An XPS study of passive films on sputter-deposited Cr-Nb alloys in 12 M HCl solution

The corrosion behavior of sputter-deposited Cr-Nb alloys in 12 M HCI solution was investigted by electrochemical and XPS methods. All of the Cr-NB alloys are spontaneously passivated in 12 M HCI solution open to air at 30°C. The open circuit potentials for chromium-rich alloys containing 42 at% or less niobium are located in the passive region of chromium and niobium, but those for niobium-rich alloys containing 52 at% or more niobuim are located in the active region of chromium. XPS analysis reveals that the passive film consists of a doubel oxyhydroxide of chromium and niobium. The formation of the passive film consisting of double oxyhydroxide of chromium and niobium is responsible for the high corrosion resistance of the Cr-Nb alloys. The passive films formed on the chromium-rich alloys are slightly rich in chromium, and are not appreciably different from the air-formed films before immersion. Angle-resolved XPS analysis indicates no concentration gradient of Cr^3- and Nb⁵⁺ ions in depth of the passive films on the chromium-rich alloys. On the other hand, the air-formed fil on the niobium-rich alloys is not stable in 12 M HCI and converted to the passive film in which niobium is slightly enriched.