Breakdown mechanism of γ-Al₂O₃ on Ni₂Al₃ coatings exposed in a biomass fired power plant

A Ni₂Al₃ coated tube was exposed in a wood firing power plant for 7100 h. The exposure resulted in protective behaviour in most areas, while corrosion attack occurred in local areas. In areas exhibiting protective behaviour, a 30−50 nm thick γ-Al₂O₃ layer was found, and a 200−250 nm K–O rich amorphous layer was identified in the corroded area. A corrosion mechanism is suggested: γ-Al₂O₃ was broken down by reaction with KCl(g) resulting in formation of potassium aluminate. Migration of Cl through potassium aluminate resulted in formation of volatile AlCl₃. KCl-AlCl₃ then led to alumina fluxing and further corrosion attack.