1

CHAPTER I

Introduction

1.1. Corrosion

Corrosion is simply defined as the destruction of materials under chemical or

electrochemical action by the surrounding environment. Corrosion entails the

conversion of a metal from the atomic to ionic state, with the loss of one or more

electrons. Rusting of iron with the formation of iron oxide is a typical example of

corrosion. Corrosion of metal and alloys lead to enormous losses to the country. The

growing need of metal in every walk of life and loss of it by corrosion has put a big

question mark before the mankind regarding how to prevent it from being corroded so

that it can be used to its maximum extent. Scientists have come out with the use of

many preventive methods. The corrosion can be prevented to a large extent if not

stopped completely. A survey carried out by US Minerals Management Service [1]

which indicates that majority of the problems are due to internal corrosion, which are

more frequent than those due to external corrosion.

1.1.1 Economical loss due to corrosion

In a study of corrosion cost conducted jointly by C.C. Technologies Inc., USA

[2], Federal Highway Agencies (FHWA), USA [3] and National Association of

Corrosion Engineers [4], the direct corrosion cost was estimated to be around 276

billion US dollars, approximately 3.1% of the national gross domestic product (GDP).

Based on an extensive survey conducted by Battelle Columbus laboratories,

Columbus, Ohio, USA and National Institute of Standards and Technology (NIST), in

1975, the cost was estimated to be 82 billion US dollars, which would have exceeded

350 billion US dollars in view of price inflation over the last twenty-five years.

2

Because of the long time involved in conducting cost structure, it is not possible to

update the information every year. However, both studies show that corrosion costs

are staggering and a figure of about 350 billions US dollars appears to be a reasonable

estimate for another two to three years. At least 35% of the above amount could have

been saved by taking appropriate corrosion control measures. In UK, the corrosion

cost was estimated to be 4-5% of GDP [5]. In Japan, the cost of corrosion was

estimated about 5258 trillion Yen per year. For most industrialized nations, the

average corrosion cost is 3.5 – 4.5% of the GDP. Below are some starting figures of

corrosion losses:

The corrosion cost of gas and liquid transmission pipelines in USA exceeds

seven billion US dollars. The figure for the major oil producing countries in

the Gulf region are not known, however the cost expected to be very high

because of highly corrosive environment in the region [4].

The corrosion-free life of automobiles in the coastal regions of Arabian Gulf

is about six months only [6]

Nearly 95% of concrete damage in the Arabian Gulf coastal region is caused

by reinforcement corrosion and consequent spalling of concrete [7].

It is estimated that 10% of all aircraft maintenance in USA is spent on

corrosion remediation [8].

Major annual corrosion losses to the tune of Euro 350 million in transport,

Euro 280 million in marine, Euro 250 million in buildings and construction

and Euro 180 million in oil and chemical industries, have been reported in

UK [9]. These are uncorrected 1971 figures.

About $ 120 billion is spent on maintenance of aging and deteriorating

infrastructures in USA.

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Automotive corrosion costs 23.4 billion US dollars annually in USA [4].

Every newborn baby in the world now has an annual corrosion debt of $ 40.

National Association of Corrosion Engineers - International India Section

(NACE) states that the loss of materials due to corrosion was about Rs.35, 000 crores

each year, nearly 2.5 - 4 % of India's GDP [10]. It was estimated that the annual cost

of all forms of corrosion to the oil and gas industries was $13.4 billion in USA for the

year 2001, of which microbially influenced corrosion accounted for about $2 billion

[11]. The importance of internal corrosion as a cause for leakage of steel tanks was

documented by various sources. A comprehensive US Environmental Protection

Agency (EPA) report documents 6-10 % of tank failures were caused by internal

corrosion. In France, it was estimated by a major company that 10 percent of

underground storage tanks leaked due to internal corrosion while other data from

France indicated 8.5% of the leakages were caused by internal corrosion. Switzerland

has reported that internal corrosion was the cause for 5% of its tank leakages. The

percentage of internal leaks was lower than that of external leakages and Sweden had

reported that half of its leaking tanks were only due to internal corrosion.

1.1.2. Theories of corrosion

Corrosion theories may be classified into 1. Homogeneous theory and

2. Heterogeneous theory

1. Homogeneous theory

Wagner and Traud [12] emphasized that the necessary condition for corrosion

is the dissolution of the metal and electronation reaction that takes place at the metal-

environment interface. Metals become unstable due to the charge transfer reaction-

taking place at the interface. Therefore it is necessary that the potential difference

across the interface be more negative than the equilibrium potential for the metal

dissolution (anodic) reaction or more positive than the equilibrium potential for the

4

electronation (cathodic) reaction. This theory is helpful in explaining the corrosion of

pure metals and uniform corrosion, as the cathodic and anodic sites interchange their

characters quite frequently.

2.Heterogeneous theory

According to this theory, the presence of impurities on the metal surface is

necessary and thus a local cell is setup between cathodic and anodic areas. Due to the

electrochemical reactions taking place at the interface between the metal and ionically

conducting films or actual electrolyte, the metal becomes unstable and hence

corrodes.

Thus a corroding metal consists of

(a) An electron sinks area where de-electronation reaction occurs.

(b) An electron source area where electronation reaction occurs.

(c) An ionic conductor to keep the ion current flowing.

This model is the basis for the local cell theory of corrosion or heterogeneous

corrosion.

1.1.3. Principles of corrosion

Principles of corrosion depends upon the following such as

1. Thermodynamic principles

2. Electrochemical principles

1. Thermodynamic principles

Thermodynamic [13] and electrochemical principles play a major role in

determining the corrosion behaviour of materials. Thermodynamics indicate the

spontaneous direction of a chemical reaction. It is used to determine whether the

corrosion is theoretically possible or not.

