MIC & Other Microbial Problems

What is MIC, and why is it a problem?
Microbiologically influenced corrosion (MIC) is a term coined by BTI Products founder, Daniel H. Pope.  MIC is any form of corrosion influenced by the presence and activities of microbes. 

MIC almost always occurs concurrently with, and is virtually impossible to separate from, other corrosion mechanisms.  This is due, in large part, to microbes helping create conditions under which other corrosion mechanisms (e.g., crevice corrosion, oxygen concentration cell corrosion, under-deposit corrosion, and acid attack corrosion) can occur.

MIC is a problem in industrial systems because the by-products of microbial activities(e.g., biofilms/slimes, deposits, acid production) cause plugging of system components, reduced porosity, souring or fouling of materials, and a form of severe corrosion (under-deposit corrosion pitting) that can cause rapid deterioration and failures of system components.


Which other forms of corrosion can accompany MIC?
The types of corrosion that may accompany MIC depends upon the microbial and chemical conditions at the location in question.  Given the proper environment, various corrosion mechanisms—including crevice corrosion, oxygen concentration cell corrosion, under-deposit corrosion, and acid attack corrosion—can all occur alongside MIC and are often difficult to distinguish from MIC.


Which problems are experienced by systems affected by microbial problems and MIC?
One or more of the following problems can occur in systems affected by MIC:  biofouling, souring, taste/odor/staining issues, reduced porosity/production rates, restricted fluid flow, plugging of system components, damage to coatings, reduction of cathodic protection efficacy, deterioration of system components, reduced component life span, improper system functioning, and system failure.


Which factors influence the rate and severity of MIC?
Environmental conditions, including the presence, availability, and activity of water, salts, oxygen, pH, microbes, nutrients, acids, and gases can influence the rate and severity of corrosion reactions, including MIC.


Which types of materials are affected by MIC?
With the exception of titanium, all metals commonly used in industrial systems are susceptible to MIC.  These include, but are not limited to:  steels (including stainless and galvanized), copper, brass, and ductile iron.  While other materials such as PVC and plastics do not suffer from MIC, per se, in that they don’t corrode or pit, MIC-related activities can cause sliming and fouling in these type systems.


Which types of microbes are involved in MIC?
Many different types of microbes are involved in MIC.  Most damage to system components is caused by the action of microbial communities composed of several different groups of bacteria working together.  The bacteria most often involved in MIC of industrial systems belong to the following groups:

• Aerobes (AERO) - Aerobic bacteria grow and live in the presence of oxygen, and are a diverse group that include slime-formers and low nutrient bacteria (see below).  Aerobes are important to MIC because they produce extracellular polymers (“slime”) that bind cells to the surface and trap particulates, forming deposits.  Aerobic slimes also regulate what permeates the deposit.  Aerobes use oxygen, preventing it from reaching the underlying surface which creates an ideal site for anaerobic bacterial growth (such as APB and SRB) and involvement of these bacteria in MIC.
• Anaerobes (ANA) - There are both obligate anaerobes, which cannot grow in the presence of oxygen and may be killed by oxygen, and facultative anaerobes, which are capable of growing in the presence or absence of oxygen.  Anaerobes include slime and acid-formers and help create conditions under which other MIC-related microbes, such as sulfate-reducing bacteria, can flourish.
• Acid-producing bacteria (APB) - There are both obligate APB, which cannot grow in the presence of oxygen and may be killed by oxygen, and facultative APB, which are capable of growing in the presence or absence of oxygen.  APB feed on organic nutrients and excrete organic acids that are very important in MIC and contribute to rapid and severe under-deposit acid attack/pitting.  The presence of APB is the best indication of “mature” MIC communities and possibly advanced pitting corrosion.
• Iron-related bacteria (IRB) - IRB are microbes capable of converting iron to insoluble deposits.  IRB gain their energy from oxidizing Fe2+ to Fe3+ and, thus, contribute to the formation of discrete deposits.  The IRB belong to many other functional groups (e.g., aerobes and anaerobes) and, therefore, can be present under a wide variety of environmental conditions. 
• Low nutrient bacteria (LNB) - LNB are microbes that grow in environments with very low concentrations of nutrients and aerobes (see above) in that they thrive in oxygenated environments.  LNB are the most common group of microbes found in fresh waters (potable waters, well waters, surface waters, condensate).  LNB start the process of MIC by forming slimes and deposits which provide places for other MIC bacteria to grow and contribute to MIC.  While LNB belong to other functional groups (e.g., aerobes, slime-formers), they are distinguished from these groups by their aptitude in thriving in low nutrient environments.
• Sulfate-reducing bacteria (SRB) - SRB convert sulfate to sulfide and are obligate anaerobes (they must grow in the absence of oxygen).  While SRB are important in MIC of soils, pipelines, and other environments high in organic materials, they are often not found in environments such as FPS and potable water systems due to the lack of organic nutrients and sulfate in these systems and because of exposure to oxygen.  SRB are indicators of “mature” MIC communities and possibly advanced pitting corrosion.  SRB are often easily detectable because of the “rotten egg” smell that results from their production of sulfide, which combines with hydrogen to form hydrogen sulfide.

