Scale formation on the Ni-base superalloys IN 939 and IN 738 LC

The scales formed on the superalloys IN 939 and IN 738 LC at 700, 900, and 1100°C in air have been investigated by optical, microprobe analysis and x-ray diffraction measurements. Both alloys form very similar scales. The main components are an outer TiO₂ layer, an intermediate Cr₂O₃ layer with dissolved Ti, and an inner layer of (Ti, Nb, Ta)O₂ with rutile structure. Beneath the scale an internal corrosion zone is formed that contained Al₂O₃ directly beneath the external scale and TiN further into the substrate.