Coupling of superduplex stainless steel and cast nickel-aluminium bronze in sea water

Tests have been performed at about 25 degrees C in both natural and chlorinated sea waters on couples of nickel-alminium bronze (NAB) and Zeron 100 (Z100) superduplex stainless steel. Two NAB/Z100 area ratios (1 : 1 and 1 : 10) were used in exposures of 60 days. The results in natural sea water showed severe localised corrosion of NAB both in the crevices and on open surfaces at both area ratios. In chlorinated sea water there was no corrosion at a 1 : 1 area ratio. The attack is explained in terms of the different cathodic reactions occurring in natural and chlorinated sea waters, and the metallurgical structure of: NAB. Service experiences with couples of NAB and superduplex stainless steel are described and recommendations are given for the avoidance of problems.