A Study on the Effect of Galvanized Bolts on External Crevice Corrosion of SS316 Gaskets in Marine Offshore Environment: (Part 1) Full-scale Exposure Tests

Abstract Recently, leakages were observed in 22Cr duplex steel ring type joint (RTJ) flange assemblies with SS316 gaskets, caused by crevice corrosion initiated externally from the marine environment. This was unexpected as such flange connections have been in operation for many decades without any issues. A comprehensive study was conducted to examine several hypotheses related to the cause of the observed change in corrosion behavior. The root cause of the leakages was identified to be a recent change in fastener material from hot dip galvanized steel (HDG) to 25Cr duplex steel. The objectives of the study were to understand why change in fastener material caused corrosion and to identify effective protective measures. Three testing programs were initiated over two years, encompassing small-scale electrochemical testing and two full-scale exposure tests in salt spray chambers with natural seawater (modification of ISO9227). This paper focuses on the full-scale salt spray exposure tests. The results from these tests are compared with the results from the small-scale laboratory tests, which are detailed in a separate paper (Part 2). Additionally, field inspection data and implementation of protective measures are discussed. A significant finding was that the galvanic protection provided by HDG fasteners successfully prevented crevice corrosion on the SS316 ring gaskets for many decades under marine atmospheric conditions.