Corrosion Problems in Coal-Fired Boiler Superheater and Reheater Tubes: Fireside Corrosion. Final Report

摘要

This program was conducted to develop improved alloys for superheater and reheater tubes in direct coal-fired steam generators in order to extend the service life of the tubes, and to evaluate materials for use at higher operating temperatures than currently in use. The program consisted of a broad literature review and laboratory evaluation involving 19 commercial and semicommercial alloys, 19 custom-melted alloys and 18 types of high-temperature coatings, subjecting them to flowing flue gas and molten alkali-iron trisulfate ash at 1250 and 1450 exp 0 F (675 and 790 exp 0 C). The commercial and semicommercial alloys suffered severe attack and pitting under the laboratory fireside test environment at 1250 exp 0 F (675 exp 0 C). In the custom-melted group, the best alloy (25%Ni-20%Cr-2%Al-2.4%Si) generated a dense, adherent scale which reduced pitting by a factor of four when subjected to the molten trisulfate. The character of the pitting attack is best described as an electrochemical mechanism where the molten trisulfate provides the electrolyte and transfers away the metal consumed at the active anodic areas. Protective scales were observed only on a few alloys of higher silicon content; these alloys are believed to offer significant improvement in useful life in steam generators firing corrosive coals. The magnesium zirconate (MgZrO sub 3 ) coating was resistant to corrosive attack, providing a self-healing mechanism by the formation of high-melting sulfates within its pores. This pore plugging effect withstood exposure cycles which would seriously damage conventional stainless steel tubes. (ERA citation 06:009116)