OPEN-CIRCUIT POTENTIALS OF THE RHENIUM ELECTRODE IN OXYGEN SATURATED CHLORIDE SOLUTIONS

The variation of the open circuit potential (E underbar) of the rhenium electorde in oxygen-saturated chloride solutions over a wide range of pH is followed till steady state (E(ss)underbar) values are established. Oxygen pressure and solution temperature are 1 atmosphere and 25-degrees-C, respectively. In hydrochloric acid solutions of concentration C underbar lower than 7.0 M underbar, E(ss)underbar. values are approahced from the positive side denoting thinning and dissolution of the atmospheric oxide. In solutions with C underbar > 7.0 M an aqueous oxide film develops. A similar behaviour to that reported for hydrochloric acid solutions has been observed in sodium hydroxide solutions. In weak acid media, the presence of the chloride ion assists the dissolution of the premmersion ion oxide, while in the weak alkaline ones, it assists the growth of the aqueous film. Frequently the E-t underbar (time) and E(ss)underbar - log C underbar curves possess critical values owing to the competition between the opposing dissolution and formation processe of surface oxide films. The mechanisms of dissolution as well as diose of passivation of rhenium in the solutions tested have been here pointed out.