Genesis and Control of MIC in Wet and Dry Type Fire Protection Systems

摘要

Pipe failures resulting from microbiologically influenced corrosion (MIC) have been widely recognized in petrochemical, gas, and nuclear power industries, but only recently has this phenomenon been associated with failures in fire protection systems (FPS). MIC results in mechanical blockages of piping and sprinkler heads as well as through-wall penetration of ferrous and non-ferrous metals. A better understanding of MIC mechanisms can lead to modifications of bulk-phase water chemistry and effective application of antimicrobials, thus mitigating the influence of microbial activity on FPS corrosion. A number of investigators have described possible mechanisms for the corrosion of stainless and mild steels. These mechanisms include development of differential oxygen cells, under-deposit (biofilm) chloride concentration, and cathodic depolarization of protective hydrogen films. Results are presented from sampling from ten randomly selected FPS systems that were subjected to a condition assessment. A diagnosis of MIC was made in 5 of the 10 cases studied. We have found that the presence of high numbers (>10~4 cfu/cm~2) of heterotrophic and other bacteria―coupled with tubercle development―can be correlated with pit formation in FPS pipe. Effective prevention and treatment requires an understanding of feedwater chemistry, fouling populations, and FPS component compatibility with treatment chemicals. Means for preventing and remediating MIC in FPS have not been fully developed; methods developed to address MIC in other types of fluid handling systems may be instructive in this regard.