Atmospheric corrosion of brass in outdoor applications: patina evolution, metal release and aesthetic appearance at urban exposure conditions.

Short (days, weeks) and long-term (months, years) non-sheltered field exposures of brass (15, and 20 wt.% Zn) and copper sheet have been conducted in three European cities (Milan, Stockholm, Madrid) to generate an in-depth time-dependent understanding of patina evolution, corrosion rates, aesthetic appearance, metal release and degree of dezincification in relation to detailed bulk and surface characteristics prior to exposure. This has been accomplished by using a multitude of surface and bulk analytical tools, chemical analysis and colorimetric investigations. Small differences in surface finish and local variations in nobility observed for the non-exposed brass alloys resulted in slight differences in corrosion initiation. Despite different kinetic behaviour and relative surface distributions of zinc- and copper-rich patina constituents, similar phases were identified with copper-rich phases rapidly dominating the outermost patina layer in Milan, compared to Madrid and Stockholm showing both copper- and zinc-rich phases. As a consequence of differences in surface coverage of copper- and zinc-rich corrosion products at the different sites, the release ratios of copper to zinc varied concordantly. The released amount of zinc to copper (Zn/Cu) was for both alloys and test sites always higher compared to the bulk composition showing a preferential release of zinc. The amount of released copper from the brass alloys was on an average 30-40% lower compared to copper sheet at all test sites investigated. Significantly lower annual total release rates of copper and zinc compared with annual corrosion rates were evident for both brass alloys at all sites.