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

Abstract 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.