The use of aluminium-enriched layers on Hastelloy X against high temperature carburization in high temperature gas-cooled reactor helium

The possibility of protecting nickel-based alloys against carburization using Ni-Al layers is discussed. Ni-Al layers were produced using the chemical vapour deposition reaction: AlCl3+(3/2)H2+Ni-based alloy → Ni-Al+3HCl. The kinetics of growth of an Ni-Al layer on nickel and the nickel-based alloys Hastelloy X, IN-738LC and IN-617 in the temperature range 1173-1373 K at total pressures ranging from 100 to 800 mbar are described. No carbon uptake occurs in the layer or in the bulk alloy, because of the formation of an α-Al2O3 oxide layer in CH4-H2 gas mixtures with carbon activities from 0.2 to 0.8 and at temperatures of up to 1273 K. Al2O3 formation is caused by the presence of oxygen as an impurity in the CH4-H2 gas mixtures. (Auth.)