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Understanding MIC (and the H2S Connection)


By Warren Brand

Understanding run-of-the-mill corrosion on steel is complex enough. I remember my confusion when first learning about the interaction between an anode, cathode, metallic pathway and electrolyte.  

To further complicate things, over the past decade or so, more and more information has emerged about corrosion caused by bacteria.

© / spawns

Our industry is gaining traction in understanding how microbes can, and do, contribute to corrosion.

Now, our industry is gaining traction in understanding how microbes can, and do, contribute to corrosion in manners complex, unexpected, costly and difficult to mitigate.

Corrosive Terms

To gain an understanding of anything, one must have an agreed-upon, common language. To that end, let’s define some of our terms:  

MIC: Either microbially induced corrosion or microbially influenced corrosion. From my experience, they are used interchangeably. MIC is the process by which corrosion takes place because some living organism is actively causing the corrosion.

Biogenic Sulfide Corrosion (BSC): A type of MIC, wherein hydrogen sulfide (H2S) gas is produced (typically in sewer systems) as a byproduct of bacteria digesting the waste. The H2S gas (which has a rotten-egg odor) is then gorged upon by another bacteria, Thiobacillus. As thiobacillus “digests” or process the gas, it produces small amounts of sulfuric acid. These bacteria tend to grow in colonies, resulting in potentially high concentrations of highly corrosive sulfuric acid.

Anaerobic and aerobic bacteria: Aerobic bacteria require oxygen in order to live and function. Anaerobic bacteria do not require oxygen, and instead, thrive on sulfur, and produce sulfuric acid, like thiobacillus.

The BSC Life Cycle

To understand this process in the simplest terms, let’s follow the process from toilet bowl to thiobacillus.

  1. Contents of the toilet head through pipes to a wastewater treatment plant.
  2. The raw sewage is treated in a variety of manners, one of which is aeration, which pumps oxygen into the sewage to help (aerobic) bacteria digest the organic sludge.
  3. When decomposition takes place anaerobically, H2S gas is formed.
  4. In the open air, H2S gas dissipates quickly, but when in a confined, damp sewer system, and in concentration, it becomes A: Dangerous for humans to breath and B: Of enough concentration to allow the formation of colonies of thiobacillus, which leads to the production of sulfuric acid.
Aeration tank
John Rostron, CC BY-SA 2.0, via Wikimedia Commons

The raw sewage is treated in a variety of manners, one of which is aeration, which pumps oxygen into the sewage to help (aerobic) bacteria digest the organic sludge.

The problem is that often these colonies of thiobacillus like dark (no UV light or oxygen) and damp—characteristics of areas that are notoriously difficult to coat. The other problem is that specifiers, engineers and coating companies don’t really understand BSC.

One of our primary functions at my firm is to design optimal corrosion mitigation solutions. One of the components of that process is to do an exhaustive condition survey so we understand exactly all of the variables, environments and microenvironments in a system. We take into account the pH, pH swings, temperatures ranges, contents (liquid, solid and gas) humidity, condition of substrate, how often the asset is cleaned, how it’s cleaned, access to the asset, etc.

This is an exhaustive and exhausting task.

Designing for BSC

When designing a corrosion mitigation solution, particularly in a wastewater treatment system, one must assume that BSC will take place and, therefore, design the system to either:

  1. Remove or prevent the buildup or presence of H2S, or
  2. Coat the interior of the sewer, pipe, sump, manhole, etc., with a coating system that is specifically designed to withstand high concentrations of sulfuric acid.

A Problem of Concepts and Procurement

There are two broad hurdles to identifying an optimal corrosion mitigation solution in any material identification: incentive of the specifying party and the knowledge base of the specifying party.

Unfortunately, most entities specifying protective coatings (coating suppliers, engineering firms, architects, contractors) are poorly incentivized to identify “optimal” solutions. That is, if installing a ventilation system is the optimal solution to prevent BSC, it’s unlikely that a coating company or coating contractor will make that recommendation, because that’s not part of their business model, or they may not know.

The solution, of course, is to identify materials that will resist contact with sulfuric acid in a high-moisture content environment. These types of coating systems (most notably chemical-resistant epoxies) are costly and difficult to apply, and some require post-curing to increase their chemical resistance. There are also acid-resistant grouts, which may be an option, but these also are difficult to apply and may not provide sufficient chemical resistance.

And the environment in which BSC takes place is often near the tops of sewers and other areas that are made of concrete and are potentially damp all the time, increasing the technical challenges of application. Further, if thiobacillus or other contaminants have been allowed to gain a foothold, then additional surface preparation, with the inclusion of biocides of some type, may be required.  

