Development of binary and ternary chromium, aluminum, carbon thin film coatings for hot corrosion prevention
Increasing the operating temperature of high temperature energy conversion systems is often very desirable in order to maximize their corresponding efficiencies. Materials selected for high temperature applications must therefore incorporate associated considerations such as creep strength, high temperature corrosion resistance, and thermal fatigue. Hot corrosion is a form of accelerated high temperature oxidation associated with the deposition of salt such as sodium sulfate (Na 2SO 4) on metals or their protective oxide surfaces. Severe corrosion develops when Na 2SO 4 reacts with metals to form eutectic (low-melting point) compositions on the surface of the metal at 700 °C (Type II Hot Corrosion) or when the surface has been wetted by molten Na 2SO 4 at 900 °C (Type I Hot Corrosion). This phenomenon reduces the useful life of materials such as nickel alloys used in high temperature gas turbines for aircraft and marine applications. In this study, a variety of binary and ternary thin film coatings using chromium, aluminum, and carbon as base elements were deposited onto Ni-201 alloy and investigated as a function of exposure to hot corrosion environments. The coatings were deposited using a magnetron sputtering system and were approximately 1.5 microns in thickness. Prototypical Na 2SO 4 was applied to the samples before exposing them for up to 250 hours in an air/SO 2 gas mixture at 700 °C and 900 °C, which simulates Type II and Type I Hot Corrosion, respectively. It was determined that all coatings reduced the specific mass gains (corrosion rates) of the samples when compared to that of an uncoated sample, indicating that they provided protection against hot corrosion. For Type II Hot Corrosion, binary Cr - Al and ternary Cr - Al - C thin film coatings held up after 250 hours of exposure; whereas the other two coatings had completely disintegrated. For Type I Hot Corrosion, all thin film coatings utilized in the study had completely dissolved after 50 hours of exposure. It was suggested that the two most effective coating materials, Cr -Al and Cr - Al - C coatings should be used for future testing and that their thicknesses should be increased to help provide better protection against Type I Hot Corrosion.