Corrosion of Steel Strings on Historical Musical Instruments Triggered by the Emissions of Degrading Cellulose Nitrate

Corrosion is a challenge for the conservation of musical instruments when metal parts are exposed to organic materials of the instrument's body. In long-term observations in the Norwegian Museum of Music, Ringve and Rockheim (Museums of Southern Trøndelag), local contact corrosion is documented on metal strings of plucked instruments when mounted close to cellulose nitrate (CN). The investigated strings are iron-based alloys, in some cases covered with a tin coating. In the gas mixture emitted from CN, the presence of corrosive compounds, such as nitrogen dioxide (NO₂), and carboxylic acids, is confirmed by an electrochemical NO₂-sensor and gas chromatography-mass spectrometry. The corrosion product on the strings consists of iron oxide nitrate hydroxide hydrate, Fe₂(NO₃)O(OH)₃(H₂O), and goethite, α-FeOOH, detected with X-ray diffraction, Raman, and infrared spectroscopy. X-ray photoelectron spectroscopy, used to investigate the surface of tin-coated corrosion products, showed the presence of adsorbed organic nitrogen. The formation of corrosion products is dominated by organic acids and moisture. The presence of nitrate in the corrosion product proves the involvement of nitrogen oxides in the corrosion mechanism. This analytical study highlights a dilemma for preserving material combinations in musical instruments and provides a basis for further research in conservation practice.