Corrosion testing program at the TVA's 20 MW AFBC Pilot Plant

Corrosion test racks containing several engineering alloys were exposed at two in-bed locations in the TVA's 20 MW AFBC Pilot Plant. The materials tested included low alloy steels, ferritic/martensitic and austenitic stainless steels, and nickel-rich alloys. Material wastage rates based on weight loss measurements were obtained for all the alloys; microstructural examinations were conducted for selected materials. Material wastage rates for the low alloy steels were very high. Among the austenitic stainless steels, wastage rates were in the 1-15 mpy (mils per year, 1 mil = 25.4 μm) a discernible correlation between wastage rate and the concentration of the major alloying additives. Subscale corrosion features, in the form of finger-like, low-sulfur penetrations and deeper sulfur-rich precipitates, were detected to depths of 50-150 μm. Corrosion scales were mainly Cr rich with little sulfur. Welded 304ss coupons revealed little or no weld-related deterioration. Of the two nickel-rich alloys tested, Incoloy 800 showed a wastage rate and microstructure comparable to those of the austenitic stainless steels. Inconel 690 experienced internal penetration to a much larger degree than the austenitic stainless steels or Incoloy 800. The ferritic, martensitic stainless steels experienced wastage rates (in the absence of erosion) not unlike those of the austenitic stainless steels