Galvanic Corrosion Cells

Abstract

Two dissimilar metals, A and C, subject to corrosion in the same electrolyte, normally are at different corrosion potentials, (ca)corr and (cc)corro The electrode system metal C/solution/metal A then is a galvanic cell with the open-circuit voltage (1Ll) We choose indices such that (cJcorr is more positive than (ca)com so that A is the anode and C the cathode in the cell. When short circuited by an external metallic connection between A and C, a cell current, j, will flow, moving positive charge carriers in the solution, and electrons in the metalllic connection, in the direction from the anode to the cathode. The current passing the anode/solution interface has the positive strength +Ijl; a current of the same strength but opposite sign,-Ijl, passes the solution/cathode interface. At both electrodes the total current, j, is, in principle, the sum of anodic and cathodic partial currents. Often, circumstances are simpler, when at the anode the cathodic partial current of the reduction of an oxidizing species, and/or when at the cathode the partial current of anodic metal dissolution are non-existent or negligible, but this is not the general case. The early assumption that, instead, the action of galvanic cells is the general principle of corrosion in fact has been a source of much confusion. In the special case of the titanium anode/platinum cathode cell described in Chap. 10, the anode is passivated by the contact with the cathode, but this and similar cases again are exceptions. The rule is that anodic polarization of the anode increases the rate of anode metal dissolution, thus accelerating corrosion; and this effect defines contact corrosion. Some aspects of the matter have already been described in Chap. 4, more will follow presently, but some complications will be further disregarded, including the possibility that the system under consideration is a galvanic chain of the type metal A/solution I1metal C/solution IIImetal A, with an electrolytic instead of a metallic short circuit. Normally, A directly contacts C, as for steel tubing welded to copper tubing, and in many other cases. This then creates conditions different from what so far has been assumed in this text, which was uniform distribution of all partial current densities, including the current density of metal dissolution, everywhere on the surface of the respective electrode. When, instead, direct H. Kaesche, Corrosion of Metals