Smart overlay coatings are a functionally gradient coating system designed to provide high temperature corrosion protection over a wide range of operating conditions.

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This paper describes work to study oxidation and hot corrosion damage in nickel disc alloy RR1000. Oxidation damage has been characterised via mass change data from thermogravimetric analyses at temperatures from 700 to 800 ºC and cyclic oxidation testing at 700 and 750 ºC. Thin oxide scales rich in Cr and Ti have been found to grow in a parabolic dependence with time. Oxidation rate constants have been correlated with temperature using an Arrhenius equation. Through the use of simplifying assumptions, mass change data have been converted to oxide thickness values. These values have been compared with observed values of oxide scale and the depth of near-surface damage, which have been measured using focused ion beam (FIB) microscopy.Hot corrosion of RR1000 has been studied in the laboratory by 'deposit' recoat experiments at 700ºC. In these tests, samples were coated with deposits of Na 2 SO 4 /NaCl salt in a gas stream of air-300 vpm SO 2 . Corrosion damage has been quantified by dimensional metrology of samples before and after exposure. This enabled metal losses, i.e. the depth of corrosion pits, to be determined. Distributions of metal loss data were then generated to produce median damage values and exceedance cumulative probabilities. Finally, the effect of contaminants on the hot corrosion resistance of RR1000 has also been evaluated.

The traditional trend towards the development and use of power plants with ever increasing efficiencies is now being coupled to the use of a wider range of fuels and technologies designed to minimise CO 2 emissions. Alternative solid fuels such as biomass and waste products, which can be classified as CO 2 neutral, are being used alone or cofired with fossil fuels. The cofiring of biomass and coal is currently the most efficient and effective method for using biomass to generate power. CO 2 capture technologies include systems for either precombustion or postcombustion CO 2 removal. Gasification of fuels (using either oxygen or steam as the oxidant) produces a gas that can be conditioned to enable precombustion CO 2 removal. Post-combustion CO 2 capture can be carried out using either solid or aqueous sorbent processes. Oxy firing of fuels is a technology that would enable more efficient post-combustion CO 2 capture. The various combinations of new fuels, novel technologies and higher temperature component operating conditions are producing challenging operating environments for components. Deposition, erosion and corrosion issues for hot gas path components in these advanced power generating systems, which are potentially life limiting, are reviewed. Reduction in heat transfer owing to high rates of deposition can significantly reduce heat transfer and increase the need for component cleaning. Depending on the system, component parts can include various heat exchangers, gas cleaning systems and gas turbines.

Citation: KARABELA, A. ... et al., 2011 AbstractOxidation damage, combined with fatigue, is a concern for nickel-based superalloys utilised as disc rotors in high pressure compressor and turbine of aero-engines. A study has been carried out for a nickel-based alloy RR1000, which includes cyclic experiments at selected temperatures (700°C~800°C) and microscopy examination using Focused Ion Beam (FIB). The results suggest that the major mechanism of oxidation damage consists of the formation of surface oxide scales and internal micro-voids and oxide particles beneath the oxide scales, which become more severe with the increase of temperature. Applying a cyclic stress does not change the nature of oxidation damage but tends to enhance the extent of oxidation damage for temperatures at 750°C and 800°C. The influence of cyclic stress on oxidation damage appears to be insignificant at 700°C, indicating a combined effect of cyclic stress and temperature. Further energy dispersive x-ray (EDX) analyses show the enrichment of Cr and Ti, together with lower Ni and Co levels, in the surface oxide scales, suggesting the formation of brittle Cr 2 O 3 , TiO 2 , NiO and Co 3 O 4 oxides on the specimen surface.Penetration of oxygen into the material and associated internal oxidation, which leads to further 2 material embrittlement and associated failure, are evidenced from both secondary ion imaging and EDX analyses.