Artificial Patination of Copper and Copper Alloys in Wet Atmosphere with Increased Content of SO2

Abstract

Natural copper patina is usually formed over several decades. This work investigates the possibility of obtaining a stable artificial patina based on brochantite in a more reasonable time. The patination process was based on patina formation from a humid atmosphere containing sulphur dioxide. The studied parameters were humidity (condensation and condensation/drying), sulphur dioxide concentration (4.4–44.3 g·m−3) and surface pre-treatments (grinding, pre-oxidation and pre-patination) prior to the patination process. Samples were evaluated by mass change, digital image analysis, spectrophotometry, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). A resistometric method was employed in order to observe the patina formation continuously during the exposure. Conditions inside the chamber were monitored during the exposure (pH of water and concentration of SO2 in gaseous phase). According to XRD, it was possible to deliberately grow a brochantite patina of reasonable thickness (approx. 30 µm), even within a couple of days of exposure. The drying phase of the condensation cycle increased the homogeneity of the deposited patina. Formation kinetics were the fastest under a condensation/drying cycle, starting with 17.7 g·m−3 sulphur dioxide and decreasing dosing in the cycle, with an electrolyte pH close to 3. The higher sulphur dioxide content above 17.7 g·m−3 forms too aggressive a surface electrolyte, which led to the dissolution of the brochantite. The pre-oxidation of copper surface resulted in a significant improvement of patina homogeneity on the surface.