Pulsed electrophoretic deposition of surfactant-free alumina suspensions for high-quality coatings on Inconel 625

Nickel-based superalloys such as Inconel 625 are widely employed in aerospace and energy systems where components are exposed to aggressive thermal and corrosive environments, necessitating durable corrosion-resistant coatings. Conventional electrophoretic deposition (EPD) of ceramic films requires organic stabilizers to prevent particle agglomeration with direct current and voltage. In this work, a surfactant-free, aqueous electrolite was developed for depositing alumina coatings on Inconel 625. A highly stable colloidal suspension of γ-Al₂O₃ nanorods (73 ± 3 nm in length and 27 ± 1 nm in diameter) was synthesized via controlled hydrolysis and condensation of aluminum isopropoxide without any organic dispersants. Using this suspension under bipolar pulsed DC conditions markedly suppressed hydrogen evolution, improved coating uniformity and increased deposition efficiency compared with constant-DC EPD. Subsequent high-temperature oxidation transformed the as-deposited amorphous alumina into dense, crystalline spinel-structured oxide layers. Electrochemical testing in 3.5 wt% NaCl at 30 °C revealed a significant increase in polarization resistance and overall corrosion protection, underscoring the suitability of these coatings for high-temperature service. This combined surfactant-free colloid synthesis and pulsed-field deposition offers a green, scalable pathway to high-quality alumina-based protective layers on nickel superalloys.