Internal Oxidation of Nickel Alloys Containing a Small Amount of Chromium

A dilute solid solution of nickel containing chromium up to 4 wt% in the form of a plate was internally oxidized at temperatures between 900° and 1300°C for various times in order to investigate the growth and structure of the subscale formed. The results obtained are as follows: (1) The relationship between the penetration depth of oxygen observed from the growth of the internally oxidized layer and the oxidation time of the alloys was well expressed by a parabolic law. It may be concluded that the activation process in internal oxidation is governed by diffusion of oxygen through the internally oxidized layer. (2) The activation energy for the growth of the internally oxidized layer decreased with increasing solute content in the alloys. (3) At high temperatures, the velocity of internal oxidation increased, and the relation between the temperature T and the penetration depth E was expressed by E∝T2. (4) The penetration depth E increased with increasing oxygen pressure PO2 in relation to the logE∝logPO2, and it was explained by Seybolt’s law. (5) The higher the solute content, the more grain boundary diffusion prevailed. (6) Diffusion constants of oxygen in the internally oxidized layer were calculated by Wagner’s equation. With increase of the Cr2O3 content, the diffusion constant of oxygen increased. This was explained by the interfatial diffusion of oxygen between nickel matrix and Cr2O3 particles. (7) From the results obtained above the diffusion constant of oxygen in pure nickel was deduced.