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2. Electrochemical principles

Electrochemical principles are extensively used to determine the corrosion

behaviour of materials. Here the corrosion reaction can be represented by partial

reactions such as metal oxidation and reduction of some reducible species of the

environment both occurring simultaneously at equal rates at the mixed potential of the

reaction. Corrosion reaction mainly occurs at the metal-environment interface.

The electrochemical nature of corrosion can be illustrated by the attack of

iron in hydrochloric acid. When iron is dipped in acid a vigorous action occurs as a

result hydrogen gas is evolved and iron gets dissolved.

Hence the reaction is

Fe + 2H+ → Fe2+ + H2 (1.1)

The above reaction can be divided into two partial reactions

Fe → Fe2+ + 2e- oxidation (anodic reaction) (1.2)

2H+ + 2e → H2 reduction (cathodic reaction) (1.3)

Corrosion is a surface electrochemical phenomenon common to all base

metals in aqueous, or at least humid, environments. Metal dissolution takes place at

anodic sites

M M2++ 2e- (1.4)

with the electrons being accepted in a cathodic reaction at separate sites. Most

commonly the reaction at the cathodic involves oxygen and occurs at neutral or

alkaline pH values,

½ O2 +H2O + 2e- 2OH- (1.5)

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Under acid conditions however, protons may be the cathodic reactant,

2H+ + 2e- 2H H2 (1.6)

giving rise first to atomic then to molecular hydrogen.

There are a number of ways in which such an electrolytic cell can become

established: for example, with two dissimilar metals in electrical contact, or within a

single metallic structure where different areas of the surface have developed different

electrochemical potentials. This can arise from inclusions in the metal, from surface

imperfections, or from the presence of differential concentration cells. Most

importantly these last often involve oxygen, where the differential concentrations

arise from surface deposits or growths that limit access of oxygen to the underlying

metallic surface. Areas of low oxygen, or other nutrient concentration beneath such

deposits, are anodic sites of metal dissolution. Because the corrosion resistance of,

for example, aluminium and stainless steel is due to the formation of an oxide

passivation film, oxygen depletion cells can have a further serious corrosive effect

with these metals.

Greene and Fontana [14] stated that the term ‘Corrosion’ includes the reaction

of metals, glasses, ionic solids, polymeric solids and composites with environments

that include liquids, gases and non-aqueous electrolytes. Corrosion is regarded as the

cancer of metals that arise from the thermodynamic instability [15]. It is necessary to

devote more attention to metallic corrosion now than earlier, due to the increased use

of metals in all fields of technology. The use of rare and expensive metals for special

applications such as atomic energy requires special precautions for preservation. A

more corrosive environment is due to the increasing pollution of air and water. Mild

steel is widely used in most of the chemical industries due to its low cost and easy

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availability for fabrication of various vessels, tanks, pipes etc. Since mild steel suffers

from severe corrosion in aggressive environments, it needs to be protected by

appropriate methodology.

1.2. Forms of Corrosion

Fontana [16] categorized eight major forms of corrosion mainly based on

appearance. Dillon [17] considered Fontana’s basic forms of corrosion and divided

them into three groups, based on their ease of identification. The three categories used

were:

Readily identifiable by ordinary visual examination:

Uniform corrosion

Pitting corrosion

Crevice corrosion

Galvanic corrosion

May require supplementary means of examination

Erosion corrosion

Cavitation corrosion

Fretting corrosion

Intergranular corrosion.

Verification is usually required by microscopy and other chemical and

electrochemical methods:

Exfoliation

Dealloying (selective leaching)

Stress corrosion cracking

Corrosion fatigue

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Microbial corrosion.

However, the corrosion of metals and alloys almost never follows a single law

nor is governed by a single mode. Even the simplest situation can lead to the

corrosion of a material by a combination of modes driven by intertwined mechanisms.

1.2.1 Uniform corrosion

Uniform corrosion is characterized by corrosive attack proceeding evenly over

the entire surface area, or a large fraction of the total area. General thinning takes

place until failure. On the basis of tonnage wasted, this is the most important form of

corrosion. However, uniform corrosion is relatively easily measured and predicted,

making disastrous failures relatively rare. In many cases, it is objectionable only from

an appearance standpoint. As corrosion occurs uniformly over the entire surface of

the metallic component, it can be practically controlled by cathodic protection, use of

coatings or paints, or simply by specifying a corrosion allowance.

1.2.2 Pitting corrosion

Pitting corrosion is a localized form of corrosion by which cavities or ‘holes’

are produced in the material. Pitting is considered to be more dangerous than uniform

corrosion damage, because it is more difficult to detect, predict and design against

corrosion as products often cover the pits. A small, narrow pit with minimal overall

metal loss can lead to the failure of an entire engineering system. Pitting corrosion,

which for example is almost common denominator of all types of localized corrosion

attack, may assume different shapes. Pitting is initiated by localized chemical or

mechanical damage to the protective oxide film. Water chemistry factors, which can

cause breakdown of a passive film, are acidity, low dissolved oxygen concentration

and high concentration of chloride, and localized damage or poor application of a

protective coating which also cause pitting corrosion. Besides, non-metallic inclusions

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also cause pitting. Pitting corrosion can produce pit with their mouth open

(uncovered) or covered with a semi permeable membrane of corrosion products. Pits

can be either hemispherical or cup shaped.

1.2.3. Crevice corrosion

Crevice corrosion is a localized form of corrosion usually associated with a

stagnant solution on the micro-environmental level. Such stagnant microenvironments

tend to occur in crevices (shielded areas) such as those formed under gaskets,

washers, insulation material, fastener heads, surface deposits, disbonded coatings,

threads, cap joints and clamps. Crevice corrosion is initiated by changes in local

chemistry within the crevice such as, depletion of inhibitor in the crevice, depletion of

oxygen in the crevice, a shift to acidic conditions in the crevice; build up of

aggressive ionic species like chloride in the crevice etc.

