A non-destructive in-situ approach to monitor corrosion inside historical brass instruments

In the frame of the project “Brass instruments of the 19th and early 20th centuries between long-term conservation and use in historically informed performance practice” [1] an interdisciplinary methodology is being developed for the evaluation and monitoring of the corrosion state inside historical brass instruments before and after being played. The efficiency of preventive conservation protocol can be established. In line with the scope of the project, non-destructive techniques that could be used inside the musical instruments to get information on the actual corrosion state and possibly on the corrosion rate were evaluated. As corrosion of metal artifacts is an electrochemical process, electrochemical techniques such as potential measurements and polarization resistance measurements are the methods of choice. In addition the internal surfaces of the instruments were characterized by optical methods (endoscope). In a first step traditional electrochemical measurements in solution were performed in order to study the influence of the alloy composition [2, 3]. As expected brass alloys with higher zinc content (e.g. alloys CuZn37) behaved more noble (more positive potentials) and showed lower corrosion rates. To perform the same electrochemical instruments inside the instru-ments a special electro¬chemical sensor was developed. The sensor combined an Ag/AgCl (pseudo) reference electrode and a small platinum grid as counter electrode, both embedded in a thin cylindrical sponge mounted on a flexible tube. The sensor had a surface of about 1 cm2 and could be pressed against the inside of the brass tubes by a small balloon that could be pumped or emptied. The sponge was wetted with a non-aggressive electrolyte (phosphate buffer pH 7) in order to ensure the electrolytic contact to the tube. Several ancient brass instruments, horns, trumpets and tuba, were characterized by endoscope, potential measurements and polarization resistance measurements. The results showed that the local surface condition at the point of measurement is more important then the type of instrument or the alloy composition. The in-situ measurements will be repeated after about half a year of playing the instruments by musicians in order to see if changes in the corrosion behavior can be observed. [1] T. Lombardo et al., Brass instruments of the 19th and early 20th centuries between long-term conservation and use in historically informed performance practice, paper presented at this conference [2] F. Cocco, Master thesis University of Cagliari (2014). [3] F. Cocco, M. Fantauzzi, B. Elsener, A. Rossi, EUROCORR’2014