Effects of sol-gel-derived silica coatings on high-temperature oxidation of Ni

Effects of sol-gel-based coatings on high-temperature oxidation of Ni were studied in the temperature range 700-1200 degreesC in air by thermogravimetry, SEM, and TEAL The oxidation kinetics are parabolic and lower than for uncoated Ni, which also shows subparabolic kinetics below similar to 1000 degreesC The temperature dependence of the parabolic rate constant for oxidation of silica-coated Ni corresponds to a single Arrhenius relation. The NiO scale formed on silica-coated Ni consists of three regions; an outer region with columnar grains and an inner region of equiaxed grains, separated by, a fine-grain, Si-rich layer. For uncoated specimens only, the outer columnar and inner equiaxed regions were encountered. The beneficial effect of the silica on the rate of oxidation has been interpreted to reflect that Si segregated along grain boundaries in the intermediate layer decreases the outward Ni flux along grain boundaries.