Gas phase aluminizing of nickel alloys with hydrogen chloride

Aluminizing of novel nickel alloy Haynes 282 and alloy 201 was performed by chemical vapor deposition. Aluminum chloride served as precursor and was generated in-situ by feeding gaseous hydrogen chloride (HCl) into the retort reacting with a solid high-melting aluminum alloy at about 1350 K. In comparison to over-pack aluminizing this process offers the advantage to control the coating growth by simply adjusting the HCl gas flow. Either continuous HCl flow or discontinuous addition was applied. Hydrogen (H2) to HCl gas flow ratios from 0:1 up to 9:1 (total flow of 50 l/h in every case) were tested at atmospheric pressure. Coating thickness was determined from glow discharge spectroscopy and metallographic cross sections. It is shown that formation of the desired β-NiAl phase is relatively insensitive to the gas flow ratio, yet forms best at hydrogen excess with a H2 to HCl flow ratio of approx. 4:1 (40:10 l/h). Discontinuous addition of HCl does not lower coating growth rate significantly in either case because the chloride introduced into the chamber is not consumed in the aluminum deposition process. The results are explained by thermo-chemical calculations showing that an excess of hydrogen chloride reduces formation of NiAl on the part to be coated. Isothermal oxidation tests revealed an improved oxidation resistance of the aluminized surfaces.