1.2.4. Galvanic corrosion

Galvanic corrosion refers to corrosion damage induced when two dissimilar

materials are coupled in a corrosive electrolyte. It occurs when two or more

dissimilar metals are brought into electrical contact under aqueous medium such as

water. When a galvanic couple forms, one of the metals in the couple becomes the

anode and corrodes faster than it would all by itself. While the other becomes the

cathode, corrodes slower than it would alone. In a bimetallic couple, the loss of noble

material will become the anode of the corrosion cell and tend to corrode at an

accelerated rate, compared to an uncoupled condition. The more noble material will

act as a cathode in the corrosion cell. Galvanic corrosion can be one of the most

common forms of corrosion and one of the most destructive.

1.2.5. Erosion corrosion

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Corrosion of metal or alloy can be accelerated when there is an abrasive

removal of the protective oxide layer. It is the increase in the rate of attack of a metal

because of relative movement between a corrosive medium and the metal surface.

Generally, this is removed from the surface either in the form of dissolved ions or in

the form of solid corrosion products, which are mechanically swept from the metal

surface.

1.2.6. Cavitation corrosion

Cavitation occurs when a fluid’s operational pressure drops below its vapour

pressure causing gas pockets and bubbles to form and collapse. This can occur in

what can be a rather explosive and dramatic fashion. In fact, this can actually produce

steam at the suction of a pump in a matter of minute. The most locations are likely to

occur such as the suction of a pump, at the discharge of a valve or regulator, at the

geometry-affected flow areas such as pipe elbows and expansions.

1.2.7. Fretting corrosion

Fretting corrosion refers to corrosion damage at the asperities of contact

surfaces. This damage is induced under load and in the presence of repeated relative

surface motion, as induced for e.g. by vibration. Pits or grooves and oxide debris

characteristics are the typical example of fretting damage, typically found in

machinery, bolted assemblies and ball or roller bearings.

1.2.8. Intergranular corrosion

The microstructure of metals and alloys is made up of grains, separated by

grain boundaries, intergranular corrosion is localized attack along the grain

boundaries, or immediately adjacent to grain boundaries, while the bulk of the grains

remain largely unaffected. This form of corrosion is usually associated with chemical

segregation effects or specific phases precipitated on the grain boundaries. Such

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precipitation can produce zones of reduced corrosion resistance in the immediate

vicinity. The attack is usually related to the segregation of specific elements or the

formation of a compound in the boundary. Corrosion then occurs by preferential

attack on the grain boundary phase, or in a zone adjacent to it that has lost an element

necessary for adequate corrosion resistance thus making the grain boundary zone

anodic relative to the remainder of the surface. The attack usually progresses along a

narrow path along the grain boundary. In any case the mechanical properties of the

structure will be seriously affected.

1.2.9. Exfoliation corrosion

Exfoliation corrosion is a particular form of intergranular corrosion associated

with high strength aluminium alloys. Alloys that have been extruded or otherwise

worked heavily, with a microstructure of elongated, flattened grains are particularly

prone to this damage.

1.2.10. Dealloying

Dealloying or selective leaching refers to the selective removal of one element

from an alloy by corrosion processes. A common example is the dezincification of

unstabilized brass, whereby a weakened, porous copper structure is produced. The

selective removal of zinc can proceed in a uniform manner or on a localized scale. It

is difficult to rationalize dezincification in terms of preferential zinc dissolution out of

the brass lattice structure; rather, it is believed that brass dissolves with zinc

remaining in solution and copper replating out of the solution.

1.2.11. Stress corrosion cracking

Stress corrosion cracking (SCC) is the cracking induced from the combined

influence of tensile stress and a corrosive medium. The impact of SCC on a material

usually falls between dry cracking and the fatigue threshold of that material. The

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required tensile stresses may be in the form of directly applied stresses or in the form

of residual stresses. One of the most important forms of stress corrosion that concerns

the nuclear industry is chloride stress corrosion.

1.2.12. Corrosion fatigue

Corrosion fatigue is the cracking as a result of the combined action of an

alternating stress and corrosive environment. The fatigue process is thought to cause

receptive or the protective passive film, upon which corrosion is accelerated. The

introduction of a corrosive environment often eliminates the normal ‘fatigue’ limit of

a ferrous alloy thereby creating a finite life regardless of stress level.

1.2.13. Microbiologically influenced corrosion (MIC)

Microorganisms, inhabitants of all natural environments, influence corrosion

phenomenon either directly by accelerating the electrochemical reactions or indirectly

by virtue of their metabolic products. However, microbiologically induced corrosion

(MIC) is not a new form of corrosion, but provides a new path way to an already

existing process [18].

Microbes can induce corrosion by creating differential aeration/concentration

cells [19], changing pH [20], depleting oxygen [21], utilizing cathodic hydrogen or by

producing hydrogen [22], removing atoms from metals [23], oxidizing or reducing

corrosion inhibitors [24], setting up of galvanic cells [25], disturbing or feeding upon

protective organic coatings and by their metabolic products [26].

1.3. Biofilms and MIC

MIC is linked to cellular adhesion and extracellular polymeric substances

(EPS) produced during biofilm formation. These adhesion processes and the

subsequent EPS production lead to an important modification of the metal solution

13

interface. Its partial or total coverage by strongly adherent biofilms produces a barrier

to the exchange of elements between the metal surface and the aqueous environment.

In addition, reactions between metabolites produced by bacteria and the metal take

place within the biofilm.

