Abstract
Samples of alloy 800 H (UNS N08810) and alloy 617 (UNS N06617) were oxidized in a simulated process gas atmosphere. The principal elements of the gaseous environment were 50% H2O, 35% H2 and 5% of CO2, CO, and CH4, respectively. The experiments were carried out at temperatures from 850°C to 950°C for exposures ranging from 600 to 5000 hours. After the exposure, the alloy surfaces showed compact oxide layers, which resulted in a depleted zone of oxidizing elements in the substrate layer. Using Electron Probe Microanalysis (EPMA), concentration profiles and the extension of the depleted zone were measured. Calculated theoretical profiles were then fitted to the EPMA-measured profiles, using Fick's second law of diffusion and resolved by a finite difference method and diffusion coefficients as fitting parameters. The diffusion coefficients for chromium may be represented as D = Do exp.(- Q / RT) with the values Do = 11.4 cm2/s and Q = 289 kJ / mol for alloy 800 H and Do = 2.1 cm2/s and Q = 280 kJ/ mol for alloy 617. The value of the resulting activation energy Q for diffusion in the investigated alloys is similar to that reported in the literature for f.c.c structures.
