Aluminum Anodic Dissolution In Chloride Solution Behavior And Mechanism

Al alloys were widely used as sacrificial anodes in the solution containing chloride ion. Although there were many investigations on the Al anodes, the electrochemical behavior of segregations in the dissolution processes of Al anodes, the effect of segregations on the dissolution mechanism of Al anodes and the relationship between microstructure and macroscopic electrochemical properties of Al anodes had important effect on the properties of Al anodes, which needed systematic investigations. Four kinds of Al anodes were prepared, such as Al-Zn-In-Sn, Al-Zn-In-RE, Al-Zn-In-Te and Al-Zn-In-Sn-Te. The effect of different solution treatment time and Sn4+ in the solution on the dissolution processes of Al anodes was investigated by potentiodynamic polarization, electrochemical impedance spectroscopy, inductively coupled plasma mass spectrometry and scanning electron microscopy with energy-dispersive X-ray analysis techniques. The dissolution mechanism of Al anodes in the NaCl solution with different concentration of CeCl3 was studied. Meanwhile, the electrochemical properties and dissolution mechanism of new Al anodes with Te as the alloying element were investigated. Furthermore, the effect of different heat reservation treatment time on the electrochemical properties was investigated.The results showed that Sn element was enriched in the hemispherical segregations at the grain boundary of Al-Zn-In-Sn anode after solution treatment. The active dissolution initiated at the hemispherical segregations and Sn as the anodic phase dissolved first. After the solution treatment, Al-Zn-In-Sn anodes exhibited higher current efficiency, lower self-corrosion of hydrogen evolution, more negative open-circuit potential, and more active surface, which were desirable for sacrificial anodes. After the overlong time of solution treatment, Al-Zn-In-Sn anodes exhibited less and broken segregations, and the enrichment of harmful Mg in the hemispherical segregations containing Sn, which caused serious self-corrosion and loss of electrochemical properties. Sn4+ enhanced the activation on the surface and the active dissolution, and improved the current efficiency of Al anodes. In the NaCl solution containing SnCl4, the Al-Zn-In-Sn anode exhibited a more even corrosion than the Al-Zn-In anode.RE as the alloy element and Ce3+ as the additive in a chloride solution, enhanced current efficiency for Al alloys. The anodic dissolution of Al-Zn-In anodes, in the NaCl solution with the addition of CeCl3, began around the hemispherical precipitations where Ce is enriched. Major hemispherical precipitations containing Ce were distributed in the Al matrix. The precipitations of Ce enhanced activation on the surface of Al anodes. Ce3+ as the additive in the solution inhibited the active dissolution of Ce as the alloying element of the Al-Zn-In-RE anode.With the addition of Te as the alloying element, the Al-Zn-In-Te anode exhibited lower harmful impurity content of Fe and Si, more negative corrosion potential, the lower self-corrosion and higer current efficiency. Al-Zn-In-Sn-Te series anodes showed more negative corrosion potential and higher current efficiency than Al-Zn-In-Te series anodes. However, excessive addition of Te led to the loss of electrochemical properties of Al anodes. The corrosion of the Al-Zn-In-Te anode initiated at the Zn and Te enriched hemispherical segregation in grain boundaries and interdendritic zones.Properly increasing the time of heat reservation treatment during the cast process could enrich the active element in the hemispherical segregations of Al anodes, enhance the activation on the surface of Al anodes, reduce the self-corrosion of hydrogen evolution, and improve the current efficiency. Al-Zn-In-Sn anodes after the overlong time (45min) of heat reservation treatment exhibited much less segregations and the inhibition of enrichment of the active element on the surface of Al anodes which led to the inactivation on the surface of Al anodes. The quantity and distribution of segregations influenced the corrosion morphology and the electrochemical performance.