Electrochemical polarization measurements have been made on a graphite laminate gasket and a super- duplex stainless steel (DSS) in deaerated lM 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.
Crevice corrosion in seawater is assumed usually to be induced by attainment of a critical solution chemistry which might be a critical pHi or, where dissolution of sulphide inclusions is perceived to be a precursoP3, a critical concentration of thiosulphate perhaps. Prevention of crevice corrosion is achieved by judicious materials selection or by cathodic protection at potentials below the critical value for sustaining crevice attack. If the solution chemistry in the crevice could be controlled to limit the evolution of an acid pH then initiation of crevice attack by this mechanism would be unlikely, and protection afforded for some systems.
For duplex and austenitic stainless steels, the kinetics of cathodic reduction of hydrogen ions and water are too small at the potentials relevant to seawater, typically about +400mV (SCE) due to biofilm activity, to have much influence on the potential or pH in the crevice, even with potential drop considerations. Preliminary modeling has shown that, in principle, stimulation of the internal cathodic kinetics to a sufficient extent would elevate the pH in the crevice. Conceptually, this could be achieved ideally by internal coupling of the stainless steel to a material contained within the crevice whose potential in acidic solutions is more noble than the external potentials.
Graphite is noted for its enhanced level of cathodic activity, particularly with respect to oxygen reduction. For that reason, coupling to graphite creates concern with respect to galvanic corrosion and is usually avoided unless there is a pressing need for its utilization, e.g. as replacement for asbestos gaskets in the offshore and chemical industries. This concern stems from conventional thinking with respect to enhancement of the oxygen reduction reaction on graphite exposed to bulk solution. The electrode potential can be increased and with it the possibility of localized attack. However, if graphite is constrained as a gasket wholly within the crevice region, within which oxygen is depleted, then enhanced cathodic reduction (of hydrogen ions and water) could be an advantage for the reasons articulated above.
In order to evaluate whether graphite gaskets are likely to prevent or enhance crevice corrosion we have conducted electroc
