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

The variation of the open circuit potential (E underbar) of the rheniu m electorde in oxygen-saturated chloride solutions over a wide range o f pH is followed till steady state (E(ss)underbar) values are establis hed. Oxygen pressure and solution temperature are 1 atmosphere and 25- degrees-C, respectively. In hydrochloric acid solutions of concentrati on C underbar lower than 7.0 M underbar, E(ss)underbar. values are app roahced from the positive side denoting thinning and dissolution of th e atmospheric oxide. In solutions with C underbar > 7.0 M an aqueous o xide film develops. A similar behaviour to that reported for hydrochlo ric acid solutions has been observed in sodium hydroxide solutions. In weak acid media, the presence of the chloride ion assists the dissolu tion 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 value s owing to the competition between the opposing dissolution and format ion processe of surface oxide films. The mechanisms of dissolution as well as diose of passivation of rhenium in the solutions tested have b een here pointed out.