Studies on polarization characteristics of aluminium and its alloys

Abstract

Polarization of aluminium and its alloys in various electrolyte was studied. The characteristics of polarization in aluminium and its alloys in chloride bearing solution are such that its cathodic polarization is predominant over anodic polarization (this is the case of so called. “cathodic control”), and that the potential change caused by cathodic current at low current density is slight, whereas the sudden change or break of potential will occur when current reaches a certain critical value. This is also true with most of the other metals in chloride solution. But in sulfate or nitrate solutions, a nodic polarization of aluminium is of the same order as cathodic polarization, and hence no potential break occurs.These features of polarization in chloride solution show that the cathodic reaction is the rate determing step, and that the factor which promotes it, such as depolarization of cathode, increases the corrosion and consequently increases the corrosion of anode.When the two dissimilar metals come in contact with each other in electrolyte solution, the corrosion current caused does not depent upon the open circuit potential difference between the two metals, but on the polarization characteristics of the couple. In the case of aluminium-copper couple in solution chloride solution, for example, copper (cathode) polarizes in a marked degree than aluminium (anode), and therefore cathode controls overall corrosion reaction.In general, the corrosion current caused by the metallic couple of aluminium and other metals in chloride solution is nearly proportional to the area of cathode of the couple and is much less dependent on the anode area.Contact corrosion of aluminium with other metals can be protected by cathodic polarization of the couple by external e. m. f., and the potential break in cathodic polarization curve for these couples can be used as criterion of protection.And this is also true in the cathodic protection of various metals in corrosion electrolyte, provided that the cathodic reaction is the rate determing step.The corrosion of aluminium alloy caused by the local current between its structural components can also be explained by the polarization characteristics of these contact corrosion.