Active Dissolution And Electrochemical Behavior Of Aluminum Alloy Anodes With High Content Of Magnesium

Cathodic protection by sacrificial anodes is one of important methods for protecting metal materials. The common anode materials inciude zinc anodes, magnesium anodes and aluminum anodes. Aluminum alloys have become popular as galvanic anodes for cathodic protection of ships and offshore structures because of their superiority in respect of high-energy capability (3.6 times of zinc, 1.35 times of magnesium), long life, light weight, and low cost compared with other competitors, principally magnesium and zinc alloys. But commercially pure aluminum is not suitable for galvanic anodes because it exhibits a relatively noble potential in corrosive environment due to an impervious Al₂O₃ film on the surface. In order to promote surface activation, aluminium is alloyed with small quantities of one or more elements. However, the aluminium alloy anodes have some disadvantage such as including poisonous elements, precarious performes and so on. So searching new green elements to improve performes of aluminum anodes has been adverted all the time.To base on the alloy phase electrochemistry theorem, pure aluminium (E_SCE=-0.78V) was alloyed with active element—magnesium ( E_SCE=-1 -5V) . This paper investigated the influence of magnesium contents on microstructure and electrochemistry properties of aluminum anodes. The main conclusions were as following:When the pure aluminium was alloyed with more than 10% quantities of magnesium, the more anode phase Mg₂Al₃ were be formed in the microstructure of aluminium. Along with the increasing of magnesium quantities, the open potential went on reduceing, and the polarization rate was also decreased. When the content of magnesium was increased to 30%, the open potential of Al—Mg anodes was reduced to -1.18V in 3.5% NaCl solution and coulid be holded this value for a long time. However, the dissolution of aluminium alloy anodes with high content of magnesium was unsymmetrical.Investigated the active role and cooperation of alloying elements in Al—Mg anodes, such as In, Sn, Bi, Zn, Sb, Mn. The results showed that In could cooperate with Sn well, and the suitable quantitie ranges of In and Sn were separately 0.02%～0.06% and 0.04%～ 0.1%. The active role which Bi and Zn acted was not notable. Sb and Mn had hardly active role to act for Al—Mg anodes.Making use of orthogonal test to produced 9 kinds of aluminium alloy anodes. Thorough compared with open potential and polarization rate, Al— (25%—30%) Mg— (0.02%— 0.06%) In- (0.04%— 0.1%) Sn was fixed on the optimum chemical composition and Al —30Mg—0.04In—0.1Sn was produced. Its closed-circuit potential was -0.9992V, and the current efficiency was 74.36%. The white corrosion production adhered to the surface of anode.Al—30Mg—0.04In-0.1Sn had been heat-treatment separately for 10h,15h, and 20h, resulted in forming more symmetrical microstructure and more electronegative potentials—the biggest reduced value was about -0.06V. The current efficiency could reach 85% and the corrosion production was easily brushed off oneself.