Mechanism of dissimilar metal crevice corrosion of superferritic stainless steels

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Mechanism of dissimilar metal crevice corrosion of superferritic stainless steels

Salamat, G. / Juhl, G.A. / Kelly, R.G.

Superferritic stainless steels (SS) can suffer from dissimilar metal crevice (DMC) corrosion when part of a crevice with certain other stainless alloys. Electrochemical measurements in these simulated crevice solutions were compared to measurements made in simple hydrochlorid acid (HCl) solutions. Accelerated crevice corrosion of alloy S44735 occurs when it is part of a crevice with type 316 SS in neutral Cl(-) solutions. The attack on alloy 316 SS has developed an active crevice site. The attack of alloy S44735 can occur at potentials as low as 0 V_SCE, which is well below E_corr values of both type 316 SS and alloy S44735 in neutral Cl(-) solutions. Therefore, normal operating conditions in a seawater heat exchanger would be sufficient to lead to attack at an alloy S44735 tube/type 316 SS tubesheet crevice, as has been observed. Analyses of the ionic content of solutions removed from active alloy S44735/type 316 SS DMC allowed the chemical composition of the active site to be established and bulk solutions designed for electrochemical testing. Polarization measurements on alloy S44735 and type 316 SS in acidified Cl(-) solutions, and the simulated crevice solutions (based upon the chromatographic analyses) demonstrated that the simulated crevice solutions are much more aggressive, widening the active-to-passive transition of alloy S44735 and increasing the dissolution rate of the type 316 SS at all potentials. DMC corrosion of alloy S44735 cannot be explained completely in terms of either a simple decrease in pH or the developement of sufficient ohmic drop. These aspects of the conditions inside the crevice are important to the developement and maintenance of crevice corrosion of alloy S44735, but do not fully rationalize the observations. The preeminent importance of the high Cl(-) content generated during the active crevicing of the type 316 SS on the developement of the attack on alloy S44735 has been demonstrated. The observed alloy specificity can be rationalized in terms of the inhibitory effects of molybdenum on the active dissolution.