ON THE MECHANISM OF DISSOLUTION OF MAGNESIUM IN AQUEOUS MAGNESIUM CHLORIDE SOLUTIONS: PART II

Abstract

Measurements have been made of the effect of pressure on the rate of dissolution of Mg at pH 2.0 in two concentrations of aqueous MgCl 2 . Over the pressure range 0.026–2.5 atmospheres, the rates in 0.3 and 3.0 molal solutions have, respectively, the pressure dependences[Formula: see text]The induction period of the reaction was studied in detail with a sensitive gas-measuring technique. Hydrogen enters the metal in the early stages, to about 1 cc per cc Mg at 27 °C. Its rate of release after the specimen is dried is about five times higher than the rate at which it enters the magnesium.X-Ray diffraction patterns of insoluble reaction products formed on the metal surface after prolonged reaction were examined. Magnesium and magnesium hydroxide are formed in both solutions, while a substantial amount of still unidentified extra solid material (not one of the hydrated hydroxy chlorides) is formed in the 3 molal solution.A mechanism is proposed which describes the pressure measurements quantitatively and is consistent with the other known facts. It involves univalent magnesium as an essential intermediate, the rate being controlled by H 2 bubble growth through the gelatinous Mg(OH) 2 film, which remains on and protects the surface in 0.3 molal solution, and by bubble nucleation in 3 molal solution, in which the film is complexed away.