DISCUSSION

It is well known that MIC commonly results when residual water is left in stainless steel systems after hydrostatic testing. Natural waters, particularly well water, contain several classes of microorganisms, which can develop a biofilm in moist environments and accelerate metal corrosion. The corrosion is in the form of selective phase attack resulting in corrosion pits in HAZ and weld zones. The failure rates are often rapid (in terms of weeks or months) and frequently occur in waters with low chloride concentrations (less than 100 ppm).

In this case pitting corrosion failures due to MIC occurred in approximately nine months, because the system had been allowed to sit stagnant with biologically active, but low?chloride, water. While the failure analysis was being performed for the Phase I system pitting failures, the Phase 2 system was inadvertently only partially drained. Inspection was immediately performed on the Phase 2 system. Inspection revealed shallow pitting had occurred in the Phase 2 system after approximately two months stagnant exposure to city water. A root cause failure analysis found MIC as the cause of the pitting.

PAGE 1 2 3 4 5 next