A study of the Cr-depleted surface layers formed on four Cr-Ni steels during oxidation in steam at 600° C and 800° C

Three commercial steels with 18% Cr-11% Ni, 19% Cr-25% Ni, and 19% Cr-35% Ni and a laboratory-made 20% Cr-35% Ni steel have been oxidized in steam at 600°C and 800°C. Two types of oxides are distinguished: a thin layer rich in Cr2O3 and patches of thick oxide of Fe, Ni, Cr spinels. The Cr concentration profile in the steels under the first type has been determined by microprobe analysis of ground and pickled or annealed specimens oxidized for various times. The variation of the Cr concentration in the alloy at the alloy-oxide interface with surface treatment, time, and temperature is discussed. It is concluded that one reason for the more extensive nodular growth on pickled or annealed specimens is their greater Cr depletion after short time oxidation. The interdiffusion coefficients in the Cr-depleted zones have been determined on ground and annealed specimens oxidized at 800°C. The acceleration of the diffusion rate due to grinding is clearly demonstrated. A fair agreement with diffusion data from diffusion couples is found.