X-ray photoelectron spectroscopy characterization of Cu compounds for the development of organic protection treatments dedicated to heritage Cu objects preservation

The preservation of bronze and copper heritage objects is challenging. Exposure to water or pollution in outdoor conditions leads to corrosion phenomena, which can highly degrade the objects or structures. We aim to develop an alternative nontoxic corrosion inhibiting treatment, based on the use of a carboxylate (HC10) treatment. Electron spectroscopies (X-ray photoelectron spectroscopy and scanning Auger microscopy) are used to determine, from micrometric to nanometric scale, the composition and chemical environments (oxidation degrees) of the copper compounds commonly found in the corrosion product layer. In the present study, we focus on the evaluation of X-ray beam irradiation damage when performing surface analysis on cuprite, brochantite, and Cu decanoate reference samples. The reduction phenomenon has already been reported, especially for Cu(II) compounds, but not clearly explained. Different behavior has been observed depending on the X-ray source used, and the nature and hydration level of the compounds. The photoreduction issue is critical, as it guarantees the reliability of the chemical information obtained and sheds light on the best analysis pathway to adopt when multitechnique analyses are implemented. An analytical procedure is employed to track the reduction of Cu(II) reference samples during XPS analysis evidencing practically instantaneous modifications of the spectra and thus, the instability of these phases except for the dehydrated cuprite sample.