Microbiological Influenced Corrosion

Introduction

Are you experiencing or repairing leaking sprinkler systems which are only a couple of years old? If you are, you are probably a victim of microbiological influenced corrosion (MIC). Some consider MIC a witch hunt, while others feel it's a major threat to the fire protection industry. Right now we don't know because there is little documentation on it for sprinkler systems.

Due to the severe consequences of MIC, AFSA wants to better define the extent of this problem. This is an introductory article discussing what MIC is and how it works. Future articles will address formal identification, means of mitigation, and consequences other than leaks.

In general, the sprinkler industry does not consider corrosion an issue. We are aware of environmentally induced corrosion affecting the exterior of the components. This has been remedied by galvanized and copper pipe and sprinklers with special coatings. We're also aware of oxidation which affects the entire surface. This typically affects the inner pipe wall and is generally not a problem (except for dry pipe systems that are overly tripped tested and even then the interval for problems is 20 plus years).

We are generally not aware of MIC and even though it may be new to us, it is not new to other industries. The petro-chemical industries, power plants, and facilities with cooling towers have been dealing with MIC for awhile. As a result, a lot is known about it.

What is MIC?

There are biological organisms (microbes) which influence corrosion. 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

As already identified, 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, as depicted by 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.

Extent of Problem

There are several factors leading to our lack of knowledge about the extent of this problem in sprinkler systems. This appears to be a relatively recent phenomenon, but since our industry is not really aware of MIC, we are not sure how long it has been occurring. Even if one is aware of MIC, its formal identification, though not difficult, can present problems. For instance, damaging the test sample can lead to a false negative. Since MIC in sprinkler systems is typically detected only as a result of leaks, it is reasonable to assume MIC is the culprit if you have a pin hole leak(s), limited overall corrosion, and the presence of nodules (which can be small). After all, if it quacks like a duck and looks like a duck, it's probably a duck. The biggest problem affecting our perception of the seriousness of MIC is the lack of communication. Unless you're involved with a MIC occurrence or are searching for cases, you don't hear about them.

Currently, Phoenix, Ariz. is an area identified as having confirmed MIC occurrences. An engineering report has identified many sprinkler systems in that area as suffering from MIC, one of which is known to be in litigation. It was an extremely severe case with over 20 leaks appearing in a six month period. On the other hand, discussion with a large sprinkler contractor in that area, indicated they have seen only occasional MIC occurrences; all with a low level of severity such that only a few pieces of pipe required replacement. Even within the same city, we are receiving conflicting reports. Thus, we don't know whether or not the sprinkler industry is facing a serious problem.

ABOUT THE AUTHOR:

Roland Huggins, PE, is AFSA's Director of Technical Services. He oversees all aspects of the AFSA technical services program, including the monthly TechTalk newsletter, weekly technical updates, and informal standards interpretations, and he represents AFSA on various NFPA technical committees. Huggins is a graduate of the University of Maryland School of Fire Protection Engineering, and is a member of the Society of Fire Protection Engineers.

EDITOR'S NOTE:

The author and editors wish to acknowledge and thank Dr. Daniel Pope of Bioindustrial Technologies Inc., Georgetown, Texas, who has published numerous reports on MIC and provided much of the information and the photographs used in this article.

Photo 1: Above photo shows multiple nodules. One nodule is broken open, showing black corrosion products inside the nodule. Pits are found under most large nodules.

Photo 2: Above photo shows a pipe cleaned of nodules. Note the pits, which were found under the nodules.

Figure 1: Phase One - Recognition of desirable sites (metallurgical feature desirable to bacteria).

Figure 2: Phase Two - Colony formation and development of crevice corrosion. It is suspected that the pH in the crevice region is above 4 during this phase.

Figure 3: Phase Three - Nodule formed over "mature" pit. It is thought that most of nodule is corrosion product - live bacteria are present in crust of nodule only.