The chemistry of copper patination

doi:10.1016/S0010-938X(98)00093-6

Corrosion Science

Volume 40, Issue 12, December 1998, Pages 2029-2050

https://doi.org/10.1016/S0010-938X(98)00093-6 Get rights and content

Abstract

The chemistry of copper patination was investigated by two series of experiments. The chemistry of an aqueous copper-sulphate solution was studied at concentrations and pH values near those predicted in an electrolyte on copper exposed to the atmosphere. The electrochemical reactions in an electrolyte in contact with cuprite were investigated in a reaction vessel which used cuprite powder in artificial rainwater to study the electrochemistry of the atmospheric corrosion and patination of copper. Typical sulphate concentrations in rainwater are sufficient to precipitate posnjakite (Cu₄SO₄ (OH)₆2H₂O)), a possible precursor to brochantite, within an hour of wetting a cuprite surface. Brochantite (Cu₄SO₄ (OH)₆), the most commonly found copper salt in natural patinas is responsible for their green appearance. Precipitation of brochantite from the electrolyte resulted from an increase in pH due to the cathodic reduction of oxygen and an increase in cupric ion concentrations by cuprite oxidation.

Introduction

Clean copper, upon exposure to the atmosphere, transforms from salmon-pink to a progressively darkening brown and finally to an aesthetically pleasing green. The surface layer causing the green colour is commonly known as patina and consists of corrosion products formed and retained on the copper surface. The copper patina compounds are produced by chemical reaction with trace elements in the atmosphere, particularly sulphate and chloride. The time scale for this transformation of clean copper to the green patina colour can be anywhere from 6–50 years.[1] In clean environments that are dry for substantial periods of time, like Adelaide in South Australia, it is not uncommon for copper roofs to be still at the black stage after 80–90 years with none of the green patina compound present. This paper aims to understand the chemistry of the patination process.

Copper patinas have been studied[1] extensively over a long period of time extending over 200 years. Our studies at the University of Queensland2, 3, 4, 5 have examined the detailed microstructure of a range of natural patinas. Fig. 1 (a) shows a typical metallurgical cross-section through the 150 year old patina from the Church of Riddarholmen, Stockholm, Sweden. This is typical of the natural patinas examined. These natural patina samples have been sourced from Brisbane, Australia (ranging in age from 7 years to 142 years), Denmark (ranging in age from 26 to 332 years), France, Sweden and the USA. Our studies have shown that copper patinas are chemically and metallurgically complex structures.2, 3, 4, 5 The copper patina typically consists of two distinct layers, a 5–10 mm layer of essentially continuous cuprite (Cu₂O) against the copper base metal and an external, porous layer of 5–40 mm of basic copper sulphate, brochantite (Cu₄SO₄ (OH)₆) or basic copper chloride, atacamite (Cu₂Cl (OH)₃). Brochantite patinas are more common; however atacamite is found in marine patinas.2, 3, 4, 5

Atmospheric corrosion develops through the action of adsorbed water; that is, atmospheric corrosion can be considered to occur by a mechanism of moist or wet corrosion. The thickness of this moisture layer depends on its origins. Surfaces wet from rain are estimated[6] to have a water layer thickness of 100 mm; surfaces covered by dew or at 100% relative humidity are covered by a water layer thickness of 10 mm and 1 mm, respectively.6, 7 Electrochemical corrosion can occur within this aqueous layer. The anodic reactions for copper are $Cu→Cu^{+} +e^{−},$ (1) $Cu→Cu^{2+} +2e^{−},$ (2) $Cu_{2} O+2H^{+} →2Cu^{2+} +H_{2} O+2e^{−}$ (3)and in a neutral aerated aqueous solution, the cathodic reaction is $O_{2} +2H_{2} O+4e^{−} →4OH^{−} .$ (4)Numerous exposure tests show cuprite (wherein copper is in the Cu⁺ oxidation state) to be the initial corrosion product. The rate of atmospheric copper corrosion follows a linear bi-logarithmic law.[8] Typically, after a few years of atmospheric exposure, an essentially continuous cuprite layer forms of about 6 mm thickness.2, 3, 4, 5 This continuous cuprite layer is consistent with the observed bi-logarithmic law, with the cuprite layer providing increasing corrosion protection with the passage of time for the copper substrate. An aqueous layer on the surface can therefore be considered to be in contact with the cuprite layer and further oxidation occurs via reaction (3).

