Electrochemical impedance spectroscopic study of dimensionally stable anode corrosion

An electrode of nominal composition Ti/Ir0.3Ti0.7O2 was prepared by thermal decomposition of the chloride precursor mixture at 450 degrees C. A system atic study of the corrosion behaviour of this anode was performed under acc elerated conditions (j = 400 mA cm(-2)) in acidic media. Simultaneously, CV and EIS measurements were done at regular time intervals until the end of the electrode service life (similar to 528 h). It was possible to identify the various stages of the electrode deactivation, and quantify the associat ed parameters. Three main steps were identified in the electrode deactivati on mechanism. During the first 170 h, the loss of the more external (porous ) part of the oxide active layer occurred. This is supported by the decreas e in the parameter-values related to the electrochemically active surface a rea of the anode (q(a) and C-dl) and an increase in the R-ct-values. The se cond stage is evidenced by a potential step afterwards remaining practicall y constant up to similar to 380 h, suggesting that the more compact and sti ll very active region of the electrode is now exposed. Finally, for t(cor) > 400 h the anode potential increases again. This behaviour, together with the R-ct against time, suggests that the most internal part of the original coating, less rich in IrO2, is now exposed to the solution and is corroded . For t greater than or equal to similar to 510 h the anode potential incre ases rapidly, the electrode being totally deactivated at t similar or equal to 528 h, when the anodically grown TiO2 film is exposed to the solution. During the complete deactivation process, the growth of a TiO2 film, due to Ti-support oxidation, in addition to that already present (IrO2 doped TiO2 film formed during the calcination step), is evidenced by the increase in the R-f-values, as well as decreasing C-f-values. However, for t(cor) < 450 h the rate of this process is not significant. Only for t(cor) > 450 h Ti- support oxidation becomes the main feature of the deactivation process.