The objective of this research by the U.S. Bureau of Mines was to determine the effects of al additions on the sulfidation resistance of fe-cr-ni alloys. A series of fe-cr-ni alloys with and without additions of 4 pct al were tested in a wet s vapor atmosphere at 677 deg c. A study of the morphology and composition of corrosion scales indicated a different corrosion mechanism for al-free alloys compared with al-containing alloys. Aluminum-containing alloys formed a compact, tightly adherent layer at the metal-scale interface that reduced corrosion rates by a factor of 3 to 10 compared with that of al-free alloys. The thin, adherent al- containing scale limited inward diffusion of o and s to the base metal and outward diffusion of ni from the base metal. Aluminum- containing alloys formed a three-layer scale on oxidizing-s environments, while al-free alloys formed a less protective two- layer scale.