Promotion of the Al2O3-Scale Formation on Ni-Cr-Al Alloys via the Fluorine Effect

The halogen effect, which involves the surface enrichment of an alloy with a halogen element to result in improved oxidation resistance, was assessed using model Ni-based Ni–Cr–Al alloys. The alloys studied varied in composition in a manner that produced sub-critical, borderline and protective intrinsic oxidation behaviours from the standpoint of being able to form a continuous Al2O3 scale. Fluorine-ion implantation allowed for a well-defined surface doping of the alloys. It was found that this doping could promote protective Al2O3-scale formation during oxidation at 900 and 1,000 °C in both dry and steam-containing atmospheres. Specifically, the beneficial halogen effect was manifested by lowering the critical Al content in Ni–Cr–Al alloys needed for Al2O3-scale formation.