Corrosion testing of high-nickel alloy composite tube materials in simulated recovery boiler lower furnace conditions

High-nickel alloys like Alloy 825 and Alloy 625 have been found very promising as substitutes for 304L austenitic stainless steel composite tubing commonly used in recovery boiler lower furnace water-wall panels. High-nickel alloys may be applied as outer layers of composite tubing or as weld overlays. As compared to austenitic stainlesss steels they possess higher yield strength at elevated temperatures and their thermal expansion coefficient is as closer to that of carbon steel. These properties make them, as compared to type 304L austenitic stainless steel, less prone to thermal fatigue or stress corrosion cracking. However, because of the high-nickel content, their sulphidation resistance has been questioned. This paper reports about recent results of comparative laboratory testing of sulphidation and stress corrosion cracking susceptibility of some candidate high-nickel alloys. Laboratory testing performed under simulated lower furnace environments at exposure temperatures of 360oC and higher indicate that corrosion and stress corrosion cracking resistance of high-nickel alloys of type 625 is superior to austenitic stainless steel of type 304L under normal lower furnace operation conditions. Corrosion resistance of Alloy 825 as compared to austenitic stainless steel of type 304 L varies depending on the detailed environmental conditions. Testing programme will be continued and extended to higher temperatures and more aggressive chemical conditions.