microbiological influenced corrosion

Lecture 25: MIC – Electrochemical Aspects and General Mechanisms

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Course Title: Advances in Corrosion Engineering

Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore

Lecture 25

MIC – Electrochemical Aspects and General Mechanisms

Keywords: Electrochemical Aspects, Direct Mechanism, Indirect Mechanism.

Eh and pH are the important environmental parameters controlling the growth and

activity of various aerobic and anaerobic organisms. The stability regions of various

types of microorganisms corresponding to optimum activity can be defined through

Eh – pH diagrams. Eh-pH diagram for sulphur – water – oxygen system wherein the

stability and growth regions of various types of microorganisms are represented will

be useful in the understanding of MIC. Iron and sulphur-oxidising acidophilic

bacteria such as Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans

grow under higher oxidising potentials and acid pH levels. Sulphur and thiosulphate

oxidising autotrophs such as Thiobacillus thioparus have optimum activity at near

neutral pH ranges and relatively higher oxidising potentials. Sulphate reducing

bacteria (SRB) grow under reducing and neutral pH environments. Iron oxidising

heterotrophs are stable and active at neutral pH and higher oxidising conditions.

Stability regions for some acidophilic chemolithotrophs and anaerobic heterotrophs

such as SRB are shown in a S-H2O – O2 diagram in Fig. 25.1. Ferric-ferrous ratios at

high acidic pH levels determine the potential limits for Acidithiobacllus ferrooxidans

where as sulfate formation from sulphide oxidation at acidic pH dictates the stability

limits for Acidithiobacillus thiooxidans. Sulfate reducing bacteria are anaerobes

having optimum growth at neutral pH ranges. Stability region for SRB corresponds

to reducing potentials at neutral to mildly alkaline pH.


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Fig.25.1 Stability regions for Acidithiobacllus and SRB in a S-H2O-O2 diagram.

Eh-pH corrosion diagrams can be readily constructed for various metal-water-

oxygen systems in the presence of micro-organisms to predict the regions of MIC,

immunity and passivation. Common Eh-pH diagrams cannot represent the corrosion

behavior of metals and alloys in the presence of micro-organisms. Superimposition

of bacterial stability regions on these diagrams may bring about significant changes

in the regions of corrosion, immunity and passivation. There are instances where

electrochemical prediction of corrosion went astray when microbial activities at the

respective Eh and pH conditions were also considered. Due to microbial growth and

biofilm formation, corrosion and protection regions in such diagrams can shift.

Principal slime forming bacteria such as Bacillus subtilis, Bacillus cereus and

species of Flavobacterium, Aerobacters and Pseudomonas are present in soil

environments. Pseudomonas can grow in systems containing hydrocarbon sources

such as oils and emulsions using hydrocarbons as energy source.


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Algae range from single cell plants to multicellular species of diverse forms and

shapes. They contain coloured pigments, the most important of which is the

chlorophyll. Algae generally grow on moist surfaces such as cooling towers, screens

and distribution systems. Some common algae groups are blue-green algae, the green

algae and the diatoms. Owing to their ability to produce corrosive organic acids,

oxygen and metabolites corrosion can be promoted.

Fungi are similar to algae but do not contain chlorophyll. Mould fungi are

filamentous in form but most of yeast fungi are unicellular. Some corrosion-causing

fungi are Aspergillus niger, Aspergillus fumigatus, Penicilium cyclospium and

Cladosporium resinae. Production of various types of organic acids such as oxalic

acid, citric acid and gluconic acid by fungal metabolism create corrosive

environment.

Direct and indirect mechanisms

Reactions involved in MIC are based on electrochemical reactions similar to general

corrosion principles.

