Corrosion performance of structural alloys in oxygen/sulfur/chlorine-containing environments.

Component reliability and long-term trouble-free performance of structural materials are essential in power-generating processes that utilize coal as a feedstock. The combustion environments encompass a wide range of oxygen partial pressures, from excess-air conditions in conventional systems to air-deficient conditions in low-NO{sub x} systems. Apart from the environmental aspects of the effluent from coal combustion, one concern from the systems standpoint is the aggressiveness of the combustion environment toward boiler structural components such as waterwall tubes and steam superheaters. The corrosion tests in this program address the individual and combined effects of oxygen, sulfur, and chlorine on the corrosion response of several ASME-coded and noncoded boiler materials exposed to air-deficient and excess-air combustion conditions. Data in this paper address the corrosion behavior of structural materials such as Type 347 stainless steel, Alloys 800, 825, 625, 214, and Hastelloy X when exposed at 650 C to excess-air combustion conditions with and without HCl. Thermodynamic calculations were made to evaluate the gas chemistries formed from coal combustion. The results of such calculations, coupled with oxygen/sulfur/chlorine thermochemical diagrams, were used to select the gas environments for the laboratory test program. Results are presented for weight change, thickness loss, microstructural characteristics of corrosion products, mechanical integrity and cracking of scales, and the mechanistic understanding gained on the role of sulfur and chlorine in the corrosion process