Atmospheric corrosion zinc, mechanisms

Peitknecht, W. (1952). Uber den Zusammenbruch der Oxydfilnre auf Metallo-berflachen in sauren Dampfen und den Mechanisms der atmospharischen Korrosion (The breakdown of the oxide films on metal surfaces in acid vapours and the mechanism of atmospheric corrosion). Chimia, 6, 3-13 (in German) Zinc Abst., 54/224. 

The protection mechanism is similar to that of metallic zinc coatings zinc is less noble than steel and protects the substrate by forming a galvanic corrosion cell, in which zinc is the anode. This mechanism is particularly active in presence of defects in the coating that provide an electrolyte path to the substrate surface. In addition, the atmospheric corrosion of zinc yields voluminous solid corrosion products (oxides, carbonates, etc.) that are capable of blocking pores or small defects in the coating, thereby reinforcing its barrier effect. 

This system of atmospheric classification is now being revised to create a new approach based on dose-response functions for steel, copper, and zinc. Because the corrosion of aluminum occurs by a pitting or localized mechanism, the traditional approach of using mass loss to determine severity of attack is often misleading. Atmospheric corrosion problems with aluminum alloys are most frequently a result of metallurgical conditions rather than environmental conditions, and the behavior of aluminum may be excluded in the upcoming revision of the ISO 9223-6 documents. 

In this work, the role of NO2 in the atmospheric corrosion of zinc was analyzed from a detailed characterization of corrosion products. Laboratory tests with exposru e parameters close to the conditions observed in real atmospheres were performed, with the aim of simulating close-to-reality corrosion mechanisms. Low-pollutant concentrations and shortterm exposmes were carried out. For these reasons, XPS were used for the analysis of very thin corrosion layers formed. 

The analysis by XPS can confirm that the ZnO was always present initially on the surface of zinc although the probe had not been exposed in the ehamber. A very thin film of this oxide forms instantaneously by chemical oxidation on the zine surfaee in contact with clean air at room temperature. This film does not affeet the later eleetroehemical corrosion process [5]. Once the humidity layer is established, zinc hydroxide is rapidly formed on the ZnO film via an electrochemical mechanism. Generally, it is considered that hydroxides are the initial compounds in zinc atmospheric corrosion studies [5, 15]. 

Access to new and more sensitive analytical techniques has resulted in substantial progress in the characterization of corrosion products formed under both laboratory and field exposure conditions. These techniques permit the determination of, e.g., thickness, chemical composition, and atomic structure of corrosion products formed at both early and later stages of exposure. When combined with environmental data, such as deposition rates of corrosion-stimulating atmospheric constituents, relative humidity, temperature, and sunshine hours, the new techniques have resulted in a more comprehensive understanding of the complex processes that govern atmospheric corrosion. In a series of papers, Graedel has summarized the corrosion mechanisms of zinc [62], aluminum [63], copper [18], iron and low-alloy steel [64], and silver [19]. It is beyond the scope of this chapter to provide... 

Zinc and zinc-coated products corrode rapidly in moisture present in the atmosphere. The corrosion process and its mechanism were studied in different media, nitrate [283], perchlorate [259], chloride ions [284], and in simulated acid rain [285]. This process was also investigated in alkaline solutions with various iron oxides or iron hydroxides [286] and in sulfuric acid with oxygen and Fe(III) ions [287]. In the solution with benzothia-zole (BTAH) [287], the protective layer of BTAH that formed on the electrode surface inhibited the Zn corrosion. 

In situ Raman spectroscopy is being used to investigate corrosion products from zinc in a humid atmosphere and sodium chloride70 and from Type 304L stainless steel in aerated water at elevated temperatures and pressures.71 The changes in detected species over time helped identify possible corrosion mechanisms and the effect of different variables on corrosion rates and mechanisms. 

In a study of zinc-coated steel covered with a polymer topcoat, the mechanism of topcoat delamination was elucidated with high spatial resolution [216]. Depending on the details of the defect and the composition of the corroding atmosphere, the rate and type of delamination could be described. A similar study with a coated iron surface has been reported [217]. A comparison of results obtained with SKP, electrochemical impedance measurements and cyclic voltammetry with respect to validity as a corrosion prediction tool has been reported [218]. 

Whilst galvanising is an excellent corrosion protection system, it is also an expensive process. Moreover the mechanism of corrosive attack, specifically in a potash environment, is fundamentally different to zinc protection in atmospheric conditions. Where zinc is exposed to normal atmospheric conditions, insoluble oxide and carbonate deposition on the zinc surface maintain the protective layer. The principle corrosion product of zinc in a potash environment is zinc chloride. This is a soluble material which will be lost to the brine. The protection of the zinc would therefore have a limited lifespan which in this case may not be a cost effective solution. 

In natural atmospheres, once the moisture layer has been established, zinc hydroxide rapidly forms on this film, in this case due to an electrochemical mechanism. The formation of a moisture layer of sufficient thickness, together with the action of atmospheric CO2, leads to the formation of basic zinc carbonates from the initially formed hydroxide [3, 5]. Both the hydroxide and the carbonates are very stable and have a protective character, and they therefore tend to inhibit zinc corrosion in atmospheres without contamination. However, if the... 

In immersion conditions, the time of protection depends on the zinc content in the film and on its dissolution rate. The mechanism is different for films exposed to the atmosphere, because after the cathodic protection in the first stage, the action is restricted substantially to a barrier effect (inhibition resistance) generated by the soluble zinc salts from corrosion by sealing the pores controlling access to water, water vapor and various pollutants. Due to the... 

T. E. Graedel, Corrosion mechanisms for zinc exposed to the atmosphere, J. Electrochem. Soc. 75d 193C (1989). 

Describe very briefly the corrosion mechanism of zinc in an atmosphere containing SO2 and chlorides. 

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