Polarization behaviour and lifetime of IrO2–Ta2O5–SnO2/Ti anodes in p-phenolsulfonic acid solutions for tin plating

Abstract

Insoluble electrodes consisting of iridium and tantalum oxides thermally prepared on a titanium substrate have received much attention as oxygen evolution anodes in industrial electroplating processes. In commercial electrogalvanizing lines, the application of the IrO2±Ta2O5/ Ti anode has increased, replacing other insoluble anodes such as the platinum-coated titanium electrode, and a lifetime of more than 10 000 h has been demonstrated under severe operation conditions of 100Admy2 and 333K in practical use [1, 2]. The IrO2-based coating provides excellent catalytic and corrosion-resistant properties, if the electrolyte is an inorganic acid solution such as a sulfuric acid-based bath for electrogalvanizing. However, iridium oxide, along with other platinumgroup metals or their oxides, shows a high consumption rate if the electrolyte contains organic compounds [3, 4]. As a result, little has been done to develop the IrO2± Ta2O5/Ti anode for commercial electroplating lines using organic acid-based baths, for example, theFerrostan bath based on p-phenolsulfonic acid (PSA) for electrotinning. By contrast, we studied the electrochemical behavior of the IrO2±Ta2O5/Ti anode in a PSA solution in order to determine the real applicability of this anode for electrotinning lines [5]. The results indicated that PSA was oxidized at potentials less positive potential than that of oxygen evolution and the oxidation of PSA produced an organic ®lm that originated from the PSA on the anode during oxygen evolution. The organic ®lm on the anode adversely a€ected the electrocatalysis of the IrO2±Ta2O5 catalytic layer and caused a drastic decrease in the lifetime of the anode. However, it was also found that the addition of SnO2 on the outermost surface of the coating layer suppressed the accumulation of the organic ®lm, thus suggesting the prolongation of the anode lifetime in the PSA solution. It is, therefore, possible that the IrO2±Ta2O5 coating layer containing SnO2 has enhanced durability for use in the PSA-based tin plating bath. In this paper, the anodic polarization behaviour and lifetime for oxygen evolution of IrO2±Ta2O5±SnO2/Ti anodes in a PSA solution are reported. The manifested advantage of the SnO2 addition in the IrO2±Ta2O5 system to improve the anode lifetime is shown.