Conditions for Codeposition of Al and Cr on Ni Base Superalloys by Pack Cementation Process

Abstract

Both theoretical and experimental studies were carried out to identify pack powder compositions and coating conditions suitable for codeposition of Al and Cr by the pack cementation process. It has been demonstrated that for pack powders containing chloride salts as activators and elemental Al and Cr as source materials, codeposition of Al and Cr occurs when the partial pressures P_AlCl and P_CrCl are comparable or when P_AlCl /P_CrCl is close to one. But, only Al or Cr may be deposited when P_AlCl /P_CrCl is much larger or smaller than one. Pack powder compositions suitable for codeposition of Al and Cr depend very much on the type of activators used. For pack powders containing 25 wt-% of source elements of Al and Cr, codeposition of Al and Cr may occur when the Al content is within the range 1·25–1·90 wt-% for the packs activated by 3 wt-%NH₄ Cl, and 1·85–1·95 wt-% for the pack activated by 4 wt-%CrCl₃ . 6H₂O. However, codeposition of Al and Cr may be not achievable with pack powders activated by NaCl and CrCl₃ . Elemental Al and Cr may be used as source materials for codeposition purposes instead of Cr–Al master alloys as conventionally recommended. The coatings formed by codeposition at 1100°C showed a complex structure consisting of three or four major distinctive layers depending on the pack powder composition and coating conditions used, suggesting that the coatings were formed by a sequential deposition mechanism. Both Al and Cr were deposited in the initial and interim stages. However, only Cr was deposited in the final stage, producing a Cr rich top surface layer.