MIC

There are biological organisms, called microorganisms or microbes, which influence corrosion. This type of corrosion is known as microbial influenced corrosion (MIC). The primary, and to many, the only concern is that this influence often results in an extremely accelerated rate of corrosion. It affects most alloys, such as ductile iron, steel (including stainless and galvanized), and copper; but it doesn't seem to affect titanium. The affect does vary between the different alloys with ductile iron corroding slower than steel. There was also a case where MIC caused the water in copper pipe to turn blue.

MIC is not caused by a single microbe, but is attributed to many different microbes. These are often categorized by common characteristics such as by-products (i.e., sludge producing) or compounds they effect (i.e. sulfur oxidizing). In a general sense, they all fall into one of two groups based upon their oxygen requirements; one being aerobic (requires oxygen) such as sulfur oxidizing bacteria, and the other being anaerobic, (requires little or no oxygen), such as sulfate reducing bacteria.

General corrosion affects the entire surface or at least the wetted surface. MIC, on the other hand, is very localized. It creates a nodule and a pit beneath the nodule. There can be only a few nodules or there can be many. Within these nodules microbes rarely work alone but operate as a mixed community of differing types and groups. The different microbes perform different functions within the community. This interaction allows a community to thrive in environments that are actually hostile to some of its members. For example, in an aerobic environment, anaerobic bacteria are generally inhibited or killed. But within a community the aerobic bacteria reside in the outer layer of the nodule consuming the oxygen in the water as it penetrates the nodule. Thus, the inner portion of the nodule experiences a reduced oxygen level allowing anaerobic bacteria to thrive.

How MIC Works

MIC operates as an individual nodule covering a pit. The development of this process occurs in three phases, which are

• Attachment of microbes.
• Growth of nodule and initial pit.
• Mature pit and nodule.

• Phase One is depicted by Figure 1. Obviously, for MIC to occur the microbes must be introduced into the sprinkler system. Even though a nodule can contain many different bacteria, they do not necessarily arrive and/or thrive simultaneously. In order for microbes to attach themselves to the inner wall of the pipe, the bacteria must find a desirable site. Such sites are defined as containing absorbed nutrients and having a metallurgical feature that the microbes can attach to. These features seem to be critical for MIC to occur and consist of irregularities in the pipe surface such as from welded connection, pipe seams, pre-existing corrosion, inclusions, etc.
• After successfully attaching to a location, Phase Two starts. As shown in Figure 2, a lot occurs at this stage. (Actually, only a fraction of the activity is shown, since there can be an immense amount of chemical interaction occurring). Among the microbe's by-products are sticky polymers which retain organic and inorganic materials aiding in the creation of the nodule. Once the nodule is formed, it allows the underlying conditions to become chemically dissimilar to the surrounding surface. This is the start of accelerated corrosion, which initially leads to crevice corrosion. Some of the characteristics of the community at this phase are: microbes are located throughout the nodule and the pH level is lowered (acidic) within the crevice, but it is still above 4. This lower pH adds to the corrosiveness of the environment, as well as stimulating the growth of certain types of bacteria. The increased acidic level is commonly initiated by acid-producing bacteria which produce organic acids as a by-product. This acid provides a nutrient source for other bacteria whose by-product results in a buildup of hydrogen protons and an even lower pH level.
• In the final phase (Figure 3), there is continued formation of the nodule over a mature pit. Such a pit not only increases in depth but also produces a tunneling characteristic. A significant condition is that the pH is less than 4. One of the factors which can contribute to the high acidic level is the weak organic acids discussed in Phase Two. These can be converted to a stronger acid by combining with chloride from the water, thus producing hydrochloric acid. As a result of the high acidity in the pit area, live bacteria are present only in the outer portion of the nodule. At this point, the bacteria could be eliminated and the corrosion would continue as a traditional electrochemical corrosion process. Because of a better understanding of the final phase, whereby the presence of the bacteria is not required, the name was changed from microbiological "induced" corrosion to microbiological "influenced" corrosion.