Mechanisms of atmospheric corrosion in tropical environments

The paper presents results from a five-nation (Australia, Indonesia, the Philippines, Thailand, and Indonesia) study into the relationship between climatic/pollutant factors and corrosion of zinc and mild steel. While a good correlation was found between the corrosion rate of mild steel and gaseous SO2 and rainwater pH, no such relationship was found between these parameters and the corrosion rate of zinc. This observation has prompted an investigation into the forms of pollutant deposition. Particulate deposition, gaseous absorption into moisture layers on metal surfaces, and deposition in fine and coarse raindrops are discussed. The discussion focuses on the rate of each process and the chemistry of the moisture formed on metal surfaces as a result of the processes. A chemical reaction simulator is used to model the changing chemistry of moisture layers under a number of environmental scenarios. These scenarios highlight the importance of the ratio of the gaseous NH3/SO2 and of oxidation of S(IV) in controlling both the acidity and composition of moisture films. The relevance of these processes to the rates of corrosion is discussed, and some hypothesis for the low corrosion rate of zinc in the five-nation study is presented.