The use of titanium plate heat exchangers is widespread for Offshore and Coastal installations where seawater cooling is required. Several years ago difficulties to supply titanium Grade 1 for this application triggered the need for qualification of other alloys for this application. Applications using passive alloys are especially susceptible to crevice corrosion when exposed to hot seawater. Since titanium is the most corrosion resistant alloy in seawater only highly corrosion resistant alloys could be considered for its replacement in plate heat exchangers. In addition since they need to be shaped into plates by cold pressing only alloys with a high formability could be selected. This narrowed even further candidate alloys for plate heat exchangers. In this study nickel chromium molybdenum alloys and a highly alloyed stainless steel were tested between 30 and 70°C in natural seawater with up to 1 ppm free chlorine. Rather than using short term electrochemical testing that is often difficult to compare with service performance long term exposure tests (up to 18 months) of real plate heat exchangers were carried out. These tests are considered to be more representative of actual service conditions. The results of these tests and their significance are discussed and compared with previous reported work.