It is widely acknowledged in the literature that biofouling starts immediately

after metal immersion in an aqueous medium. A thin film (approximately 20-80 nm

thick), due to the depositon of inorganic ions and high relative molecular mass

organic compounds, is formed in the first stage. This initial film is able to modify the

electrostatic charge and wettability of the metal surface [27], facilitating its further

colonization by microorganisms. Hitherto, no visible biofilm will have settled at the

metal results in the development of a biofilm consisting of bacterial cells, their EPS

and, occasionally, some entrapped particulate material. Thus, a biofilm is the result of

a surface accumulation that is not necessarily uniform in time or space [28].

A dynamic system is formed at the biofouled interface, and different transport

processes will take place through the biofilm [29]. This is a consequence of the

biofilm structure, characterized by a high degree of hydration, with water comprising

nearly 90% [30].

In this way, microbial colonization of metals drastically modifies the classic

concept of the electrical interface commonly used in electrochemical studies.

Important changes in the type and concentration of ions, pH and redox condition are

induced by the biofilm, altering the passive behavior of the metal substratum and its

corrosion products, as well as the electrochemical parameters used to assess corrosion

rates [31].

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In MIC research, the organisms mainly implicated and primarily studied are

the sulphate reducing bacteria (SRB), which are distinguished by their sulphate

reduction capacity [32]. The other important groups involved are sulphate-oxidizing

bacteria which belong to genus Thiobacillus sp. [33] that are concerned with the

oxidation of elemental sulphur and other sulphur compounds as well as the oxidation

of iron. In spite of the initial unawareness that existed about MIC, seriousness of this

menace is slowly being recognized throughout the world in the past few decades,

which is well evinced by the ever increasing case histories [34]. MIC is accused for

failures in cooling water [35], industrial process plants [36], underground sub-tanks

[37], oil storages [38], alcohol industries [39], nuclear power stations [40], ocean

going tankers, off-shore concrete storage cells [41], heat exchangers [42] etc.

1.3.1. Economic losses due to MIC

It may seem surprising to look at the percentage of contribution of MIC

towards total corrosion failures. The pipeline corrosion was estimated in 1996 to cost

the gas industry about $840 million/year [43], and in 2001 it was estimated that the

annual cost of all forms of corrosion to the oil and gas industries was $13.4 billion, of

which MIC accounted for about $2 billion [44]. It has been estimated that 40% of all

internal pipeline corrosion in the industry can be attributed to microbial corrosion

[45].

1.3.2. Microorganisms associated with MIC

Microorganisms are thought to be primarily responsible for many metallic

corrosion failures and are classified by metabolic groups as follows: Those microbes

using inorganic material as a carbon source is called autotroph and those using

organic material as a carbon source called heterotroph. Microorganisms, which use

chemical as an energy source, are called as chemotroph and those using light as an

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energy source are called phototroph. Microbes those using inorganic compounds as

electron donor are called as lithotroph and organic compounds as an electron donor

are called as organotroph. Most of the microorganisms associated with MIC come

under the group chemotrophs referring to those which get their energy from a

chemical source rather than a light source. They are as follows:

Sulphate reducing bacteria (SRB)

Sulphur oxidizing bacteria (SOB)

Iron oxidizing/depositing bacteria (IOB)

Manganese oxidizing / depositing bacteria (MOB)

1.3.2.1 Sulphate reducing bacteria

Sulphate reducing bacteria (SRB) require strictly anaerobic conditions for their

growth. Dissimilatory sulphate reduction is always associated with a respiratory

chemiosmotic type of energy conservation. Several species of sulphate reducing

bacteria obligatorily depend on sulphate or another sulphur compound as electron

acceptor. They are generally curved rods (ranging from Vibrio, spirilloid, semi lunar

straight or coccoid) but their morphology is influenced by age and environment.

The production of hydrogen sulphide in aquatic systems was recognized as a

biologically mediated reduction of sulphate. Beijerinck [46] demonstrated microbial

sulphide production which resulted in the first isolation of a sulphate reducing

bacterium, a strict anaerobe that can be called Spirillum desulphuricans. Van Delden

[47] was the first to grow sulphate-reducing bacteria on lactate.

1.3.2.2. Sulphur bacteria

Sulphur bacteria are found in soils rich in sulphur, sulphur springs, acid mine

water and marine resources. They are small rod shaped cells (0.5 x 1.0 – 4.0 µm) with

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some species mobile by means of polar flagella. They are gram negative and obligate

aerobes. Energy is derived from the oxidation of reduced sulphur compounds

including sulphides, elemental sulphur, thiosulphates, polythionates and sulphites and

the final oxidation product is sulphate generally. Optimum temperature ranges from

28 to 30°C and pH between 2.0 to 3.5. Upper limit of growth is near pH.6.0 and

lower limit usually near 0.5.

1.3.2.3. Iron Bacteria

The iron bacteria are a diverse group of bacteria that are able to oxidize and /

or deposit iron oxides extracellularly or, sometimes, intracellularly. Iron bacteria are

chemolithotrophs deriving energy from the oxidation of ferrous iron Fe (II) with the

formation of ferric hydroxide Fe (III). Iron reducing bacteria can modify the

protective oxide film that forms over a mild steel surface causing the surface to

depolarize [48].

1.3.2.4. Reduction

The redox cycling of iron plays a major role in the biogeochemical cycling of

many elements in natural systems [49]. Within the reductive side of the iron redox

cycle, dissimilatory microbial systematic reduction of Fe (III) oxides has an extremely

broad range of influences on the aqueous solid phase geochemistry and behaviour of

natural and contaminant compounds in non sulfidogenic subsurface sedimentary

environments. The microbiology and environmental significance of bacterial Fe (II)

oxide reduction have been extensively reviewed over the last ten years [50-58].

Facultative anaerobes of Pseudomonas sp. capable of using ferric ions and sulphide as

terminal electron acceptors for anaerobic respiration use low molecular weight

compounds such as lactate as carbon source. Some of these bacteria attached to mild

17

steel coupons and removed a passive γ - Fe2O3 film and replaced it with a biofilm

under which pits developed.

