Coal-Ash Corrosion of a Chromium-Molybdenum Steel Superheater Tube
The top tube of a horizontal superheater bank in the reheat furnace of a steam generator ruptured after seven years in service. The rupture was found to have occurred in the ferritic steel tubing (2.25Cr-1Mo steel (ASME SA-213, grade T-22)) near the joint where it was welded to austenitic stainless steel tubing (type 321 stainless steel (ASME SA-213, grade TP321H)). The surface temperature of the tube was found to be higher than operating temperature in use earlier. The ferritic steel portion of the tube was found to be longitudinally split and heavily corroded in the region of the rupture. A red and white deposit was found on the sides and bottom of the tube in the rupture area. The deposit was produced by attack of the steel by the alkali acid sulfate and had thinned the tube wall. It was concluded that rupture of the tube had occurred due to thinning of the wall by coal-ash corrosion. The thinned tubes were reinforced by pad welding. Type 304 stainless steel shields were welded to the stainless steel portions of the top reheater tubes and were held in place about the chromium-molybdenum steel portions of the tubes by steel bands.