A design of experiment to study the influence of aggressive ions in wet & dry depositions on the atmospheric corrosion of weathering steel

During environmental exposure, weathering steel naturally develops a protective patina, which makes this material more resistant to atmospheric corrosion than common carbon steel. As known from literature, the composition, stability and consequently the protective ability of the patina are strongly influenced by exposure and environmental conditions, such as microclimate and local composition of the atmosphere. Several laboratory studies were performed to investigate the action of sulphur dioxide and sulphate or chlorides, the species recognized as the most affecting the formation of the patina; however the relatively recent changes in pollutants concentrations (e.g. reduction in SO2 levels and increasing in the relative NOx and particulate matter concentration) induce to consider more complex atmospheric conditions. The aim of this study is to investigate the influence of the main aggressive inorganic anions contained in wet and dry atmospheric depositions (i.e chlorides, sulphates and nitrates) on weathering steel corrosion in relation to their concentration and to possible synergic effects. In order to consider the effect of different ion mixtures, a multivariate approach of Design of Experiment (DoE) was applied to plan and optimize the experimental procedures. The experimental domain in terms of ions concentrations has been selected in order to simulate both mild and quite aggressive atmospheric conditions. The effects of the considered anions on weathering steel corrosion were evaluated in controlled laboratory conditions, by performing alternate immersion accelerated ageing tests and by analysing the following parameters: appearance and composition of corrosion products formed on the surfaces, alloying metal release and corrosion rates.