Corrosion, galling and wear testing of duplex and austenitic stainless steels for seawater valve applications

As part of a larger programme, resistance to various forms of degradation affecting seawater valves has been investigated for 37 different alloys. Among these were 12 cast and wrought stainless steels. This paper reports on the performance of seven wrought stainless steels, UNS S31603, 521000, S31254, S32654, N08367, S32750 and S32760, that have been identified as candidate trim materials for a new generation of seawater valves. In addition to galling and wear tests, a number of seawater corrosion tests were conducted to assess these stainless steels' resistance to general and localised pitting and crevice corrosion, as well as to galvanic interactions with dissimilar metals. Also investigated was their resistance to erosion-corrosion caused by cavitation and direct impingement. Except for the cavitation test which utilised ASTM substitute ocean water, all of the other tests, including the galling and wear tests, were conducted in or with natural seawater. A brief description of each test is provided along with a summary of results. Overall, S31603 was the least resistant stainless steel tested. Alloy S32654 was the only stainless steel found to be resistant to crevice corrosion in a 6 month quiescent exposure at 30°C. For the most part, comparable behaviour was exhibited by the other superaustenitics and superduplexes tested. These materials exhibited erosion-corrosion resistance on. par with titanium, and were found to offer enhanced wear resistance over titanium. Although inferior to the galling resistance identified for Co-based alloys and M (Pb-Sn) bronze, all of the stainless steels benefited from pre-exposure to seawater and the formation of biofilms which greatly elevated their galling threshold stress levels.