Urban or Industrial Atmospheres

An aesthetic problem is the gradual darkening and eventual blackening that occurs if there is no regulation on smoke emissions. A noted example is the performance in U.S. cities, such as Pittsburgh, before and after smoke control regulations. This discoloration problem is not limited to aluminum and occurs on other metals, masonry, and stone. 

Almost all of the aluminum alloys, at least with appropriate protective measures, will provide usable corrosion resistance in urban/industrial environments. Choice of alloy, therefore, is generally made on other engineering requirements, plus cost and availability. 

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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 . 

Both the acids and the salts stabilize the aerosol particles, which eventually settle from the air or dissolve in larger raindrops. Nitrogen dioxide, besides causing the formation of ozone, is a primary cause of haze in urban or industrial atmospheres because of its participation in the process of aerosol formation. 

The protection afforded in rural atmospheres is greater than that in urban or industrial atmospheres. In the latter area there is a greater concentration of industrial pollutants. The air in these areas is contaminated with various sulfur compounds, which together with the moisture in the air convert the normally impervious corrosion-resistant zinc carbonate and zinc oxide layer into zinc sulfate and zinc sulfite. These water-soluble compounds have poor adhesion to the zinc surface and therefore are washed away relatively easily by rain. This exposes the underlying surface to attack by oxygen in the air. 

Urban or industrial atmospheres are those atmospheres that are high in CO and CO2, sulfates and sulfites, and possibly various N,0 gases. The corrosion rate is largely the same for unalloyed aluminum as with most conventional aluminum alloys (alloys with Zn, Mg, Si, and/or Mn). Aluminum alloys with copper will exhibit corrosion rates 4-20 times higher. When the SO2 concentration is less than 0.01% by volume, the atmospheric corrosion rate, even at a relative humidity of 98%, is negligibly affected by the presence of the SO2. This is probably due to the low adsorption tendency for SO2 on aluminum surfaces. When the SO2 concentration exceeds 0.01% by volume, severe corrosion effects occur. 

When exposed to the atmosphere over long periods of time, copper will form a coloration on the surface knovm as patina, which in reality is a corrosion product that acts as a protective film against further corrosion. The length of time required to form the patina depends on the atmosphere because the color is due to the formation of copper hydroxide compounds. Initially, the patina has a dark color, which gradually turns green. In urban or industrial atmospheres, the compoimd is a mixture of copper/hydroxide/sulfate and in marine atmospheres it is a mixture of copper/hydroxide/chloride. It takes approximately 7 years for these compoimds to form. When exposed to... 

The air emissions of fossil fuel combustion are dispersed and diluted within the atmosphere, eventually falling or migrating to the surface of the Earth or ocean at various rates. Until recently, most attention was focused on the so-called primary pollutants of fossil fuel combustion that are harmful to human health oxides of sulphur and nitrogen, carbon monoxide, suspended particles (including soot), heavy metals, and products of incomplete combustion. These pollutants are most concentrated in urban or industrialized areas close to large or multiple sources. However, the primary pollutants may interact with each other, and with atmospheric constituents and sunlight, forming secondary pollutants that disperse far beyond the urban-... 

As can be seen from the table, a G90 sheet, which has a 1 mil zinc coating, cannot be used for a roof having a durability of 10 years in any atmosphere except in a rural area. Were this sheet to be used in an urban, marine, or industrial atmosphere, it would have to be painted for protection. 

In years gone by zinc coating of steel (galvanizing) was considered a satisfactory corrosion-resistant material for structures in outdoor atmospheres. The corrosion rate was only 0.5-1.0 xm/year. However, the corrosion rate has been increasing in many urban or industrial areas as a result of increasing pollution by SO2. Corrosion rates have reached 5 xm/year or more in many areas. Because of this it is often necessary to apply to galvanized steel a coat of anticorrosion paint for added protection. 

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. 

All the atmospheric Hg data collected for this study are summarized in Table L Atmospheric Hg values reported by other groups are also included for comparison. The atmospheric Hg data measured at CC and SC were similar to those measured at other urban or industrial areas. At a certain site, both RGHg and Hg-P exhibited higher temporal variability compared to THg, as indicated by the ratio between the standard deviation and the mean value. Detailed analyses of our data are presented in the following sections. 

Area sources of either a selected chemical or a precursor present a common problem for modeling. In particular, the rich and complex patterns of hydrocarbon emissions from general urban and industrial sources either include or might produce through atmospheric photochemical reactions some of the species on the analysis list. The treatment of such species in photochemical airshed modeling is difficult (8, 9). The effort required for any one such exercise is substantial, and the effort required for a comprehensive analysis of all urban regions relevant to this program would be prohibitive. 

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. 

The chemical composition of groundwaters depends on many causes rainwaters have a notable chemical charge, especially in marine salts (M. Schoeller, 1962) and in elements which originate from atmospheric dusts (silts, clays, pollen and bacteria) or from urban and industrial activity (Crozat, 1978 Dessfevre Delepoule, 1978). 

A high sulfuric acid content of industrial and urban atmospheres shortens the life of metal structures (see Tables 9.2 and 9.3). The effect is most pronounced for metals that are not particularly resistant to sulfuric acid, such as zinc, cadmium, nickel, and iron. It is less pronounced for metals that are more resistant to dilute sulfuric acid, such as lead, aluminum, and stainless steels. Copper, forming a protective basic copper sulfate film, is more resistant than nickel or 70% Ni-Cu alloy, on which the corresponding hlms are less protective. In the industrial atmosphere of Altoona, Pennsylvania, galvanized steel sheets [0.381 kg zinc per m, 0.028 mm thick (1.25 oz zinc per ft, 1.1 mil thick)] began to rust after 2.4 years, whereas in the rural atmosphere of State College, Pennsylvania, rust appeared only after 14.6 years [19]. 

Atmospheres are usually classified subjectively (Table 2.1) as industrial, urban, or rural, with the classification usually corresponding to high, medium, or low sulfur dioxide, respectively. The numerical value relates both to the geographical area considered and the year environmental controls worldwide are leading to diminution in the amount of sulfur dioxide in the air. A fourth category, marine or coastal, reflects the presence of chlorides from the sea and should be subdivided into tropical and temperate areas (because of the difference in seawater composition and temperature) as well as industrial, urban, or rural. 

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