Material substitution is of limited value since, as noted, MIC affects almost all industrial metals. There are, however, several materials which are impervious or resistant to MIC where cost and compatibility justify their use. These materials are generally extremely expensive and in some cases, such as titanium, require specialized fabrication methods. In the case of underground pipelines and other fluid transport and storage systems, alternate non-metallic materials such as PVC have significantly limited MIC where these materials can be substituted. Local building codes, however, often exclude this option in structural applications.

Design to minimize low-flow areas, crevices, welds, etc. can reduce the likelihood of MIC but there are severe limitations to how far this approach can be taken in the design and manufacture of practical systems. Biocides are widely used to treat incoming water. These, however, are highly toxic and expensive, and require regular monitoring of concentration. Their toxicity and potential contaminative effect precludes their use in any food products system and with many process fluids.

The parameters in which MIC can occur are extremely varied and include multiple bacteria species, a broad range of affected materials and almost endless environmental diversity. As a result, MIC prevention and mitigation is equally varied. Accurate analysis of the cause and effects of each individual MIC failure is an essential first step in selecting from this range of solutions.

Microbiologically Induced Corrosion (MIC) Failures

MIC Failure Example 1

This sequence shows several steps in the analysis of pitting corrosion in stainless steel tubing from a water bottling plant. The plant processes purified water, normally a media relatively immune to MIC. However, hydrostatic testing, performed during installation of the process piping, introduced anaerobic bacteria which adhered to several tube ID welds and adjacent areas, resulting in MIC and perforation of the tubes (above).

The perforations were examined using a Scanning Electron Microscope (SEM) which revealed biological adhesions in and around the pits. Several entries leading to apparent sub-surface voids were also observed (shown at arrows).

Micro-chemical analysis of the biological adhesions, by Energy Dispersive Spectroscopy (EDS), identified high levels of carbon (C), oxygen (O) and sulfur (S). These elements are consistent with sulfur reducing and oxidizing anaerobic bacteria species implicated in MIC.

Polished cross sections through the pits revealed internal cavities in the 0.060” thick tube wall, again, a hallmark of anaerobic bacteria which adhered to the tube ID surface and migrated to these oxygen depleted cavities formed by corrosive attack from their acidic bi-products. Because MIC usually initiates at the ID of tubing, extensive corrosion and eventual perforation occur before any visible evidence of attack is apparent externally.

MIC Failure Example 2

“Weeping” of fluid from systems is a precursor to full blown perforation by MIC. The source of this “weeping” is often a subtle discoloration of the tube or vessel surface as shown at the center of the circled area.

Examination of these features by Scanning Electron Microscopy reveals fine micro-pitting and a “sponge” like morphology as the interior MIC attack nears the outer surface.

Probing of this “sponge” like surface collapsed the thin crust of remaining metal, exposing the sub-surface cavity created by anaerobic bacteria and their sulfuric acid bi-products.

A cross section of the discolored feature reveals the extent of corrosive MIC damage which has penetrated completely through the tube wall thickness.

MIC Failure Example 3

Pitting and general corrosion are both associated with MIC, sometimes in the same corrosion failure. The interior of this carbon steel storage tank exhibits extensive general corrosion.

Examination by Scanning Electron Microscopy revealed numerous tubercles on the corroded tank ID surface. Tubercles are found in association with MIC producing iron oxidizing anaerobic bacteria.

The interface of the tubercle with the metal substrate beneath it offers an oxygen depleted environment that is ideal for anaerobic MIC bacteria. Ultrasonic cleaning of a section of the corroded tank to remove the tubercles revealed small deep pits suggesting connected sub-surface cavities consistent with MIC by sulfur reducing bacteria.

Cross sections of the tank confirm anaerobic MIC bacterial activity by the presence of characteristic sub-surface voids. This failure demonstrates the symbiotic relationship often found between two or more MIC implicated bacterial species, producing two corrosion modes (general and pitting) in a single corrosion failure.

See Part 1 of this series – Biological Corrosion of Metals

See Part 2 of this series – Microbiologically 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