The evolution of outdoor copper patina

A general reaction scheme has been deduced which describes the evolution of the copper patina, including altogether eight compounds, formed upon exposure to outdoor atmospheric environments. The scheme is based on data obtained from altogether 39 exposure sites, using a recently developed method for quantitative X-ray powder diffraction analysis of up to 8 year old patina still adhering to the copper substrate. In all cases, cuprite forms initially and continuously throughout the atmospheric exposure. Three main sequences have been identified on sheltered copper, representing different reaction routes in sulfur- or chlorine-dominated environments. In less sulfur-polluted environments, posnjakite forms on the cuprite as a precursor to brochantite. In more sulfur-polluted environments, strandbergite is a precursor to antlerite. Both reaction routes may operate simultaneously and the main route may change from the more sulfur-polluted to the less polluted route, as a result of decreased pollutant levels. In chlorine-dominated environments the initial cuprite formation is followed by nantokite and atacamite. By far the most dominating sequence on unsheltered copper includes cuprite, followed by posnjakite and brochantite formation.     

The chemistry of copper patination

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.     

Copper patinas: an investigation by Auger electron spectroscopy

Auger electron spectroscopy in conjunction with Ar⁺ sputtering has been used to investigate four samples of copper that have been patinated by exposure to the atmosphere for periods ranging from 1 to 100 years. The elemental depth profiles were qualitatively similar for all samples. The patinas were covered with a thin layer of carbon containing species and consisted primarily of copper, sulfur, chlorine, and oxygen. The sulfur is present primarily near the surface of the patina and the chlorine is distributed throughout the patina. In two samples, 44 and 100 years old, significant amounts of chlorine were found in the portion of the patina nearest the bulk copper. A simple mass balance shows that the surface of the patina (beneath the carbon containing layer) is composed primarily of the mineral brochantite, Cu₄(SO₄)(OH)₆, as expected thermodynamically. A thermodynamic analysis also shows that atacamite, Cu₂Cl(OH)₃, the principal chlorine containing mineral found in the patinas, can be stable with respect to cuprite, Cu₂O, particularly in the immediate vicinity of unoxidized copper.     

Morphological study of 16-year patinas formed on copper in a wide range of atmospheric exposures

Much information is available on the atmospheric corrosion of copper and patina formation mechanisms in the short, mid and even long term. However, studies of the structure and morphology of patina layers are less abundant and mostly deal with patinas formed in the atmosphere over a small number of years. The present study concentrates on the structure and morphology of corrosion product films formed on copper after long-term atmospheric exposure (13-16 years) in five Spanish atmospheres of different types: rural, urban, industrial and marine (mild and severe). Characterisation has been performed by X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). Long-term copper corrosion is higher in industrial and marine atmospheres and lower in rural and urban atmospheres. In all cases a decrease in the corrosion rate with exposure time is observed. The formation of antlerite [Cu₃SO₄(OH)₄] is seen in more acidic conditions and in specimen areas subject to a high time of wetness. The presence of nantokite (CuCl), which is not generally mentioned in field studies, has been detected under the cuprite layer very close to the base copper.     

The characterization of patina components by X-ray diffraction and evolved gas analysis

Fourteen patinated copper specimens, seven each from the Statue of Liberty, New York Habor and from roofs at AT&T Bell Laboratories in Murray Hill, NJ, ranged in atmospheric exposure from 1 to 100 years. X-ray diffraction showed the presence of cuprite, Cu₂O, and brochantite, Cu₄(SO₄)(OH)₆, in all specimens and antlerite, Cu₃(SO₄)(OH)₄ (up to 0.7 times brochantite), atacamite, Cu₂Cl(OH)₃ (up to 1.6 times brochantite), and/or posnjakite, Cu₄(SO₄)(OH)₆ · 2H₂O (up to 5.2 times brochantite) in some. Posnjakite has been previously reported as a patina component only once during short term exposures in Eurasia. It appears to be an early corrosion product which subsequently converts to brochantite. Mass spectrographic examination of gases emitted from heated patinas provides further information on patina composition, in particular on the presence of both carbonate and oxalate in widely varying ratios.     

