Accelerated Atmospheric Corrosion Testing Using a Cyclic Wet/Dry Exposure Test: Aluminum, Galvanized Steel, and Steel

Overhead electrical power transmission conductors, constructed from aluminium wires centrally reinforced by galvanized steel strands, have been found, in a few isolated instances, to suffer from internal corrosion, which is associated with the presence of chloride ion, and external corrosion, which is associated with sulfate ion. A cyclic wet/dry exposure test, with salt spray solutions containing appropriate ratios of sulfate and chloride ion, was used to simulate these observations. The parameters of the test are explained, and the performance of the exposed metals are reported in terms of weight loss and are compared with available data on the atmospheric corrosion of similar materials at industrial and marine locations. An acceleration of corrosion in the chamber of between 25 to 50 times was observed. The results are discussed in terms of the solution chemistry of the salt spray as it acts on the metal surface, with particular regard to the buffering capacity of ammonium and bicarbonate ion. The reality of the situation, with regard to atmospheric attack, is considered. It is clear that a true laboratory model should include a consideration of the chemistry on the surface in conjunction with dilution and concentration by surface-adsorbed moisture (wetting and drying) in order to represent the outside environment adequately.