Revealing microstructure and the associated corrosion mechanism of Al/ amorphous Al2O3/Al tri-layer coating deposited on depleted uranium by magnetron sputtering

As important energy materials, uranium and its alloys have been widely used in the fields of material sciences and nuclear industrial applications. Applying metal and/or ceramic surface protective coatings can effectively retard easy corrosion of uranium with extremely high chemical activity. In principle, ideal surface protective coatings should possess (1) lowered density of elongated diffusion pathways (i.e. grain boundaries and growth defects) and (2) improved interfacial bonding to substrate by preventing corrosion medium from reacting with uranium. In this work, we have demonstrated that by magnetron -sputtering amorphous Al2O3 (a-Al2O3) interlayers within Al coatings on depleted uranium (DU), the corrosion resistance is significantly improved as indicated by corrosion potential datum DU substrate (-645 mV), DU with mono -layer Al coating (-610 mV) and DU with Al/a-Al2O3/Al tri-layer coating (-550 mV). Diffusion pathways are elongated with misalignment of columnar grain boundaries in Al coatings around a-Al2O3 interlayers. Meanwhile, density of diffusion pathways is lowered since a-Al2O3 does not contain conventional crystal defects, and can suppress accumulation and facilitate outgassing of Ar bubbles as major growth defects. The effect of coating/substrate interfacial bonding on corrosion behavior is investigated by introduction of intermixing layers between Al- and UO2-rich interfacial regions. A strategy to improve anti -corrosion stability of Al/a-Al2O3/Al tri-layer coating by optimizing degree of intermixing is proposed. The present findings may shed lights on compositional and microstructural design of anti -corrosion coatings on uranium and its alloys.