Environment/atmosphere metal dusting

Metal dusting of heat-resistant alloys (Ni-Cr-Fe) occurs in carbonaceous atmospheres at temperatures above 800°F (426.7°C), with very high activity in the temperature range of 1450°F to 1550°F (787.7° to 843.3°C) and above 1700°F (926.6°C). In metal dusting environments, a black sooty residue (graphite) usually covers the surface in most of the wasted areas. The residual metal is usually thinned and contains multiple rounded pits. The originally austenitic matrix... 

In strongly carbon-reducing atmospheres (e.g., carbon monoxide) at high temperatures, carburization of stainless steel takes place. In oxidizing atmospheres, such as steam or carbon dioxide, carbon may be selectively removed (decarburization). Usually, complex gas mixtures are involved and the net result of the H2/H2O and CO/CO2 is critical. Under some conditions of environment and temperature, a pitting-t) e phenomenon called "metal dusting" occurs. 

Al-Rajhi, M.A., al-Shayeb, S.M., Seaward, M.R.D., and Edwards, H.G.M. 1996. Particle size effect for metal pollution analysis of atmospherically deposited dust. Atmospheric Environment 3Q y. A5- 53. 

The name dust , is used in a variety of ways, and with different meanings. These range from the material that accumulates on the earth s surface, such as on streets and in living and working environments, to the particulate material suspended in the atmosphere. In this paper I wish to consider these two materials in terms of their chemical composition, sources and relationship between them. The names used for the two materials will be surface dust and atmospheric dust . The word aerosol may also be used for atmospheric dust but it more properly applies to the finer particles of atmospheric dust and includes liquid aerosol (i). Botfi surface and atmospheric dusts are increasingly seen to be a hazard to human beings as they are a source of intake of toxic materials such as heavy metals. For this reason study is important of the composition and sources of the dusts. 

Strontium is a naturally occurring element found in rocks, soil, dust, coal, and oil. Naturally occurring strontium is not radioactive and is referred to as stable strontium. Stable strontium in the environment exists in four stable isotopes, " Sr (read as strontium 84), Sr, Sr, and Sr. Twelve other unstable isotopes are known to exist. Its radioactive isotopes are Sr and °Sr. Strontium is chemically similar to calcium. It was discovered in 1790. The isotope Sr is a highly radioactive poison, and was present in fallout from atmospheric nuclear explosions and is created in nuclear reactors. Atmospheric tests of nuclear weapons in the 1950s resulted in deposits and contaminations. °Sr has a half-life of 28 years and is a high-energy beta emitter. Its common cationic salts are water soluble it forms chelates with compounds such as ethylenediaminetetraacetic acid strontium coordination compounds are not common. Powdered metallic strontium may constitute an explosion hazard when exposed to flame. 

Atmospheric corrosion is an electrochemical process with the electrolyte being a thin layer of moisture on the metal surface. The composition of the electrolyte depends on the deposition rates of the air pollutants and varies with the wetting conditions. The factors influencing the corrosivity of atmospheres are gases in the atmosphere, critical humidity and dust content. Two rural environments can differ widely in average yearly rainfall and temperature and can have different corrosive... 

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. 

Outdoor storage of aluminium, even under a tarpaulin, is generally not desirable for long periods of time this varies with the alloy, the end product, and the local environment. Moisture can collect on the surface, sometimes at relative humidities helow the dew point, because of the hygroscopic nature of the dust or particles that deposit on the metal from the atmosphere. 

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