Abstract
Large water wall panels and convection modules were chromized in a pilot-scale facility, using the pack cementation process. Variations in chromium content and thickness of the coating were determined by optical microscopy and electron microprobe. The chromium content of the coating increased with thickness. Minimum coating thickness was about 200 μm and minimum chromium content was 23-25%, which was found to provide adequate sulfidation resistance in laboratory tests at 400°C. Chromium carbide precipitation, especially in heat-affected zones of welds, caused chromium-depleted zones, with chromium contents as low as 16%. These zones sulfidized in laboratory corrosion tests. The use of Nb-stabilized, low-alloy steels as substrates prevented excess carbide precipitation and eliminated intergranular sulfidation. Welding of chromized steels did not cause cracking of the coating. Exposure to acid condensates during down-time caused severe intergranular corrosion and pitting of chromized T-11 steel, but much less damage in chromized Nb-stabilized T-22. It is concluded that chromized, carbon-stabilized, low-alloy steels show promise for use in coal gas environments at temperatures up to 500°C, provided their chromium content is 27% or higher and exposure to acid condensates during shutdowns can be avoided.