The reaction of simulated rain with copper,copper patina,and some copper compounds

Rain samples of varying acidities related to the New York City metropolitan area environment for the epochs 1890, 1950, and 1980, as well as two additional variants, were used to study the accelerated reaction with copper, existing patina specimens, and 13 copper compounds (minerals). Cuprite (cuprous oxide) and brochantite (a basic cupric sulfate) formed rapidly under a wide range of conditions. Posnjakite (a hydrated basic cupric sulfate) forms under more limited conditions, sometimes simultaneously with brochantite, but subsequently converts to brochantite. Most copper compounds degrade under low pH (<2.5) conditions. The opposing roles of the reaction of copper and patina with precipitation to increase the pH and the evaporation of the precipitation to decrease the pH were also investigated. While there is considerable evidence that the chemistry of precipitation has changed markedly during the last century as a result of anthropogenic activity, the results show that it is unlikely the changes in the rain composition will have any significant effect on copper patinas. However, the more highly acid fogs do have the potential to produce substantial materials degradation.     

Corrosion of copper in seawater and its aerosols in a tropical island

A complete characterization of copper corrosion behavior has been carried out under permanent immersion, water line, splash zone and at the atmosphere (near and far from the sea) at the tropical Cuban archipelago. No significant differences have been determined for corrosion of copper under complete immersion for test sites representative of Cuban archipelago. The maximum corrosion rate was observed on the line of water, related to the partial removing of the corrosion products layer due to water movement (waves) and a higher availability of oxygen. Patina composition was characterized using XRD, IR, EDS and SEM techniques. Paratacamite (Cu₂(OH)₃Cl) was the main component of the patina formed under complete immersion, on the line of water and in the splash zone. In poorly polluted atmospheric marine environments also atacamite (another structural modification of Cu₂(OH)₃Cl) was found. When environmental SO₂ reaches a competitive level with the chloride aerosol the patina formed is a complex mixture of basic cupric chlorides (paratacamite and atacamite) and basic cupric sulfates, antlerite (Cu₃(SO₄)(OH)₄) and brochantite (Cu₄(SO₄)(OH)₆). Brochantite and basic cupric chlorides are detected at inland rural sites. The patina morphology reveals details about the local environment in which it is formed and shed light on its more or less protective role for the metal. The relatively large corrosion rate under complete immersion and on the line of water is related to the formation of a patina with poor adherence to the metal surface and to a porous layer of cuprite formed by relatively large octahedral crystals.     

Long term corrosion behaviour of copper in soil: A study of archaeological analogues

The long term corrosion behaviour of copper in soil environment has been addressed by studying two archaeological copper samples. The microstructures of the material of construction of a Chalcolithic (2350BC‐1800BC) copper chisel from Balathal and an OCP period (2650BC‐800BC) Cu anthropomorphic object have been first characterized by microscopy and the features understood by stereological methods. The equiaxed grain size, coring effects in the grains and the relatively soft matrix of the OCP copper object indicated that it was manufactured by casting. The deformed grains near the surfaces and variation in the microhardness of the sample at different points suggests that the Chalcolithic copper chisel was processed by cold deformation after casting of the square cross section chisel. The surface patina on the two archaelogical copper objects has been characterized by X‐ray diffraction. In the case of OCP copper, the green surface patina was analyzed as a mixture composed mainly of cuprite, and minor amounts of malachite and brochantite. In the case of Chalcolithic copper, the patina was composed of sulfates and oxysulfates in the outer layers while the inner layers were rich in copper oxides. The electrochemical behaviour of both the archaeololgical coppers has been characterized and compared with that of a modern Cu sample by potentiodynamic polarization studies. The corrosion rate, determined by Tafel extrapolation technique in 3.5% NaCl solution, of Chalcolithic Cu was only marginally higher than that of modern and OCP Cu. The higher rates of corrosion in case of archaeological coppers have been attributed to the presence of second phase sulfide inclusions.     

