NH3 probing of the surface acidity of passive films on chromium

Abstract The acid–base properties of passive films formed on chromium have been investigated with a probe molecule. The adsorption of NH3, a Lewis base, has been studied by X-ray photoelectron spectroscopy (XPS) on passive films formed on Cr(1 1 0) in 0.5 M H2SO4. A single N 1s signal resulted from the exposure of the passive film to gaseous ammonia (25°C, pNH 3 =20 mbar ). The comparison with NH3 probing of dry oxide films reveals that the oxide surface reactivity is markedly reduced by the layer of OH. The Lewis acid sites of the oxide surface are not directly available on the passive film. The data indicate that NH3 reacts with Bronsted acid sites consisting of hydrogen termination of the hydroxyl or hydroxide groups on the passive film surface. The implications of these findings for the interactions of chloride ions with passive films (passivity breakdown and initiation of pitting) are discussed.