Internal oxidation and phase transformations of multi-phase Fe–Ni–Al and Fe–Ni–Al–Cr alloys induced by KCl corrosion

The corrosion of two multiphase Fe–Ni–Al and Fe–Ni–Al–Cr alloys is studied at 650 °C in KCl-contaminated air. The oxidation rate of the alloys in air alone is low. When KCl is introduced, the corrosion is accelerated, producing a thick external scale of iron oxides, an intermediate layer of spinel, and a region of internal oxidation of Al. Potassium chromate is detected on Fe–Ni–Al–Cr surface that accounts for the degradation of protective chromia. An Al-depleted single phase region is observed in the front of the internal oxidation, due to the selective consumption of Al via an “active oxidation” process.

► The aggressiveness of KCl on two multi-phase Fe–Ni–Al and Fe–Ni–Al–Cr alloys was investigated and characterized.
► Accelerated corrosion was observed including the large formation of non-protective external oxide scale and a deep internal oxidation.
► The multi-phase structures in the corrosion front were changed into a single phase zone.
► The mechanism was discussed in terms of a preferential outward diffusion of Al via an “active oxidation” process.