The Influence of Alloy Composition and microstructure on the corrosion behaviour of Cu-Ni alloys in seawater

The aim of the study was to relate the general corrosion behaviour of CuNi 10-alloys in seawater with its iron content and microstructure. To that end the microstructure of four commercial alloys with various Fe-contents (1.20-1.78%) and there laboratory melted alloys with various Fe-contents (1.5-2.5%) were investigated in the as delivered state and after three different heat treatments. Samples of these alloys were exposed to flowing natural seawater. During exposure electrochemical measurements were performed. After exposure the weight loss was determined. Moreover, for a number of samples the corrosion products were analysed by means of electron microprobe analysis. Irrespective of the Fe-content, the presence of discontinuous precipitates in the laboratory melted alloys turned out to be detrimental with regard to general corrosion. To a less extent this also holds for alloys with high concentrations of continuous precipitates, as observed for iron contents of 2 and 2.5%. The precipitate free alloys as well as the ones containing low concentrations of continuous precipitates because of low Fe-concentrations (<1.5%Fe) do show a good corrosion behaviour. The corrosion behaviour of commercial CuNi 10 Fe in the as received condition was generally comparable with that of laboratory melted alloys with continuous precipitates. The corrosion rates of the former do not exceed those of the latter, which are very low for practical applications. Finally the corrosion behaviour of five laboratory melted CuNi5 alloys, containing up to 4% fe in solution, was investigated. It turned out that their reciprocal polarisation resistance in the steady state situation is lower than those for the CuNi 10 Fe alloys.