Upon exposure to the atmosphere, copper develops a protective film called patina, and its composition depends on varying regional atmospheric conditions.
In industrial and urban atmospheres it consists mainly of basic copper sulphate, and in non-urban environments it consists of basic copper carbonate. These copper salts have chemical compositions similar to those found in natural minerals, and once the patina has developed, no further copper corrosion occurs under normal conditions, As it is self-healing, any superficial mechanical damage is repaired by the renewed formation of patina.
The patina, consisting of green copper salts, is often described as verdigris which is inaccurate, as verdigris is caused by the chemical reaction of copper with acetic acid.
In contrast to copper salts, which form a natural patina, verdigris is water soluble and is visually recognised by its strikingly green colour.
Atmospheric corrosion of copper occurs at 2-3 µm per year depending on the environment, but this rate is applicable only during the first few years and with time it decreases until it reaches zero after 70 years.
Copper components exposed to the atmosphere undergo various stages of discolouration from the time of installation to the development of the natural patina. Minor marking will become invisible as copper develops its primary protective film, a uniform brown oxide, after a few weeks due to the reaction with atmospheric oxygen. The intensity of the brown colouration increases with time until the patina develops as a secondary layer of various shades of green.
This is caused by various copper salts and depends on prevailing local atmospheric conditions, exposure to moisture and air pollutants, the pitch of copper roof or wall areas, and on time. The composition of the atmosphere dictates the rate of patina development and the following periods are considered normal for the formation of the protective patina film:
- Moderate — 18 years.
- Industrial — 10 years.
- Marine — 5 years.
In mild environments, it may take over 30 years to turn green and in some dry environments, it may never turn colour.
Strength properties and the degree of purity of the copper do not affect the rate of patina formation.
In some locations or positions, the slope of the roof, or vertical surfaces or soffits, can affect the development of the patina to the degree that copper may never turn green.
Copper can be pre-patinated or patinated after installation, and these field methods may provide rapid patination, but the resulting colour can vary significantly.
Patination can be affected by any streaking, marking, or soiled areas or by perspiration caused by handling, which can be avoided by the use of cotton gloves during installation.
Water run-off from copper can visibly stain light-coloured building materials, such as concrete, brick and stone.