Coal-ash Corrosion of Alloys for Combustion Power Plants

A program on coal-ash 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. Candidate alloys are also exposed in a small-scale coal-fired combustor at the National Energy Technology Laboratory in Pittsburgh. Experiments in the present program, which addresses 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 ≈1850 h. Fe-base alloys selected for the study included HR3C, 310TaN, HR120, SAVE 25, NF709, modified 800, 347HFG, and HCM12A. In addition, 800H clad with Alloy 671 was included in several of the exposures. Ni-base alloys selected for the study included 600, 601, 617, 690, 625, 602CA, 214, 230, 45TM, HR 160, and 693. Data were obtained on weight change, scale thickness, internal penetration, microstructural characteristics of corrosion products, mechanical integrity of the scales, and cracking of scales. Results showed that the relationship of corrosion rates to temperature followed a bell-shaped curve for Fe-base alloys, 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 sulfatecontaining 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.