Corrosion resistance and formation mechanism of oxide film formed on 70Cu-30Ni alloy under different pre-filming processes

Abstract

The oxide films were prepared on 70Cu-30Ni alloy tubes under three different pre-filming processes including the film-forming in still seawater, in flowing seawater at the velocity of 0.5 m/s, and in flowing 200 mg/L FeSO₄ solution (pH = 6.0) after 3 days of pretreatment in flowing seawater, separately for 20 days. The property, composition and structure of the film were characterized and compared by using electrochemical measurement, XPS analysis and OM/SEM/TEM observation. Based on the corrosion resistance of the oxide film, the superiority sequence of pre-filming processes is: FeSO₄ solution > flowing seawater > still seawater. Different from the common understanding, the film formed in seawater consists of four layers with alternating n-p semiconductor types: the NiO/Ni(OH)₂ inner layer, the Cu₂O/CuO/Cu₂(OH)₃Cl intermediate layer, the FeOOH/Fe₂O₃ outer layer, and the outermost layer of nanoscale Cu particle deposition. Acidic FeSO₄ solution inhibits the growth of Cu₂O layer, but promotes the rapid accumulation of FeOOH/Fe₂O₃ adsorption layer. There are distinct differences in the film growth mode and corrosion defense mechanism of 70Cu-30Ni alloy in seawater and FeSO₄ solution, which was also discussed.