Failure Analysis

Microbiologically Induced Corrosion

Posted on 03. Jun, 2010 by Rob in Corrosion Analysis

Microbiologically Induced Corrosion (MIC) occurs as both general corrosion and pitting corrosion, though localized pitting is the more definitive form and more likely to result in dramatic system failures. Low flow areas in circulating systems such as heat exchangers and process piping are particularly susceptible since these “stalled flow” locations provide bacteria with the opportunity to attach to the tube or pipe surface. At both microscopic and macroscopic features, fluid flow “stalling” occurs at any crevice, joint, weld, or imperfection and these are typical locations for MIC. Interrupted flow in circulating fluid systems such as weekend, over night, or even brief maintenance shutdowns, also provides the opportunity for bacterial adhesions and the initiation of MIC. Once the bacteria are established, the corrosion process will proceed even after flow is restored. Hydro-static testing, in which a system is filled with fluid, pressurized, leak tested and drained – but often not completely dried – is a sequence repeatedly seen in the initiation of MIC failures. This testing usually immediately precedes placing the system in service, and failure may not occur for several months. When failure does eventually occur, the hydro-static test and stagnant fluid residue are often overlooked and the cause of failure is misdiagnosed as chloride induced corrosion.

Static fluid systems such as sumps and storage tanks are receptive environments for MIC. Corners, fittings, joints and welds are again vulnerable and in the case of fuels and non-water soluble fluids, the interface between the fluid and any water contaminant is particularly susceptible. MIC in underground storage tanks and pipelines, particularly in moist clay soils, has been widely observed despite protective tar, asphalt or polymeric coatings. While effective in preventing conventional corrosion, any de-lamination or bond failure of the coating provides an ideal bacterial growth environment.

Virtually all industrial metal alloys are subject to MIC, with the exception of titanium alloys. Testing suggests that the few stainless steel alloys containing molybdenum at levels of 6% or more are also highly resistant to MIC. These limitations severely restrict material substitution as a strategy to resolve MIC failures.

Carbon Steels – Generally more susceptible to conventional corrosion processes, carbon steels are also widely affected by a broad range of MIC implicated bacteria. Considerations of cost and ease of fabrication make carbon steel the material of choice in many water storage and transport applications, as well as the most widely reported material in MIC failures. Protective coatings generally have limited preventive value.

Stainless Steels – These alloys develop tough chromium oxide surface layers from which they derive their corrosion resistance. Once the oxide layer is breached, however, they are particularly vulnerable to both conventional and MIC corrosion. Welds are highly susceptible due to potential alloy inhomogenaity. Highly stressed components are potential initiation sites for MIC induced stress corrosion cracking.

Aluminum Alloys – One of the earliest high profile cases of MIC was of aluminum jet aircraft fuel tanks in the 1950’s. Water contamination in the kerosene based fuel and condensation in the tanks provided the media in which the bacteria multiplied. Research indicates some bacteria species may utilize kerosene and other fossil fuels as a nutrient source. Since this landmark case, MIC has been widely recognized as a significant problem in both tank and structural aircraft components.

Copper Alloys – Typically, higher alloy content lowers the corrosion resistance of copper alloys, although relatively pure copper is also susceptible to MIC. Copper and copper alloys are effected by a wide range of microbial bi-products including carbon dioxide, hydrogen sulfide, and organic and inorganic acids. Cold worked or stressed copper alloy components are especially susceptible to stress corrosion cracking from ammonia and the bacteria that generate it. Selective corrosion, such as de-zincification in brass alloys, has also been observed in MIC failures.

Nickel Alloys – These alloys are often used in high pressure, high flow rate applications such as pumps, turbine blades, valves and evaporators. Nickel alloy components in these systems are vulnerable to MIC during shut down intervals and stagnant water conditions. Nickel-chromium alloys exhibit a degree of resistance to MIC.

See Part 1 of this series – Biological Corrosion of Metals

See Part 3 of this series – Microbiologically Induced Corrosion Prevention and Analysis

Tags: biological corrosion, MIC, Microbiologically Induced Corrosion