An Oxygen Potential Gradient as a Possible Diffusion Driving Force

Oxygen-active elements such as Y, Zr and Hf are added to alumina-forming alloys to improve the adhesion of the external α-Al 2 O 3 scale formed during high temperature oxidation. During oxidation at 1000°-1500°C, many such elements are observed to diffuse from the alloy through the scale and form oxide particles at the scale-gas interface. Once nucleated, the volume fraction of these particles increases with oxidation temperature and time. The continued increase in volume fraction with time suggests that diffusion is not merely driven by a concentration gradient of the oxygen-active elements. Pre-coating with a dense, 1 μm-thick, plasma-deposited alumina layer prior to oxidation did not prevent the nucleation and growth of these types of particles at the gas interface of the coating. The driving force for this diffusion phenomenon is attributed to the oxygen potential gradient across the metal-oxide-gas system and the high oxygen affinity of these elements.