An investigation into the surface layers formed on oxidised copper exposed to SO2 in humid air under hypoxic conditions

Preoxidised Cu surfaces and powdery Cu oxides were exposed to SO2 vapour in an oxygen-deprived humid atmosphere. The reaction products were investigated by XRD and SEM-EDX. Chevreul’s salt, Cu2SO4·CuSO4·2H2O results to be the only stable product. Cu(0) is detected in experiments on Cu(I) oxide. Based on the competition between Cu(I) disproportionation and Cu(II) reduction by SO2, a reaction pathway is proposed. Initially formed Chevreul’s salt exhibits orthorhombic symmetry and a phase transformation to the stable monoclinic structure occurs upon ageing at room temperature. A mechanistic analogy with bronze disease of archaeological artefacts has been pointed out.

► Chevreul’s salt was formed by exposing a range of Cu oxides to hypoxic, humid SO2.
► The role of tenorite and cuprite in Chevreul’s salt formation has been pinpointed.
► Starting from tenorite, Cu(II) is directly reduced to Cu(0) without Cu(I) formation.
► Starting from cuprite, disproportionation of Cu(I) yields sulphite.
► Orthorhombic Cu2SO4·CuSO4·2H2O initially forms and changes to monoclinic upon ageing.