Spontaneously passivated films on sputter-deposited Cr-Ti alloys in 6M HCl solution

Among sputter-deposited Cr-Ti alloys, 30-65 at% Ti alloys are amorphized. The Cr-Ti alloys with 22 at% or less Ti are bcc single phase alloys. Regardless of the crystallinity, the open circuit potentials of sputter-deposited Cr-Ti alloys are located in the passive regions of both titanium and chromium in 6 M HCl solution open to air at 30°C, and all Cr-Ti alloys are spontaneously passivated. X-ray photo-electron spectroscopy (XPS) analysis reveals that the spontaneously formed passive film as well as air-formed film is slightly rich in titanium ions mainly because of preferential oxidation of titanium. According to angle-resolved XPS measurement, no concentration gradient of Cr³⁺ and Ti⁴⁺ ions has been detected in depth of the passive film. Analysis of binding energies of core electrons of Cr³⁺ and Ti⁴⁺ cations shows that Cr³⁺ and Ti⁴⁺ cations are located very closely in the film so as to show the electronic interaction, and the film is not composed of a heterogeneous mixture of chromium and titanium oxyhydroxides but of homogeneous double oxyhydroxide consisting of Cr³⁺ and Ti⁴⁺ cations. The formation of the homogeneous double oxyhydroxide film containing both Cr³⁺ and Ti⁴⁺ cations is responsible for extremely higher corrosion resistance of the homogeneous single phase Cr-Ti alloys in 6 M HCl open to air at 30°C in comparison with the corrosion resistance of chromium and titanium metals.