Hot corrosion in industrial gas turbines.

Future gas fired power stations are likely to use gas turbines operating both at higher temperatures to improve thermal efficiency, and fire cheaper fuels containing more deleterious contaminants. Additional changes relate to potential new methods of CO₂ reduction including pre-combustion capture. These developments in industrial gas turbine technologies present new materials challenges that must be understood. This research aims to further scientific understanding of type II hot corrosion and its effect on a range of candidate alloys for use in industrial gas turbines under novel exposure conditions. Selected parameters have been isolated under those conditions and their effects investigated. Laboratory-based hot corrosion tests using the “deposit recoat” technique were used to evaluate the following variables on candidate materials (GTD 111, Rene 80, PWA 1483, MarM 509, RT22 and SC2464): exposure atmospheres including SO₂ (3.6 to 300 ppm) and water vapour content (0 to 20 vol.%); deposit flux (1.5 and 5 µg/cm²/h of 80% Na₂SO₄ 20% K₂SO₄ ); and the addition of molybdenum particles to deposits. Qualitative assessment of hot corrosion morphologies used a scanning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) spectroscopy capabilities. The effect of water vapour was found to be dependent on SO₂ concentration. Increasing water content increased hot corrosion in low (3.6 ppm) SO₂ atmospheres, but was found to reduce it at higher (300 ppm) SO₂ concentrations. Mixed mode corrosion features have been identified by SEM/EDX analysis, on all samples that progressed from hot corrosion incubation to propagation. These features are attributed to changes in the deposit melt’s chemistry allowing for both acidic and basic fluxing to occur. Refractory metal carbide precipitates were found to exacerbate degradation and acted as nucleation points for hot corrosion damage. It is proposed that this is, at least in part, alloy induced acidic fluxing occurring concurrently with gas phase induced hot corrosion.