The structure and high temperature corrosion behavior of pack aluminized coatings on superalloy IN-738LC

Three aluminized coatings (simple aluminized, Rh-electroplated aluminized, and Pd-electroplated aluminized coatings) are prepared to raise the resistance of high temperature oxidation and hot corrosion of superalloy IN-738LC in this study. The rhodium or palladium is electroplated on the superalloy IN-738LC specimen prior to the conventional pack aluminization process. The kinetics of the pack aluminization process was evaluated while the structure and composition of the aluminized coatings were thoroughly analyzed. Experimental results show that there is more aluminum deposited on the superalloy specimen of the Rh-plated or Pd-plated aluminization process than that of the simple aluminization process. The thickness of each aluminized coating containing NiAl and Ni2Al3 is uniform and good adherent to the substrate, and no voids or cracks are observed in the coatings. Vickers hardness test shows that the microhardness of the aluminized coatings with Rh- or Pd-plating is almost equivalent to that of the simple aluminized coating. The uncoated superalloy and those with various aluminized coatings are subjected to isothermal oxidation at 1000, 1100, and 1200 °C in static air, and hot corrosion at 900 °C in molten salt Na2SO4. The resistance of high temperature oxidation and hot corrosion of all tested specimens, from best to worst, is as follows: Rh-plated aluminized, simple aluminized, Pd-plated aluminized, and uncoated superalloy IN-738LC samples according to the analyses of the oxidation and corrosion kinetics as well as the degradation of coatings and scales.