Atmospheric corrosion environmental factors

In addition, upon exposure to atmospheric and environmental factors, even an SRB-induced corrosion may produce corrosion products that are in colors other than black (such as reddish brown, which could be hematite-ferric hydroxide). Figure 4.34 shows a case of SRB-induced MIC where orange tubercles were the main corrosion products available. 

Chlorides are often found as the salt aerosols of the atmosphere, and consequently may strongly influence the corrosion performance of structures and plant, particularly in marine or coastal situations. This influence on corrosivity reduces proportionately with distance from the seawater surface, though local environmental factors such as prevailing wind direction, level... 

The results obtained with this equipment show that the corrosion rate in the rig is about four times that encountered in an industrial UK atmosphere. This acceleration, however, is not achieved by accentuating any of the environmental factors, but rather by holding them near to the worst natural conditions for as long as possible. The procedure used ensures that the rust film is completely dried for short periods, thus simulating the conditions that bring out the beneficial effects of protective rust films on the steels under study. 

The four important areas of application of carbon steels are (i) atmospheric corrosion (ii) corrosion in fresh water (iii) corrosion in seawater and (iv) corrosion in soils. The atmospheric corrosion of steel is caused by major environmental factors such as (i) time of wetness as defined by ISO 9223-1992 (ii) sulfur dioxide in the atmosphere due to the combustion of fossil fuels and (iii) chloride carried by the wind from sea. The equations for corrosion rates of carbon steel by multiple regression analysis have been obtained.1... 

The influence of mainly SO on the corrosion rate of several materials has been shown in numerous national exposure programs. During the last decades a number of empirical relations have been derived from measurements of atmospheric corrosion rates of the most important structural metals and from measurements of environmental factors. The results are usually presented in form of equations including pollution and meteorological parameters (5.). 

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. 

The environmental factors that tend to accelerate metal loss include high humidity, high temperature and proximity to the ocean, extended periods of wetness and the presence of pollutants in the atmosphere. The small amount of carbon dioxide normally present in the air neither initiates nor accelerates corrosion. Vernon [57] was the first to study the corrosion rate of steel coupons in the presence of well-defined atmospheres. Atmospheric gases such as CO2, SO2, NO2, HCl, etc. after getting dissolved in the moisture layer on the metal surface, these gases result in a number of ions and ionic species like H", CF, COa , NOa , S04 , etc. They measured corrosion rate was as a function of time, relative humidity and... 

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. 

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. 

S. D. Cramer, S. A. Matthes, B. S. Covino Jr., S. J. Bullard, and G. R. Holcomb, Environmental factors affecting the atmospheric corrosion of copper, ASTM Special Techn. Publ. 1421 245 (2002). I. Odnevall Wallinder, P. Verbiest, W. He, and C. Leygraf, Effects of exposiue direction and inclination of the runoff rates of zinc and copper roofs, Corros. Sci. 42 1471 (2000). 

It has long been recognized that local environmental characteristics influence the rates of material corrosion. After two years of measurements at 39 sites in Europe and North America, significant relationships have been shown between corrosion rates of building materials and atmospheric pollutants( 5). While direction of exposure relative to weather and other factors such as frequency and duration of wetting significantly influence corrosion, Kucera (46) has shown that sulphur oxides are strongly correlated with deterioration of structural materials. 

A project was undertaken to perform retrospective reconstruction of environmental histories at the sites of previous long term atmospheric metal exposures. The effort required development of appropriate emission information and dispersion modeling capabilities on both the regional and urban spatial scales. The development of useful urban scale emission inventories dating back several decades proved to be a limiting factor. At present, therefore, the retrospective reconstruction of environmental histories is not possible for the large number of sites in the metals corrosion data base. This precludes derivation of damage functions at this time. 

A critical but poorly understood factor related to atmospheric exposure is the connection between nominal environmental conditions (e.g., humidity, ultraviolet radiation, pollution, atmospheric particles, and so on) and the actual chemistry of the environment on the material s surface. Time-of-wetness is known to be an important parameter in outdoor exposure, given that water associated with precipitation or condensation is critical to the corrosion processes. However, accurate prediction of corrosion rates depends on knowing how the water on the surface affects the concentration of all the important corrosive species. 

Atmospheric exposure consists of placing specimens such as coupons, parts, components, on racks at stationary test sites [53]. The example in Fig. 7 shows bumpers imder test at the marine splash and spray facility of the LaQue Center for Corrosion Technology at Wrightsville Beach, North Carolina. The nature of the site varies depending on geographic area and specific location within an area. Factors such as environmental chemistry (chlorides, pollutants. 

There is a wide variety of these experiments, which can range from the simplest, conducted in cabinets with a constant temperature and humidity, to other more sophisticated ones which involve the incorporation of factors that speed up corrosion, such as UV radiation, temperature cycles, wetting and drying, aerosol spraying, incorporation of salts, changes in pH, etc. Thus, the appropriate selection ofthe accelerated experiment and ofthe environmental conditions inposed in the cabinet makes it possible to develop the corrosion process that one wishes to study. Table 19-3 shows a set of normalized experiments that give a correlation between the results obtained in real atmospheres and in the laboratory for different materials (Haynes, 1995). 

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