Synthesis and multi-analytical characterization of corrosion layers on CuSn6 bronze: Implications for cultural heritage conservation

: The natural formation of a thick corrosion layer (CL) on bronze of cultural heritage usually takes several decades. Creating representative CL is essential for elaborating model systems that can be used to validate and improve conservation treatments. This study presents a controlled method for creating “green” thick CL on CuSn6 bronze alloys using accelerated wet/dry cycling in a climatic chamber with Na 2 SO 4 solution (pH 3.2) within 15 days. Multi-analytical characterization combining XRD, Raman spectroscopy, SEM, and TEM revealed a stratified structure consisting of a thin inner cuprite Cu 2 O layer, an intermediate SnO 2 layer, and a thick external brochantite Cu 4 SO 4 (OH) 6 layer, closely matching the composition and morphology of natural corrosion layers. Electrochemical impedance spectroscopy (EIS) shows that the external layer of brochantite has a negligible effect on the electrochemical behaviour of the CL, whereas the internal Cu 2 O layer increases the cathodic activity of the interface because of its semiconducting properties. Then the porous SnO 2 layer seems to constitute a permeable membrane that reduces the ionic diffusion. To prevent degradation of such corroded bronze, the goal of future conservation treatments should aim to block the semi-conducting properties of the Cu 2 O layer.