Chlorine Adsorption at a Titanium Surface Segregation from the Bulk Crystal Interaction with Chlorine Gas

Abstract

Experiments have been conducted with a single crystal of Ti [exposing the (0001) face] in an ultrahigh vacuum system (∼10⁻¹⁰ Torr) using Auger spectroscopy, ellipsometry, and surface potential difference measurements to monitor adsorbed chlorine on the surface. It was discovered that although the impurity level of chlorine and sulfur in the bulk crystal are < 1 ppm, these elements segregate from the bulk to the crystal surface at elevated temperatures with a segregation coefficient ∼10⁵ to 10⁶. Above 600°C sulfur displaces the chlorine from the surface. Segregation coefficients and diffusion coefficients are measured for chlorine as a function of temperature. Adsorption of chlorine gas on a clean Ti surface occurs as a nonactivated process with a sticking coefficient ∼0.05. The adsorption isotherms are consistent with dissociation of molecular chlorine to atomic chlorine. The change in work function of the Ti surface with a monolayer of Cl with respect to a clean surface is +1.2V. The index of refraction of the film is about 2.4 and the thickness of a monolayer is approximately 4.5Aå. Ellipsometer data show that more than a monolayer of chlorine will form at room temperature and a chlorine pressure of ∼10⁻⁷ Torr. It is concluded that at room temperature the diffusion coefficient and segregation coefficient are much too low to allow chlorine to segregate to the surface at a crack tip during stress corrosion cracking unless the stress and increased local temperature are sufficient to lower the diffusion activation energy.