The appearance and barrier properties of the patina changes with time.
What is patina?
As soon as a metal surface interacts with the atmosphere, a layer of corrosion products starts to form. On copper-based materials, these corrosion products are often referred to as the patina.
How is patina formed?
Its composition and growth depend on the prevailing environmental conditions and pollutant levels. Repeated daily dry and wet conditions result in dissolution-re-precipitation processes that gradually build up the patina. The rate of patina formation is determined by the corrosion rate, which reflects the total amount of oxidised metal per surface area and time unit.
At most atmospheric conditions copper and copper-based alloys form protective patinas with time that act as efficient barriers that reduce the corrosion rate and extend the service life of the material. The low solubility and protective properties of the patina explain for instance why roofs and facades on ancient churches require no maintenance and remain intact for centuries without being replaced.
The aesthetic appearance of the patina predominantly depends on the atmospheric pollutant levels of sulphur dioxide (SO2) or sulphates (SO42-) and of chlorides (Cl-), as schematically illustrated in the figure below. The patina consists of an inner layer predominantly consisting of copper(I)oxide and an outer layer of different copper(II)hydroxylsulphates and/or chlorides. These corrosion products are poorly soluble and very different from water soluble copper salts.
- The evolution of outdoor copper patina, A. Krätschmer, I. Odnevall Wallinder C. Leygraf, Corrosion Science, 44(3), 425 (2002)
- Atmospheric Corrosion, 2nd edition, C. Leygraf, I. Odnevall Wallinder, J. Tidblad, T.E. Graedel, John Wiley & Sons (2016)