ENVIRONMENTAL-EFFECTS ON METALLIC MATERIALS - RESULTS OF AN OUTDOOR EXPOSURE PROGRAM RUNNING IN SWITZERLAND

As part of the project ''Environmental Effects on Materials'' a field exposure programme was begun in 1993 in Switzerland. The exposure site s were chosen near the stations of the National Air Pollution Monitori ng Network (NABEL), where climatic data are also recorded. This work p resents 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 materia l loss due to corrosion for the different test sites that could be cor related with the environmental parameters. A mass balance calculation revealed that for steel, zinc and galvanized steel a considerable port ion of corrosion products was lost during the exposure period dependin g on the environmental conditions at the test site. The losses could b e 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 d uring rainfall. This is supported by a good correlation of zinc corros ion product loss with the amount of rainfall and the fact that shelter ed specimens did not exhibit significant losses of corrosion products. These specimens, on the other hand, showed a considerable accumulatio n of sulphates, chlorides and nitrates indicating the important role o f dry deposition for the adsorption of gaseous pollutants on the metal surface. Generally the corrosive attack was relatively severe and mor e localized for the sheltered samples although the average corrosion l oss was smaller than for those with open exposure. Specimens that had been covered with snow for a longer period of time (exposure site in t he 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 u nexpected behaviour can be explained by a longer effective time of wet ness (compared to the calculated value) and the accumulation of pollut ants in the snow layer and on the metal surface. The statistical analy sis of the corrosion loss data and the environmental parameters yielde d good correlations with the average SO2 concentration for the differe nt test sites especially when the time of wetness and wind velocity we re also taken into account. Sulfur dioxide plays a dominant role among the gaseous pollutants although its concentration could be reduced su bstantially during the last decade due to various measures taken to re duce sulfur emission. The effect of atmospheric nitrogen compounds (ma inly nitrogen dioxide) and ozone for the corrosion of the different me tals and the interaction of the different air pollutants is not yet co mpletely understood and is still under investigation under defined con ditions in a test chamber. The corrosion products found on different m etals surfaces have been analysed using various techniques. Their comp osition could in some cases be related to the corresponding environmen tal conditions at the test sites.