Corrosion product film formed on the 90/10 copper–nickel tube in natural seawater: Composition/structure and formation mechanism

• The film is a mixture of Cu2(OH)3Cl, Cu2O, γ-FeOOH, Fe, NiO/Ni(OH)2, Ni, Cu grains.
• Highest enrichment of Fe is in the outer layer and Ni in the inner layer.
• The filmed sample shows higher corrosion resistance than the unfilmed sample.
• A new understanding of formation process of the Cu2O film is expounded.
• NiO in the inner layer contributes a lot to the good corrosion resistance.

Composition/structure of the film formed on 90/10 copper–nickel tube immersed in seawater for one month were investigated using EPMA, Raman spectroscopy, XPS, etc., revealing enrichment of Ni and Fe and depletion of Cu in the film with highest enrichment of Fe (γ-FeOOH and metallic Fe) in the outer-layer and Ni (NiO/Ni(OH)2 and metallic Ni) in the inner-layer, and Cu-rich grains in the mid-region. Higher corrosion resistance of the film than the unfilmed sample was demonstrated by EIS and polarization resistance. Growth modes of the Cu2O film, existence of Cu-rich grains, and enrichment mechanisms of Ni and Fe were discussed.