Environmental effects on metallic materials – Results of an Outdoor exposure programme running in Switzerland

Abstract

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 SO,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 Im Rahmen des Projektes ,,Umwelteinflusse auf Werkstoffe" der EMPA wurden Anfang 1993 Freibewitterungsversuche an verschiedenen Standorten in der Schweiz gestartet. Diese wurden bei den Meljstati...

Atmospheric corrosion data of weathering steels. A review

“…The upper surface of steel specimens is generally expected to be wet for a shorter time than the lower surface, which receives no sunshine. In this respect, Oesch and Heimgartner [74] noted that rust on the lower side of a WS exposed to an atmosphere polluted with SO 2 presented a higher concentration of sulphate nests, while the rust formed on the upper side was more compact with fewer flakeoffs.…”

Long-term atmospheric corrosion of mild steel

“…Water-soluble compounds have been analysed by Oesch and Heimgartner [34] in Switzerland using ion chromatography with aqueous extracts from plate specimens.…”

“…side of the panels is usually thinner and finer than on the rougher groundward side. This roughness is due to the large rust particles and easy spallation of large flaky rust [34]. Figure 4c shows an example of porosity of a rust layer.…”

Atmospheric corrosion of mild steel

“…This may be because the amounts in question are so small that they are below the detection limit of the instrument, or because they are obscured by the complexity of the oxide scale after prolonged exposure. Water-soluble compounds have been analysed by Oesch and Heimgartner [32] in Switzerland using ion chromatography with aqueous extracts from plate specimens. Pollution of the surfaces by ionic species varied greatly between the different test sites.…”