High-Temperature Corrosion Mechanisms for Selected Iron and Nickel-Based Alloys Exposed to Sulfur and Chlorine-Containing Environments

A comprehensive research project consisting of pilot-scale combustion testing and long-term laboratory corrosion study has been performed. A pilot-scale combustion facility was modified to enable burning of pulverized coals under the conditions typical for coal-fired utility boilers. Eight United States (U.S.) coals were selected for this investigation, with the test conditions for all coals set to have the same heat input to the combustor. In addition, the air/fuel ratio was controlled so that approximately 3% excess oxygen was attained in the combustion gas at the furnace outlet, a location equivalent to the furnace exit of commercial boilers. During each pilot-scale combustion test, extensive online measurements of the flue gas compositions were performed. In addition, deposit samples were collected at the same location for chemical analyses. This information characterized well the combustion environments adjacent to the superheaters and reheaters of utility boilers burning different U.S. coals. The gas and deposit compositions were then simulated in a series of 1,000-h laboratory corrosion tests in which the corrosion performances of different alloys were evaluated at 704 degrees C (1,300 degrees F). Results of this laboratory study led to better understanding of the corrosion mechanisms operating on superheaters and reheaters in coal-fired boilers due to the coexistence of sulfur and chlorine in coal. A new corrosion mechanism, i.e., active sulfide-to-oxide corrosion mechanism, accounts for the rapid corrosion attack on low-alloy ferritic steels and lower-grade stainless steels resulting from the formation of iron chloride (FeCl2) vapor and associated cycling reactions. For higher alloys containing sufficient chromium, the attack is dominated by hot corrosion in the presence of a fused salt. In addition, two stages of the hot corrosion mechanism have been elucidated here in detail. The initiation of hot corrosion attack induced by molten sulfate may involve Stage 1 acidic fluxing and re-precipitation of the protective scale formed initially on the deposit-covered alloy surfaces, followed by Stage 2 Hot Corrosion dominated by basic fluxing and re-precipitation after the scale is locally penetrated by the fused salt.