Anodic: M = M++

+ 2e

Cathodic: O2 +4H+ + 4e = 2H2O (aerated, acidic)

O2 + 2H2O + 4e = 4OH- (aerated, neutral and alkaline)

2H+ + 2e = H2 (in the absence of oxygen in acid solutions)

Microorganisms, very often contribute towards corrosion without being solely and

directly responsible for the failure. Both direct and indirect mechanisms are

involved. Microorganisms can play both direct and indirect roles. In direct attack

mechanisms, the organisms interlinks an electrode reaction (anodic or cathodic)


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through metabolism, while indirect mechanisms involves indirect microbial

contribution to corrosion through creation of corrosive environments, such as

differential aeration cells, acidic reaction products and other metal solubilising

bioreagents

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General mechanisms can therefore be seen in different perspectives:

Changes in dissolved oxygen levels through microbial growth leading to

formation of concentration cells.

Biodegradation of additive reagents present in lubricants and emulsions.

Biogeneration of corrosive products and hydrogen consumption.

Microbiological breakdown or disruption of organic paint coatings, plastic

fittings and linings, protective films and inhibitors.

Typical examples of some of the corrosive metabolic products are illustrated below:

Both organic and inorganic acids can be produced by microbial metabolism.

Oxidation of inorganic sulphur compounds by Acidithiobacillus group of bacteria

to produce sulfuric acid.

Oxidation of iron sulphides by Acidithiobacillus ferrooxidans to produce acidic

ferric sulfate.

In the presence of organic carbon such as sucrose, fungi such as Aspergillus

generate oxalic, citric and gluconic acids.

Exopolysaccharides and bioproteins secreted by Bacillus species.

Several bacterial enzymes are electrocatalysts.


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At environmental pH, the following bacterial mechanisms are relevant.

Corrosion by cathodic depolarization attributable to Sulfate Reducing Bacteria

(SRB) which contain the enzyme, hydrogenase.

Corrosion by differential aeration cells due to deposits and biofilms formed by

iron bacteria and other slime bacteria.

Corrosive products such as organic sulphides, mercaptides, amines, ammonia,

phosphorous compounds and surfactants.

Organic corrosion inhibitors such as diamines and aliphatics are used as nutrients by

bacteria. For example, Nitrosomonas and Nitrobacter oxidise ammonia and amines

to nitrite and nitrate, destroying the inhibition properties of several inhibitors. Ferric

oxide coatings are degraded by Pseudomonas, exposing the base metal for corrosion.

Iron sulphide films are broken down by Sulphate Reducing Bacteria. Protective

aluminium oxide layers (passive film) on aluminium and its alloys could be

destroyed by the fungus, C. resinae.

Bacterial attachment and Biofilms.

Under environmental conditions, submicroscopic bacterial cells can be considered as

living colloids. Bacterial suspensions as in water and soil exhibit colloidal behavior.

At natural pH, bacterial surfaces are negatively charged. Bacterial cell walls contain

many types of cationic, anionic and nonionic polymeric substances such as

polysaccharides, phospholipids and proteins. Cell surface hydrophobicity and

hydrophilicity depends on cell wall architecture. Surface – chemical characteristics

of microorganisms are important since they govern their adhesion behavior to solid

substrates. Bacterial adhesion and biofilm formation on metals and alloys are initial

events in ultimate metallic corrosion. Forces of bacterial adhesion (attachment) need

to be understood to get an insight into biofilm formation mechanisms. A fully –

developed microbial biofilm may consist of both micro – and macroorganisms


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along with their metabolic products and chemical reaction products. It should be

understood that initial stages in biofilm formation invariably involves only bacterial

attachment. Attached and colorized bacterial cells prepare the foundation on which

macroorganisms subsequently attach and grow. Under the circumstances, it becomes

imperative to understand forces and mechanisms of bacterial attachment to metals

and alloys in different environments (water, air and soil).

Attachment of Acidithiobacillus organisms on (A) aluminium (B) mild steel and (C)

stainless steel are illustrated in Fig. 25.2.

Fig. 25.2. Scanning electron micrographs illustrating attachment of Acidithiobacllus sp on (A) aluminium, (B) mild

steel and (C) stainless steel surfaces.


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Attachment of sulfate reducing bacteria such as Desulfovibrio and

Desulfotomaculum on titanium surfaces is illustrated in Fig 25.3

Fig. 25.3 Scanning electron micrographs showing SRB attachment and biofilm formation on titanium surfaces

microbiological influenced corrosion

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