Some principal results

Before going into a more detailed description of the results obtained from exposures of individual metal plates (copper sheet, galvanized steel, stainless steel), a few key observations from the runoff studies will be presented. It is well-known that the corrosion rate for both copper and zinc changes quite conspicuoulsy with time, showing an initial high rate that successively levels off. In contrast, the runoff rate is much more constant in time, at least seen at time scales of one to several years (Odnevall Wallinder and Leygraf, 2002). Since this behaviour has been repeatedly observed with both copper and zinc, in several different atmospheric environments, and in spite of the significant changes in chemical composition the corrosion products undergo with time, it was concluded that the runoff rate of copper and zinc is basically governed by precipitation parameters, including air quality and rain pH.

In the beginning of the exposure to the atmosphere, the runoff rate of both metals is lower than the corresponding corrosion rate, leading to a continuous accumulation of metal in the corrosion products, remaining on the panel. Eventually, the runoff rate will equal the corrosion rate, which is reflected by the discontinuation of the growth in mass of corrosion products. This happens on zinc after about a few years exposure in the Stockholm atmosphere, and probably after a few decades for copper (Odnevall Wallinder and Leygraf, 2002). The age of corrosion products on copper and zinc does not seem to have any major influence on the runoff rate, at least for up to 140 years old copper sheet (Odnevall Wallinder et al., 2002a) and 40 years old zinc sheet (He et al., 2002).

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