Evolution of patinas on copper exposed in a suburban area

Copper plates were exposed under sheltered outdoor conditions for up to one year, starting in September 2001 in Musashino City, Tokyo, a suburban area. Following various periods of exposure, the patinas on the plates were characterized to investigate their evolution by using X-ray fluorescence analysis, X-ray diffraction, field emission scanning electron microscopy, and glow discharge optical emission spectroscopy. The difference in the roles of sulfur and chlorine in the early stages of copper patination were identified by analyzing the depth profiles of these two elements. Sulfur was found on top of the patina as cupric sulfates such as posnjakite (Cu₄SO₄(OH)₆ · H₂O) or brochantite (Cu₄SO₄(OH)₆). Brochantite appeared only after 12 months of exposure. In contrast, chlorine was found on the surface after only one month of exposure. It gradually penetrated the patina as the exposure period lengthened, forming copper chloride complexes. Chloride ions accumulated at the patina/copper interface, forming nantokite (CuCl), which promoted corrosion.     

Copper patinas formed in the atmosphere—III. A semi-quantitative assessment of rates and constraints in the greater New York metropolitan area

Because of the ability of copper to interact with a variety of atmospheric species and to retain the signatures of those interactions in a stable patina layer, copper is a particularly appropriate material for a study of the rates of corrosion processes. The rate of formation of copper patinas in a specific geographical location is dependent upon the atmospheric concentrations of corrosive species, their degree of interaction with the copper surface, and the mechanisms and rates of the processes that govern interaction. A semi-quantitative model of patina growth is developed for samples exposed for up to several decades to the atmosphere in the greater New York City metropolitan area. In contrast to previous studies of patina chemistry, which have dealt with equilibrium conditions, the present work treats the problem from a kinetic standpoint. Under modern atmospheric circumstances, it is shown that neither the supply of atmospheric water, the supply of incorporated corrosive species from the atmosphere, nor the rate of oxidation chemistry are limiting factors in patina growth. Rather, the growth is controlled by the rate of cementation of patina components at early stages of the growth process and by the supply of diffusing copper ions at later stages. To assess patina formation on a historical basis, estimates are made of atmospheric and precipitation chemistry in the greater New York City metropolitan area over the past century. The rapid formation of modern patinas, compared with those formed a few decades ago, is shown to be a consequence of increased atmospheric levels of strong inorganic acids, particularly H₂SO₄, perhaps in combination with increased concentrations of atmospheric oxidizing species and organic compounds.     

Corrosion of copper and silver plates by volcanic gases

Corrosion products that had been formed on copper and silver plates exposed in Miyake Island, where suffered a volcanic eruption in 2000, were analyzed by X-ray techniques to get better understanding of copper and silver corrosion in harsh environment. The exposure experiment was carried out from September 2004 to April 2005. Many kinds of patina were found on copper such as cuprite (Cu₂O), posnjakite (Cu₄SO₄(OH)₆ · H₂O), brochantite (Cu₄SO₄(OH)₆), antlerite (Cu₃SO₄(OH)₄), and geerite (Cu₈S₅). For silver, silver chloride (AgCl) and silver sulfide (Ag₂S) were formed. Although the volcanic activity had greatly subsided, the atmospheric corrosion of copper and silver plates exposed on Miyake Island was mainly affected by volcanic gases, wet-dry cycles in the environment, and sea-salt aerosols.     

Copper patinas formed in the atmosphere—I. Introduction

The protective green patina which forms on copper when it is exposed to the atmosphere is complex and has been poorly characterized. Using modern analytical techniques, more than 15 samples of patina of varying ages and exposures have been examined in detail. A number of new components of copper patinas have been revealed, and the results, combined with atmospheric chemical information, have been used to deduce detailed mechanisms of patination. The previously established physical and chemical characteristics of copper patinas are reviewed. The design of the investigations reported upon in subsequent papers is described.     

