Type I and II hot corrosion attack has been observed on certain marine gas turbine engines in different areas of the hot section turbine hardware. In order to improve the service life and resistance to hot corrosion of these turbine blades, laboratory testing of the current baseline coating and several candidate replacement coatings was initiated using a low-velocity, atmospheric-pressure burner-rig (LVBR). This paper examines the LVBR results of these high temperature coatings of various chemical composition and applied by several processes on one alloy substrate to determine the hot corrosion resistance relative to the baseline coating. The goal of the research is to replace the current baseline coating with a better quality, more resistant, and better performing coating system to implement on hot section blades of marine gas turbine engines used in the U.S. Navy.
Marine gas turbine engines serve as primary and auxiliary power sources for several current classes of ships in the U.S. Navy. It is desired that marine gas turbine engines have a mean time between removal of 20,000 hours. While some engines have approached this goal, others have fallen significantly short. The main reason for this shortfall is hot corrosion (Type I and Type II) damage in the hot section turbine hardware due to intrusion of salts from the marine air and/or from sulfur in the gas turbine combustion fuels. Type I hot corrosion usually occurs at metal temperatures ranging from 850 to 950 °C (1560-1740 °F) while Type II, low temperature hot corrosion occurs in the temperature range of 650 to 750°C (1202-1382 °F).
Heavy 1st turbine blade under platform corrosion attack has been observed on engines with as low as 10,000 hours. To date one engine failure (18,000 hours), has been a direct result of corrosion attack at this blade location. A mandated fleet wide borescope inspection has led to several engines being removed from service due to compromised blade integrity.
Two failed blades operated between 5000 and 10000 hours from two marine gas turbine engines and ten unfailed blades removed from another marine gas turbine engine after operating for 18000 hours were examined. Each of the unfailed blades displayed heavy deposits; corrosion was present under the platform of the double-stemmed blade of some blade samples. Figure 1 shows two blades with the heavy under-platform deposits. The blade substrate was chemically analyzed and was found to conform to Alloy M247 chemical requirements. Metallographic examination showed that the coating thickness under the platform and in the curved area of transition between the platform and the blade stem was generally very thin (0 to 1.6 mils {0 to 40 µm thick}). The specified MCrAlY coating, when present, usually was porous or had entrained contamination under the platform due to lack of adequate spray deposition in these non-line-of-sight areas as shown in Figure 2a. MCrAlY coating thickness at other sites along the blade stem was 1.4 to 4.1 mils (35 µm to 105 µm). This area of transition between the platform and the blade stem also corresponded to severe corrosion. Corrosion that was observed under- the-platform was caused by Type II, low-temperature hot corrosion which occurs in the temperature range of 650-750°C (1202-1382 °F).
