Unless metallic materials are spontaneously passive in a given environment, the metallic materials cannot be used practically. This paper reviews our work based on characterization of passive films by x-ray photoelectron spectroscopy (XPS) for tailoring alloys that passivate spontaneously in very aggressive environments. The formation of passive chromium oxyhydroxide (CrOx[OH]3−2x·nH2O) film is responsible for the corrosion resistance of the alloys whose corrosion resistance is caused by the presence of chromium. Molybdenum added to alloys suppresses anodic dissolution, forming a passive tetravalent molybdenum oxide film in the potential region where chromium and iron dissolve actively. In the passive region of alloys, molybdenum enhances the corrosion resistance by constituting the inner tetravalent oxide layer, which acts as the barrier to the outward diffusion of cations. Binary alloys consisting only of corrosion-resistant elements show high corrosion resistance by spontaneous passivation in concentrated hydrochloric acids. In particular, Cr-Ta alloys are immune to corrosion in 12 M hydrochloric acid, forming the passive double oxyhydroxide film, Cr1−xTaxO1+xOH, which has a particularly higher stability than the films formed on alloy component metals. On the basis of these studies, extremely corrosion-resistant bulk amorphous alloys were prepared for prevention of dew-point corrosion in an exhaust gas of a waste incinerator.