voltammetry and

Corrosion System, with V3 Studio software. Two graphite rods were used as counter

electrode and a saturated calomel electrode (SCE) as reference electrode. The latter was

connected via a Luggin’s capillary. Electrochemical corrosion tests were performed at the

end of 1 h of immersion at 303 K. Impedance measurements were made at corrosion

potentials (Ecorr) over a frequency range of 100 kHz – 10 mHz, with a signal amplitude

perturbation of 5 mV. Potentiodynamic polarization studies were carried out in the

potential range 250 mV versus corrosion potential (in 1 M HCl, which did not show

any evidence of passivation) and from -700 to 1600 mV (in 0.5 M H2SO4). The scan rate

in either case was 0.333 mV s-1. Each test was run in triplicate to verify the

reproducibility of the systems.

SURFACE ELECTRON-MICROSCOPY (SEM)

1cm×1cm coupons were dipped in blank solution and in best inhibitor concentration for

1-2 days. The coupons were retrieved after 1-2 hours, washed lightly with bristle brush

under running water, and then dried thoroughly in acetone and warm air. After which

they are packaged and tagged.

FTIR

Plant materials were obtained dried and grounded to fine powder. This powdered

sample was further divided into two with one subjected to analysis. The second part

55

of the dried sample was extracted using 1M HCl and 0.5M H2SO4 solution. 20ml of

the extract was collected sun-dried and the powdered residue collected for analysis.

Two coupons were dipped seperately into 200ml of the highest inhibition

concentration from the two acid solutions for 2-3 days to form an adsorbed layer.

After which they were retrieved, dried scrape with sharp blade and the scrapings

collected for analysis.

Evaporate extract solution to dryness (no heat), collect powered residue

Dip 3-4 coupons in extract solution for 2-3 days to form extract adsorbed layer.

Retrieved, dry, scrape with sharp blade and collect the scrapping.

Three different samples for FTIR

Appendix

3.1.4 Preparation of Stock Solution for 1M HCl

Basis = 100g. Average assay = = 36.5

Volume of HCl = . . . . . . . . . . . . . . 3.1.4a

56

cm3

84.75cm3of stock contains 36.5g HCl

1000cm3 will contain 430.68g

Therefore, mass concentration of HCl is g/dm3

Molarity (mol/dm3) = . . . . . . . . . . . . . . . 3.1.4b

11.81 mol/dm3

This indicates that the stock concentration of HCl is 11.81M.

To obtain the volume of 1M HCl needed for the experiment. The dilution formular was

used thereafter;

. . . . . . . . . . . . . . . . . . . . . . . 4.1.4c

Where

C1 = stock concentration (11.81 mol/dm3)

V1 = volume of stock acid required.

C2 = Concentration of acid required (1M)

V2 = Volume of flask used for dilution (2dm3)

ml

3.1.5 Preparation of Stock Solution for 0.5M H2SO4

Volume of HCl = . . . . . . . . . . . . . . . . . . . . 3.1.5a

= 54.35cm3

57

Therefore, 54.35cm3 of the stock contains g H2S04

1000cm3 will contain g

Molar concentration (mol/dm3) = =18.40 mol/dm3

To obtain the volume of 0.5M H2SO4 needed, the dilution formular was also used

. . . . . . . . . . . . . . . . . . . . . . . . . 3.1.5b

Where all parameters used are as applied to HCl.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.5c

Given that C1 = 0.5M, V2 = 2 dm3 and C1= 18.40 mol/dm3

Hence, V1 0.054347 mol/dm3 = 54.35ml.

54. 35ml of stock H2SO4 was therefore used to prepare a 2dm3 solution of 0.5M H2SO4.

58