You must always design the entire system to withstand the most aggressive element.

The solution, while often costly and logistically challenging, is to treat the project as if one were lining the interior of a sulfuric acid tank that's going to contain sulfuric acid at high concentrations.

Once that concept is embraced by all parties, decisions begin to flow more easily and in a commonsensical order. That is, the surface must be clean and dry, although there are primers which can be used on damp surfaces. The coating system must be applied properly, and be pinhole-free and of the right thicknessagain, no easy task on existing concrete, which is often pock-marked and may have aggregate and/or rebar exposed.

Design to the Most Severe Circumstances

We’ve consulted on everything from circuit boards to grain silos, interiors of sulfuric acid tanks, six-story-tall national historic sculptures and swimming pools for the rich and famous. I even recall more than 30 years ago, having to clean out the interior of a tank at a pharmaceutical complex which contained hundreds of gallons of sludge residue from pig pancreases, which was a key ingredient in making anti-coagulants. And about 10 years ago, I specified the coating system for all of the drinking water tanks in the tallest building in the world, The Burj Kalifa, in Dubai.

The one constant is taking the time and cultivating the discipline to view each situation as new and to not approach the challenge with preconceived notions. And, in this case, it’s to design to the most severe circumstances.

As William Shakespeare said, “Tis best to weigh the enemy more mighty than he seems.”

Editor's note: This post was edited Aug. 20 to correct inaccurate statements regarding aerobic and anaerobic bacteria. PaintSquare apologizes for the error.


Warren Brand

Warren Brand’s coatings career has ranged from entry-level field painting to the presidency of two successful companies. Over nearly three decades, he has project-managed thousands of coating installations and developed specs for thousands of paint and coating applications. NACE Level 3 and SSPC PCS certified, Brand, an MBA and martial-arts instructor, now heads Chicago Corrosion Group, a leading coatings consultancy. Contact Warren.



Tagged categories: Coating Materials; Consultants; NACE; Protective Coating Specialist (PCS); Protective coatings; Specification writing; SSPC; Asia Pacific; Corrosion; EMEA (Europe, Middle East and Africa); Latin America; Microbiologically Induced Corrosion (MIC); non-potable water; North America; Quality Control; Sewer systems; Waste Processing Plant; Wastewater Plants

Comment from Donald Wroble, (8/17/2017, 9:26 AM)

Excellent article Warren.

Comment from Warren Brand, (8/17/2017, 10:02 AM)

Thank you, Don.

Comment from Mike Jeffers, (8/17/2017, 3:16 PM)

Great article, Mr. Brand. I did have one question, though, regarding your description of the H2S generation process. Unless I'm mistaken (highly probable, I confess), the generation of H2S gas is primarily through the reduction of sulfate ions by SRB's (sulfate-reducing bacteria), which typically reside in the anaerobic, submerged section of a sewer pipe in a slime layer. Once the gas is generated and subsequently released to the atmosphere (aerobic, vapor phase region of the pipe), it can be oxidized by Thiobacillus, thereby producing sulfuric acid. Sewer systems that are oversized or experience low flow would be more prone to this form of attack. Am I way off base?

Comment from Lee Wilson, (8/17/2017, 3:24 PM)

Mike you are very close to base just look at the actions of Desulfovibrio Vulgaris, a species of Sulphate Reducing Bacteria (SRB) that are mostly found living in the absence of oxygen or namely anaerobic condition which is an imminent threat to pipeline long-term integrity. Great article Warren as usual!

Comment from Warren Brand, (8/17/2017, 5:07 PM)

Hi Mike and Lee. Thanks for the comments. I apologize as I think I did miss the mark. You are both correct in that the production of H2S is formed in anaerobic conditions, not aerobic. Thank you very much for calling that to my attention. One of the reasons I enjoy writing this blog is to improve my base knowledge for the benefit of our industry. I trust my conclusions in terms of remediation remain on point.

Comment from GIL DOLON, (8/18/2017, 2:09 AM)

I have lectured for many years in my certification classes to the contrary about H2S as from anaerobic reaction. I was about to post the same question too until I reach the very end of this trail. Good, informative article and prompt correction on the 'miss-the-mark'.

Comment from Warren Brand, (8/18/2017, 9:53 AM)

Thanks Gil. We'll be reposting the blog with the correction shortly.

Comment from gavin bowman, (8/23/2017, 11:33 AM)

Excellent informative article Warren

Comment from bart de cremer , (9/22/2017, 3:04 AM)

very interesting stuff Warren . Positive article

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