Hot corrosion resistance of coated single crystal superalloys under marine conditions

Abstract

Uncoated and coated nickel base superalloys designed for use in gas turbine blades and vanes were tested at 900°C in a high velocity burner rig. Diesel fuel, with an injection of 5 ppm sea salt, was used to simulate marine engine conditions. The uncoated alloys, namely, single crystal SRR99, CMSX-2, SX60A, and MMT143 and polycrystalline MAR-M002, IN738 LC, and IN-100, were all severely attacked by oxidation and corrosion after only 100 h of the cyclic test. A high chromium level in the alloy improved the hot corrosion resistance, whereas high molybdenum and tungsten contents were detrimental. The degradation in the 900°C tests was much greater than that experienced in previous tests at 1100°C using aviation kerosene fuel. Simple diffusion aluminide, platinum aluminide RT22, and four overlay coatings (air plasma sprayed LCO22, LCO22 plus aluminising, sputter ion plated Ni–Cr–Al–Si, and electron beam physical vapour deposited Ni–Co–Cr–Al–Y) all protected the alloys. The Ni–Co–Cr–Al–Y coating failed after 400 h but the other coatings were still protective after 1000 h. A rank order for the coatings has been obtained via metallographic studies.

MST/1700