CT-05:   Microbiologically Influenced Corrosion

Microbiologically Influenced Corrosion, commonly termed MIC, is a problem in many commercial and industrial properties simply due to the fact that microbiological communities are such common inhabitants of 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 provides the same function as a giant air scrubber by washing large quantities of particulates, organic material, and microbes into the water where it is then circulated throughout the piping system.  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, 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.

• Different MIC Organism Types Exist

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 and are not intended nor are capable of sterilizing the problem.  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 microbiological organisms 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 40 MPY and greater – penetrating through some metal surfaces in just a few 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.

Microscopic Views

• Most Piping Vulnerable

Microbiological activity can exist in most piping systems except steam or steam condensate service.  Whether a simple 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 within short periods of time.  Due to the optimal temperatures maintained in hot domestic water systems, the possibility of encountering MIC is slightly higher – although 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.

MIC can impact fire protection systems although it is commonly confused or misinterpreted where basic cell corrosion activity is to blame; systemwide failure then blamed on invisible bacterial rather than basic steel pipe corrosion.  Field test kits for MIC are less reliable than a formal laboratory analysis, with a full metallurgical examination of the failure cavity often required to confirm an MIC condition.

• Testing The First Action

An understanding of any corrosion problem is an extremely important first step prior to attempting any corrective actions.  This requires a thorough assessment of remaining pipe condition, and most importantly – the identification of any weak and vulnerable 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 strongly 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 for.

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 the 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.  For an excellent source of MIC identification by DNA culturing please contact CorrView.

• Prevention

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 45 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, 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.  A thorough ultrasonic investigation is always advised prior to taking any steps which may exploit a weakness in the system.

• Cleaning The System

Resolving an MIC problem is a matter of repeated cleanings and sterilization, followed by testing to confirm its resolve.  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.

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 appropriately.

• Long Term Maintenance Problem

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.  Also, the extended use of high concentrations of strongly oxidizing chemicals such as chlorine and bromine leads to further metal damage.

Often, a multi-stage program of repeated heavy duty chemical cleanings and high dosage level short duration 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, although its use being entirely dependent upon the system’s physical layout and configuration,  may be applicable in some specific examples and is extremely effective in producing immediate results.

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.

• Treatment Options

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 corrosion authorities hold differing viewpoints in addressing an MIC problem – with five generalizations presented below:

• • • Prevention

The preferred view, obviously, is to prevent an MIC infection from even beginning.  Attention to a strict water treatment program is critical, as well as is a totally automated chemical feed and bleed system.  Regularly performing laboratory cultures of the water is important to verify biocide or chlorination effectiveness.  Testing for anaerobic microbes, while technically difficult, is strongly advised in dead or low flow areas.

Periodic cleaning and sterilization of the tower is recommended at least twice annually.  Water filtration is almost mandatory, as it greatly reduces the particulate volume known to contribute to any MIC growth problem.  While an indication of biological activity can be easily determined by simple dip slides, they can not show what may be attached and growing at the interior pipe wall surface.  In such cases, electronic biofilm monitors may offer added information.

Also quite valuable, 3 in. or 4 in. spool pieces offer an inside look into the piping system and provide opportunity to sample any interior deposits for microbiological and specifically MIC analysis.

• • • Elimination

Once established, eliminating the MIC problem altogether is the preferred choice.  Aside from being an extremely difficult task, this is often not feasible due to the damage already caused to the piping system, and due to the potential for any cleaning action to cause further leaks and piping failures.  Some of the largest piping failures we are aware have been caused by acid cleanout procedures performed on weakened pipe.

In many cases, extensive repairs must be made to the system before any cleanout is even attempted – especially to the most vulnerable threaded pipe.  This greatly delays any remedial measures and allows even further damage to occur.  Once any vulnerable pipe is replaced, eliminating an MIC problem becomes an expensive exercise of repeated chemical cleaning, sterilizing and draining the system.  High pressure water jet cleaning is an excellent option in many cases, and will remove both microbiological growths and the deposits in one quick action.

The use of ozone to sterilize the system is another excellent option.  Although much more difficult to apply since it requires on-site generation, ozone will effectively sterilize an MIC condition assuming any existing deposits have been removed.

• • • Inhibit Its Growth

Another view is to identify the corrosion mechanism involved and inhibit the corrosion process to the best degree possible.  Identifying a specific MIC organism responsible is often difficult, although new developments in DNA analysis will provide most answers.  Identifying the corrosion mechanism is more difficult, though necessary in order to plan its remediation.  By many authoritative opinions, however, removing an MIC infection completely, once it has been firmly established, is nearly an impossible task.

Of all sterilizing agents, ozone likely offers the highest probability of success for any piping system having a severe MIC condition.

• • • Minimize The Damage

The fourth view assumes the impossibility of eliminating MIC once present, and instead focuses on minimizing its corrosive damage.  In many cases, the higher 15-20 MPY corrosion rates can be significantly reduced to extend system life, though random pockets of microbiological growths may produce periodic pipe failures.

Many corrosion and water treatment authorities consider that a piping system cannot be returned to normal conditions once MIC has established itself system wide – a view we generally share.  Multiple chemical sterilizations and high expense can be assumed necessary in any such cleaning effort.

• • • Replace Pipe

In many cases, a piping system seriously infected with MIC will require replacement.  This occurs usually only after MIC damage has resulted in multiple failures and the cost of another major failure is deemed to be an unacceptable risk.

Replacing less then the entire piping system, without good reason to believe that any MIC infection in those remaining areas has been eradicated, will generally reintroduce the microbiological agent into the new piping and begin the problem all over.  Intense chemical treatment and monitoring may reduce such a threat to any new piping installed.

In short, our obvious recommendation is to take the necessary precautions 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, pipe replacement and monitoring services, etc. in the tens of thousands of dollars.

© Copyright 2005 – 2025 – William P. Duncan, CorrView International, LLC