IMPACT OF GRAPHITE GASKET/DUPLEX STAINLESS STEEL COUPLES ON CREVICE CHEMISTRY AND LIKELIHOOD OF CREVICE ATTACK IN SEAWATER

摘要

Electrochemical polarisation measurements have been made on a graphite laminate gasket and a super-duplex stainless steel (DSS) in deaerated 1M NaCl solution over a range of pH. The open circuit potential of the graphite is significantly more noble than that of the duplex stainless steel and the kinetics of the hydrogen ion reduction are greater at potentials more positive than about 0.0 V SCE. The data were used as input to a model of crevice chemistry and predictions made for potentials up to +0.4 V SCE. For crevices of parallel plates of DSS -DSS and DSS - Plastic, the usual acidic conditions were predicted but for a DSS - Graphite combination the pH was predicted to be alkaline. The latter is a consequence of the enhanced kinetics for cathodic reduction of hydrogen ions and water on the graphite which, when confined within the crevice, act to elevate the pH. The predictions suggest that coupling to graphite, contained within the crevice, may act to prevent crevice corrosion initiation in contrast to the usual perception of behaviour when coupling to more noble materials. In practice, there have been significant crevice corrosion failures of a super-duplex stainless steel associated with graphite gaskets. However, in all cases, the failures were in chlorinated systems for which the corrosion potentials are particularly high and beyond the range for which a benefit from graphite could be anticipated.