Fireside and steamside corrosion of alloys for USC plants

A program on fireside corrosion is being conducted at Argonne National Laboratory to evaluate the performance of several structural alloys in the presence of mixtures of synthetic coal ash, alkali sulfates, and alkali chlorides. Experiments in the present program, which address the effects of deposit chemistry, temperature, and alloy chemistry on the corrosion response of alloys, were conducted at temperatures in the range of 575–800∘C for time periods up to ≈1850h. Alloys selected for the study included both ferritic and austenitic steels and Ni-base alloys. Data were obtained on weight change, scale thickness, internal penetration, microstructural characteristics of corrosion products, mechanical integrity, and cracking of scales. Results showed that the relationship of corrosion rates to temperature followed a bell-shaped curve, with peak rates at ≈725∘C, but the rate itself was dependent on the alloy chemistry. Several Fe-base alloys showed acceptable rates in the sulfate-containing coal-ash environment; but NaCl in the deposit led to catastrophic corrosion at 650 and 800∘C. Ni-base alloys generally exhibited less corrosion than the Fe-base alloys under similar exposure conditions; however, they were susceptible to localized corrosion in the form of pits. Experiments were also conducted to evaluate the corrosion performance of several Fe-base alloys in a steam environment. Results, presented on both the fireside and steamside corrosion, can be used to compare the performance of the various Fe-base alloys. Attaining adequate creep strength, fireside corrosion resistance, and steamside corrosion resistance is still a challenge in materials development for advanced steam-cycle applications.