Microbial corrosion
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Microbial corrosion, or bacterial corrosion, is a corrosion caused or promoted by microorganisms, usually chemoautotrophs. It can apply to both metals and non-metallic materials.
Some sulfate-reducing bacteria produce hydrogen sulfide, which can cause sulfide stress cracking. Acidothiobacillus bacteria produce sulfuric acid; Acidothiobacillus thiooxidans frequently damages sewer pipes. Ferrobacillus ferrooxidans directly oxidizes iron to iron oxides and iron hydroxides; the rusticles forming on RMS Titanic wreck are caused by bacterial activity. Other bacteria produce various acids, both organic and mineral, or ammonia.
In presence of oxygen, aerobic bacteria like thiobacillus thiooxidans, thiobacillus thioparus, and thiobacillus concretivorus, all three widely present in the environment, are the common corrosion-causing factors.
Without presence of oxygen, anaerobic bacteria, especially Desulfovibrio and Desulfotomaculum, are common. Desulfovibrio salixigens requires at least 2.5% concentration of sodium chloride, but D. vulgaris and D. desulfuricans can grow in both fresh and salt water. D. africanus is another common corrosion-causing microorganism. The Desulfotomaculum genus comprises of sulfate-reducing spore-forming bacteria; Dtm. orientis and Dtm. nigrificans are involved in corrosion processes. Sulfate-reducers require reducing environment; the electrode potential of at least -100 mV is required for them to thrive. However, even a small amount of produced hydrogen sulfide can achieve this shift, so the growth, once started, tends to accelerate.
Layers of anaerobic bacteria can exist in the inner parts of the corrosion deposits, while the outer parts are inhabited by aerobic bacteria.
Some bacteria are able to utilize hydrogen formed during cathodic corrosion processes.
Bacterial colonies and deposits can form concentration cells, causing and enhancing galvanic corrosion. [1]
Bacterial corrosion may appear like pitting corrosion. Anaerobic corrosion is evident as layers of metal sulfides and hydrogen sulfide smell. On cast iron, a graphitic corrosion selective leaching may be the result, with iron being consumed by the bacteria, leaving graphite matrix with low mechanical strength in place.
Various corrosion inhibitors can be used to combat microbial corrosion. Formulas based on benzalkonium chloride are common in oilfield industry.
Microbial corrosion can also apply to plastics, concrete, and many other materials.
Hydrocarbon utilizing microorganisms, mostly Cladosporium resinae and Pseudomonas aeruginosa, colloquially known as "HUM bugs", are commonly present in jet fuel. They live in the water-fuel interface of the water droplets, form dark black/brown/green, gel-like mats, and cause microbial corrosion to plastic and rubber parts of the aircraft fuel system by consuming them, and to the metal parts by the means of their acidic metabolic products. They are also incorrectly called algae due to their appearance. FSII added to the fuel acts as a growth retardant for them. There are about 250 kinds of bacteria that can live in jet fuel, but only less than a dozen is meaningfully harmful. When HUM bugs are detected in fuels and fuel oils, a biocidal additive (eg. BIOBOR) has to be added.
