High temperature characteristics of iridium-modified aluminide coatings for nickel-base superalloys

Oxidation and hot corrosion properties of an iridium-modified aluminized (Ir-Al) coating was investigated. The coatings, obtained by an electrodeposition of pure Ir followed by a conventional low-activity pack-cementation aluminizing process on the nickel-base single crystal superalloy TMS-75, consist of a triplex layer structure, beta-(Ir,Ni)Al layer/beta-NiAl/Al-diffusion zone in the order from the surface. The cyclic oxidation tests at 1373 K in air, and the cyclic hot corrosion tests at 1173K in a mixture of molten salts (Na2SO4 + NaCl 25wt.%) were conducted for the Ir-Al coated TMS-75 and the uncoated, simply aluminized TMS-75 for comparison. These results confirmed that the Ir-Al coated TMS-75 has both good oxidation and hot corrosion resistances. The presence of Ir may promote the formation of dense and adherent oxide scale and thus retard the growth of thermally grown oxides and progress of the pitting corrosion.