Atmospheric corrosion urban environment

Industrial environments contain sulfur compounds, nitrogen compounds, and other acidic agents that can promote the corrosion of steels. In addition, industrial environments contain a heavier loading of airborne particles, which also contribute to corrosion. Urban environments are comparable with industrial, but the amount of pollution will be less intense. Marine environments are characterized by the presence of chloride, an ion that is particularly detrimental to the corrosion resistance of steels. Rural and indoor environments are the least corrosive of the atmospheric environments. 

Atmospheric corrosion results from a metal s ambient-temperature reaction, with the earth s atmosphere as the corrosive environment. Atmospheric corrosion is electrochemical in nature, but differs from corrosion in aqueous solutions in that the electrochemical reactions occur under very thin layers of electrolyte on the metal surface. This influences the amount of oxygen present on the metal surface, since diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Atmospheric corrosion rates of metals are strongly influenced by moisture, temperature and presence of contaminants (e.g., NaCl, SO2,. ..). Hence, significantly different resistances to atmospheric corrosion are observed depending on the geographical location, whether mral, urban or marine. 

Corrosion products such as the oxides, hydroxides, carbonates, sulfates, basic sulfates hydroxy carbonates, hydroxy chlorides are formed in the various environments (marine, urban, rural, industrial) and the initially loosely bound products may become adherent in the course of time. Corrosion can occur in the pores of the corrosion product layers. The low corrosion rate observed in atmospheric corrosion, R has been expressed as ... 

The susceptibility of zinc to sulfur dioxide, a common poUutant found to cause atmospheric corrosion, was studied by Veleva et al. [67]. Galvanized steel and zinc plates were subjected to a humid tropical environment, inducing atmospheric corrosion during 2 years in rural and urban atmospheres. Runoffsamples were taken from each of the plates and analyzed to compare the results. Runoff samples taken from the galvanized steel... 

TABLE 9.1. Atmospheric Corrosive Gases in Outdoor Urban Environments [1 ]... 

The corrosion of zinc in industrial atmospheres, being essentially similar to that in an urban environment, is relatively more severe because the concentration of sulfur dioxide in the air is higher. However, the distinction... 

Whether these corrosive gas tests are reaUstic for materials other than those used for connectors or for operating electronic equipment is not clear. The test should be carried out, but the observation of no failures should not be taken to mean there will be no field failures in typical urban environments. Similarly, any failures that are observed should be carefully evaluated to ensure that the same mechanism would he operative in field situations. Connectors tire a somewhat unique part of an electronic assembly in that the active part is frequently a noble mettil and the sensitivity of the mated surfaces to failure may be lower thtin many other parts of electronic assemblies. Most failures in electronic assemblies attributable to the environment are due to ionic particle contamination in conjunction with atmospheric moisture. In 20 years of evaluating field failures in the United States, the author has never seen a failure that could be attributed to the effects of SOj, has seen a few caused by H2S or HCl, has heard of a few caused by NOx, and has seen several hundred that were caused by ionic contamination. Clearly, valid accelerated testing of electronic components, circuit boards, and assemblies must include ionic contamination. Emerging methods are discussed in the Fine Particle Testing section in this chapter. 

Atmospheric contaminants often responsible for the rusting of structural stainless steels ate the chlorides and metallic iron dust. Chlorides can originate from concrete (CaCl2) and spraying of salt on the road or from exposure to industry and marine locations. Chlorides promote pitting or crevice attack on stainless steels. As discussed above, the corrosivity of different atmospheres can differ and must be considered when materials are selected. Rural and urban environments without pollutants (chloride) do not corrode stainless steel, even in areas with high humidity. 

In iiny urban environment, the sources of trace-element contamination are manifold. As we have seen in the previous chapter, fallout from atmospheric pollution and wind-blown dusts are important sources whose effects are concentrated in urban and industrial areas. There is also incidental contamination resulting from the corrosion of metal objects and from the accidental dispersion of refuse and litter, and there is contamination resulting from the deliberate addition of waste products to the soil, such as soot, cinders, pulverised fuel ash, sewage sludge, municipal compost, or even untreated domestic rubbish. 

With the increasing concern about acidifying pollutants and their influence on atmospheric corrosion rates, scientists interest has been focused on NO2 as an additional gaseous corrosion stimulant. Whereas the SO2 concentration has shown a significant decline over the past decades in many urban and industrial areas, estimated emission of NO2 has shown a continuous increase in the same type of environment [91]. Studies in laboratories using synthetic air have provided unambiguous evidence of increased corrosion rates when NO2 is added to air containing SO2 (Fig. 11). 

The severity of atmospheric corrosion tends to vary significantly among different locations, and, historically, it has been customary to classify environments as rural, urban, industrial, marine, or combinations of these. These types of atmosphere have been described as follows ... 

In a restricted sense, corrosion is considered toconsistof the slow chemical and electrochemical reactions between melals and their environments. From a broader point of view corrosion is the slow destruction of any material by chemical agents and electrochemical reactions. This contrasts with erosion, which is the slow destruction of materials by mechanical agents. The character of the atmospheres to which materials arc exposed may he classified as rural, urban, industrial, urban-marine, industrial-marine, marine, tropical, and tropical-marine. In addition to these general kinds of environments, corrosion is of particular concern in the environments of chemical, petrochemical, and otherprocessing and manufacturing environments where extremely corrosive substances may be encountered. 

Corrosion of a particular metal may change in extent and mechanism by changing the environment. Although we may quote a given environment in discussing rates of corrosion, in reality environments may be a continuum. For example, the environment experienced by a car component may vary from salt spray to urban atmosphere to polluted industrial atmosphere or, at any one time, may be a "mix of environments. For an environment may not fit into a single compartment and may also vary in time and space. 

Laboratory corrosion tests for exfoliation corrosion susceptibility are a necessary tool for research and quality control engineers however, the validity of such accelerated tests depends on their relationship to realistic service conditions and their sensitivity to various degrees of susceptibility. The tests must be discriminating and yet not so severe as to be unrealistic. For the majority of engineered structures, exposure to outdoor atmospheres provides a baseline that is representative of many service conditions, except for structures that are subjected to unusual chemical environments. Experience has shown that seacoast conditions are more corrosive to aluminum alloys than inland urban and industrial conditions (see Fig. 2), and seacoast atmospheric exposure tests have been particularly useful for the validation of accelerated exfoliation tests [9]. 

The majority of aluminum products are used in what would be considered an outdoor atmosphere. Outdoor atmospheres are classified as seacoast, urban or industrial, and rural. Corrosion of aluminum is self-limiting in all of these environments and the rate of corrosion becomes very slow within 5 years, except for alloys and tempers that incur exfoliation. 

Investigators have found that the corrosion rates of both t) pes of materials were greater in a marine environment than in urban or rural atmospheres. The corrosion rate of galvanized steel in the marine atmospheres was found to high at the first months of exposure compared to the other sites. 

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