Effects of chromium silicon and yttrium on the stability of protective oxides of FeCrAlY for use in syngas coolers

Seven steels: Fecralloy, MA956, and five experimental Fe-Cr-Al alloys, were exposed to a simulated low Btu coal gasifier atmosphere at 4500C, and kinetic measurements were carried out. An assessment was made of the relative corrosion resistance of the alloys, and it was found that chromium, aluminium, and silicon improve resistance to sulfidation whereas yttrium has a variable effect. A more detailed characterization using transmission electron microscopy, was carried out on surface films stripped from three of the alloys: Fecralloy; MA956 and an experimental alloy with a high chromium content. The surface films were found to be a chromium-rich spinel with very little sulfur in the early stages of corrosion. The sulfur level in the film increased with exposure time to a maximum and after 15 minutes chromium-iron, sulfides formed