Copper is known as a “living” metal because it and its alloys (e.g., brass, bronze) in most atmospheric conditions can form one or many protective layers that act as barriers against corrosion. These layers are corrosion products that are generated gradually and contain many compounds that determine the overall corrosion behavior and appearance of the exposed metal. They form as a result of different chemical, electrochemical and physical processes that occur slowly and spontaneously during the interaction between the metal and its environment.
Formation of a Copper Patina
During the atmospheric corrosion of copper or copper-based materials (alloys), the corrosion products are often referred to as a patina (copper patina), which in its most stable form is a green patina. The term patina can refer to any fading, darkening or other signs of age, which make buildings, statues and sculptures covered with a patina a one-of-a-kind symbol of cultural heritage that is hard to replicate.
In urban areas, the patina consists of basic copper sulfate (CuSO4.3Cu(OH)2), although in marine environments it may contain chloride, or carbonate in industrial areas. The patina layer consists of cuprite (cuprous oxide, Cu2O), copper(II) oxide (cupric oxide, CuO) and CuOxH2O forms being the cuprite the main component. As the initial corrosion product, this layer is formed next to the metal, changing the copper surface color to opaque brown and then after some time to almost black.
After years or even decades of exposure this layer continues to grow and may reach a thickness on the order of several tens of micrometers. Depending on the environmental conditions (high/medium/low pollution, seacoast, rainy area) a bluish or green patina layer can grow over the Cu2O layer. Later, after the patina layer has formed with several products of corrosion, it is relatively stable and acts as a protective barrier for the underlying copper.
In environments rich in sulfur dioxide (SO2), the most likely primary constituents of the patina layer are basic copper sulfates: posnjakite (Cu4SO4(OH)6.H2O), brochantite (Cu4SO4(OH)6) and antlerite (Cu3SO4(OH)4); some copper chloride and carbonate are also found.
Basic copper sulfates are the dominant phases in most copper patinas. However, in some environments (e.g., chloride-rich marine atmospheres), alternative products may be found such as the polymorphs atacamite (Cu2(OH)3Cl) and paratacamite (Cu3(Cu,Zn)(OH)6Cl2) in combination with various hydroxy sulfates and cuprite. In rain-sheltered conditions, the corrosion product layers tend to be black, while in unsheltered areas where the metal is frequently washed with rain, they tend to be light green (Figure 1).
Figure 1. Copper patina, light green in an unsheltered area (wash-off effect). (Source: Diego Grandi/Dreamstime.com)
Occasionally, other copper minerals can be observed in the copper patina constituents, including tenorite (CuO), malachite (Cu2CO3(OH)2) and gerhardtite (Cu2NO3(OH)3). Tenorite occurs in the oxidized zone associated with deeper primary copper sulfide ore bodies and may be found as a sublimation product on lava. Malachite, one of the first ores used to produce copper metal, is often found with azurite, goethite and calcite, and is typically associated with copper deposits around limestone. Gerhardtite is a rare copper nitrate mineral and has been detected in vein deposits with a long-time accumulation.
Corrosion of Copper Alloys
The corrosion rate of copper and its alloys usually decreases with time. In general, copper alloys are very suitable for atmospheric exposure. High-copper alloys, silicon bronze and tin bronze corrode at a moderate rate, while brass, aluminum bronze, nickel silver and copper nickel corrode at a slower rate. (Related reading: 11 Uses for Cupronickel and Why You Should Be Using It Now.)
These alloys influence the rate of formation and characteristics of the patina layer. Some investigations have found the average thickness for the copper patina to range from 1 to 10 µm for cuprite and 10 to 40 µm for brochantite/antlerite. The thickness of each sublayer and the ratio between brochantite/cuprite sublayers can vary depending on the purity of the copper in the material and the size of inclusions found in the layers. The depth of pits (if they occur) on surfaces exposed for long periods of time range from 50 to 70 µm, but also from 100 to 105 µm in some cases.
Cleaning and Preservation of Copper
The cleaning and preservation of copper and its alloys can be done using various techniques, both physical and chemical. (For more on this topic, read Protecting Public Art Against Corrosion.) Patination and properties of artificial patinas including their transformation after atmospheric exposure and their conservation. Physical techniques include water blast cleaning to eliminating soluble parts of corrosion crust and layers, and mechanical abrasive cleaning to remove crusts, deposits and growth from the surface while retaining a thin layer of patina. Chemical cleaning includes drawing-off (use of chelation solutions) and pickling (use of a pre-treatment and application of various types of protective coatings). (Related reading: Using Pickling and Passivation Chemical Treatments to Prevent Corrosion.)
Artificial patination is not suitable for big outdoor surfaces such as copper roofs or copper parts of building cladding because this process takes too long. This technique is mainly used when a desired surface finish needs to be reached quickly. Mixtures of different chemical substances are used for patination, but the creation of the aesthetically-desirable even surface layers of artificial patina is overly complicated. Brown and black patina is created in water solutions of sulphurated potash and ammonium sulfide (amorphous mixture of polysulfides).
A thin layer of copper sulfide is formed on bronze surfaces. Green patina is usually created in water solutions of copper nitrate or chloride together with other oxidants and complex forming agents. There are also industrial processes for patination of monuments or semi-products (e.g., sheets).
The conservation of copper and bronze is achieved mainly by the treatment of surfaces with various types of waxes. Conservation should produce a waterproof, hydrophobic, chemically stable, solid, elastic and even protective layer.
Regular re-conservation at one to two year intervals can significantly reduce the formation of patina on bronze and preserve the original look of the material.
Conservation agents are applied to objects covered with an artificial green patina to protect them from washout and deterioration. The protective effect of waxes is the result of a barrier effect, primarily its low permeability for humidity and gaseous pollutants. Waxes provide only short-term protection (two years maximum) depending on the type of wax, the coating thickness and the corrosivity of the atmosphere. Waxes gradually grow old and lose their elasticity and flexibility. They then need to be removed and the surfaces re-conserved or treated with another protective system.
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 C.A. C. Sequeira. Corrosion of Copper and Copper Alloys. Uhlig’s Corrosion Handbook, Second Edition, 729, 2000, 42.
 T. Chang, I. O. Wallinder, D. De la Fuente, B. Chico, M. Morcillo, J. M. Welter, C. Leygraf. Analysis of historic Copper Patinas. Influence of Inclusions on Patina Uniformity. Materials 2017, 10(3), 298.
 D. Knotkova, K. Kreislova. Atmospheric corrosion and conservation of copper and bronze. WIT Transactions on State of the Art in Science and Engineering, Vol 28, © 2007 WIT Press.
 G. Turner-Walker. A Practical Guide to the Care and Conservation of Metals. Headquarters Administration for Cultural Heritage Council for Cultural Affairs, 2008.