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.

Testing of pilot 2 m3 exposure chamber for formation of brochantite based patina on copper and copper alloys – objects of practical dimensions

“…The previous results showed [17,18] that the drying phase is very important in a patination cycle. Therefore this work started with testing of ventilation period after condensation.…”

“…According to results from previous work, an automatic pilot exposure chamber was constructed (Fig. 1) [17]. It allows temperature control (40 °C) using rod heater and sensors, drying phase using fan placed inside ventilation system of the chamber, humidity (100 % RH) and the volume of SO 2 gas (8.9-17.7 g m 3 ) dosed at the beginning of exposure.…”

“…This work is focused on patina based on sulphates and extends the knowledge from previous experiment in a small exposure chamber (30 dm 3 ) [17]. The experiment includes observation of conditions in the exposure chamber and patination of copper sheets and bronze statue.…”

Long-term outdoor exposure of artificial copper patina based on brochantite

“…In the exposure chamber described in the previous article, samples with artificial patina were prepared, which had different surface pretreatment [19]. Samples dimension 5 × 10 cm with a ground surface using a brass brush (SFx marking), samples pre-patinated in solution -100 ml 25% NH 3 , 33 g Na 2 S and 250 ml H 2 O (marking RxSF) and pre-oxidized samples were tested.…”

Corrosion of silver in environment containing halides, pseudohalides, or thiourea

“…This allows the probes to be designed for application with a broad spectrum of conditions in terms of corrosion aggressivity. [ 27–29 ]…”