Influence of Hafnium Additions and Preoxidation Treatment on the Metal Dusting of Ni–Al Alloys

Abstract

Alloys of γ-Ni(Al), γ–γ′ Ni(Al), γ′-Ni₃Al and β-NiAl, each containing 0.5 wt% hafnium, were exposed in 1 h cycles to a carbon supersaturated CO–H₂–H₂O atmosphere (a _C = 36.7, $p_{{{\text{O}}_{2} }}$ = 2.83 × 10⁻²⁶ atm) at 650 °C and an overall pressure of 1 atm. The alloys accumulated surface coke deposits and, to varying degrees, underwent metal dusting. While morphologies of the reacted samples were found to be very similar to those of equivalent hafnium-free alloys, the weight uptake kinetics were faster for alloys containing 0.5 wt% Hf. The hafnium-containing alloys were preoxidised in a H₂–H₂O mixture at 1,100 °C to form α-Al₂O₃ prior to metal dusting exposure. This treatment led to external oxide formation on all alloys except the low Al γ-Ni(Al) alloy, which formed internal oxides. Cyclic metal dusting resistance of all alloys was improved significantly by the preoxidation treatment, as alumina forms a barrier to inward carbon diffusion, and hafnium improves the quality of the barrier oxide.