1.3.2.5. Oxidation

Iron oxidizing bacteria which oxidize ferrous ion to ferric ion, sometimes

promote the corrosion of iron and stainless steel pipes in aerobic environments.

Corrosion products composed of ferric hydroxides and other metal salts form

tubercles which accumulate on the inner surface of the pipes. The area beneath the

deposit becomes anaerobic due to the oxygen diffusion barrier created by the

precipitate and the respiratory activities of the bacteria. The oxidized iron will either

oxidize sulphur compounds to form H2SO4 or induce anodic reaction directly.

13.2.6. Manganese Oxidizing Bacteria

Manganese oxidizing bacteria has the ability to catalyze the oxidation of

divalent, soluble Mn (II) to insoluble manganese oxides of the general formula MnOx.

It resulted in the accumulation of conspicuous and easily detectable extracellular

deposits of insoluble brown-black manganese oxides. Many different organisms have

the ability to catalyse manganese oxidation, including a diverse array of bacteria,

fungi, algae and even eukaryotes [59, 60]. Among the Prokaryotes, the ability to

oxidize manganese is also quite widespread [61,62] included are members of many

physicochemical and physiological groups of Cyanobateria, a diverse heterotrophic

rods, cocci, sheathed bacteria (Leptothrix), budding bacteria (Hypomicrobium) and

Pseudomonas sp and still the controversial Metallogenium sp.

Manganese distribution and chemical specification is kinetically controlled,

thus allowing for the inter convention of microbes and microbial products into system

are shown in Fig. 1.1. The possible mechanism of Mn (II) oxidation by bacteria is

broad

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18

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19

produce major changes in the solution chemistry at an immersed stainless steel

surface in freshwater [66, 67]. Biofilms are also capable of significantly displacing the

corrosion potential of stainless steels in the noble direction. This phenomenon known

as “ennoblement” has been observed in seawater environments by several researchers.

Considering the environmental relevance of these experiments, direct oxidation of

Mn(II) to Mn(IV) by microbes [68-70] is most likely a common process in natural

environments. The biofilms formed in seawater known as natural population biofilms

because they comprise a variety of bacteria and algae, have been shown to raise the

corrosion potential up to or above the pitting potential [71-73] for types 304 and 316L

stainless steel. The ennoblement of the corrosion potential is thought to be caused by

the development of a low oxygen condition together with the formation of hydrogen

peroxide [65, 74] or by the microbial deposition of MnO2 [69,75].

Manganese dioxide deposited on stainless steel coupons, was reduced

electrochemically to divalent manganese, Mn2+, obtaining two electrons from the

metal substratum. Manganese oxyhydroxide (MnOOH) was determined as an

intermediate product in this reaction. The presence of biomineralized manganese

dioxide as a cathodic reactant may increase the corrosion rate and/or the probability of

active corrosion. The manganese oxidizing bacteria, active in the process of

manganese dioxide biomineralization, may hypothetically use the product of MnO2

reduction, Mn2+, making the cycle perpetual. Recently, the electrochemical behaviour

of manganese depositors was explained by various researchers [76-80]. Cathodic and

anodic polarization curves were explained along with manganese oxidizers. During

cathodic polarization, the reduction peaks for iron and MnO2 was routinely observed

for the n-type stainless alloy, while they were consistently absent for the p -type.

1.4.1. Theories proposed on ennoblement

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1.4.1.1. Manganese oxidizing bacteria (MOB)

Dickinson et al [65]; Olesen et al [71] and Linhardt [81] have observed that the

first to identify the deposition of MnO2 as an agent responsible for ennoblement.

Passive film chemistry on stainless steel and conducting carbon surface is ennobled

by biomineralized manganese [75, 82]. The consequence of ennoblement on

chemistry of passive films on stainless steel was quantified using surface receptive

analytical techniques. Under well-defined laboratory conditions Stainless Steels were

ennobled to +450 mVSCE by biofilms of manganese oxidizing bacterium Leprothrix

discophra SP-6[69, 83]. Because the ennobled coupons, which may hypothetically,

contribute to their susceptibility to localized corrosion.

Many natural waters can support growth of MOB, when manganese-oxidizing

biofilms accumulate on surfaces of passive metals, there was a potential for

manganese redox cycling on the metal surface. This process was initiated by

depositing manganese oxidizing bacteria (MOB) and then reduced by the electrons

derived from anodic dissolution of the metal, which was corroding, and the

manganese oxides are reduced to divalent manganese ions [83]. However, since the

manganese, ions were immediately reoxidized, and the cycles continue.

The ennoblement of stainless steel by the manganese-depositing bacterium

Leptothrix discophora was studied to establish a chemical mechanism for

ennoblement in which manganese dioxide acts as a galvanic cathode. The relationship

among surface colonization, manganese deposition and open circuit potential for

stainless steel coupons exposed to batch cultures of L.discophora in nature. It can

cause several industrial problems, surface MnOx deposits shift Ecorr to the reduction

potential for the MnOx phase and depositions induces this process and confirm that

21

manganese biomineralization by L.discophora results were ennoblement process [83,

84].

Proposed Manganese mechanism:

The proposed half reaction that fixes Ecorr is

MnO2 +H2O +e- → MnOOH + OH-

Soluble Mn(II) microorganism⎯⎯⎯⎯⎯→ Insoluble manganese oxides(MnOx); X = 1,2.

(Insoluble brown or black manganese oxides)

Mn2+ + 1/2O2 + H2O microorganism⎯⎯⎯⎯⎯→ MnO2 + 2H+

The oxidation of Mn(II) to Mn(IV) is thermodynamically favored under aerobic conditions.

