CODEPOSITED CHROMIUM-ALUMINIDE COATINGS .1. DEFINITION OF THE CODEPOSITION REGIMES

Aluminum and chromium were deposited simultaneously onto the nickel-base alloy Rene 80 H using a pack-cementation process. The pack mixtures contained a Cr-Al source alloy and the activator salts tested were NaCl, NH4Cl, YCl3 . 6H2O, and mixtures of CrCl3 . 6H2O and NaCl + NH4Cl. Two deposition temperatures of 1100 and 1120-degrees-C were studied. Only aluminum deposited in packs activated by NaCl, only chromium deposited in packs activated by CrCl3 . 6H2O or NH4Cl, and both metals deposited simultaneously in packs activated by YCl3 . 6H2O or NaCl + NH4Cl mixtures containing no more than 2 weight percent (w/o) NH4Cl. The effects of varying the relative amounts of NaCl and NH4Cl in the mixed-activator packs, together with the Cr-Al alloy composition, on the type of coating obtained were also investigated. In all cases, the tendency to deposit chromium was related to the calculated equilibrium partial pressure ratio p(CrCl2)/(p(AlCl) + P(AlCl2)) and the magnitude of p(CrCl2). Codeposition of the two metals occurred when this ratio was between ca. 0.01 and 0.1. Adjustments to pack compositions were used to control chromium concentrations in the product coating, and a maximum chromium level of 25 w/o was achieved.