A new insight on the mechanisms of corrosion deactivation of a typical Ti/IrO2 + RuO2 +TiO2 coating in the presence of Ta2O5 in chlor-alkali medium

Abstract

Mixed metal oxide (MMO) anodes are commonly used in chlor-alkaline industry. They have a longer lifetime than conventional graphite anodes due to the presence of valuable and expensive metal oxides such as iridium and ruthenium. However, they can suffer from pitting corrosion. This makes them to lose their stability, which necessitates the application of stabilizers to delay this process as one of the most accepted preventive measures. Hence, this study to aims to increase the life of MMO anodes, by altering corrosion mechanisms from localized to uniform corrosion. To achieve this objective, electrodes of different nominal compositions were prepared by adding 2 and 5 wt% of tantalum oxide into the base coating composition (Ti/(38IrO2-36RuO2-(26-X)TiO2-XTa2O5)). Accelerated lifetime and electrochemical impedance tests were employed in order to evaluate the corrosion pathways at different time intervals. Furthermore, morphological properties and composition of the anode were addressed via scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analysis. In addition, cyclic voltammetry test was performed to investigate electrochemical properties. It was understood that the addition of tantalum oxide leads to increment in active surface area and current density of the anode reaction. More importantly, tantalum oxide was found to be an effective inhibitor of the pitting corrosion while altering the dominant corrosion pathway from pitting to uniform dissolution in ternary coatings. However, it resulted in the service life of the anode to be reduced in the accelerated life test.