Combinatorial Evaluation of Corrosion Resistance of Ru-Based Alloys in Supercritical Acidic Fluids

Ruthenium (Ru) and its alloys exhibit high corrosion resistance in acidic environments; however, their corrosion behavior in supercritical acidic fluids remains unclear. In this study, the corrosion resistance of Ru–Mo–W and Ru–Mo–Fe–Cr alloy systems in supercritical nitric acid was evaluated using a combinatorial approach. Alloy thin-film libraries with a thickness of approximately 300 nm were fabricated on (001) sapphire substrates by multi-target magnetron sputtering. Composition and thickness were evaluated by X-ray fluorescence (XRF) before and after corrosion testing. The composition-dependent corrosion behavior was compared in a 0.2 M nitric acid solution at 400 °C and 30 MPa. In the Ru–Mo–W system, most Ru-rich compositions exhibited corrosion rates below 0.2 mm/y, and at comparable Ru contents, Mo addition provided higher corrosion resistance than W addition. In the Ru–Mo–Fe–Cr system, significant dissolution and film delamination were observed on the low-Ru side. These results demonstrate that the combination of combinatorial thin-film libraries and XRF-based thickness evaluation is an effective first-stage approach for identifying promising alloy composition regions in supercritical acidic environments.