Development of cleaning procedures of copper corrosion products by using “green” deep eutectic solvents

Abstract

Abstract: Selection and refinement of a method for copper corrosion removal were undertaken in an environmentally friendly way by using deep eutectic solvents (DES) integrated in a cellulose-based membrane. DES is a potential “green” solvent based on natural compounds. In this thesis, a DES composed of choline chloride and ascorbic acid in a 2:1 ratio, was prepared and used for copper corrosion cleaning. The DES was diluted with water and the DES to water ratio was optimized in terms of efficiency, time consumption, and selectivity of cleaning. It was found that DES solutions, having a concentration higher than 70 wt.% were suitable for dissolving copper corrosion products. Meanwhile, they exhibited a supressed dissolution ability of CaCO3 which is a common compound in many of the supporting structures used for copper artefacts. These solutions were then used for swelling cellulose membranes, and the swelling capacity was monitored for a week. The swollen membrane was put on an electrochemically corroded copper sheet for 24 hours. By removing the membrane, the green malachite layer was successfully removed from the copper sheet while preserving a reddish-brown cuprite layer. This was confirmed by performing X-ray diffraction analysis before and after cleaning the copper sample. For heritage objects, the defacing copper corrosion products of Cu (II) need to be removed apart from protective layer of cuprite. We concluded that the cellulose membranes containing DES solutions are promising materials for this purpose.