Patina on copper

Table 2. Composition of Green Patina on Copper from Different Locations ...

However, in this section emphasis is placed upon damp and wet atmospheric corrosion which are characterised by the presence of a thin, invisible film of electrolyte solution on the metal surface (damp type) or by visible deposits of dew, rain, sea-spray, etc. (wet type). In these categories may be placed the rusting of iron and steel (both types involved), white rusting of zinc (wet type) and the formation of patinae on copper and its alloys (both types). 

Secondly, absorbent particles such as charcoal and soot are intrinsically inert but have surfaces or infrastructures that adsorb SO, and by either coadsorption of water vapour or condensation of water within the structure, catalyse the formation of a corrosive acid electrolyte solution. Dirt with soot assists the formation of patinae on copper and its alloys by retaining soluble corrosion products long enough for them to be converted to protective, insoluble basic salts. 

Lewin, S. Z. (1973), A new approach to establishing the authenticity of patinas on copper-base Artifacs, in Application of Science in the Examination of Works of Art, Proc. Seminar, June 15-19, 1970, Museum of Fine Arts, Boston, pp. 62-66. 

Metals develop a natural corrosion-resistant film when exposed to the environment. Examples include the rusting of iron, tarnishing of silver, and the formation of the patina on copper. These passive films help prevent further corrosion. However, films do not provide complete resistance to chemical attack and are destroyed by various corrosive agents. 

Rust of iron (the most abundant corrosion product), and white rust of zinc are examples of nonprotective oxides. Aluminum and magnesium oxides are more protective than iron and zinc oxides. Patina on copper is protective in certain atmospheres. Stainless steels are passivated and protected, especially in chloride-free aqueous environments due to a very thin passive film of Cr2C>3 on the surface of the steel. Most films having low porosities can control the corrosion rate by diffusion of reactants through the him. In certain cases of uniform general corrosion of metals in acids (e.g., aluminum in hydrochloric acid or iron in reducible acids or alkalis), a thin him of oxide is present on the metal surface. These reactions cannot be considered hlm-free although the him is not a rate-determining one.1... 

Dissolution of steel or zinc in sulfuric or hydrochloric acid is a typical example of uniform electrochemical attack. Steel and copper alloys are more vulnerable to general corrosion than other alloys. Uniform corrosion often results from atmospheric exposure (polluted industrial environments) exposure in fresh, brackish, and salt waters or exposure in soils and chemicals. The rusting of steel, the green patina on copper, tarnishing silver and white mst on zinc on atmospheric exposure are due to uniform corrosion.14... 

Producing verdigris artificially on a copper object isn t an easy task. Artists usually resort to using acrylic paints to mimic the natural reaction of copper with acidic compounds in the air. By the way, the patina on copper roofs is a mixture of copper carbonate and copper sulfate. 

The reaction with CO2 leads to oxy-carbonates. The reaction with sulfur can lead to sulfide layers, e.g., on silver ware. A very prominent example of secondary corrosion products is also the green patina on copper sheets. Details can be found in the special literature on this subject. 

If the green patina on copper alloys is desired for aesthetic reasons, pretreatment of the surface with appropriate passivating solutions is recommended. If oxidation by sulfur compounds precedes the desired reaction, the surface will present only a dark brown color for many years. The behavior of copper and copper alloys in three typical atmospheres is summarized in Table 9.6. 

---