Protection of Superalloys from Corrosion
Earlier Superalloys were supposed to offer high strength and adequate resistance to oxidation. It was obtained with super alloys comprising of about above 20% chromium. Oxidation resistance at high temperature up to 982oC or 1800oF was considered outstanding. Although to improve the design versatility of nickel based super alloys, magnitude of chromium was lowered to add more content of hardener. Simultaneously some conditions were employed in applications such as marine that increased oxidation extensively or iron induced corrosion like hot corrosion. The service temperatures to which the alloys are subjected in the popular elevated temperature conditions have added to the strength increase of the alloys.
With decrease in temperature lower than 927oC or 1700oF, the extent of hot corrosion severity decreases, however in conditions at temperatures even lower than 760oC or 1400oF, corrosion may increase steeply with lowering temperature. In this condition, the region of land based gas turbines, the corrosion is prevented by producing chromium oxide on the material’s surface. As a result, the alloys for this condition and operation are particularly developed to comprise of higher magnitudes of chromium such as 20% or more. Many super alloys are made for suitable hot corrosion resistance at temperature limits more than 760oC or 1400oF.
At the elevated temperatures, chromium oxide produced while earlier heat processing is less safe and doesn’t reproduce while subjecting to the high temperature conditions. Ordinary oxidation and intergranular oxidation with the grain boundaries in materials are a trouble with chromium secured superalloys. Although the trouble is not much complicated as at first expected obtaining security nature of aluminum oxide produced in higher magnitudes by larger magnitudes of aluminum of second and third version gamma’ hardened superalloys. However the general oxidation still takes place and results in decreased areas, hence significantly increasing stress in the residual alloy. The blend of these conditions is a major concern and to secure the alloys from them, varnishing is done.
Associated with Superalloy, secured coating technique is the production of ceramic known as thermal protection coating that secures alloy components by decreasing surface temperature to many hundred degrees, allowing a superalloy to perform at low temperature conditions in a place where they may possess more strength. Thermal protection coatings are useful in several alloys. They implement overlay coating technique. A thin overlay coat acts as a bond interface among the ceramic and base metal.
Stripping and recoating of turbine airfoils are a common restoration and repair method for super alloys. Oxidation and corrosion resistant coating may be reemployed to recover resistance of superalloy against heat and gaseous media. Welding is commonly used for missed, lost, damaged and routed material. Welding of solution hardened nickel base alloys like Hastelloy X and low strength precipitation toughened nickel alloys is readily performed. Although high strength alloys like Inconel 718 may be fully weldable. The welding of cobalt based turbine airfoil and sheet products is also done readily. Machining is done after welding then heat processing and painting.