Where do the microbes involved in MIC come from?
The microbes involved in MIC can be found in a variety of environments and in a variety of materials.  MIC bacteria exist in waters (including the water phase of petrochemicals and in ultrapure waters), soils, sediments/particulates, dust, air, cutting oils, and certain antifreeze solutions (glycol).  MIC bacteria are typically introduced into a system by one or more of these vehicles.


Which chemical factors influence the occurrence of MIC and other forms of corrosion and help to diagnose MIC?
The following chemical factors can greatly influence the occurrence of MIC and other forms of corrosion and are helpful in diagnosing MIC:

• The amount of water available is very important since it is required for microbial growth and activities and corrosion reactions.  Without water, microbial growth and corrosion reactions cannot proceed.
• Oxygen (O2) is very important to the growth and activities of many MIC bacteria and drives several important corrosion reactions at accelerated rates.  Reducing or eliminating oxygen can greatly reduce corrosion rates, including MIC.
• The pH of the environment can have a dramatic effect on the type of corrosion and corrosion rates.  Lower pH (increased acidity) tends to promote corrosion while elevated pH (more alkaline) tends to reduce some corrosion rates.  Raising the pH of water in a system will not, by itself, stop MIC.
• Alkalinity is a measure of the amount of substances, primarily carbonates (CO3 2-) and bicarbonates (HCO3-), in the water capable of neutralizing acid (also known as “buffering”).  Alkalinity does not refer to pH but instead refers to the ability of the water to resist change in pH (also known as its “buffering capacity”).  Therefore, waters with higher alkalinity values have more buffering capacity than waters with lower alkalinity values.  Alkaline materials, such as carbonates, combine with calcium and magnesium (water hardness), especially at alkaline pH, to promote scaling.  Elevated alkalinity and scale may help to protect metals from general forms of corrosion but cannot, by themselves, prevent MIC.
• Water hardness is an indication of the amount of calcium (Ca2+) and magnesium (Mg2+) present.  Waters with high hardness values are referred to as “hard,” while waters with low hardness values are “soft.”  Calcium and magnesium react with alkaline materials (such as carbonate-type compounds), especially at alkaline pH, to form scale.  Hardness and scale may help protect metals from general forms of corrosion but cannot, by themselves, prevent MIC.
• Salts, especially those containing chloride (Cl-), accelerate corrosion by causing corrosion products to flake off metal surfaces, leaving new metal exposed to the corrosive environment.  Chloride also reacts with weak acids formed by bacteria (like the acetic acid in vinegar) to form hydrochloric acid (HCl), which is a strong acid.  This strong acid drops the pH from slightly acidic to strongly acidic and greatly accelerates corrosion rates.  Chloride promotes pitting corrosion, causing much more rapid penetration of metals than would be the case for general corrosion.
• Sulfides (S2-) may be present in deposits.  Sulfides are an indication of microbial activity and, therefore, are important in diagnosing MIC.  The bacteria most often involved in sulfide production are SRB.  Since SRB are involved in more mature MIC communities, the presence of sulfides can be an indicator of advanced MIC.  Because sulfides form a toxic gas (hydrogen sulfide) upon contact with acid, it is very important to know if sulfides are present before choosing cleaning or treatment methods.
• Particulates and other substances found in oils, dirt, dust, and water of poor quality provide nutrients and, in most cases, protective habitats for microbes.  This allows the microbes to grow and cause MIC at a much faster rate.  These substances can often be detected by inspecting samples from the system.
• Iron (Fe) is a very important nutrient for bacteria.  It also comprises a large percentage of deposits and corrosion products in irons and steels.  Iron is an important indicator of corrosion in iron or steel systems.
• Build-up of gas may be an indication of microbial breakdown of chemicals (e.g., glycol, corrosion inhibitors/biocides) or other materials used in the system.
• Zinc (Zn) is an important indicator of corrosion in galvanized systems.
• Copper (Cu) is an important indicator of corrosion in systems containing copper.

