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Jul-2014

Microbiological causes of corrosion

Many of the most serious problems with corrosion of refinery equipment have a microbiological basis

JAYA RAWAT, NEHA SHARMA and APOORVE KHANDELWAL
Bharat Petroleum Corporate R&D Center
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Article Summary
Microbiologically induced corrosion has a potential impact on a wide range of industrial operations, including refining. Problems associated with MIC afflict water handling operations and manufacturing processes in oil and gas production, pipelining, refining, petrochemical synthesis, and other industrial sectors. Most of the commercially used metals and alloys such as stainless steels, nickel and aluminium based alloys and materials such as concrete, asphalt and polymers are readily degraded by microorganisms. Protective coatings, inhibitors, oils and emulsions are also subject to microbial degradation. Quality research and analyses of corrosion failure have provided deeper understanding of the causes of microbial corrosion.

Microbiologically induced corrosion is not in itself a form of corrosion, but rather a process that can influence and even initiate corrosion. It can accelerate most forms of corrosion, including uniform corrosion, pitting corrosion, crevice corrosion, galvanic corrosion, intergranular corrosion, dealloying, and stress corrosion cracking. MIC deteriorates pipes, tanks or vessel surfaces by pitting corrosion. The formation of slime or tuberculation nodules can cause blockages or reduce flow. Low flow or stagnant conditions make systems more susceptible to microbial growth. Microbiologically induced corrosion can degrade or cause to fail many different types of system. The ultimate effect is the premature failure of metal components. To understand the causes and effects, it is necessary to understand the chemical, metallurgical and microbiological aspects of microbial corrosion.

Microbial activities act as a driving force for biocorrosion. Microbiologically induced corrosion causing organisms are sulphate reducing bacteria (Desulphovibrio, Desulphotomaculum, and Desulphomonas sp.), iron reducing bacteria (Gallionellea ferrugine and Ferrobacillus sp.), acid producing bacteria (Pseudomonas, Aerobacter, and Bacillus), and sulphur oxidising bacteria (Thiobacillus sp.).

Microorganisms are ubiquitous in nature and grow at very rapid rates in soil, water and air. They show extreme tolerance to varying environmental conditions such as acidic and alkaline pH, low and higher temperatures as shown in Table 1. Microorganisms and their metabolic products, including enzymes, exopolysaccharides, organic and inorganic acids, and volatile compounds, such as ammonia or hydrogen sulphide, can alter electrochemical processes at the biofilm-metal interface. Both aerobic and anaerobic organisms play an important role in the initiation, propagation, and inhibition of corrosion in different ways by their different energy deriving pathways.

Fungi and algae may also be involved in metal deterioration. In fuel and oil storage tanks, fungal species such as aspergillus, penicillium and fusarium may grow on fuel components and produce carboxylic acids which corrode iron.

Role of biofilms
The term biofilm refers to the development of microbial communities on submerged surfaces in aqueous environments. Biofilm influences the physic-chemical interactions between metal and environment, frequently enhances corrosion, and leads to deterioration of the metal. Biofilm induces many changes in the type and concentration of ions, pH values, and oxidation reduction potential.
Parameters affecting the development of biofilms include:
• Temperature of the system or ambient temperature
• Water flow rate past the surface
• Nutrient availability
• Surface of the substratum
• pH of water in the system
• Effectiveness of biofouling remedial measures.

Biofilm formation is the result of an accumulation process – not necessarily uniform in time or space – that starts immediately after immersion of metal in the aqueous environment. The growth of biofilm is considered to be a result of complex processes involving transport of organic and inorganic molecules and microbial cells to the surface, adsorption of molecules to the surface and initial attachment of microbial cells followed by their irreversible adhesion facilitated by production of extracellular polymeric substances (EPS). Once attached, the organisms begin to produce material termed extracellular biopolymer, or ‘slime’ for short. The amount of EPS produced can exceed the mass of the bacterial cell by a factor of 100 or more. The extracellular polymer that is produced provides a more suitable protective environment for the survival of the organism. The EPS in biofilm consists of lipids, polysaccharides, proteins and nucleic acids. The content of these macromolecules in EPS varies, depending on bacterial species and growth conditions. One of the important properties of EPS is their ability to complex with metal ions. This initial film is able to alter the electrostatic charges and wetability of the metal surface, facilitating its further colonisation by bacteria. 
For example, in a marine environment the presence of a biofilm can accelerate corrosion rates of carbon steel by several orders of magnitude. After colonisation and formation of biofilms, maintenance and operational problems arise, including a reduction in flow, heat transfer rates, fouling, corrosion, and scale.

Anaerobic sulphate-reducing bacteria, such as Desulphovibrio sp., is the most often considered bacteria for microbial corrosion. Anaerobic regions develop beneath the biofilm, even in aerobic bulk water environments, thus allowing sulphate reducing bacteria a very favourable environment for growth. This organism will seek out and colonise areas deficient in oxygen, such as those found within porous corrosion tubercles, within biofilms, and under debris. This bacterium is responsible for severe metal loss in industrial water systems. This type of corrosion is easily recognisable from the characteristic sulphide by-product present within the corrosion cell. Sulphate reducing bacteria primarily cause corrosion by utilising the molecular hydrogen produced at the cathode, thereby depolarising it.

The mechanisms of microbial corrosion of metals may be site specific and tend to vary with the environment, type of organism, type of metallurgy and the surface characteristics of the metal. Microorganisms may induce corrosion processes directly or indirectly. Various mechanisms have been proposed to explain the active participation of microbes in corrosion. Some microbes can produce metabolites that are acidic (for instance, sulphuric acid by sulphur oxidising bacteria) or facilitate the local depassivation or dissolution of the protective films or corrosion products on the metal surface (for instance, biogenic sulphides destabilising the copper oxide film on Cu-Ni alloys, a marine Vibrio reducing insoluble corrosion product to soluble Fe2+).

Microorganisms can consume substances in biofilm and lead to formation of concentration gradients of chemical species that are important for their metabolic activities, which are also electrochemical reactants (oxygen and protons) relevant to the underlying substratum. Mechanism on the basis of type of organisms can be classified into two categories.

Anaerobic corrosion
Pipelines, offshore oil platforms and underground structures are found to be quite vulnerable to microbial corrosion which is assumed to be mediated with different groups of microorganisms respiring with oxidised compounds such as sulphide and nitrite. Sulphate reducing bacteria are proposed to be mainly responsible for anaerobic corrosion, specifically in environments with high sulphate concentrations such as seawater. Von Wolzogen K¨uhr and van der Vlugt in 1934 first postulated the most widely accepted theory for the mechanism of corrosion of iron and steel by cathodic depolarisation. These organisms reduce sulphate to sulphide. Cathodic hydrogen formed on a metal surface by active corrosion can specifically promote growth of organisms, including sulphate reducing bacteria that are able to use hydrogen in their metabolism. Severe corrosion cells develop as sulphide, produced by the microbial reduction of sulphate, combines with ferrous ions, released by the corrosion process, to produce insoluble black iron sulphides.
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