Copper Alloys and Weathering Steel Used in Outdoor Monuments: Weathering in an Urban-marine Environment

We exposed metal coupons to natural weathering for 18 months at the Experimental Marine Station (SMS) inside Genoa Harbour.Four different compositions related to metals used in outdoor monuments were selected: copper (CU), ternary bronze (TB), quaternary bronze (QB), and weathering steel (WS). For each alloy, subsets of samples were monitored in situ for colour variation at regular intervals. To fully characterise the natural urban-marine patina growth and its chemical-physical properties, several analytical techniques have been used. For a more effective comparison with patinas on metal monuments and ornamentations, both portable Non-destructive Techniques (NdT) and classical laboratory methods were adopted. First results showed a generally slower growth rate and evolution of the properties of patinas after 12-14 months. The patina on bronzes and pure copper showed the early formation of cuprite followed by Cu2Cl(OH)3 polymorphs and copper sulfates, mainly brochantite on CU. On WS a thicker corrosion layer than on copper alloys had grown, mainly composed of iron oxyhydroxides lepidocrocite and akaganeite, with a higher corrosion rate in respect of copper alloys. The patina composition on 18 months weathered coupons corresponds to the main composition generally reported for outdoor artworks. These coupons would thus be useful to test the performance of treatments on complex patina layers.