Influence of chromium carbides on the high temperature oxidation behavior and on chromium diffusion in nickel-base alloys

Two simple cast nickel alloys Ni-30Cr-0.2C and Ni-30Cr-0.8C were oxidized at 1,000, 1,100 and 1,200 degrees C. Their behaviors were characterized using thermogravimetry techniques and their parabolic and chromia volatilization constants were determined by analyzing the m center dot dm/dt -m curves. The constants obtained were generally in good agreement with experimental mass-gain kinetics and chromium-balance aspects. A higher carbon content i.e., a higher carbides density leads to higher values of the transient linear-oxidation constant K (l) , of the parabolic constant K-p , and obviously also of the volatilization constant K-nu v . Chromium diffusion coefficients through the zone affected by oxidation were calculated from the oxidation kinetics and chromium gradients. They increase when the carbide density increases.