A multi-technique approach for characterizing patina formed on the surface of brass used for musical instrument of the XIX century

Scientific reports on the characterization of patinas formed on brass alloys used for the fabrication of musical instruments are scarce despite the interest of musicians in the execution of music using instruments built at the same age of the compositions. It is widely acknowledged that the sound depends on the manufacturing characteristics of the instrument. Very often the surface of the instruments in contact with saliva are covered by a biofilm and corroded: there is on one hand the need of characterizing the patina, of choosing how to remove it and clean the brass surface without compromising the performances of the instrument, and last how to effectively maintain the instrument upon time. In this investigation, electrochemical methods which include potentiodynamic polarization curves, open circuit potential (OCP) and linear polarization (LP) were applied together with light optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy dispersive X-ray analyser (SEM-EDX), X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES). These complementary analytical techniques allowed characterizing the brass surfaces of five different brass alloys, whose zinc content ranges from 18 wt% to 37 wt%, before and after exposure to saliva solutions at pH 7.4 and at ambient temperature. Two alloys with 1 and 2wt% of Pb were also studied for comparison. Electrochemical results The results of the electrochemical measurements showed that the corrosion rate of all the alloys after one hour of immersion is relatively high (35 – 85 µm/year), with the formation of the patina it is expected to decrease after prolonged immersion. The alloys can be separated in two groups: alloys with low zinc content (Cu18Zn, Cu28Zn) show lower corrosion rates and more positive corrosion potentials, the alloys with higher zinc content exhibit higher corrosion rates. The anodic and cathodic Tafel slopes were determined from potentiodynamic polarization curves, indicating βa = 70 mV/dec and βc = 130 mV/dec. XPS surface characterization Analysis of brasses by XPS is particularly challenging as it is known that the XPS signals of Cu2p3/2 and Zn2p3/2 do not exhibit chemical shifts. Thus in this work emphasis was put on the analysis of the x-ray induced Auger signals and the Auger parameter with the Wagner chemical state plot. After immersion in artificial saliva for one hour the XPS survey spectra revealed the presence of nitrogen, phosphorus and sulfur in addition to the components of the alloy. The detailed spectra showed a clear shift of both binding energies and kinetic energies of the x-ray induced signals. On all the brass alloys studied after immersion in artificial saliva an inorganic surface film com¬posed of CuSCN and zinc orthophosphate has been revealed by the application of the Auger parameter concept and the Wagner chemical state plot. Composition of the surface films formed in the different conditions will be reported at the conference.