The use of model alloys to study the effect of alloy composition on steam and fireside corrosion

One future objective is to develop advanced ultrasupercritical coal-fired boilers that incorporate oxy-firing to facilitate carbon capture. The steam condition goals of 760°C/34.5 MPa will require major materials changes throughout the boiler, including Ni-base alloys in the hottest sections. While a variety of commercial Ni-base alloys are being evaluated in steam and fireside laboratory experiments, exposure of model alloys also is needed to understand the roles of alloying elements such as Cr, Al, Ti, Mo and Co in these alloys. Fireside exposures were conducted at 700° and 800°C in simulated oxy-firing conditions for 500h. Alloy additions of Al and Ti were not detrimental to the metal loss under these conditions. However, reducing the Cr content from 22% to 18% had a substantial effect. Effects of Mo and Co additions were different at 700° and 800°C. Steam exposures were conducted at 800°C in 17bar steam for up to 5,000h. Internal oxidation increased with increasing Ti and Al+Ti additions, similar to observations in commercial alloys.