Development of Ni-Cr-Si Coatings to Resist Type II Hot Corrosion, 20–R8377
Inclusive Dates: 03/20/13 – 09/14/13
Background — Nickel-based gas turbine material is subject to Type II hot corrosion attack that is manifested by a localized pitting corrosion attack. Hot corrosion is an accelerated, often catastrophic, surface attack of superalloy gas turbine components in the temperature range of 650 to 1,000°C. This type of accelerated attack is considered to be caused primarily by deposits of sodium sulfate (Na2SO4), which act as a flux to damage an otherwise protective oxide scale. The Na2SO4 can be ingested in the gas turbine intake air or can be produced by a reaction between sodium chloride (NaCl) in the air and sulfur (S) as an impurity in the fuel. The corrosive effect may be further intensified in marine and other industrial turbines where the alloys may be contaminated with other impurities as well as Na2SO4.
The subject of hot corrosion is divided into two sub-types: Type I — high-temperature hot corrosion above about 900°C where pure Na2SO4 is above its melting temperature (884°C), and Type II — low-temperature hot corrosion (LTHC) between about 600 and 750°C, where a low melting eutectic such as Na2SO4-CoSO4 (melting point 565°C) is formed on the metal surface. The Type I hot corrosion is characterized by accelerated oxidation and sulfide formation in the alloy matrix. Type II hot corrosion is characterized by pitting corrosion. Prolonged exposure of a gas turbine superalloy to low-melting eutectic salts can seriously degrade to the durability of turbine components.
Most conventional protective coatings are based on either alumina (Al2O3) formation or chromia (Cr2O3) formation for high-temperature oxidation and corrosion protection. Theoretical consideration and laboratory tests indicated that both the alumina-forming and chromia-forming coatings are susceptible to Type II hot corrosion. In this project, a new coating that forms a combination of chromia (Cr2O3) and silica (SiO2) was developed. The oxidation resistance of the coating was demonstrated in furnace testing in air. The Type II hot corrosion resistance of the coating was evaluated in pilot testing in a gas flow environment.
Approach — An innovative coating that resists Type II hot corrosion attack of the disk alloy was proposed and evaluated in this project. The coating is innovative in two aspects: the coating contains only the three elements Ni, Cr and Si, and the coating was deposited at SwRI using the plasma-enhanced magnetron sputtering (PEMS) method. The coating compositions were selected from the single phase field of the Cr-Ni-Si ternary phase diagram. The alloy selected for this study was a superalloy named Alloy 10. The alloy has a composition of Ni-14.9Co-10.2Cr-3.69Al-3.9Ti-1.87Nb-2.73Mo-6.2W-0.9Ta-0.03C-0.03B-0.10Zr in wt. % nominal. Three Ni-Cr-Si coating compositions with silicon concentrations 6 atomic % (at. %), 9 at.% and 12 at.% were selected for development and evaluation. The hot corrosion tests were conducted at the Honeywell facility in Morristown, N.J.
The Ni-Cr-Si coatings were produced by a dual-gun PEMS process. The PEMS technique uses an electron source, such as a hot filament, and a discharge power supply to generate a plasma, in addition to the magnetron plasma, in the entire vacuum system. Using the PEMS technique, the measured ion flux to the sample surface can be up to 25 times higher than without the filament generated plasma.
Accomplishments — A new Ni-Cr-Si coating that resists Type II hot corrosion was developed in this project. Four Ni-Cr-Si coating deposition runs were completed using the plasma-enhanced magnetron sputtering process in a vacuum chamber. The coating microstructure was characterized, and a uniform, compact and adherent coating was produced. High-temperature oxidation resistance of the coating was evaluated in air at 700°C. The oxide morphology was examined using optical metallography and scanning electron microscopy with associated energy dispersive X-ray analysis. After exposure to 700°C for 40 hours in air, the coating surface formed a thin (0.5 µm) chromia and silica-rich layer, which acted as a barrier to resist high-temperature oxidation of the remaining coating and the substrate.
The Type II hot corrosion resistance of the Ni-Cr-Si coating processes was evaluated in the Honeywell Morristown hot corrosion pilot testing facility. In the Type II hot corrosion test, a salt paste consisting of 60 wt.% Na2SO4 and 40 wt.% MgSO4 was deposited on the coated surface. The Type II hot corrosion exposure conditions involved a flowing gas consisting of a mixture of SO2/air with a gas flow rate of two liters (L)/min of air and 2.6 mL/min SO2. Temperature was measured by thermocouples in the furnace prior to testing and controlled by standard furnace thermocouples. The samples were exposed to the hot corrosion conditions at 700°C for 24 hours. Under these hot corrosion test conditions, the uncoated Alloy 10 formed numerous corrosion pits with pit depths up to 50 µm. With the Ni-Cr-Si only approximately 1 µm of the coating was consumed to form the protective barrier layer. The Alloy 10 substrate was totally protected against Type II hot corrosion.