Improving the Corrosion Resistance of Aluminum Alloys by Cathodic Polarization in Aqueous Media

Corrosion of aluminum alloys is often closely related to the type, amount, and properties of intermetallic phases present in the matrix. The phases that contain iron as a component are especially detrimental to corrosion resistance. These phases can be dissolved preferentially, thereby achieving a relatively particle-free surface, by various chemical and electrochemical means. An efficient method appears to be cathodic polarization within a certain potential range in unbuffered salt solutions of neutral pH. It is shown that the method leads to an appreciable improvement of the corrosion resistance of alloys in the 1000,3000, and 6000 series. The particles are removed physically as a result of crevice corrosion of the matrix adjacent to their surface. The rate of removal depends on the extent to which the cathodic reaction can be depolarized on the particles relative to the substrate. The mechanism is investigated by electrochemical and microanalytical means.