The mechanism of the electrode kinetics of copper in acidic copper sulphate solutions proceeds in two steps, CutCu^+tCu² where the redox reaction between Cu⁺ and Cu² is rate-controlling, whilst Cu⁺ exists in reversible equilibrium at the electrode surface.[9] Limited electrochemical data exists on the oxidation of cuprite in rainwater. However, the oxidation of cuprite under atmospheric conditions must be slower than that of copper otherwise the intermediate cuprite would not exist.

Patina, which is predominantly comprised of brochantite, forms in the water film which remains on the surface of cuprite after a rain shower. Patina forms in the aqueous layer by the oxidation of cuprite and the incorporation of trace impurities, namely sulphates, by the following overall reaction. $2Cu_{2} O+SO^{2−}_{4} +4H^{2} O→CuSO_{4} ·3Cu(OH)_{2} +2H^{+} +4e^{−} .$ (5)Observations of copper roofs indicate that patina grows preferentially in areas washed by rain and areas which experience water films thicker than those associated with dew or high relative humidity. Corrugations in the dome featured in Fig. 1 (b) show regions of thicker patina growth. This is more obvious on the vertical tiles where only slight patination has occurred on raised areas as opposed to the thicker patina in troughs which experience more rain water.

Sulphates can be supplied from the following three sources, (1) directly from rain water, (sulphate concentrations in rain water can vary between 0.1–15 ppm), (2) via adsorption and oxidation of sulphur dioxide within the aqueous layer and (3) from sulphate particulates including aerosols. Typical SO₂ deposition rates6, 7, 8 for rural atmospheres are 0–10 mg/ (m²day), for urban atmospheres 10–100 mg/ (m²day) and for industrial atmospheres 100–200 mg/ (m²day). However, deposition rates depend greatly on the nature and geometry of the surface.[10] Adsorption of SO₂ on copper surfaces is strongly influenced by the relative humidity. Maximum adsorption occurs at 100% relative humidity, while remarkably low adsorption capacities have been measured at relative humidities less than 90%.10, 11, 12, 13, 14, 15

The aim of this present work was to investigate the chemistry of cuprite oxidation and brochantite formation in order to develop a deeper understanding of the processes involved in patination. The chemistry of patination was investigated by two series of experiments. First, the chemistry of an aqueous copper-sulphate solution was studied at concentrations and pH levels near those predicted in an electrolyte on copper exposed to the atmosphere. Second, the electrochemical reactions in an electrolyte in contact with cuprite were simulated within a reaction vessel containing artificial rainwater.

Section snippets

Experimental

In the first set of experiments, the formation of compounds detected in patinas,2, 3, 4, 5, 16, 17, 18, 19, 20 namely posnjakite (CuSO₄3Cu (OH)₂2 (H₂O)), brochantite and tenorite (CuO), were investigated by titration of copper sulphate solutions with sodium hydroxide. Five 400 ml copper sulphate solutions of 500, 200, 7, 2.8 and 1.4 ppm Cu² at 25°C were titrated with sodium hydroxide supplied at a constant rate by a peristaltic pump. The pH was recorded with a glass membrane pH electrode

Copper sulphate solutions titrated with NaOH

Copper sulphate solutions titrated with sodium hydroxide resulted in the precipitation of posnjakite, brochantite or tenorite depending on the temperature and concentration. Titrations of 500 ppm and 200 ppm Cu² (copper sulphate) resulted in the formation of brochantite and posnjakite at a pH of 5 and 5.4, respectively. However, titrations of 7 ppm and 2.8 ppm Cu² produced only posnjakite at a pH of 6.6.

Brochantite and posnjakite precipitates were observed within minutes of reaching their

Conclusion

Cuprite oxidises in an aqueous layer as a result of rain. Concentrations of 53 ppm of Cu² were measured in a simulated aqueous layer on cuprite after 4 hours at 25°C. This corresponds to an oxidation rate of 13.25 ppm Cu²/hour.

The oxidation of cuprite increases the pH of the aqueous layer while the formation of brochantite and the presence of weak atmospheric acids, i.e., formic, acetic oxalic and carboxylic acids, act as buffers and the pH remains at the equilibrium pH for brochantite or

Acknowledgements

The support of the Australian Research Council and Copper Refineries Pty Ltd is gratefully acknowledged.

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