Composition of Surface Films on Nickel Base Superalloys

The composition and thickness of the passive film formed on three nickel base superalloys (Inconel 718, Incoloy 925, and MP35N) in an acetate buffer (1 mol l −1 acetic acid/1 mol l −1 sodium acetate) was investigated using the technique of surface analysis by laser ionization (SALI). The thickness of the films was found to be similar for the three alloys and was estimated to be . The films were all enriched in Cr but were depleted in Ni and Fe (where relevant). Mo enrichment occurs with the Inconel, but only slightly with the Incoloy and MP35N. The degree of Mo segregation appears to be related to the Ni content of the alloy. The film is basically a Ni‐Cr‐Fe oxide on Inconel 718, a Ni‐Cr‐Fe‐Cu oxide on Incoloy 925, and a Ni‐Cr‐Co oxide on MP35N. The high Cr content of the oxides accounts in large part for the high corrosion resistance of these alloys. MP35N further benefits from a relatively high level of Mo in the film. The smaller amounts of Mo in the oxides on the Inconel and Incoloy are compensated to some exent by enriched levels of Nb and Cu, respectively.