Acceleration of pitting corrosion of 70Cu–30Ni alloy in seawater by NH4+ and stress

The 70Cu–30Ni alloy has excellent resistance to seawater corrosion, but corrosion perforation still occurs during use for various reasons. Copper can suffer severe ammonia corrosion in NH4+ containing solutions and copper alloys are susceptible to stress corrosion in ammonia environments. Therefore, the effect of NH4+ and stress on the corrosion of 70Cu–30Ni alloy in seawater cannot be ignored. The main objective of this study was to investigate the accelerating effect of NH4+ and stress on the pitting of 70Cu–30Ni alloy. The effect of NH4+ and stress on the pitting of 70Cu–30Ni alloy in seawater was investigated using laser confocal scanning microscopy (LCSM), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results show that in seawater with NH4+ the depth and diameter of the pits and the number of pits in the specimens increase. NH4+ destroys the oxide film on the surface of the 70Cu–30Ni alloy and inhibits the repair process of the oxide film on the surface of the alloy, thus reducing the amount of protective Cu2O and NiO in the corrosion products of the specimen. In seawater with NH4+, the film capacitance CPEf and dispersion coefficient of the 70Cu–30Ni alloy interface are significantly lower, the oxide film is thinner and there are more local defects and poor protection. After loading stress, the pitting depth of the specimen increases further, showing stress corrosion cracking. Stress and NH4+ have a synergistic effect on the pitting corrosion of the alloy.