An interdisciplinary methodology is being developed for the evaluation and monitoring of the corrosion state inside historical brass wind instruments of the 19th and early 20th centuries before and after being played. In a first part the focus is on the development of a non-destructive electrochemical sensor for the in–situ determination of the corrosion state and rate inside historical brass wind instruments, allowing also insight into the degradation mechanism which affects the artifacts. The electrochemical sensor consists of a combined Ag/AgCl solid-state reference electrode and a small platinum grid as counter electrode, both embedded in a thin sponge soaked with a diluted phosphate buffer solution, pH 7, with 10-3 M chlorides. XPS surface analysis has shown that the test solution does not alter the surface composition of the brass alloys during the contact. The electrochemical measurements with the sensor have to be related to surface state, requiring surface analysis of the brass alloys. This is challenging, because for both Cu and Zn the chemical state identification based on the binding energy values of Cu2p and Zn2p photoelectron signals is not possible: the chemical shifts between Zn(II) and Zn(0) and between Cu(I) and Cu(0) in their photoelectron signals are very small. A novel analytical strategy has been proposed for the identification and the quantification of Cu and Zn when different chemical states are simultaneously present. The chemical state identification is based on the Auger signals (XAES) and on the determination of the multicomponent signals of Cu and Zn metal and oxides on reference compounds. The intensities obtained from curve fitting the XAES spectra had to be converted to XPS photoelectron signals. This analytical approach has been tested on the Cu37Zn model brass alloy and it has proven to be successful. As a result it was applied for the identification and characterization of brass model alloys with different surface states. In the third part of the electrochemical and corrosion behavior of brass model alloys after aging in a phosphate buffer solution pH 7 and an artificial saliva solution (pH 7.4), were studied by means of open circuit (OCP), linear polarization (Rp) and Electrochemical Impedance Spectroscopy (EIS) measurements. In the phosphate buffer solution the effect of immersion time was small and the EIS data indicated a charge transfer control. In artificial saliva both the OCP and the Rp increased markedly and the EIS data indicated a resistive control by a surface film. Surface analysis by XPS clearly showed the formation of a thick, protective film composed of CuSCN and Zn3(PO4)2 on brass alloys in artificial saliva. In buffer solution only a very thin Cu(OH)2/CuO/ZnO film was formed. Thus it was possible to correlate the electrochemical and corrosion data with the surface composition of the aged samples. A mechanistic explanation based on the oxygen reduction current that is catalysed in presence of Cu(OH)2/CuO couple on the surface could be given and explain the much higher corrosion rates in the phosphate buffer solution. Finally, the electrochemical sensor was used on samples from ancient brass instruments and inside the tuning slides of brass wind instruments of the 19th century to check the efficiency of the preventive conservation protocol. A plot of log(Rp) vs OCP allowed to compare all the results obtained on the brass model alloys and ancient samples. Knowing the surface state from the surface analysis, the electrochemical results could be correlated to the surface state of the brass samples studied. So, the condensed representation of the electrochemical results showing distinct groups of log(Rp) vs OCP data can be used both for diagnostic purpose and for mechanistic interpretation. The interdisciplinary approach combining electrochemical and surface analytical techniques at the forefront of research allowed reaching successfully the research goal.
An interdisciplinary methodology is being developed for the evaluation and monitoring of the corrosion state inside historical brass wind instruments of the 19th and early 20th centuries before and after being played. In a first part the focus is on the development of a non-destructive electrochemical sensor for the in–situ determination of the corrosion state and rate inside historical brass wind instruments, allowing also insight into the degradation mechanism which affects the artifacts. The electrochemical sensor consists of a combined Ag/AgCl solid-state reference electrode and a small platinum grid as counter electrode, both embedded in a thin sponge soaked with a diluted phosphate buffer solution, pH 7, with 10-3 M chlorides. XPS surface analysis has shown that the test solution does not alter the surface composition of the brass alloys during the contact. The electrochemical measurements with the sensor have to be related to surface state, requiring surface analysis of the brass alloys. This is challenging, because for both Cu and Zn the chemical state identification based on the binding energy values of Cu2p and Zn2p photoelectron signals is not possible: the chemical shifts between Zn(II) and Zn(0) and between Cu(I) and Cu(0) in their photoelectron signals are very small. A novel analytical strategy has been proposed for the identification and the quantification of Cu and Zn when different chemical states are simultaneously present. The chemical state identification is based on the Auger signals (XAES) and on the determination of the multicomponent signals of Cu and Zn metal and oxides on reference compounds. The intensities obtained from curve fitting the XAES spectra had to be converted to XPS photoelectron signals. This analytical approach has been tested on the Cu37Zn model brass alloy and it has proven to be successful. As a result it was applied for the identification and characterization of brass model alloys with different surface states. In the third part of the electrochemical and corrosion behavior of brass model alloys after aging in a phosphate buffer solution pH 7 and an artificial saliva solution (pH 7.4), were studied by means of open circuit (OCP), linear polarization (Rp) and Electrochemical Impedance Spectroscopy (EIS) measurements. In the phosphate buffer solution the effect of immersion time was small and the EIS data indicated a charge transfer control. In artificial saliva both the OCP and the Rp increased markedly and the EIS data indicated a resistive control by a surface film. Surface analysis by XPS clearly showed the formation of a thick, protective film composed of CuSCN and Zn3(PO4)2 on brass alloys in artificial saliva. In buffer solution only a very thin Cu(OH)2/CuO/ZnO film was formed. Thus it was possible to correlate the electrochemical and corrosion data with the surface composition of the aged samples. A mechanistic explanation based on the oxygen reduction current that is catalysed in presence of Cu(OH)2/CuO couple on the surface could be given and explain the much higher corrosion rates in the phosphate buffer solution. Finally, the electrochemical sensor was used on samples from ancient brass instruments and inside the tuning slides of brass wind instruments of the 19th century to check the efficiency of the preventive conservation protocol. A plot of log(Rp) vs OCP allowed to compare all the results obtained on the brass model alloys and ancient samples. Knowing the surface state from the surface analysis, the electrochemical results could be correlated to the surface state of the brass samples studied. So, the condensed representation of the electrochemical results showing distinct groups of log(Rp) vs OCP data can be used both for diagnostic purpose and for mechanistic interpretation. The interdisciplinary approach combining electrochemical and surface analytical techniques at the forefront of research allowed reaching successfully the research goal.
Sustainability in cultural heritage: from diagnosis to the development of innovative systems for monitoring and understanding corrosion inside ancient brass wind instruments
COCCO, FEDERICA
2017-03-16
Abstract
An interdisciplinary methodology is being developed for the evaluation and monitoring of the corrosion state inside historical brass wind instruments of the 19th and early 20th centuries before and after being played. In a first part the focus is on the development of a non-destructive electrochemical sensor for the in–situ determination of the corrosion state and rate inside historical brass wind instruments, allowing also insight into the degradation mechanism which affects the artifacts. The electrochemical sensor consists of a combined Ag/AgCl solid-state reference electrode and a small platinum grid as counter electrode, both embedded in a thin sponge soaked with a diluted phosphate buffer solution, pH 7, with 10-3 M chlorides. XPS surface analysis has shown that the test solution does not alter the surface composition of the brass alloys during the contact. The electrochemical measurements with the sensor have to be related to surface state, requiring surface analysis of the brass alloys. This is challenging, because for both Cu and Zn the chemical state identification based on the binding energy values of Cu2p and Zn2p photoelectron signals is not possible: the chemical shifts between Zn(II) and Zn(0) and between Cu(I) and Cu(0) in their photoelectron signals are very small. A novel analytical strategy has been proposed for the identification and the quantification of Cu and Zn when different chemical states are simultaneously present. The chemical state identification is based on the Auger signals (XAES) and on the determination of the multicomponent signals of Cu and Zn metal and oxides on reference compounds. The intensities obtained from curve fitting the XAES spectra had to be converted to XPS photoelectron signals. This analytical approach has been tested on the Cu37Zn model brass alloy and it has proven to be successful. As a result it was applied for the identification and characterization of brass model alloys with different surface states. In the third part of the electrochemical and corrosion behavior of brass model alloys after aging in a phosphate buffer solution pH 7 and an artificial saliva solution (pH 7.4), were studied by means of open circuit (OCP), linear polarization (Rp) and Electrochemical Impedance Spectroscopy (EIS) measurements. In the phosphate buffer solution the effect of immersion time was small and the EIS data indicated a charge transfer control. In artificial saliva both the OCP and the Rp increased markedly and the EIS data indicated a resistive control by a surface film. Surface analysis by XPS clearly showed the formation of a thick, protective film composed of CuSCN and Zn3(PO4)2 on brass alloys in artificial saliva. In buffer solution only a very thin Cu(OH)2/CuO/ZnO film was formed. Thus it was possible to correlate the electrochemical and corrosion data with the surface composition of the aged samples. A mechanistic explanation based on the oxygen reduction current that is catalysed in presence of Cu(OH)2/CuO couple on the surface could be given and explain the much higher corrosion rates in the phosphate buffer solution. Finally, the electrochemical sensor was used on samples from ancient brass instruments and inside the tuning slides of brass wind instruments of the 19th century to check the efficiency of the preventive conservation protocol. A plot of log(Rp) vs OCP allowed to compare all the results obtained on the brass model alloys and ancient samples. Knowing the surface state from the surface analysis, the electrochemical results could be correlated to the surface state of the brass samples studied. So, the condensed representation of the electrochemical results showing distinct groups of log(Rp) vs OCP data can be used both for diagnostic purpose and for mechanistic interpretation. The interdisciplinary approach combining electrochemical and surface analytical techniques at the forefront of research allowed reaching successfully the research goal.
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PhD Thesis FC.pdf
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