Environmental effects on metallic materials – Results of an Outdoor exposure programme running in Switzerland
Authors: Oesch S. , Heimgartner P. 
Published: May 2005

As part of the project “Environmental Effects on Materials” a field exposure programme was begun in 1993 in Switzerland. The exposure sites were chosen near the stations of the National Air Pollution Monitoring Network (NABEL), where climatic data are also recorded. This work presents the results for unalloyed and weathering steel, zinc, hot dip galvanized steel, copper and aluminium specimens after 1 year outdoor exposure, i.e. in an early stage of material degradation.

Gravimetric evaluation of the test specimens revealed large differences in material loss due to corrosion for the different test sites that could be correlated with the environmental parameters. A mass balance calculation revealed that for steel, zinc and galvanized steel a considerable portion of corrosion products was lost during the exposure period depending on the environmental conditions at the test site. The losses could be attributed to flake‐off processes for the steels. This was concluded from SEM micrographs. For the zinc samples the most likely mechanism is removal by dissolution and washing away of the corrosion products during rainfall. This is supported by a good correlation of zinc corrosion product loss with the amount of rainfall and the fact that sheltered specimens did not exhibit significant losses of corrosion products. These specimens, on the other hand, showed a considerable accumulation of sulphates, chlorides and nitrates indicating the important role of dry deposition for the adsorption of gaseous pollutants on the metal surface. Generally the corrosive attack was relatively severe and more localized for the sheltered samples although the average corrosion loss was smaller than for those with open exposure.

Specimens that had been covered with snow for a longer period of time (exposure site in the mountains) exhibited higher material losses than could be expected from the air pollution situation and climatic data. They also showed a remarkable accumulation of corrosive species on their surface. This unexpected behaviour can be explained by a longer effective time of wetness (compared to the calculated value) and the accumulation of pollutants in the snow layer and on the metal surface.

The statistical analysis of the corrosion loss data and the environmental parameters yielded good correlations with the average SO2‐concentration for the different test sites especially when the time of wetness and wind velocity were also taken into account. Sulfur dioxide plays a dominant role among the gaseous pollutants although its concentration could be reduced substantially during the last decade due to various measures taken to reduce sulfur emission. The effect of atmospheric nitrogen compounds (mainly nitrogen dioxide) and ozone for the corrosion of the different metals and the interaction of the different air pollutants is not yet completely understood and is still under investigation under defined conditions in a test chamber.

The corrosion products found on different metals surfaces have been analysed using various techniques. Their composition could in some cases be related to the corresponding environmental conditions at the test sites.