Mn2+(l) → Mn3+ (intermediate) →Mn4+

(S) (or)

Mn2+(l) → MnOOH (intermediate) →MnO2(S)(Mn4+ is adhered in metal surface)

Finally, Mn2+(l) → MnO2(S) (hydrothermal vent plume)

Reduction of deposited MnO2 takes place by electrochemical steps as given in reaction

Step1

MnO2 + H+ + e- → MnOOH (1.10)

Step 2

MnOOH + 3H+ + e- → Mn2+ + 2H2O (1.11) ----------------------------------------------------------------------------------

MnO2 + 4H+ + 2e- ⎯⎯→←⎯⎯ Mn2+ + 2H2O (Overall electrochemical reaction) (1.12)

-------------------------------------------------------

It has been reported that the manganese oxidation state for MnOx formed by L.

discophora increases as the oxide ages, from a value of +300 mV after 24 hrs to +500

mVSCE after 42 days [85, 86]. Such a change would increase the oxidizing power of

the oxide and thus shift the reduction potential to more positive values. Partitioning of

MnOx between suspended cells and sessile cells was a key issue in determining the

effectiveness of natural populations of manganese oxidizing microorganisms in

22

promoting ennoblement. However, the tendency for biofilms to form under these

conditions is undisputed, and it has been reported elsewhere that sessile growth

greatly enhances the manganese oxidation rate [87]. Manganese oxidizing bacteria

were widely distributed in nature and a variety of industrial problems ranging from

dirty drinking water to fouled heat exchangers. Numerous reports during the past 2

decades have also associated these bacteria with stainless steel corrosion, but the

biological role in the corrosion process has never been established [88]. Over

generally the same period, the ennoblement phenomenon and its corrosive

consequences have been recognized [89]. They take apart observations to a common

cause, biominerlization of MnOx and support the biological mechanism of

ennoblement. Based on these findings, ennoblement and the associated risk of

corrosion must be added to an already extensive list of industrial problems caused by

manganese oxidizing bacteria [71, 73].

1.4.1.2. Oxygen reduction theory

The oxygen plays a vital role in ennobled process on stainless steel surface. The

effect of the biofilm on a more resistant stainless steel will be to shift the corrosion

potential towards the noble direction. If the critical pitting potential for the alloy was

more active than reversible oxygen potential under exposure conditions, the corrosion

potential may shift nobly enough to initiate pitting corrosion [90]. The explanation for

the increase in cathodic activity depicted as being an increase in the exchange current

density for the oxygen reaction. An increase in current density for the oxygen reaction

was possible, considering that the extracellular slimes produced on stainless steel

surface by the bacteria known to bind heavy metals, and that some of these organo-

metallic complexes of Mn, Fe, Co, and Ni can serve as oxygen reduction catalysts

[91]. The mechanism of the noble shift in potential in the presence of bacterial film

23

probably involves a change in the kinetics of the oxygen reaction. Assuming that a

new oxide with rate enhancing properties towards oxygen reduction developed, at

noble potentials, oxygen reduction occurred on a modified oxide surface an enhanced

rate [65]. It was speculate about the ennoblement mechanism involving catalytic

enhancement of the oxygen reduction reaction by organo-metallic complexes [90].

1.4.1.3. Peroxide theory

Another proposed theory of ennoblement process influenced by hydrogen

peroxide [66]. One of the hypothesis is based on the presence of hydrogen peroxide

(H2O2), generated by microorganisms in the native biofilm. The concentration of

H2O2 in native biofilms formed on the coupons was detected after the exposures, and

OCP for the specimens were determined. During the summer season, the

concentration of H2O2 was higher than 10 ppm and OCP were nobler than +600

mVSHE. Other hand, during the winter season, the H2O2 concentration was lower than

2 ppm and OCP were less than +300 mVSHE [74]. The ennoblement of OCP was

counteracting by the decomposing H2O2 .The value of OCP for type 316L with H2O2

free biofilms was as less noble as that with no biofilm growth. A H2O2 free state was

reached by the addition of H2O2 decomposition enzyme (catalase or peroxidase) to

water and control of the water temperature [74]. The OCP shift is due to presence and

absence of H2O2. Its hypothesis also expressed ennoblement process on stainless steel

due to not only H2O2 concentration and also biomineralization of MnO2 [86].

1.4.1.4. Enzyme theory

The process of ennoblement has been correlated with enzyme theory such as extra

cellular polysaccharides (EPS) and biogenic manganese dioxide deposition. Enzyme

theory proposed to find out the role of cations and polymers on ennoblement process.

Ethylene diamine tetra acetic acid (EDTA) has been used to eliminate all cations in

24

biofilm. The ennoblement could not be observed in EDTA treated biofilm. It reveals

that without the presence of cations in the biofilm ennoblement does not take place on

passive materials [92]. The process of ennoblement was connected to organometallic

catalysis of the oxygen reduction reaction. The ennoblement of stainless steel in

seawater was due to the role of enzyme theory, produced by the biofilm causing

microbes [93]. The biofilm matrix has both anodic inhibitor and cation like

manganese which accelerates the corrosion of metals by ennoblement. Hence, it

concludes that without biogenic cations, there is no ennoblement, which contradicts

enzyme theory proposed who pointed out that enzymes trapped in EPS of biofilm

only contribute to ennoblement. Finally, it was proposed that a contributing factor to

ennoblement comes from relations between biogenic MnO2 and bacterial polymers in

the biofilm.

1.4.1.5. n / p - type semiconducting theory

Based on photo electrochemical studies, the oxide films on the conducting

materials were classified as n-type or p-type semiconductors based on the direction in

which their OCP shifts [94]. A negative shift in OCP is characteristics of an n-type

passive film. While a positive shift indicates a p-type film. The nature of n-type films

has excess cations with an abundance of free electrons, while p-type films have excess

anions with electron holes. The nature of cathodic curves was various n/p type semi

conducting oxide films in presence of biofilm. They used variety of metal in

constraint time of exposure take place only few metals are n or p- type semiconductor.

