Oxide Scale Development in a Ni-16 Cr-4.5 Al Alloy for Short Exposure Times

Nickel-base alloys are used in high-temperature applications due to their favorable mechanical properties and oxidation behavior. Some of these alloys are designed to form a protective aluminum oxide scale to achieve oxidation resistance. In some oxidizing environments, water vapor is also present. However, only limited data are available regarding the effects of water vapor on the oxidation behavior of alumina-forming alloys (“alumina formers”), especially for early stage oxidation. Moreover, the currently available mechanisms for dry oxidation of alumina formers propose different pathways leading up to the final morphology of the oxide scale. In this study, the early oxidation behavior of an alumina former (UNS N07214) in dry and humid air was conducted at 1000°C for different exposure times (1 min to 100 h). Detailed examination of the surface of this alloy reveals that an alumina film is initially (1–10 min) formed in both dry and humid conditions. For longer exposure times (1–10 h), this initial alumina film is disrupted by the formation of chromia and nickel oxide/nickel chromite islands in both cases. Subsequently, for 100 h exposures, a continuous alumina scale is re-established. A mechanism to explain the observed phenomena is proposed.