Factors Affecting Atmospheric Corrosion

Ceramics are subject to the same general forms of corrosion as metals and the same factors of atmospheric corrosion that affect metals also affect ceramics. Table 11.2 lists the atmospheric corrosion resistance of selected glasses and Table 11.3 does likewise for other selected ceramics. 

Shaw, T. R. In "Atmospheric Factors Affecting the Corrosion of Engineering Metals" ASTM SPP 646, 1978. 

Atmosphere corrosion is the general term for all of corrosion phenomena occurring in air. The vaporized water (humid component) forms very thin water films on materials surfaces and the electrochemical reactions leading to corrosion proceed in the thin water film. There are many environmental factors existing in thin water films and they affect the corrosion mechanism. 

W. Ailor, in Atmospheric Factors Affecting the Corrosion of Engineering Metals, S. Coburn, editor, STP 646, American Society for Testing and Materials, Philadelphia, 1978, p. 129. 

Metallurgical factors affect metal loss and tend to corrode at a lower rate with higher alloy content. Atmospheric corrosion resistance of steel was improved by alloying with Cu, P or Cr to form passive oxide layer [50]. Studies have shown that these steels show superior corrosion resistance in particular during atmospheric exposure but not so much for immersed exposure as in seawater and close to the coastline in the presence of high chloride concentrations. Alloying elements like... 

The control of atmospheric corrosion is due to the formation of films comprising basic salts, notably carbonate (Tables 2.16 and 2.17). The most widely accepted formula is 3Zn(OH)2 2ZnC03, which may be written Zn5(0H)6(C03)2- Environmental conditions that interfere with the formation of such films, or conditions that lead to the formation of soluble films, may cause quite rapid attack on zinc. One most important factor affecting the corrosion of zinc in the atmosphere is the duration and frequency of moisture contact. 

German, G. (1978). Corrosion prevention with thermal-sprayed zinc and aluminum coatings. Proc. Anniversary Symposium, Atmospheric Factors Affecting the Corrosion of Engineering Metals, ASTM STP 646. ASTM, Philadelphia, pp. 74-82. 

The initial corrosion rate of a copper coating is dependent on atmospheric conditions such as time of wetness and type and amount of pollutants. Time of wetness is the most important factor affecting the corrosion rate of copper. The corrosion rate of copper usually obeys parabolic law ... 

Guttman, H. and Sereda, P. J., "Measurement of Atmospheric Factors Affecting the Corrosion of Metals, Metal Corrosion in the Atmospheres, ASTM STP 435, ASTM International, West Conshohocken, PA, 1968, pp. 326-359. 

Atmospheric—Atmospheric corrosion is responsible for a large fraction of the total corrosion in the world. Factors that affect the atmospheric corrosion of materials in a marine environment are the time of wetness, temperature, material, atmospheric contaminants and pollutants, solar radiation, composition of the corrosion products, wind velocity, and biological species [fO]. Atmospheric corrosion of a passive alloy tends to be localized. For electrochemical processes related to corrosion to occur, an electrolyte must be present to allow current to pass via diffusion and electrochemical migration of cations and anions. Seawater is a very conductive electrolyte. The severity of corrosion in an atmospheric environment is related to the time of wetness during which electrochemical processes and corrosion take place. There is also a direct relationship between atmospheric salt content and measured corrosion rates [/O]. 

As a final precaution, whatever the test atmosphere, single data points emd single analytical measurements can be potentially highly misleading. Corrosion rates and modes vary with time. Data as a function of extended time are vital to define true performance in engineering applications. Because of the multiplicity of corrosion modes that can occur simultaneously in many environments, it is important to use as memy analytical techniques as practical to fiiUy understand all factors affecting performance. 

The main environmental factors that govern the corrosion rate of steel in the atmosphere are temperature, time-of-wetness, and type and amount of chemical contamination. Time-of-wetness includes not only time when the steel is wet from precipitation or dew, but also time above a certain critical humidity," which can vary from about 50-70 % relative humidity, depending upon the contaminants present in the air [3]. For most inland sites, the most important chemical contaminant affecting atmospheric corrosion of steel is sulfurdioxide. At coastal sites, and locations where de-icing salts are used, chloride content is most important. 

One of the most important factors affecting atmospheric corrosion is the presence of specific pollutants. In areas having low atmospheric pollution, corrosion rates are correspondingly low. The presence of atmospheric pollutants such as the various oxides of nitrogen, sulfur-containing compounds, chlorine-containing compounds, and other less common pollutants will stimulate corrosion. 

Atmospheric corrosion differs from the action that occurs in water or underground in that a plentiful supply of oxygen is always present. In this case, the formation of insoluble films, and the presence of moisture and deposits from the atmosphere become the controlling factors. The presence of contaminants such as sulfur compounds and salt particles also affects the corrosion rate. Nevertheless, atmospheric corrosion is mainly electrochemical, rather than a direct chemical attack by the elements. The anodic and... 

In the previous chapters the factors affecting the mechanisms of atmospheric corrosion have been discussed. However, these factors and mechanisms have different effects and react in different manners with different metals and alloys. Each metal forms its own protective film of corrosion products, some of which offer greater protection than others. These will be discussed in this chapter. 

As discussed in Chapter 2, there are several factors that affect the atmospheric corrosion rate. The most important are the time of wetness and the pollution of the atmosphere, primarily SO2, and in marine locations chlorides. 

The same factors that promote atmospheric corrosion of metals, time of wetness, atmospheric types, initial exposure conditions, sheltering, wind velocity, and the nature of corrosion products also affect the atmospheric corrosion of ceramics. 

Based on the differences between the indoor and outdoor factors affecting atmospheric corrosion rates, it follows that the corrosion rate of many metals is lower indoors than outdoors. This has been verified by examining... 

The deterioration of metals, commonly referred to as corrosion, is a critical factor affecting the useful life of metals in facilities. Most importantly, the rate of corrosion affects how long a particular metal component wiU function in its intended use. Some metals corrode at a very slow rate, which makes them good candidates for certain applications. For example, aluminum has been found to be an excellent material for hatch covers in atmospheric exposures. On the other hand, the use of 304 stainless steel for pipe hangers in piers over salt water has resulted in failures within a year. The application, the environment, and the intended service are critical to proper material selection. 

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