A STUDY ON CONTACT CORROSION OF ALUMINIUM AND ITS ALLOYS WITH DISSIMILAR METALS

Abstract

Some of the fundamental problemes on the contact corrosion of aluminium and its alloys (99.99%Al, 99.8%Al, 99.5%Al, 99.0%Al, 3S, 52S) with dissimilar metals (Pb, Cu, Ni, Brass, Fe, Cd, Mg, Zn, 18-8 stainless steel, 13 Cr stainlees steel) in the aqueous solution containing 5.85% sodium chloride and 0.3% hydrogen peroxide were studied. The results are summarized as follows.(1) Although the higher purity aluminium has the superior corrosion resistance to the lower purity one in the corrosion test by itself (as shown in Fig. 2, Photo. 1), the lower purity aluminium is superior to the higher purity one in the contact corrosion test with copper. (See, Fig. 3, Photo. 3)(2) Zinc protects aluminium cathodicallly from corrosion, but magnesium does not protect aluminium cathodically although it makes itself the anode.Ragarding to the relation beween the penetrations of aluminium and the relative potentials and the solutional currents, nickel, 18-8 stainless steel and 13 Cr stainless steel have the special features. Iron has not only the poor corrosion resistance to this solution by itself, bus gives considerably severe contact-corrosion attack on aluminium.13 Cr stainless steel gives severer attack to alminium than iron, while 18-8 stainless steel attacks weaker than iron. (See, Fig. 4, Photo. 4)(3) When aluminium is corroded by this solution contacting with copper, the surface-area of copper defines the quantity of dissolved aluminium. (See Fig. 7, 8, 9)If the distance between aluminium and copper varies from zero as frequently encountered in the actual application, the corrosion pits are concentrated around the copper. (See, Photo. 7, 8.) (4) The values of solutional current and relative potential are not always constant throughout the whole testing period, but they vary with the submerging time. (See, Fig. 7.)(5) One of main factors dominating the contact corrosion is not the surface area of aluminium but that of dissimilar metals.