Effect of Y₂O₃ on Oxidation Resistance of Ni-Al Composite Coatings Before and After Low-Temperature Chromizing

Ni-Al/Ni-Al-Y₂O₃ composite coatings with 10.5 wt% Al were first developed by the codeposition of Ni and Al/Y₂O₃ particles, then chromized at 700°C for 8 h to produce a novel Y₂O₃-modified-/free-Al-containg chromium coating. The oxidation resistance of Ni-Al/Ni-Al-Y₂O₃ composite before and after chromization was compared at 900°C for 50 h. The results indicated that Y₂O₃ slightly decreased the scaling rate of the electrodeposited alumina-forming Ni-10.5 wt% Al composite coatings, and the reduction effect increased with the extended oxidation time. The inward diffusion of Cr and outward diffusion of Al caused the formation of the surface Cr₂Al layer above an Al-depleted area. At the same time, the gradual enrichment of Al in the inner area caused the formation of γ’-Ni₃Al inner layer, which inversely inhibited the inward Cr diffusion due to lower Cr solution in γ’-Ni₃Al. Mixture oxides of (Cr_1−xAlx)₂O₃ formed on the surface Cr₂Al spell at 5 h due to lower spallation resistance. After that, Y₂O₃ significantly decreased the scaling rate of chromium coatings. The effects of Y₂O₃ on the chromium coating formation and the oxidation behavior of the as-deposited alumina-forming Ni-Al composite coatings before and after chromization are discussed in detail.