Environmental and compositional factors often influence the hot-corrosion behavior of alloys and coatings used in gas turbine engine applications. This paper examines the effects of environmental factors such as salt deposition rate, gas composition, and extent of oxidation on the hot-corrosion resistance (Type I-900°C and Type II-704°C) of Ni-based alloys and coatings. It also evaluates the effects of Pt-modified ß-NiAl alloy composition on hot-corrosion resistance. Both types of hot corrosion conditions were simulated by depositing Na2SO4 salt on the test samples and then exposing those samples to a catalyzed O2:SO2 atmosphere. It was found that Type I hot corrosion resistance decreased with increase in salt deposition rate up to 8 mg/cm2; however, no measurable hot corrosion was observed when the sample was completely buried in a salt. It was also found that the Type II hot corrosion resistance of Pt-modified Ni-based alloys is very sensitive to both alloy and gas composition.
Advanced aero, industrial, and marine gas-turbines can face harsh (i.e. with aggressive deposits and mixed gas mixture) operating conditions at elevated temperatures; hence, surface degradation by high temperature oxidation (>1000°C) and hot corrosion (~ 850-1000°C for Type I and 600-800°C for Type II) may occur [1]. Of particular concern in this study is salt-induced hot corrosion (HC), which is an accelerated degradation process induced by the presence of a thin layer of molten alkali salts on the sample or component surface. Hot corrosion can be a particularly significant degradation mechanism in marine gas turbines and jet engines operating near a sea environment. Hot corrosion is more frequently observed in the low-pressure turbine (LPT) sections because they operate at lower temperatures, thus allowing a significant amount of corrosive deposits to accumulate on the surface [2]. It has also been observed that hot corrosion is usually greatest at the hottest point on the concave (pressure) surface of the LPT blade [3]. Recently, with a progressive increase in operating temperatures of gas turbines, cooler sections such as under the platform of the airfoil are susceptible to experiencing accelerated hot corrosion attack, thus severely reducing longevity of gas turbine components. There are typically two different modes of hot corrosion reported in the literature [1-4]: Type I (~900°C) and Type II (~700°C). Which of these two modes occurs depends upon temperature which, in turn, affects microstructural appearance. The temperature dependence and degradation rates of these two modes of hot corrosion are compared with oxidation rate in Figure 1 [4]. It is seen that hot corrosion is more aggressive than oxidation in the temperature range 650-1000°C; while, above 1000°C salt vaporizes and oxidation becomes the significant degradation process. Type I HC occurs above the melting temperature of external salt deposit and Type II HC occurs below the melting temperature of external salt deposit; however, there is localized liquid formation associated with Type II HC. This localized liquid formation is primarily due to a reaction between the salt deposit and alloy underneath. Superalloys are often protected from hot corrosion and/or oxidation by application of an Alenriched diffusion or overlay type coating.
