Activation of water reduction in the presence of REM salts in aqueous solution

When rare earth metal (REM) cations are added into an aqueous solution, the mechanism of the cathodic process on steel is deeply modified. In air-saturated solutions, the potentiodynamic cathodic curves present four distinct zones: a short current plateau due to oxygen reduction, then a straight current increase with a slope value close to 0.12Vdec-1. Then the cathodic reaction becomes once again diffusion limited before water reduction occurs. In the present work, the mechanism of the cathodic process observed in the second and the third regions is discussed from potentiodynamic experiments performed in 0.5 M sodium chloride solutions, under different hydrodynamic conditions using a rotating steel disk electrode, and as a function of the concentration of REM(III) in the solution, for three different REM cations (yttrium, cerium, and lanthanum). It clearly appears that in the presence of REM(III) in the solution, the reduction of bonded water in the REM(III)–aqua complex is observed at a potential close to the equilibrium potential for water reduction. A simple thermodynamic model is used to estimate the onset potential for water–ligand reduction as a function of the solubility product of the corresponding REM hydroxide.