Reactions of copper patina compounds - I. Influence of some air pollutants

Aged copper or bronze objects in outdoor environments exhibit patina layers containing several constituents. The influence of air pollutants on some of these compounds was studied, including cuprite (Cu2O), tenorite (CuO), brochantite (Cu-4(OH)(6)SO4), antlerite, (Cu-3(OH)(4)SO4), Cu-2.5(OH)(3)SO4 . 2H(2)O, and a mixture of atacamite/clinoatacamite (Cu-4(OH)(3)Cl). The compounds were exposed to air containing ppb-levels of SO2, O-3 and NO2. The products were characterized by X-ray diffraction (XRD) after four weeks' exposure. Deposition of SO2 and consumption of O-3 were Studied using on-line gas analysis. Mechanisms are suggested for the interaction of air pollutants with the patina compounds and the reactions are discussed in relation to outdoor conditions. The results are in agreement with observations from the field. Tenorite reacted rapidly with SO2 in humid air, acting as an ideal absorber. This observation is in agreement with the rare occurrence of this corrosion product in outdoor environment. In the case of cuprite, sulfates were produced in a humid SO2 + O-3 environment while no sulfate formed when SO2 was the only pollutant or when SO2 and NO2 were combined. However, active carbon on the cuprite surface enhanced the sulfation markedly in SO2 + NO2 atmosphere. Cu-2.5(OH)(3)SO4 . 2H(2)O, brochantite and antlerite formed on copper oxides. Cu-2.5(OH)(3)SO4 . 2H(2)O, is suggested to be a metastable precursor in brochantite and antlerite formation and is slightly more soluble than these. Brochantite and antlerite did not react in any of the environments studied. In contrast, atacamite/clinoatacamite reacted rapidly with SO2 in humid air, forming soluble CuSO4 . xH(2)O and a CuCl2 . xH(2)O solution. (C) 1998 Elsevier Science Ltd. All rights reserved.