Abstract
Microbiologically influenced corrosion (MIC) has become well recognized as a major contributing factor in industrial corrosion. Not only are MIC related failures costly in terms of repair and replacement costs, they can compromise the safety related systems in nuclear power plants.1 A variety of bacterial organisms are involved in the MIC phenomenon. Thiobacillus and others are acid producing bacteria (APB) that produce mineral acids which attack mild steel and other piping/heat exchanger materials. Gallionella, Sphaerolitus and others are iron oxidizing or iron depositing bacteria that consume ferrous iron and create voluminous iron (ferric) oxide deposits. The constant removal of soluble ferrous iron from the metal surface accelerates the corrosion rate. The thick deposits lead to underdeposit corrosion. Sulfide producing bacteria (a.k.a. sulfate reducing bacteria or SRB) such as Desulfovibrio and Desulfomaculum metabolize sulfate and produce corrosive sulfide.
Not all biological activity in piping systems is detrimental. Most are basically benign if accumulation is not excessive. In a few cases, biological organisms can offset the effects of the more harmful bacterial species.
This paper will describe a variety of both detrimental and beneficial effects of microbiological organisms found in actual cooling water case histories. One situation is where a particular biological microorganism helped protect the system from MIC. It will also describe several case histories where corrosion of various metals resulted from the growth of the more harmful microorganisms.
