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A plate heat exchanger with plates made of two highly alloyed stainless steels and a nickel-base alloy has been tested at different temperature using chlorinated seawater (2 ppm continuous chlorination) as the coolant. The superaustenitic 6Mo steel SMO (UNS S31254) and the nickel-base Alloy C-276 (UNS N10276) were attacked by crevice corrosion at 45 Grad C which was the lowest temperature investigated. The superaustenitic 7Mo steel 654 SMO (UNS S32654) was completely resistant even at 70 deg C, which was the highest temperature tested. Three laboratory tests were evaluated in terms of their correlation to this practical results. Conventional plate coupons, made of the same alloys and equipped with MCA washers, were also exposed in the 45 Grad C water. However, the results of this test were not in accordance with the results of the heat exchanger test and cannot be used for predictions. The critical pitting (CPT) and crevice corrosion (CCT) temperatures of the plate materials were determined in 6 % ferric chloride. Only the CCT values show promise for prediction of the behaviour in a plate heat exchanger.