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: $\text{AlCl}{}_3\text{+}\text{3}\text{2}\text{H}{}_2\text{+}\text{Ni-based alloy}\text{→}\text{NiAl}\text{+3}\text{HCl}$ 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.