Metallographic studies of the copper patina formed in the atmosphere

The green coating (patina) which forms on copper has been metallurgically investigated by examining 14 samples exposed to the atmosphere for periods of 1–100 years. Various samples in cross-sectioned form are shown to have a layered structure with numerous voids throughout. Oxides and sulfates predominate in the patinas, and small amounts of chlorine and phosphorus can also be found. Extremes in patina growths manifested as color variations are shown to result from variations in grain structure brought about by different heat treatment prior to exposure of the copper to the elements.     

Outdoor atmospheric corrosion of copper in Saudi Arabia

Abstract

Copper specimens have been exposed to the action of marine, marine industrial, urban and rural atmospheres of Saudi Arabia. Environmental factors such as average temperature, average relative humidity and deposition rates of atmospheric pollutants (i.e. Cl^- and SO₂) was investigated. By applying the standard ISO 9223, the aggressiveness of the atmospheres corresponding to the different test stations has been determined. Calculations of corrosion rates were made via loss of weight after one, two and three years of exposure and characterisation of the corrosion products formed on samples have been analysed using X-ray diffraction. Three main sequences have been identified on outdoor copper, representing different reaction routes in chloride dominated environments. Many kinds of patina were found on copper specimens such as cuprite (Cu₂ O), atacamite (Cu₂ Cl(OH)₃), paratacamite (Cu₂ (OH)₃Cl), copper amine nitrite hydroxide, gerhardite, copper hydroxide nitrate and copper amine nitrate.     

A study of copper runoff in an urban atmosphere

Initiated by the concern in several countries regarding the release of copper from, e.g., roofs, facings and other outdoor constructions, the present study aims to compare runoff rates with corrosion rates during exposure of copper in an urban atmosphere. The copper runoff rate turns out to be relatively stable during the 2 year period studied, with an average rate per year of around 135 μg Cu cm⁻². This stable runoff rate is associated with the formation and dissolution properties of cuprite (Cu₂O), which is the dominating copper patina phase throughout the 2 year period. The copper corrosion rate, on the other hand, is highly time-dependent. It exhibits an initially high value and decreases with exposure time. As a consequence, the ratio between copper runoff and copper mass loss is very low in the beginning, around 7% after 1 month, and increases with time to reach around 22% after 2 years. With prolonged exposure this ratio eventually reaches 100%, corresponding to a copper patina thickness that does not change any further with time.     

The influence of copper upon the atmospheric corrosion of iron

The atmospheric corrosion of pure iron and the binary alloy Fe-0.5Cu has been analyzed by a simultaneous measurement of the anodic current density of the metal dissolution and the cathodic current density of the O₂ reduction reaction during several wet/dry cycles using a magnetic and a gas volumetric technique, respectively. The results show three typical stages of the atmospheric corrosion: stage 1 (wetting of a dry surface): rapid corrosion with rust reduction as cathodic process; stage 2 (wet surface): slow corrosion with O₂ reduction as cathodic process; and stage 3 (drying out of the surface): very rapid corrosion with O₂ reduction as the cathodic process during critical wetting of the surface. The effect of copper is restricted to stage 3, where the corrosion rate is much smaller for the Fe-0.5Cu alloy than for pure iron. Two models are discussed to explain these results.     

Corrosion-induced copper runoff from naturally and pre-patinated copper in a marine environment

Corrosion-induced copper runoff has been monitored for copper sheet, naturally patinated copper and pre-patinated copper, with and without surface treatments, in a marine environment during one year. The study comprises solution measurements on total copper runoff rates, sulphates and chlorides released from the patina, and parallel surface analytical studies on patina formation, combined with electrochemical impedance measurements on changes in barrier properties during exposure. Bioassay tests and model predictions were applied to elucidate copper bioavailability at the immediate release situation. The runoff rate of copper was significantly lower compared to the corrosion rate throughout the exposure period. At comparable rain quantities, copper runoff rates were significantly lower at the marine site compared to similar data obtained in an urban environment. The bioavailable concentration of released copper was significantly lower compared to the total copper concentration.     

