Abstract
The effect of variation of pack activators, compositions, temperature, and time on the thickness and structure of aluminide coatings formed on the nickel‐base superalloy IN‐100 was studied in one‐step packs containing aluminum at unit activity. Times were varied from 4 to 24 hr and temperatures were varied from 982° to 1149°C in ‐activated packs. The other halides of sodium and the ammonium halides were primarily used to activate 1093°C, 16‐hr packs. In addition, an analysis of the thermodynamics and kinetics of aluminizing was carried out. The mechanism of coating formation in each pack was established from agreement between observed coating weights and predictions based on a gaseous diffusion model and solid‐state diffusion considerations which used published diffusion data for the Ni‐Al system. Pack temperature rather than pack aluminum activity controls the principal coating phase formed. The halides ranked according to aluminum weight gain in (weight per cent) Al packs are . Solid‐state nickel diffusion controlled the rate of coating formation in fluoride‐activated packs. Gaseous diffusion controlled the rate of coating formation in bromide‐, iodide‐, and ‐activated packs containing 1 w/o Al. In ‐activated packs containing the ability of the substrate to supply nickel appeared to be in balance with the ability of the pack to supply aluminum. However, the observed rate constant and activation energy indicated that solid‐state diffusion controlled coating growth. Increasing pack aluminum content from 1 to 5 w/o shifted control of coating formation from the gas phase to the solid state in the 16‐hr, 1093°C, ‐activated pack. Regardless of the rate‐controlling step, the kinetics of coating formation were parabolic. The activation energy for coating formation controlled by solid‐state diffusion was . Similar coating microstructures and weight gains were obtained for each halogen regardless of whether its source was a sodium or ammonium halide.
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