Formation of protective and coating layers

When an installation comprising copper tubes starts to operate, initially the natural reaction of copper with the oxygen dissolved in the water leads to the formation of a protective layer of copper(I) oxide (chemical composition Cu₂O) on the inner surface of the tubes.

Then, in drinking water the surface of the protective layer will react with the gases and salts which are contained in the water. Above the protective layer, a coating layer is formed which predominantly consists of basic copper carbonates (predominantly malachite). Depending on the composition of the water, the colour of this coating layer may vary from a turquoise blue to a deep green; it is usually of a greenish colour. As a result, these desired coating layers are often confused with verdigris. However, verdigris is a salt of acetic acid, i.e. a product of the reaction between copper and acetic acid. Since there is not generally any vinegar in drinking water pipes, it is also impossible for any verdigris to form in these pipes.

Furthermore, the colour of the coating layer is dependent on the oxygen content of the water, and therefore generally adopts a brown shade in hot-water pipes. Therefore, in copper tube installations, the formation of a coating layer is a natural, useful and, above all, desired process.

To support the formation of the protective and coating layers, Wieland tubes for domestic water and heating applications, marketed as the SANCO, WICU and cuprotherm brands, are provided with protection against corrosion on the working side.