蠕墨铸铁中性盐雾腐蚀行为及机理研究

目的揭示蠕墨铸铁的大气腐蚀行为,阐明其腐蚀规律及腐蚀机理。方法采用室内加速中性盐雾腐蚀实验,并用失重法、SEM\EDS、XRD、电化学的方法来表征实验现象。结果蠕墨铸铁在中性盐雾环境中锈层截面具有明显的分层现象,前期腐蚀速率为0.53 mg/(cm~2·h),后期腐蚀速率在波动中总体趋于稳定,为0.36mg/(cm~2·h)。蠕墨铸铁带锈试样的自腐蚀电位(Ecorr)在-680～-600mV之间先减小后增大,极化电阻(Rp)变化趋势与自腐蚀电位(Ecorr)一致,自腐蚀电流(Icorr)大小在整个腐蚀周期内具有明显的波动。蠕墨铸铁在中性盐雾环境中的腐蚀产物为Fe(OH)_3、Fe_2O_3、FeOOH及少量Fe_3O_4和金属碳化物Fe_2C。结论蠕墨铸铁在中性盐雾环境中腐蚀84 h后发展为全面腐蚀,形貌呈沟壑状,腐蚀产物微观形貌呈团簇状和片层状。腐蚀早期,基体表面发生电化学腐蚀形成一层氧化膜,腐蚀介质沿石墨侵蚀基体从而产生内应力,导致外部锈层断裂,同时蠕虫状石墨处腐蚀产物呈疏松团簇状,二者共同构成介质传质通道,使腐蚀更容易发展。     

Determination of instantaneous corrosion rates and runoff rates of copper from naturally patinated copper during continuous rain events

Instantaneous corrosion rates of naturally patinated copper of varying age (16 months, 138 and 145 years) have been determined during continuous rain events in the laboratory with electrochemical impedance spectroscopy using a two-electrode cell. The two-electrode cell was found to yield the same information in bulk rainwater as a conventional three-electrode cell.Relatively constant corrosion rates, between 0.2 and 0.6 μm/y, were determined for samples having a two-layer structure with an inner brownish layer of cuprous oxide and an outer greenish layer of basic copper salts (138, 145 years). Samples with cuprous oxide as the dominating phase of the patina (16 months) showed higher and somewhat increasing corrosion rates during a rain event (from 0.6 to 1.2 μm/y). During a continuous rain event, corrosion rates were found to be approximately 10 (brownish patina) and 25 times (greenish patina) lower than corresponding instantaneous runoff rates. The first flush phenomena of the runoff process, with an increased concentration during first flush and a relatively constant concentration during steady state, was indirectly seen as an increase in solution conductivity during the first rain volume followed by relative constant value. The contribution of the concentration in the first flush to the total annual runoff rate was significant for panels having a greenish layer (138, 145 years) whereas it was negligible for panels having a brownish layer (16 months).     

Characterisation of corrosion products formed on copper exposed at indoor and outdoor sites with high H2S concentrations

Abstract

Corrosion products formed on copper exposed indoors and outdoors at sites with high hydrogen sulphide (H₂S) concentrations were characterised using several analytical techniques. The crystalline corrosion products that formed on the copper exposed indoors were chalcocite (Cu₂S) and cuprite (Cu₂O), while those that formed on the copper exposed outdoors were chalcocite, cuprite and basic copper sulphates. Surface analysis by X-ray photoelectron spectroscopy revealed differences between the copper exposed indoors and outdoors that are explained by the composition, localisation and oxidation of the corrosion products. The surface morphologies of the corrosion products also differed. Elemental depth profiling by glow discharge optical emission spectroscopy revealed that the corrosion products that formed indoors were mainly chalcocite with cuprite only at and near the surface. In contrast, the corrosion products that formed outdoors were a mixture of chalcocite and cuprite. These differences in corrosion products are attributed to the differences in relative humidity during exposure.