Hot corrosion tests on corrosion resistant coatings developed for gas turbines burning biomass and waste derived fuel gases

This paper reports on results of hot corrosion tests carried out on silicon–aluminide coatings developed for hot components of gas turbines burning biomass and waste derived fuel gases. The corrosion tests of the silicon–aluminide coatings, applied to superalloys IN738LC and CMSX-4, each consisted of five 100 h periods; at 700 °C for the type II tests and at 900 °C for the type I tests. Deposits of Cd + alkali and Pb + alkali were applied before each exposure. These deposits had been previously identified as being trace species produced from gasification of biomass containing fuels which after combustion had the potential to initiate hot corrosion in a gas turbine. Additionally, gases were supplied to the furnace to simulate the atmosphere anticipated post-combustion of these biomass derived fuel gases. Results of the type I hot corrosion tests showed that these novel coatings remained in the incubation stage for at least 300 h, after which some of the coating entered propagation. Mass change results for the first 100 h confirmed this early incubation stage. For the type II hot corrosion tests, differences occurred in oxidation and sulphidation rates between the two substrates; the incubation stages for CMSX-4 samples continued for all but the Cd + alkali high salt flux samples, whereas, for IN738LC, all samples exhibited consistent incubation rates. Following both the type I and type II corrosion tests, assessments using BSE/EDX results and XRD analysis confirmed that there has to be remnant coating, sufficient to grow a protective scale. s study, the novel silicon–aluminide coating development was based on coating technology originally evolved for gas turbines burning natural gas and fossil fuel oils. So in this paper comparisons of performance have been made with three commercially available coatings; a CoCrAlY overlay, a platinum-aluminide diffusion, and triple layer nickel–aluminide/silicon–aluminide-diffusion coatings. These comparisons showed that the novel single-step silicon–aluminide coatings provide equal or superior type II hot corrosion resistance to the best of the commercial coatings.