Laboratory simulation of atmospheric corrosion by SO2—II. Electrochemical mass loss comparisons

Abstract

Mass losses of zinc, steel and rolled Zn-55%Al alloy over the range 0–1 ppm [SO2], obtained by weighing, or calculated using atmospheric corrosion monitors, ACMs, from galvanic current, Ig, charge-transfer resistance, Rm, polarization resistance, Rp, or corrosion current, Ik have been compared. The ability of Zn and Zn-55%Al alloy coatings to protect steel under these conditions is discussed. Although similar trends in mass loss vs [SO2] are generally apparent for each of the methods of obtaining mass loss, there are differences in the magnitude of mass loss, which in some cases are substantial. The reasons for these differences are discussed in detail. One reason, that of partial short circuits between adjacent ACM plates in the presence of [SO2], thought due to electron-conducting corrosion products, leads to high values of calculated mass loss. Partial short circuits appear to occur at random and may or may not occur in apparently identical tests for ACM3 (Zn) and ACM8 (Fe) but not ACM4 (rolled Zn-55%Al). The effect of these partial short circuits can be modelled by inclusion of a corrosion product resistance, Rep, in parallel with Rt and Warburg impedance, Zω. The scope of the work with these ACMs is wider than previous work in the literature. This includes a modified method of manufacture; a more sophisticated method of measuring ACM corrosion currents; and a wider correlation of electrochemically calculated mass loss with weighed mass loss.