Most iron containing alloys tend to be n-type and their surface are dominated by ferric

oxides. It is also well known that biogenic MnO2 is an n-type semiconductor. The role

of biologically deposited MnO2 is on stainless steel surface with n-type semi

conducting film [76]. And then the bacteria for maintenance of a physiological carbon

25

: phosphorus ratio(40:1) during the growth of the cell, brings out the too little carbon

and too much phosphorus from the cell in the form of glucose -6-phosphate anion and

some amount of individual sugars in microfouling material. The proposed mechanism

is that the anion of organic phosphate combines with excess cations of n-type oxide

film leading to both strengthening of oxide film and a positive shift in the corrosion

potential [94].

1.5. Corrosion control and Prevention

1.5.1. Materials selection

Stainless steel is the almost an exclusive choice of pipelines designers. This is

for pipeline systems that are used to cooling water system; is also true for those

pipelines that are used to process industries. It is also the case for piping systems that

are used to distribute natural gas, water, water-refined liquids, and so on, to the end

user [95]. Metallic corrosion can be prevented by either changing the metal (or)

altering the environment or by separating the metal from the environment (or) by

changing the electrode potential of the metal. A large number of corrosion failures are

due to improper design of equipment and the corrosion control can be therefore

warranted at the design stage itself. Corrosion can be prevented by changing the

electrode potential by taking the metal to the immune region (or) passive region.

According to Pourbaix diagram this can be accomplished by making the potential of

the cathode equal to the open circuit potential of anode.

1.5.2. Corrosion inhibitors

1.5.2.1. Introduction

An inhibitor is a chemical substance which, when added to an environment in

small concentration, effectively checks, decreases or prevents the reaction of the

metal with the environment [96]. Corrosion inhibitors are added to many systems

including, cleaning pads, cooling systems, various refinery units, chemical operations,

26

steam generators, ballast tanks, oil and gas production units. Corrosion inhibitors are

used to protect metals from corrosion, including temporary protection during storage

or transport as well as localized protection required for example to prevent corrosion

that may result from accumulation of small amounts of an aggressive phase. One

example is brine, in a nonaggressive phase, such as oil. An efficient inhibitor is

compatible with the environment, is economical for application and produces the

desired effect when present in small concentrations.

1.5.2.2. Classification of inhibitors

Inhibitor selection is based on the metal and the environment. A qualitative

classification of inhibitors is presented in Fig. 1.2. Inhibitors can be classified into

environmental conditioners and interface inhibitors.

1.5.2.3. Environmental conditioners (scavengers)

Corrosion can be controlled by removing the corrosive species from the medium.

Inhibitors that decrease corrosivity of the medium by scavenging the aggressive

substances are called environmental conditioners or scavengers. In near-neutral and

alkaline solutions, oxygen reduction is a common cathodic reaction. In such

situations, corrosion can be controlled by decreasing the oxygen content using

scavengers [97].

1.5.3. Interface inhibitors

Interface inhibitors control corrosion by forming a film at the metal /

environment interface. Interface inhibitors can be classified into liquid and vapour

phase inhibitors.

1.5.3.1. Liquid phase inhibitors

27

Liquid phase inhibitors are classified as anodic, cathodic or mixed inhibitors

depending on whether they inhibit the anodic, cathodic, or both electrochemical

reactions.

1.5.3.2. Anodic inhibitors

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

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

form, or facilitate the formation of passivating films that inhibit the anodic metal

dissolution reaction. Anodic inhibitors are often called passivating inhibitors.

1.5.3.3. Cathodic inhibitors

Cathodic inhibitors control corrosion by either decreasing the reduction

process (cathodic poison) or by precipitating selectively on the cathodic areas

(cathodic precipitators). Cathodic poisons, such as sulfides and selenides, are

adsorbed on the metal surface; whereas in near neutral and alkaline solutions,

inorganic anions, such as phosphates, silicates, and borates, form protective films that

decrease the cathodic reaction rate by limiting and diffusion of oxygen to the metal

surface. Cathodic precipitators increase the alkalinity at cathodic sites and precipitate

insoluble compounds on the metal surface. The most widely used cathodic

precipitators are the carbonates of calcium and magnesium.

1.5.3.4. Mixed inhibitors

About 80% of inhibitors are organic compounds that cannot be designated

specifically as anodic or cathodic and are known as mixed inhibitors. 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 on the type of electrolyte [98]. Mixed inhibitors protect the

metal in three possible ways: physical adsorption, chemisorption and film formation.

28

Physical (or electrostatic) adsorption is a result of electrostatic attraction between the

inhibitor and metal surface. When the metal surface is positively charged (anionic)

inhibitors is facilitated (Fig. 1.3). Positively charged molecules acting in combination

with a negatively charged intermediate can inhibit a positively charged metal. Anions

such halide ions, in solution adsorb on the positively charged metal surface, and

organic cations subsequently adsorb on the dipole (Fig. 1.4a -1.4b).

Physically adsorbed inhibitors interact rapidly, but they are also easily

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

physically adsorbed inhibitor molecules. The most effective inhibitors are those that

chemically adsorb (chemisorb), a process that involves charge sharing or charge

transfer between the inhibitor molecules and the metal surface. Chemisorption takes

place more slowly than physical adsorption. As temperature increases adsorption and

inhibition also increases. Chemisorption is specific and is not completely reversible

[99].

1.5.3.5. Vapour phase inhibitors

Temporary protection against atmospheric corrosion, particularly in close

environments can be achieved using vapour phase inhibitors (VPI). Substances having

low but significant pressure of vapour with inhibitive properties are effective [100].

