Effect of Zirconium Addition on High-Temperature Cyclic Oxidation of Diffusion Chromo-Aluminized Ni-Base Superalloy

Nimonic 75 alloy was coated with two different types of coatings: chromo-aluminized coating and Zr-doped chromo-aluminized coating. Diffusion coating was carried out by pack cementation process at 1000 degrees C for 8h. Cyclic oxidation tests of Nimonic 75 and its coated specimens were conducted at 900, 1000 and 1100 degrees C in air for a total period of 100h. During the first 30-h period, 5-h cycle was applied, and during the 40-100-h period, 10-h cycle was applied. The structures of the coated Nimonic 75 alloy before and after high-temperature oxidation were investigated using light microscopy, scanning electron microscopy/energy-dispersive spectroscopy and X-ray diffraction characterization techniques. The results indicated that Zr-doped chromo-aluminide coating is more effective than chromo-aluminide coating in increasing the oxidation resistance of Nimonic 75 alloy. The parabolic oxidation rate constant k(p) for cyclic oxidation of uncoated , Cr-Al-coated and Zr/Cr-Al-coated alloys are 1.7, 0.77 and 0.61 10(-6)mg(2)cm(-4)s(-1) at 900 degrees C, and 10.08, 5.2 and 3.91 10(-6)mg(2)cm(-4)s(-1) at 1000 degrees C, and 41.27, 26.69 and 18.13 10(-6)mg(2)cm(-4)s(-1) at 1100 degrees C. The improvement in oxidation resistance by zirconium addition to the chromo-aluminized coating is attributed to its effect on achieving better control of interdiffusion fluxes occurring between coating and substrate so as to reduce Kirkendall effect and voids formation, hence enhancing adhesion, in addition to reducing the outward diffusion of aluminum and stabilizing aluminum oxide scale.