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To understand the formation of structural isomers of alumina (Al 2 O 3 ) on the surfaces of Fe-Cr-Al alloys, cathodoluminescence (CL), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) were used. The alloys were annealed under oxygen at a low partial pressure to focus on the initial formation of Al 2 O 3 . XRD revealed that corundum (α-Al 2 O 3 ) was predominantly formed on the alloy surfaces after annealing at approximately 1 100°C, whereas γ-Al 2 O 3 and θ-Al 2 O 3 were formed at 1 000°C. CL peaks around 700 nm, which originated from Cr 3 + ions in Al 2 O 3 , was drastically increases by rising the annealing temperatures from 1 000 to 1 100°C because of the increase of the chromium content in the Al 2 O 3 scales that was revealed by XPS. EPMA revealed that the surficial α-Al 2 O 3 had striped patterns, which were similar to those observed using CL imaging, indicating that CL can be conveniently used for imaging α-Al 2 O 3 and analyzing the inhomogeneous distribution of α-Al 2 O 3 on the alloys.

A Ni–Pt alloy porous layer was formed by electrodepositing Pt using Ni as the substrate sample, followed by Al-depositing and Al-dissolving. The Pt was electrolyzed using an aqueous solution as the medium, and the Al-depositing and Al-dissolving were treated using a molten salt as the medium. The molten salt used was NaCl–KCl with 3.5 mol% AlF3 added. It was found that Pt electrodeposition formed on the surface had a finer structure. Furthermore, it was clarified that the lower the electrodeposition potential, the thicker the Ni-Pt alloy porous layer. The cathode polarization curve was measured in KOH solution, and the hydrogen gas was determined when a constant voltage electrolysis was performed with a hydrogen detection gas sensor using a tubular yttria-stabilized zirconia (8 mol% Y2O3–ZrO2).