INTRODUCTION

The term (MIC) is used to designate corrosion resulting from the presence and activities of microorganisms within biofilms. MIC is often misdiagnosed as attack caused by conventional chloride crevice/pitting corrosion.

The corrosion literature contains numerous articles in the past few years describing costly pitting failures of austenitic stainless steel components. The austenitic stainless steel MIC failures are often characterized by rapid failure rates with extremely localized attack at weld regions. Although stainless steel is subject to localized crevice corrosion in chloride bearing waters, the relatively low chloride concentration of some waters combined with the rapid failure rates does not fit classical stainless steel crevice corrosion.

The cooling water system was an all welded design constructed of AISI Type 304L stainless steel piping and was designed to operate with corrosion-inhibited water in a closed loop mode servicing a series of heat exchangers. During construction (Phase 1) the system was successfully hydrotested with city water (potable). Due to construction scheduling errors, the hydrotest water was not drained before the Phase 1 piping system was put into lay-up. Numerous pipe leaks associated with welded regions were detected during a re-hydrotest after a nine?month lay-up. Phase II piping system components, which were subjected to only two months of lay-up in the same city water also exhibited corrosion in welded regions.

The system was constructed in two phases; Phase 1 construction was completed and hydrotested with city drinking water in January 1998. Phase 2 was completed, joined to the Phase 1 piping and hydrotested with city water in September1999. At that time failures due to through?wall pitting were discovered in the Phase 1 piping. At that time it was realized that Phase 1 was put into lay-up while full of hydrotest water and not drained and dried as planned.

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