Patina formation

Fig. 19.13 Equipment for studying patina formation on low-alloy steels (after Pourbaix " )...

MIC of Materials. Many cases have been documented of the biodeterioration by bacteria and/or fungi of architectural building materials, stonework, fiber-reinforced composites, polymeric coatings, and concrete.66 Biodeterioration then proceeds by the processes of staining, patina formation, pitting, etching, disaggregation, and exfoliation. (Dexter)5... 

Other Cu-based alloys Dealloying phenomena have also been discussed for Cu alloys from the Cu—Ni, Cu—Mn, and Cu—Sn systems [45, 84, 85]. In the case of long-term corrosion of Sn-bronze (a-Cu—Sn) in natural environments, which is obscured by complex patina formation, it has been shown that the relevant dealloying process is decuprification rather than destannification (as formerly assumed). 

Kihira, H., Ito, S., Murata, T. The behavior of phosphorous during passivation of weathering steel by protective patina formation, Cotr. Sci. 31 383-388 (1990)... 

This is the most firequently observed form of atmospheric corrosion, where the water layers or pockets are formed on the metal surface, and the metal surface remains constantly in contact with water. The rate of corrosion would depend on the solubility of the corrosion product Higher solubility means a higher rate of corrosion, because the dissolved ions increase the electrolytic conductivity. In case of alternate dry and wet conditions, the dry corrosion product film may absorb moisture fi om the air which increases the rate of corrosion of the metal by bringing the moisture in contact with the metal surface. Patina formation on copper, such as brochantite, and corrosion of iron and steel structures are common examples of corrosion caused by wet atmosphere. 

On exposure to the atmosphere, copper and copper alloys are known to form a thin layer of greenish blue or brownish green corrosion product, called patina. It has initially a dark color which turns eventually green with a greater exposure period. The composition of patina is very complex. The time scale of patina formation varies with environment. 

The chemistry of patina formation in copper is highly complex. With the limits in the existing knowledge and the complexity of the process it is not easy to completely understand the patina formation mechanism. 

Patina Formation of corrosion products, such as green colored brochantite on copper. 

Coloring. Of the three basic processes for accelerated patina formation, one uses a sulfate solution and two utilize chloride salts. 

Another ak pollutant that can have very serious effects is hydrogen sulfide, which is largely responsible for the tarnishing of silver, but also has played a destmctive role in the discoloration of the natural patinas on ancient bronzes through the formation of copper sulfide. Moreover, a special vulnerabihty is created when two metals are in contact. The electromotive force can result in an accelerated corrosion, eg, in bronzes having kon mounting pins. 

Bronze disease necessitates immediate action to halt the process and remove the cause. For a long time, stabilization was sought by removal of the cuprous chloride by immersing the object in a solution of sodium sesquicarbonate. This process was, however, extremely time-consuming, frequentiy unsuccesshil, and often the cause of unpleasant discolorations of the patina. Objects affected by bronze disease are mostiy treated by immersion in, or surface appHcation of, 1 H-henzotriazole [95-14-7] C H N, a corrosion inhibitor for copper. A localized treatment is the excavation of cuprous chloride from the affected area until bare metal is obtained, followed by appHcation of moist, freshly precipitated silver oxide which serves to stabilize the chloride by formation of silver chloride. Subsequent storage in very dry conditions is generally recommended to prevent recurrence. 

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. 

Copper and the Copper Alloys. Copper and its alloys are relatively resistant to corrosion dry, unpolluted air rarely affects them at normal temperatures surfaces of the metal or its alloys exposed to polluted air, even under ordinary atmospheric conditions, however, are tarnished by pollutants such as hydrogen sulfide and/or carbon dioxide. Given sufficient time, the activity of the pollutants result in the formation of a usually green layer, known as patina, which coats and surrounds the bulk of the metal or alloy (see Fig. 40). If the patina is chemically stable, that is, if it is hard, is non-porous, and covers the entire surface of an object, it protects the underlying metal core from further corrosion. Such a patina consists mostly of basic... 

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. 

Robbiola L, Blengino J-M, Eiaud C (1998) Morphology and mechanisms of formation of natural patinas on archaeological Cu-Sn alloys. Corros Sd 40 2083-2 111. 

Pure aluminum has a chemical property in common with copper and iron. It reacts with oxygen in air to form a different substance with different properties. This substance is called aluminum oxide. Copper has the same chemical property. The substance that results when copper reacts with oxygen is called a patina. Similarly, iron reacts with oxygen to form rust. Do research to compare the properties and uses (if any) of aluminum oxide, copper patina, and rust. What technologies are available to prevent their formation What technologies make use of their formation ... 

Recent surface enhanced Raman scattering (SERS) measurements, carried out during the reduction of CO2 on the Cu surface [29], provide some insight into a series of events associated with deactivation of the electrode. These include (i) the time-dependent decay of the SERS bands corresponding to adsorbed CO paralleled by (ii) the increase of a new band attributed to the formation of a "patina" like species (including copper oxide, hydroxide and carbonate). At this stage, the latter surface compound appears as the most likely poisoning species. 

It is a well known fact that formation of the green patina takes a substantially shorter time in urban than in rural atmosphere, where often very long time elapses before the surface is covered by patina or in very pure atmospheres the surface remains covered by a black oxide layer. Also the corrosion rate in rural atmosphere is usually lower (<1 um/year) than in urban or industrial atmospheres (1-3 /Lim/year) (8 . 

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