Internal oxidation-nitridation of ferritic Fe(Al) alloys in air

Exposure of undoped Fe(Al) and Fe(Al)+Cr ferritic alloys in laboratory air at 900-1,000 °C resulted in significant internal attack after 5,000 h, including oxides and underlying nitrides. In the most severely attacked alloys, kinetics based on mass gain and maximum penetration depth were linear; also, the deepest penetrations were a significant fraction of the specimen thickness, and were thickness-dependent. Little internal attack was observed at 700-800 °C where these compositions may be used as coatings. The extent of internal attack did not decrease with increasing Al or Cr content which may indicate that rather than classical internal oxidation this attack is related to the permeation of N through a defective external scale. No internal attack was observed in alloys doped with Y, Zr, Hf or Ti where the substrate-alumina scale interface was flatter.