1.5.4. Mechanistic aspects of corrosion inhibition

1.5.4.1. Environmental conditioners (scavengers)

In near-neutral solution, the common cathodic reaction is oxygen reduction:

O2 + 2H2O + 4e- ↔ 4OH- (1.13)

Scavengers deplete the oxygen by chemical reaction: for example, hydrazine removes

oxygen by the following reaction:

29

5O2 + 2(NH2-NH2) ↔ 2H2O + 4H+ + 4NO2- (1.14)

1.5.4.2 Anodic inhibitors (Passivators)

The mechanism of anodic inhibition can be explained using the polarization

diagram of an active-passive metal [101]. In the absence of inhibitors, the metal

corrodes in the active state at a rate corresponding to point A in Fig.1.4. As the

concentration of inhibitor is increased, the corrosion rate also increases until a critical

concentration and a critical corrosion rate are reached. At the critical concentration,

there is a rapid transition of the metal to the passive state, and the corrosion rate is

decreased.

1.5.4.3. Cathodic inhibitors

In acid solution, the cathodic reaction is typically the reduction of hydrogen ions

to hydrogen, which combine forming hydrogen molecules:

H+ + e- ↔ H (1.15)

2H ↔ H2 (1.16)

In alkaline solution, the cathodic reaction is typically oxygen reduction [eq.(1.13)].

Cathodic inhibitors impede reduction reactions. Substance with high over potential for

hydrogen and those that form precipitates at the cathode are effective in acid and

alkaline solution, respectively. In this case, the slope of the anodic polarization curve

is unaffected, but the slope of the cathodic polarization curve is changed [102].

1.5.4.4. Mixed inhibitors (Adsorption)

Adsorption occurs as a result of electrostatic forces between the electric charge

on the metal and the ionic charges or dipoles on the inhibitor molecules. The potential

at which there is no charge on the metal is known as the zero-charge potential (ZCP).

The charge on a metal surface in a given medium can be determined from the

30

corrosion potential (Ecorr) and zero-charge potential. When the difference, Ecorr - ZCP,

is negative, the metal is negatively charged and adsorption of cations is favored, when

Ecorr - ZCP is positive, the metal is positively charged and adsorption of anions is

favored.

The charge on inhibitors depends on the presence of loosely bound electrons,

lone pair of electrons, π-electrons cloud and functional groups containing elements of

group V and VI of the periodic table. Most organic inhibitor possesses atleast one

functional group, regarded as the reaction center or anchoring group. The strength of

adsorption depends on the charge on this anchoring group [rather on the hetero atom

(i.e., atoms other than carbon including nitrogen and sulphur) present in the anchoring

group]. The structure of the rest of the molecule influences the charge density on the

anchoring group [99]. Water molecules adsorb on the metal surface immersed in an

aqueous phase. Organic molecules adsorb by replacing the water molecules:

[Inhibitor]soln + [nH2O]adsorbed ↔ [Inhibitor] adsorbed + [nH2O]soln (1.17)

where n is the number of water molecules displaced by one inhibitor molecule.

The ability of the inhibitor to replace water molecules depends on the

electrostatic interaction between the metal and the inhibitor. On the other hand, the

number of water molecules displaced depends on the size and orientation of the

inhibitor molecule. Thus, the first interaction between inhibitor and metal surface is

nonspecific and involves low activation energy. This process, called “Physical

adsorption”, is rapid and in many cases, reversible [103].

The adsorbed molecules involved in chemical reaction (chemisorption) from

bands with the metal surface. Chemisorption is specific and is not reversible. The

bonding occurs with electron transfer or sharing between metal and inhibitor. Electron

transfer is typical for transition metals having vacant, low energy electron orbitals.

31

Inhibitors having relatively loosely bound electrons transfer charge easily. The

inhibition efficiency of the homologous series of organic substances differing only in

the heteroatom is usually in the following sequence: P > S > N > O. On the other

hand, electro negativity, that is, the ability to attract electrons, increases in the reverse

order.

32

Fig. 1.3. Adsorption of negatively charged inhibitor on a positively charged metal surface.

Fig.1.4(a). Positively charged inhibitor molecule does not interact with

positively charged metal surface.

Fig. 1.4(b). Synergistic adsorption of positively charged inhibitor molecule

and anion on a positively charged metal surface.

33

1.6. Biocides

Biocides are the most underused, misunderstood, and misapplied chemical

products in the process industry for many reasons. Biocides are used to combat a

problem that is subtle and difficult to detect. In general, biocides are needed to control

the activity of the bacteria in a system. However, biocides alone cannot solve a

microbiological problem. Very few sample and reliable means of monitoring are

available to the supplier or the end user, and the benefits of biocide use take a long

time to become evident. Finally, once bacteria are well established it is nearly

impossible to control without drastic measures [104]. Hence, identification of

microbes and also the role of inhibitors on microbial growth are to be studied in

detail.

1.7. Summary

This chapter mainly deals with the definition of corrosion followed by various forms

of corrosion. Special emphasize is given to microbiologically influenced corrosion

and the economic losses caused by MIC. The effect of various corrosion causing

microorganisms such as sulphate reducing bacteria, iron oxidizing bacteria and

manganese oxidizing bacteria and their morphology, biochemistry of each

microorganisms are discussed in detail. The mechanistic aspects of corrosion of

passive metals caused by ennoblement process are also dealt with. Some recent

research in the area of microbiologically influenced corrosion and some open

problems for further research have been also discussed in this chapter. Besides,

control the corrosion, an efficient corrosion inhibitor should be able to bind

irreversibly to the metal surface, provide good surface coverage and at the same time

with very good biocidel effect. It explains the brief history of inhibitor of corrosion

and the importance of corrosion inhibitors along with their potential applications in

cooling water / transporting pipelines.

34

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