Microbiologically Influenced Corrosion (MIC) is a problem in many commercial and industrial properties simply due to the fact that microbiological communities are such common inhabitants in our environment. MIC is most commonly found in open condenser water and process cooling loops, although its presence has been identified in most piping systems - from domestic water and fire sprinkler lines, to those serving hot water heating systems.

     For open systems, the main entry point for MIC is via the cooling tower - which acts similar to a giant air scrubber by washing large quantities of particulates, organic material, and microbes into the water. For closed systems, the microbes present in the make-up water usually provide the initial source of the problem. Under favorable conditions, even a small initial contamination can produce significant end result.

     MIC based corrosion is extremely aggressive, and in its worst form, will lead to piping failures within a short period of time. Once established, MIC is extremely difficult to eliminate, and may elevate into a chronic maintenance and operating problem for years following. The failure to totally remove MIC from deep pits and the furthermost branches and dead legs of a piping system generally results in reinfection by the same microorganisms within a short period of time.

     Most alloys including steel, cast iron, copper, and even stainless steel are known to be susceptible to MIC corrosion - meaning that MIC can attack any piping system given the proper conditions. Of the many potential corrosion problems which can plague any building or plant property, MIC is unquestionably the most feared, as well as the most difficult to identify and correct.

     When a metal surface is exposed to water, the microorganisms typically resident in the water quickly attach themselves to the surface to form a biofilm - which is a living biological mass composed of bacteria, algae and other microorganisms. Those microorganisms grow, break free, and distribute throughout the piping system. Chemical biocides are generally applied to prevent the growth of such microorganisms, although they are not always effective. Even under well controlled conditions, MIC can develop within a short period of time due to a variety of factors. Once MIC has gained a solid presence in the system, the reliance on biocides alone as a corrective measure becomes worthless.

     Many forms of MIC types exist to present different levels of threat. Some microorganisms are capable of producing metal dissolving metabolic by-products such as sulfuric acid, and are often identified within a classification termed sulfur reducing bacteria, or SRB. Whereas normal condenser water corrosion rates may range between 1 to 5 mils per year (MPY), MIC attack often results in accelerated corrosion rates exceeding 20 MPY and more - causing penetration of some metal surfaces in as little as one or two years.

     The below close-up photographs well illustrate the deep penetration typical of an MIC infection. In many examples, the surrounding area suffers only moderate deterioration, or little metal loss at all. We offer a number of excellent resources with additional information regarding MIC in our reprints section.

     Microbiological activity should be assumed to exist to some degree in anything but a steam piping system - an excellent indicator of which is always plate count monitoring. Whether a microbiological presence turns into a severe corrosion loss, however, depends upon a number of special factors related to the piping system and service involved.

     MIC can be found in domestic cold water systems comprised of copper pipe, and will similarly produce pinhole leaks in short periods of time. Due to the optimal temperatures maintained in hot domestic water systems, the possibility of encountering MIC is slightly higher - though still not a common occurrence. While MIC is a concern due to its potential for damaging domestic water piping, it is still of secondary importance to other pathogenic microorganisms such as Legionella Pneumophila - which can cause acute sickness to humans, and in isolated cases, even death.

     An understanding of any corrosion problem is an extremely important first step prior to attempting any cleanout procedure. This requires a thorough assessment of remaining pipe condition, and most importantly - the identification of any weak areas of the piping system.

     For most MIC problems, the greatest threat always exists at the threaded joints, at fixtures such as temperature wells and pressure gauges, and at lower floors where higher pressures exist. Installing sufficient shut-off valves to isolate critically weakened areas is well recommended in the event a chemical cleanout produces further leaks - an always present danger. Initiating a chemical cleanout program that results in producing an overhead lawn sprinkler system is a nightmare no building owner or operator wants to ever be responsible.

     Corrosion coupons, ultrasound and other nondestructive testing methods are generally ineffective at showing an MIC condition. Therefore, a full metallurgical and biological analysis of multiple representative samples of pipe becomes another prerequisite step. Viable cell culture tests can determine both the types and approximate volume of microbes present in the system. This is an extremely important tool since the presence of specific microbes and their metabolic by-products are indicative of MIC. For example, the presence of ferrous iron, sulfide, and low pH at the corrosion site would support a diagnosis of SRB or sulfur reducing MIC.

