Studies on Al-Zn-In Alloy Anode for Cathodic Protection

Al-Zn-In alloy is taken up as a galvanic anode and studied fundamentally elucidating the anodic behaviors in artificial sea-water in the laboratory. Experiments are continued for several hundred hours at ambient temperature while maintaining the anodic current density around 1.0mA/cm2. Throughout the test anodic potentials are observed regularly and weight decreases of anodes are finally determined. Al-Zn alloy anodes are tested similarly for comparison. The results revealed are:1) Superior performance characteristics can be obtained with Al-Zn-In anode containing 0.01-0.02% of In and 0.5-5% of Zn.2) The anodic potential of the Al-Zn-In anodes is about -1.1V referred to SCE and shows little tendency of rise during the test. On the other hand, the Al-Zn anode gives the potential of -0.95V.3) The anodic current efficiency of the Al-Zn-In anode exceeds 80% at the current density above 1mA/cm2, whereas that of the Al-Zn anode is no more than 60%.4) Small amount of Fe and Si contained in 99.85% grade aluminum is not only effective in improving the anode performance but it somewhat increases anodic current efficiency, while 0.01% of Cu gives rise to marked deterioration of the performance.5) The Al-Zn anode shows appreciable pitting corrosion and slime formation which can scarcely be observed in case of the Al-Zn-In anode.6) Some explanations of the corrosion process and the activation mechanism of the anodes are attempted from microscopical and electronmicroscopical observations.Further, an actual marine test at a pier by which the superior performance characteristics of the Al-Zn-In alloy anode compared with that of the Al-Zn anode (Alcoa's Al anode composition) has been established is described. The former shows about four times as much of current output as the latter.