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. In order to improve the life of hotsection superalloy components, they are often coated with an aluminide. In this study, the hot-corrosion resistance of different types of â-NiAl aluminides and CoCrAlY-based coatings on nickel-based superalloys 247 and 792 was assessed. The coatings were tested under cyclic high-temperature (HTHC- 900oC) and low-temperature (LTHC-705oC) hot corrosion conditions using a laboratory-based Dean rig. The hot corrosion conditions were simulated by depositing Na2SO4 salt on the coated samples and then exposing the samples to a O2 + 0.1%SO2 flowing gas environment. HTHC testing was performed for up to 500 hours, while LTHC testing was performed for up to 200 hours. The effect of pre-oxidation on LTHC resistance was also studied. Coating performances under both HTHC and LTHC conditions were ranked by assessing the extents of attack around the circumference of each coated sample, together with maximum depth of attack. The corrosion products and phases present in the as-received and corroded coatings were characterized. Finally, was found that pre-oxidizing at 1050oC improved the resistance of the coatings to LTHC.
The surface degradation of marine gas-turbine components can occur by high temperature oxidation (>1000ºC) and/or hot corrosion (~ 850-1000ºC for type I and 600-800ºC for type II) 1. Salt-induced hot corrosion is an accelerated mode of surface degradation in marine environments. The severe environments encountered from hot corrosion in turbines generally require that the nickel-base superalloy components be protected by diffusion or overlay coatings2. The most widely used diffusion coatings are based on the aluminide â-NiAl, while overlay coatings are typically based on an MCrAlY composition in which M represents Ni, Co or Ni + Co. The performance of coatings depends upon their chemical composition and microstructure. Hot corrosion resistance of aluminide coatings is often improved by the addition of modifying elements like Pt, Cr and Si 3-10.
Addition of Pt to aluminide coatings improves HTHC and oxidation resistance, but it is not as beneficial at low temperatures. Indeed, addition of Pt tends to reduce surface spallation by promoting the slow growth of extremely adherent á-Al2O3 scale11. Haynes et al.12 and Zhang et al.13 found that, along with improved scale adhesion, Pt also helped in decreasing detrimental effects of high S levels in the substrate and reducing the amount of voids at the scale/metal interface. Addition of Cr has been shown to improve hot-corrosion resistance at both low and high temperatures3, 4. In addition, Cr can be beneficial to oxidation resistance by decreasing the amount of Al in the alloy required to form a protective Al2O3 scale layer14, 15. As discussed by Rapp16 the basic dissolution of Cr2O3 and Al2O3 results in the formation of CrO4 2- and AlO2 - ions respectively, and produces a positive solubility gradient thereby inhibiting further basic fluxing.