     New advances in DNA technology now allow the identification of the specific types of bacteria within a MIC tubercular deposit and provide unquestionable proof of exactly what is causing the problem. See Technical Bulletin # C-8 about new DNA identification methods for microbiological growths.

     Prevention of MIC depends on constant vigilance and awareness of the many conditions that contribute to its formation. Deposit covered metal surfaces, low flow conditions, interior surface pitting, high bacterial counts, the absence of (or improperly applied) water treatment, as well as various other conditions contribute to the growth of bacteria - thereby placing the entire system at risk. A measured corrosion rate exceeding 10 MPY always suggests the possibility of MIC, while a rate of over 25 MPY almost confirms it.

     A fully automated chemical feed and bleed station is absolutely mandatory for any condenser water or open process water system today. In addition, regular monitoring for correct inhibitor level, biological characterization, testing for microbiological cell count, frequent visual inspection of any pipe access points, and the use of multiple CorrView ^® corrosion monitors are all highly recommended as a guard against MIC.

     Once it has been positively determined that a system is infected with MIC, the first decision that must be made relates to the method of cleaning. This is an often difficult decision which must take into account the remaining condition of the pipe wall, physical layout of the piping system, deposit buildup, the relative level of MIC infection, and system operating conditions, among other factors.

     Resolving an MIC problem is a matter of repeated cleanings and sterilization, followed by testing. Generally, microbiological growths exist hidden within other deposits in a stratification of layers. Removing only the surface deposits, therefore, will not provide an effective solution, and it is necessary to clean the pipe down to the bare metal if any success is expected. See Technical Bulletin # C-15 about an effective but rarely employed solution to many MIC problems.

     Establishing a spool piece at a section of larger 3 in. to 6 in. pipe is well advised in order to periodically evaluate cleanout effectiveness. Due to the high volume of rust and particulates typically associated with an MIC problem, and the physical volume of material returned into solution through any cleanout procedure, an effective filtration system is always recommended.

     Following the elimination or control of an MIC condition, added attention to the system is mandatory since under deposit corrosion and pits will have provided the ideal environment for new microorganisms to collect and grow. For any system which has undergone a vigorous cleaning down to the base metal, it is imperative to increase the inhibitor level in order to discourage new corrosion activity while the surface metal is being passivated. Biocides should be added regularly.

     Because the microbiological agents causing MIC are generally found at the boundary layer between the pipe and interior deposits, it is often difficult to physically solve the problem with sterilizing chemicals alone. Increased biocide use alone is generally useless, as they are only designed to suppress microbiological growths, not kill and eradicate them. And the extended use of high concentrations of strongly oxidizing chemicals such as chlorine leads to further metal damage.

     Often, a multi-stage program of repeated heavy duty chemical cleanings and high dosage level sterilizations must be established. The use of chemical dispersants and chelating agents are some additional methods which may be employed to remove the attached deposits. Mechanical cleaning using a high pressure water jet may be applicable in some specific examples. See Technical Bulletin # M-3 about high pressure water jet pipe cleaning.

     The benefits of any proposed aggressive cleaning program must always be weighed against the potential damage caused to the piping itself. Yet, it is important to realize that the failure to aggressively address an established MIC problem will lead to advanced pipe failure anyway! Due to the fact that MIC produces intensive corrosion rates at localized sites, it is critically important to first establish the extent throughout the piping system and the depth of surface pitting prior to any cleaning program.

     While the elimination of an MIC problem is always preferred, it may not be possible for a variety of reasons. In many cases, a severe MIC problem cannot be solved and will be recognized as such - therefore requiring some consideration of alternative options. Different authorities hold differing viewpoints in addressing an MIC problem - with five generalizations presented below:

     In short, our obvious recommendation is to take the necessary precautions now to ensure that an MIC condition does not begin in the first place. Aside from operating problems and equipment damage, an MIC infection is an extremely costly - producing expenses from pipe testing, lab tests, maintenance overtime, chemicals cleanings, and monitoring and services, etc. in the hundreds of thousands of dollars.

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