We were called in by an engineering contractor responsible for installing the topsides facilities on a North Sea gas production facility. Frequent failures of the immersion heaters in the calorifiers had been resulting in interruptions to the domestic hot water supply. This in turn was contributing to deteriorating industrial relations because the workers were denied hot showers at the end of their shifts.
The failures were found to be caused by cracks in the heater sheaths. Investigation showed that they were type 304 austenitic stainless steel (UNS S30400). A metallographic examination (see figure) showed inter-granular and trans-granular branched cracks. These are typical of chloride induced stress corrosion cracking (SCC) of these materials. Analysis of the water supply revealed chloride levels of over 100 mg/l, confirming the suspected failure mechanism. In this instance the problem was rectified by ordering replacement heaters sheathed with nickel alloy (UNS N06625).
SCC is a very common form of corrosion failure in austenitic stainless steels. Equipment is at risk if it operate at above about 50oC in waters containing very small levels of chloride and dissolved oxygen. The majority of such failures arise from a basic lack of corrosion awareness on the part of designers and specifiers.