Model Protective Films on Cu-Zn Alloys Simulating the Inner Surfaces of Historical Brass Wind Instruments by EIS and XPS

Abstract

The present work focuses on the characterization of brass surfaces after contact with artificial saliva solution at pH 7.4 and phosphate buffer solution at pH 7 simulating two extreme conditions that might occur when playing ancient brass wind instruments in the context of historically informed performance practice. The composition and the morphology of the film formed following the contact with the solutions for 1, 3, and 16 h were investigated by ex situ X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to shed a light on the surface changes upon time. In situ electrochemical impedance spectroscopy (EIS) was used to study the mechanism of corrosion and protection of the alloys. The results could be interpreted using a reliable equivalent electrical circuit; they provided evidence that the alloys behave differently when in contact to the various solutions. In saliva solution the formation on the brass surface of a thick surface film was observed, composed of crystallites of about 200 nm size mainly composed of CuSCN and Zn 3 (PO 4) 2. This layer hinders the alloy dissolution. The contact of the alloys with the buffer solution originated a much thinner layer composed of Cu 2 O, ZnO, and a small amount of Zn 3 (PO 4) 2. This film is rapidly formed and does not evolve upon time in a protective film.

X-rays investigations for the characterization of two 17th century brass instruments from Nuremberg

“…The study of the structural features gave access to the manufacturing processes, to identifying the damages, the presence of some reparations or restorations. Even the alterations commonly found on the inner surface of tubes caused by the playing were put into evidence [23]. RX investigation suggested that the inv.878 (Figure 1c) was characterized by a lamina with a uniform thickness joint by a nipped tooth joint between the edges (Figure 2a).…”

The effect of the saliva formulation on brass corrosion

“…The exponential increase of Rp with the OCP (straight line in the log Rp vs. OCP diagram) is similar for all the model solutions here examined, and also for the Tani-Zucchi solution studied in previous works. [2][3][4][5][6] The values measured for short immersion times F I G U R E 4 Diagram log Rp versus OCP for CuZn37 alloy exposed for 1, 3, and 16 h to Darvell, Carter-Brugirard, and SALMO. For comparison, the results previously obtained with Tani-Zucchi saliva [2,3] are also reported.…”

“…Model solutions of human saliva are used for investigating the corrosion behavior of various materials from steels to brass alloys in a wide range of applications, which includes orthodontics and cultural heritage. [1][2][3][4][5][6] The use of model solutions is required since the natural saliva is a complex mixture of organic and inorganic compounds, strongly variable from person to person as its composition depends on environmental, physiological, and pathological factors. [7][8][9][10][11][12] In addition, its composition changes throughout the day and, due to its instability outside the oral cavity, it is not usually used for in vivo and in vitro studies.…”

“…One of the most common is Tani-Zucchi's formulation, [13] which was recently employed for the investigation of the corrosion behavior of steels exploited in the production of orthodontic appliances [1] and for highlighting the surface modifications induced when musicians play brass wind instruments of historical-artistic interest, belonging to museum collections. [2][3][4][5][6] Tani-Zucchi has one of the simplest saliva formulations reported in the literature, it contains KSCN, NaHCO 3 , KCl and NaH 2 PO 4 , and urea and α-amylase as organic compounds. It is known that organic compounds can form complexes with the metal cations and thus there is the need to ascertain whether the presence of other inorganic compounds and/or if the simultaneous presence in the solution of organic compounds that are also typically revealed in natural human saliva, might play a role in the stability of metal alloys.…”

“…The results obtained by some of the authors showed that in Tani-Zucchi's solution, the dissolution mechanism of brass alloys was under anodic control, the corrosion rate decreased with prolonged exposure time, and the dissolution rate was controlled by the resistance of the protective film formed on the surface, mainly constituted of CuSCN and Zn 3 (PO 4 ) 2 . [2][3][4][5][6] In this study the effect of the composition of various model solutions on the corrosion behavior of a CuZn37 alloy was investigated. The solutions employed in this study are: Darvell (D), [14] Carter-Brugirard (C-B), [15] and SALMO (S), [16] which are characterized by the presence of chlorides, thiocyanates, phosphates, carbonates, and urea.…”