Characterisation of corrosion products formed on surface of synthetic 90/10 Cu–Ni alloy containing alloying additions in marine environment

Alloying elements including aluminium, chromium, cobalt, indium, molybdenum, titanium and vanadium were added to a synthetic 90/10 Cu–Ni alloy and its behaviours in natural sea water and in artificial saline solutions (containing 2·86 wt-%NaCl and 0 or 2260 ppm sulphate) were investigated. Alloying elements included the addition of 2 wt-% of one of the elements such as aluminium, chromium, cobalt, indium, molybdenum, titanium and vanadium. The passive film formed on most of the synthetic alloys is uniform in thickness (̃100 μm when the sample was exposed to the solution for 20 h) and consists of more than one layer when the alloy was exposed to artificial saline solutions or N.S. sea water for a relatively long time. The corrosion film formed on 90/10 Cu–Ni synthetic alloy surfaces after 20 h exposure in three different electrolytes (0 ppm sulphate artificial saline solution, 2260 ppm sulphate artificial saline solution and N.S. sea water) was characterised by visual observation, field emission scanning electron microscope, electron probe microanalysis, energy dispersive spectroscopy, wavelength dispersive spectroscopy, and X-ray diffraction. The stability and protection provided by the corrosion film depend on the alloying element added to the alloy. Cobalt was detected to be the most effective alloying element and aluminium may be used as an alloying element only in the absence of sulphate. Chromium could be used to some extent while the use of molybdenum, indium, vanadium or titanium as alloying elements did not improve the corrosion properties of 90/10 Cu–Ni alloy.