Study of hot corrosion of single crystal superalloys and platinum-aluminide coatings

摘要

At the present time, combined cycle systems for power generation (e. g.IGCC), offer increased efficiency of power generation and lowerenvironmental emissions, specifically C02, SOxg, and NO,,, as well as beingadaptable to most fossil fuels. Economic factors, such as the cost of thematerials must be considered. Materials influence the service lifetime in therequired operational environment. Solid fuels like coal and biomass producedifferent combustion environments containing a range of contaminants that,when they reach their melting points, may cause accelerated corrosion,affecting directly the service life time of the gas turbine constructionalmaterials. This accelerated corrosion is known as Hot Corrosion.The aim of this study was to develop, an understanding of the influenceof these environmental factors on rate of hot corrosion of modem turbinematerials, i. e. the single crystal alloys CMSX4 the SC2,both uncoated andPtAl coated that are needed for a gas turbine blade and vanes operating in arange of hot corrosion environments expected in an lGCC plant.To achieve this aim, a series of laboratory corrosion tests was plannedto simulate the same corrosion environment as in industrial high temperaturegas turbine operation. Following established procedures for corrosion testing,samples were exposed in a controlled atmosphere furnace to a mix of gases(air/SO241CI) with a cyclic exposure time of 50 and/or 100h duration. Eachcycle, samples were removed to be recoated with an alkali salt mixture to atotal exposure time of 500h and or 1000h. Cross sections were examined bySEM/EDX to identify the mode of hot corrosion attack. To quantify the rate ofcorrosion, samples were measured pre-exposure and post-exposure, and thiscorrosion data was statistically assessed.Finally, from this quantitative data, life prediction models weredeveloped to describe/predict the onset of hot corrosion and the corrosion ratesobserved under different gas compositions, and various deposition fluxes, bothat typical type I and type II hot corrosion temperatures in terms of incubationand propagation periods. Separate models have been developed for the twosingle crystals superalloys: CMSX4 and SC2, in both the uncoated andplatinum aluminide coated condition. The goodness of fit as defined by theregression coefficient varies from 0.88 to 0.99 for the propagation models at700 and 900'C. The incubation models are as precise at 7001C but less preciseat 9001C with regression coefficients of 0.78-0.94.I