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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. [ABSTRACT FROM AUTHOR]

Abstract: Heat treating metals at high temperatures trigger diffusion processes which may lead to the formation of oxide layers. In this work the diffusion of Cr, Fe and Ti into an alumina coating applied to Inconel-718 is being investigated. Mass gain measurements, UV–vis spectroscopy and transmission electron microscopy were applied in order to study the evolution of the diffusion process. It was found that mainly Cr as well as minor amounts of Fe and Ti are being incorporated into the alumina coating upon prolonged heat treatment at 700°C. It could be shown that alumina coatings being void of Cr have the same oxidation related mass gain as uncoated samples. However, incorporation of Cr into the alumina coating decreased their mass gain below that of uncoated substrates forming a Cr oxide scale only. [Copyright &y& Elsevier]

Abstract: Alumina coatings with differing phase compositions were deposited on Inconel-718 (IN-718) Ni-base superalloy substrates using sol–gel processing. Mass gain measurements served for studying the oxidation behavior of coated metal surfaces if exposed to 800 °C and 900 °C for up to 4000 h in static air. It was found, that alumina coatings significantly reduce the oxidation related mass gain of IN-718 even after heating to 900 °C for 4000 h. Transmission electron microscopy (TEM) studies revealed diffusion of elements from the metal substrate (IN-718) into the alumina coatings and — to a much lower degree — from the coating into the substrate. These diffusion processes are greatly influenced by the phase composition of the coatings, especially by the presence of alpha-alumina. [Copyright &y& Elsevier]