Microbes have the potential to create conditions under which corrosive processes, such as pitting or crevice corrosion, can occur.¹ Microbial-induced corrosion (MIC) is recognized as a significant problem in the pharmaceutical and food processing industries² and is a potential problem in other critical componentor device manufacture.

Before yelling “microbe” in a crowded process-development meeting, given the typical conditions for most controlled, critical cleaning processes, MIC would not be expected to be a problem. Most MIC occurs under conditions of long exposure, such as in liquid-containing pipes or long submersion. An example is the hull of the Titanic where the steady decomposition due to microbialcolonies created structures called “rusticles.”³

However, as performance criteria increase and size decreases, any cause of corrosive products should be minimized. Effects of corrosive processes may not always be visible. Since bacteria are very small, small pits and cracks can provide a haven. Many manufacturers may not be aware of the potential for MIC and assume another cause for corrosive action. We are compiling case histories of MIC in critical manufacturing; if readers are aware of any such cases, please let us know.

CAUSES OF MICROBIAL-INDUCED CORROSION
While MIC does not produce a unique type of corrosion, understanding the dynamics of microbes is a key to avoiding a problem. Microbes thrive under the same conditions as present in many aqueous cleaning systems. They need heat and a food source to provide energy, moisture, and sometimes oxygen (although some more troublesome bacteria associated with MIC are anaerobic, i.e., neither requiring nor tolerating oxygen). Conditions that may be considered to be safer for workers, such as moderate temperatures and near neutral pH, can, at the same time, be more favorable towards microbial growth, and, therefore,increase the possibilities for MIC.

For the microbes (usually bacteria) that are involved in MIC, their food can be organic compounds, including oils, or in certain cases, the host metal itself. Bacteria known as “iron bacteria” use iron as an energy source and can attack stainless steel. Sulfur-reducing bacteria can attack sulfur-containing fluids, including lubricants,releasing sulfuric acid that attacks metal surfaces.

Some microbes create biofilms that produce a cocoon-like condition. Regions beneath these films are shielded from normally aerated water and can become oxygen deprived. Anaerobic bacteria may feed on the underlying metal or other food. Even if anaerobic bacteria are not present, these oxygen-deprived areas become anodic compared with the surrounding metal, allowing corrosive processesto proceed.

WHAT ABOUT MICROBES FOR BIOREMEDIATION?
The presence of microbes does not de facto mean that corrosive processes will take place. There are many cases where microbial presence has no known significant effect on the corrosion rate. For example, in certain industrial cleaning processes, microbes are added to the process bath to provide bioremediation or bioaugmentation; they digest the oils that have been removed from components. This both prolongs the useful life of the cleaning bath as well as facilitates waste disposal. These microbes are specifically selected for their ability to utilize the oils as food; the metal is not part of their diet. It could even be hypothesized that, by decreasing the soil loading of the bath, the cleaning action is more efficient and the likelihood of corrosive cells isdecreased.

In a subsequent column, we shall discuss methods to detect and avoid corrosive effects, including those associated with MIC.

References:

1. B. Kanegsberg & E. Kanegsberg, “Must it Rust?” Controlled Environments, May 2007.
2. B. Bavarian, California State University Northridge, private communication.
3. R. Cullimore & L. Johnston, “Biodeterioration of the RMS Titanic,” available at
   http://www.encyclopedia-titanica.org/item/1478/.

We appreciate the comments of Vince Scuilla (Osprey Biotech-nics), Art Gillman (Unique Equipment), Jim Unmack (Unmack Corporation), and Dr. Behzad Bavarian (CSUN).

Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical and precision cleaning,surface preparation,and contamination control.They are the editors of The Handbook for Critical Cleaning,CRC Press.Contact them at BFK Solutions LLC.,310-459-3614;info@bfksolutions.com;www.bfksolutions.com.