Microstructure of Bare and Sol–Gel Alumina-Coated Nickel-Base Alloy Inconel 625 After Long-Term Oxidation at 900 °C

Abstract

Though Ni-based superalloys show a high oxidation and corrosion resistance, coatings can still improve these properties, especially if used at temperatures up to 1000 °C. Here, a coating was prepared by applying a boehmite-sol via dip-coating and a subsequent heat treatment at 600 °C for 30 min. To evaluate the coating, the oxidation behavior of bare and alumina-coated Ni-base alloy Inconel 625 in air at 900 °C was studied for up to 2000 h. Electron microscopy studies of sample surfaces and cross sections showed that (1) in the 3.5–6.3 µm-thick scale formed on the bare alloy, Fe and Ni are located as fine precipitates at the grain boundaries of the chromia-rich scale, (2) Ni and Ti are concentrated to a minor degree at the grain boundaries of the scale, too; and for the coated sample: (3) the only 1.8-µm-thick sol–gel alumina coating slows down the formation of chromia on the alloy surface and reduces the outward diffusion of the alloy constituents. The protective effect of the coating was evidenced by (1) diminished chromium diffusion at grain boundaries resulting in less pronounced string-like protrusions at the outer surface of the coated IN 625, (2) formation of a Cr-enriched zone above the alloy surface which was thinner than the scale on the uncoated sample, (3) lower extension in depth of Cr depletion in the superficial zone of the alloy surface of the coated sample in comparison with that region of the uncoated one, and (4) a narrower zone of formation of Kirkendall pores.