A Study on the Effect of Galvanized Bolts on External Crevice Corrosion of SS316 Gaskets in Marine Offshore Environment: (Part 2) Small-scale Electrochemical Tests

Duplex flange assemblies on an offshore platform started to experience crevice corrosion on the SS316 gaskets after the bolting material was changed from a hot-dip galvanized (HDG) steel to a corrosion resistant alloy (25Cr SDSS). This paper serves as Part 2 in an investigative study that aims to understand the crevice corrosion prevention that has been imparted by the galvanized fasteners in a marine atmosphere environment. The work that is presented in this paper focuses on the small-scale electrochemical testing component of the study in which two sets of experiments were carried out to examine underlying causes. The first set of experiments examined the galvanic interaction between SS316 and anode materials such as zinc and HDG steel in sea salt films at various humidity levels. The second set of experiments involved different polarization techniques to conservatively assess the crevice repassivation potentials of the stainless steel alloys used as gaskets in the flange assemblies. It was found that the galvanic potentials obtained in salt films were significantly more electronegative compared to the repassivation potentials of the various stainless steels. Even under relatively dry conditions, in which the salt films had a high impedance (~3 kΩ∙cm2), the galvanic couples achieved potentials that were far below the repassivation potentials.