This study compares and ranks the low-temperature (i.e., 705°C/1300°F) hot corrosion resistance of CoCrAlY and Pt-modified CoAl coatings, together with Pt-modified â-NiAl coatings. A laboratory scale Dean rig was used to simulate the hot corrosion conditions. Coating performances under the low- temperature (Type II) conditions were ranked by assessing the cross-sectional SEM images of the corroded samples. It was found that the CoCrAlY coating had better resistance than the Pt-modified CoAl coating for the range of times tested (up to 200 h), while the Pt-modified â-NiAl coating had the best resistance overall.
An aluminide-based coating is often applied to the superalloy hot-section components of a turbine in order to extend the service life of those components. The aluminide coating can confer protection by forming a continuous and adherent thermally grown oxide (TGO) scale of Al2O3.
Salt-induced hot corrosion is an accelerated mode of degradation that is known to occur in various high temperature engineering applications, including marine gas turbines. Such degradation involves the deposition of salts (e.g., sulfates) from the surrounding environment (e.g., combustion gas) to the surface of hot components, followed by destruction of the protective TGO scale by a fluxing mechanism. The gas turbine engine components exposed to marine environments are apt to encounter two modes of hot corrosion: high-temperature hot corrosion (Type I) in the temperature range 850- 1000ºC and low- temperature hot corrosion (Type II) in the range 600-800ºC. The lab-scale simulation of both modes of hot corrosion can be achieved by depositing sulfate(s) on the sample surface and controlling the gaseous SO3 content in the test atmosphere. Of particular concern in this study is low-temperature hot corrosion resistance (705ºC) of CoCrAlY and Pt-modified CoAl coatings on the cobalt-based superalloy MAR-M509 (superalloy 509). The CoCrAlY coating is currently used in first stage vanes of gas turbine engine components; while Pt-modified CoAl is a candidate replacement coating that is deposited by a proprietary process. The CoCrAlY-based coating is known to have relatively good resistance to high-temperature Type I hot corrosion; however, their low-temperature Type II hot corrosion depends upon the Cr content. Luthra showed that Cr-rich CoCrAlY coatings have better Type II hot corrosion resistance than conventional CoCrAlY coatings.
Hot corrosion resistance of coatings often depends on chemical composition, microstructure and deposition method. Platinum addition has been found to improve Type I hot corrosion and oxidation resistance of â-NiAl coatings. One contributing benefit is that Pt addition to â-NiAl coating reduces surface spallation by promoting the formation of a slow growing, adherent and continuous á-Al2O3 scale. Haynes et al. and Zhang et al. found that Pt helped in decreasing the detrimental effects of high sulfur levels in the substrate by reducing the amount of voids in the metal at the scale/metal interface. The benefits of Pt addition to â-NiAl coatings are well known, but the same is not true for Pt addition to Co-based aluminides.
The primary purpose of this study was to compare and rank the resistance of the selected Cobased aluminide coatings (CoCrAlY and Pt-modified CoAl) and Pt-modified â-NiAl coating to Type II hot corrosion. In an attempt to simulate hot corrosion (HC) in actual marine gas turbine service conditions, a laboratory-based Dean rig was used in which the test samples are subjected to a continuous deposition of Na2SO4 salt in an O2: SOx atmosphere. Coating performances under the Type II conditions were ranked by assessing the cross-section
