| Abstract Scope |
Single-phase γ-Ni of composition (in at.%) Ni-6.3Al-5.4Cr is borderline between forming alumina internally or externally. This alloy was oxidized in air and 1%SO<SUB>2</SUB>+O<SUB>2</SUB> at 1000°C for 20 hours. In air, the alloy was oxidized in a mixed mode, with regions forming a non-protective product of internal Al<SUB>2</SUB>O<SUB>3</SUB>/NiAl<SUB>2</SUB>O<SUB>4</SUB> and external NiO. In 1%SO<SUB>2</SUB>+O<SUB>2</SUB>, a complete external Al<SUB>2</SUB>O<SUB>3</SUB> scale formed. Thus, a small amount of sulfur in the atmosphere can promote the transition from internal to external Al<SUB>2</SUB>O<SUB>3</SUB> formation. In a parallel study, single-phase γ'-Ni<SUB>3</SUB>Al of composition (in at.%) Ni-5Cr-20Al-3Pt-0.1Hf-0.05Y was oxidized at 900°C for 20 hours in air, 0.1%SO<SUB>2</SUB>+O<SUB>2</SUB>, and with a Na<SUB>2</SUB>SO<SUB>4</SUB> deposit. For all conditions, external alumina scales formed. ϴ-Al<SUB>2</SUB>O<SUB>3</SUB> formed when oxidation took place in air, whereas α-Al<SUB>2</SUB>O<SUB>3</SUB> formed during oxidation in 0.1%SO<SUB>2</SUB>+O<SUB>2</SUB>, and with a Na<SUB>2</SUB>SO<SUB>4</SUB> deposit. Thus, sulfur from the salt deposit or gas atmosphere promoted the transformation from ϴ-Al<SUB>2</SUB>O<SUB>3</SUB> to α-Al<SUB>2</SUB>O<SUB>3</SUB>. |