Microbiologically Influenced Corrosion is the deterioration of metals as a result of the metabolic activity of microorganisms. Microbiologically influenced corrosion of carbon steels, stainless steels, aluminum alloys and copper alloys caused by the vital activities of bacteria(microorganism) in waters and soils with pH 4~9 and temperature 10oC~50oC. These bacteria can be classified as aerobic (requires oxygen to be active) or anaerobic (oxygen is toxic to the bacteria). Sulphate reducing bacteria or SRB, is anaerobic, which is responsible for most instances of accelerated corrosion damages to ships and offshore steel structures. Iron and manganese oxidizing bacteria are aerobic and are frequently associated with accelerated pitting attacks on stainless steels at welds.

Carbon steels may experience random pitting, general corrosion, or severe degradation in flow as a result of MIC. Tubercles (comprised of corrosion products, microbes, the sticky exopolymer associated with both living and dead cells, and debris) often form on carbon steel pipes and other components. The tubercles create a hydraulic resistance to cooling water flow as well as sites for additional microbial activity. Tubercles can grow together, eventually becoming a severe impediment to cooling water flow. Pitting is also often observed beneath tubercles as mechanical and chemical conditions are established that encourage localized corrosion effects. Once tubercles have formed, microbial activity at the metal surface (i.e., beneath the tubercle) is effectively insulated from the bulk fluid, its level of aeration, and most water treatments. A wide variety of bacteria, from the aerobic acid formers to the more well known anaerobic sulfate reducing bacteria, can contribute to tuberculation and pitting of carbon steels.

Stainless steels are also subject to localized attack as a result of MIC. Occlusion of pipes is generally not observed, however, tubercles (much smaller than those associated with carbon steels) are often present. Rusty streaks appear on piping or tanks, often running both up and down the surfaces.

As shown in the scanning electron micrograph to the right, MIC of stainless steel weld metal often produces through-wall pits as essentially all of the austenite is removed from the duplex microstructure, leaving behind a skeleton of delta ferrite.

Microbiologically Influenced Corrosion of stainless steels most often produces pitting at welds, generally in the weld metal itself. Closed pits, typified by tiny entrance and exit holes, with a cavernous subsurface defect, are often observed as illustrated below.

Copper-based alloys are also subject to MIC despite copper's reputation as a material toxic to most organisms. Failures in these materials are generally manifested by pitting, erosion-corrosion, and occasionally by stress corrosion cracking.

Industries are affected by MIC as

• Chemical processing industries: stainless steel tanks, pipelines and flanged joints, particularly in welded areas after hydrotesting with natural river or well waters.
• Nuclear power generation: carbon and stainless steel piping and tanks; copper-nickel, stainless, brass and aluminum bronze cooling water pipes and tubes, especially during construction, hydrotest, and outage periods.
• Onshore and offshore oil and gas industries: mothballed and waterflood systems; oil and gas handling systems, particularly in those environments soured by sulfate reducing bacteria -produced sulfides.
• Underground pipeline industry: water-saturated clay-type soils of near-neutral pH with decaying organic matter and a source of sulfate reducing bacteria.
• Water treatment industry: heat exchangers and piping.
• Metal working industry: increased wear from breakdown of machining oils and emulsions.
• Sewage handling and treatment industry: concrete and reinforced concrete structures.
• Marine and shipping industry: accelerated damage to ships and barges.
• Highway maintenance industry: culvert piping.
• Aviation industry: aluminum integral wing tanks and fuel storage tanks.

Mechanisms
MIC is caused by specific genera of bacteria. These bacteria feed on nutrients and other elements found in waters and soils. Sea water is a primary source of sulphate reducing bacteria. The biological activities modify the local chemistry (acid-producing) and render it more corrosive to the metals.

For example, iron-oxidizing bacteria can perforate a 5mm thick 316 stainless steel tank in just over a month.

Prevention
MIC can be prevented by the following methods

• Regular mechanical cleaning if possible
• Chemical treatment with biocides to control the population of bacteria
• Complete drainage and dry-storage