It is important to understand the basic stoichiometric equations of corrosion and the associated electrochemical reactions to comprehend the resultant degradation in the different types of metals that may be affected. All corrosion processes, including MIC, are electrochemical processes. Corrosion is a transference of positive and negative ions to change oxidation states which causes a removal of structural or base material and the formation of a compound either helpful or hurtful to the system. It can be autocatalytic (growing without external help) or systemdependent (needing to be “fed” with nutrients).

In MIC, some types of microbial reactions create ammonia. Fittings and piping systems are designed to operate at stress levels below that of the yield strength for the composition alloy. However when some copper or brass alloys are used in the presence of ammonia, the ammonia sensitizes the metal so that residual and/or service stress levels are exceeded and intergranular stress corrosion cracking occurs. Ammonia attacks at the grain boundaries, sensitizing them and allowing slow crack growth.

Copper, stainless steel, aluminum and some other less commonly used alloy surfaces rely on passive films or specific elemental atomic layers to combat degradation and corrosion. Microbes and their chemical byproducts are very destructive to some alloys. Sometimes the microbes associated with MIC may come and go without detection but leave surface characteristics that disrupt the passive layer on these alloys. Brasses, bronzes, and copper systems, such as piping, fittings and heat exchanger systems are more susceptible than other alloys in this group.

Copper-Base Alloys
These alloys are most susceptible to MIC because they are soft and can incur impingement that can cause erosion and other wear problems if their passive layer is interrupted. Newly installed products of these alloys typically do not build up their protective layer for a period of time. If MICrelated bacteria are present, or there are elements like chlorides or sulfides in the initial solutions contacting the metal, the passive or protective layers form other complex chemical compounds rather than copper oxide. The problem with these complex chemical compounds are their strength; they are typically much weaker than copper oxide and break down at lower flow rates allowing a variety of mechanisms to attack or degrade the substrate. If systems with copper-based systems coated with these poor passive films (the complex chemical compounds) are cleaned off, further attack is inevitable, through a reiterative process.

Stainless Steel and Aluminum
These alloys have tough substrates and they develop tough passive layers. Aluminum is not as strong as stainless steel, but it is still much stronger than most bronzes. Although each of these alloys is durable in the applications where oxygen is plentiful, they are not sturdy when in an oxygen-starved environment. These alloys are not robust when around anaerobic, aerobic or other types of bacteria as the passive layer is easily penetrated by them. Thus, in the presence of MIC-related bacteria, systems constructed of these alloys are susceptible to MIC and MIC related intergranular stress corrosion cracking. The most common point of susceptibility is welded areas and highly stressed areas that exceed the threshold stress level for the intergranular stress corrosion cracking.

Steel
This material develops less dense passive layers than other materials. It is penetrated immediately by microbes present unless there is some sort of insulating (barrier) aid such as a coating, conversion chemical, calcium carbonate in water, or cathodic protection. MIC is found in these iron-base alloys more often than any other alloy system.

MIC related bacterial growth typically occurs in systems within specific temperature ranges, depending on the type of bacteria; an “ideal” range is often reported as 4° to 49°C. Bacterial growth typically hibernates below 4°C. Some types of bacteria favor other temperature ranges; for example, most common strains of SRB grow best at 25° to 35°C. A few thermophillic types of SRB grow more efficiently at more than 60°C, and one type is capable of growing at more than 100°C. While MIC more favorably grows in their typical temperature ranges, growth in other temperature ranges should not be discounted. MIC has been found in extremely cold environments such as freezers or piping systems of Alaskan villages north of the Arctic Circle.

If MIC is suspected due to observation of slime, restrictions in flow, or leaks/pinhole leaks in pipes, the testing for the presence of MIC is warranted to determine:

• Whether bacteria related to MIC are present and, if so, their relative concentration
• The extent of the corrosion present
• The source of the corrosion
• Limits of the MIC affected system components

Comprehensive testing is performed by collecting a number of samples at various locations in a system and sampling the makeup water. Depending on the system configuration, visual observations, and problems experienced at the facility, sampling during one or more time intervals may also be appropriate. The samples should be cultured on media for the presence (and relative concentration) of low nutrient bacteria, sulfatereducing bacteria, iron-related bacteria, and aerobic bacteria. Timely culturing of the samples is very important as MIC-related bacteria become dormant when the environmental conditions are altered. Scaling or other chemical conditions in the water affect system corrosion and the interpretation of MIC sampling results; therefore, chemical testing of each sampling location and sampling interval is also useful. The results of the water chemistry testing can also be beneficial in ascertaining how far along the corrosion is, due to MIC-related bacteria.

Where leaks are present, appropriate sampling may also include metallurgical analysis of system components. The metallurgical engineer analyzes the component using electron microscopes to ascertain the nature of all corrosion and failures present. Microbiologically influenced corrosion, like other corrosion, causes degradation, deterioration and failures. Understanding the causes, effects, and appropriate investigational methods is the first step in addressing MIC related problems.