Growth and Microstructure of α ‐ Al2 O 3 on Ni‐Al Alloys: Internal Precipitation and Transition to External Scale

Oxidation properties of Ni‐2 and 6 w/o Al alloys were investigated in 1 atm oxygen at temperatures in the range 1273°–1573°K. Ni‐2 w/o Al oxidized parabolically, one order of magnitude more rapid than pure nickel, to a duplex scale by metal diffusion consisting of outer and inner layers of Al‐doped and , respectively, and an internal zone of rodlike precipitates by oxygen diffusion through the Al‐depleted alloy. Oxidation kinetics of Ni‐6 w/o Al alloy were irreproducible due to early‐stage formation of an imperfect scale containing nodules. Impingement of favorably oriented rodlike precipitates growing beneath the nodules accompanied with lateral diffusion of aluminum to react with oxygen at the precipitation front eventually led to transition from internal oxidation to development of a multilayered scale.