Preparation and hot corrosion behaviour of two Co modified NiAl coatings on a Ni-based superalloy

• Two Co modified NiAl coatings have been prepared by a novel two-step process.
• The addition of Co improves the corrosion resistance in sulphate salts.
• The addition of Co reduces the corrosion resistance in chloride salts.
• The enhancement in sulphate salt is due to the high melting point of Co-CoxSy.
• In chloride salt the formation of volatile CoCl2 speeds up the corrosion rate.

Two Co modified aluminide coatings with different Co contents were prepared by a combined method of pack cementation and chemical vapour deposition. The type-I hot corrosion tests of the two Co modified coatings together with the simple aluminide coating were carried out at 900 °C. Results indicate that the addition of cobalt could improve the corrosion resistance of the coatings in the sulphate salts, but exacerbate the degradation of the coatings when chloride salt gets involved. The formation mechanism of the Co modified aluminide coatings and the corrosion mechanism during the corrosion test have been discussed as well.