The hyper duplex grade UNS S32707 has, together with other duplex stainless steel grades, been tested in a model heat exchanger with chlorinated seawater on the tube side. The test periods have been 6 to 18 months with a maximum outer skin temperature of 170°C, giving an inner skin temperature of 95°C. The test results show that standard duplex of type 2205 andsuper duplex stainless steel of type 2507 develop pitting corrosion at an inner skin temperatures of 50°C and 70°C, respectively. The hyper duplex alloy S32707 does not show corrosion under any of the tested conditions. This shows that hyper duplex S32707 is a viable stainless steel option for demanding seawater cooler applications, being able to replace expensive nickel base alloys and titanium solutions.