THE ROLES OF THE RUTHENIUM CONCENTRATION PROFILE, THE STABILIZING COMPONENT AND THE SUBSTRATE ON THE STABILITY OF OXIDE COATINGS

Electrocatalytic oxide coatings with variable concentration profiles of RuO2 as the active component were obtained through a combination of separately applied layers of RuO2, TiO2, IrO2, RuO2+TiO2 and RuO2+IrO2 on titanium and glassy carbon substrates. The stability of the samples was examined by accelerated tests performed at high anodic current densities. Electrochemical techniques, cyclic voltammetry for assessing the charge associated with the coating, polarization measurements for assessing the electrocatalytic activity, and Auger electron spectroscopy to register surface composition of the coatings, were applied to follow changes due to the stability experiments. The stability and the charge depended strongly on the sequence of layers in the RuO2-TiO2 coating, with the samples having the RuO2+TiO2 layer facing the electrolyte exhibiting the highest values for both properties. In contrast to this, the stability of the RuO2-IrO2 coatings, besides being lower than the stability of RuO2-TiO2 coatings, showed no dependence on the sequence of the applied layers. Much lower stability was exhibited by the coatings applied on glassy carbon rather than on titanium. A mechanism of the stability of the coatings based on the interaction of lower than four valency state titanium with higher than four valency ruthenium, proposed for single-crystal surfaces, is corroborated. Finally, during the thermal treatment a diffusion of titanium originating in the titanium substrate through the coating was established.