 
What are the physical indicators of MIC?
Slimes, discrete deposits, under-deposit pitting, and pinhole leaks are telltale signs of MIC in industrial systems. 

What are the different stages of MIC?
When relevant MIC factors (e.g., susceptible metal, water, MIC-related bacteria, nutrients which are present in water and sediments, oxygen which is present in water and air) exist in a system, MIC occurs in the following stages:

1. MIC-related bacteria live on metal surfaces and form communities of bacteria containing many different types of interdependent bacteria.  These bacterial communities produce biofilms (a.k.a., slimes), among other things.  These biofilms can cause fouling or souring of materials and plugging of some system components, as well as reduced production rates.  Biofilms often prevent corrosion inhibitors and biocides from reaching corrosion sites, they help protect microbes (thereby making their elimination more difficult), and trap debris and help scale and deposits to form.
2. The growth of bacteria and their by-products results in the formation of discrete deposits (a.k.a., tubercules, carbuncles).  Discrete deposits can impede liquid flow and can plug system components, causing them to fail.  In addition, discrete deposits allow for the build-up and concentration of bacterial by-products (such as acids, alcohols, and gases), which changes the underlying metal surface in many ways.  Discrete deposits are even harder than biofilms to penetrate with biocides and chemical treatments and they reduce cathodic protection efficacy, which makes eliminating MIC-bacteria and protecting the metal beneath deposits very challenging.
3. MIC-related bacteria create conditions that promote very rapid localized pitting corrosion beneath the discrete deposits (under-deposit pitting corrosion).  MIC-related bacteria do this principally by producing acids and consuming oxygen.  Under-deposit pitting deteriorates the integrity of system components, often lessening their life span.
4. Under-deposit pitting often results in pinhole leaks, which sometimes occur within months of system installation.  Leaks resulting from under-deposit pitting cause improper system functioning and system failures.

 
How do we know if our system has MIC or other microbial problems?
When causative agents are unknown, it is important to get the most complete microbiological and chemical information possible.  Even in situations where there appear to be definitive signs of MIC (e.g., deposits or pinhole leaks), it is important to identify the extent of microbial involvement and whether or not other corrosion mechanisms are potentially contributing to the problem.  Diagnostic testing is one of the fastest and least expensive ways to determine microbial involvement in problems and to identify which types of corrosion may be occurring and the locations in the system that are most likely affected.  Once this information is obtained, more detailed investigations can be done on targeted areas.  BTI Products’ On-Site Diagnostic Facility Assessments and Diagnostic Test Kits can aid in determining which problems are affecting the system.