RAPID OXIDATION AND SULFIDATION OF FEAL INTERMETALLICS IN NACL-(NA,K)2SO4 MELTS AT 800-850-DEGREES-C

The corrosion of FeAl both pure and containing Zr or B additions has b een studied in (Na, K)2SO4-NaCl melt at 800-850-degrees-C in air. The hot-corrosion resistance of pure FeAl is rather poor due to the absenc e of a continuous protective layer of alumina and to the presence of F eS in the scales. A strong intergranular attack, denoted as grain-boun dary pest, produces oxide and sulfide precipitates in the alloy, showi ng that oxygen and sulfur can diffuse rapidly along the grain boundari es. These internal precipitates produce compressive stresses which det eriorate the bonding between grains and lead to the formation of crack s, resulting eventually in a complete sample disintegration over most of its surface. Similar but less important pest attack is also observe d for FeAl containing Zr, but not in the case of B addition. The benef icial effect of B in our experimental conditions may be ascribed to it s ability to promote the formation of protective alumina scales, to in hibit scale cracking and to hinder the penetration of oxygen and sulfu r along the grain boundaries in the alloy. Zr has a smaller effect on these processes.