Corrosion materials purity

Concentrations over 8 wt % are classified as corrosive Hquids by the Department of Transportation (DOT). The Bureau of Explosives regulation (132) classifies all solutions containing 20 wt % and greater as oxidizers and corrosives. The product containers must have identifying labels (dmms) or placards (tank cars, tank trailers) indicating that the contents are an oxidizer and corrosive material, UN 2014 or UN 2015. Bills of lading must also be so identified. Tank cars and tank trailers are constmcted from high purity aluminum or 300 series stainless steel. 

This input to design refers to the long-term stability of the raw material sources for the plant. It is only of importance where the raw materials can or do contain impurities which can have profound effects on the corrosivity of the process. Just as the design should cater not only for the norm of operation but for the extremes, so it is pertinent to question the assumptions made about raw material purity. Crude oil (where HjS, mercaptan sulphur and napthenic acid contents determine the corrosivity of the distillation process) and phosphate rock (chloride, silica and fluoride determine the corrosivity of phosphoric acid) are very pertinent examples. Thus, crude-oil units intended to process low-sulphur crudes , and therefore designed on a basis of carbon-steel equipment, experience serious corrosion problems when only higher sulphur crudes are economically available and must be processed. 

SBA is a colorless, high boiling point (212°F) liquid with a pleasant odor. TBA, on the other hand, is a white solid (melting point is 78°F) with a camphor-like odor. Both alcohols are traded as technical grade (99% purity) and need a hazardous (corrosive) materials label. 

Design material of corrosion not for structural integrity material of corrosion for purity of product corrosion allowance and special processing for corrosion resistance. 

Importance of materials purity in improving the corrosion resistance... 

The evolution of stainless steel composition can be used to illustrate the importance of materials purity in reducing corrosion susceptibility. Chromium and molybdenum are the key elements in promoting resistance to pitting and crevice attack of stainless steels, but high chromium and molybdenum concentrations are not sufficient to ensure an adequate corrosion resistance. Low concentrations of impurities, like carbon, silicon, phosphorous and sulphur, are required. Type 316L and 316LVM stainless steels are commonly employed to fabricate a variety of fracture fixation devices. They both have low carbon concentration, below 0.03 wt%, which is indicated by the letter L. VM stands for vacuum-melted, a technique... 

The indirect hydration, also called the sulfuric acid process, practiced by the three U.S. domestic producers, was the only process used worldwide until ICI started up the first commercial direct hydration process in 1951. Both processes use propylene and water as raw materials. Early problems of high corrosion, high energy costs, and air pollution using the indirect process led to the development of the direct hydration process in Europe. However, a high purity propylene feedstock is required. In the indirect hydration process, C -feedstock streams from refinery off-gases containing only 40—60 wt % propylene are often used in the United States. 

Acid corrosion presents a problem in isopropyl alcohol factories. Steel (qv) is a satisfactory material of constmction for tanks, lines, and columns where concentrated (>65 wt%) acid and moderate (<60° C) temperatures are employed. For dilute acid and higher temperatures, however, stainless steel, tantalum, HasteUoy, and the like are required for corrosion resistance and to ensure product purity (65). 

The use of the 99-8% and 99-99% grades is usually confined to those applications where very high corrosion resistance or ductility is required. The chemical industry can advantageously use these purities for handling some products, but because of their low mechanical strength they are sometimes used as a cladding material for a stronger substrate. 

With increasing purity of aluminium, greater resistance to corrosion is developed. On high-purity materials, however, any pits which develop are likely to be deeper though fewer in number than those formed in more impure metal. In some special applications, notably in contact with ammonia solutions or pure water at elevated temperatures and pressures, the iron and silicon present in commercial-purity metal are beneficial and retard corrosion. Up to about 5% magnesium improves the corrosion resistance to sea-water. 

Contamination in this context refers specifically to the debasing of the purity of steam and condensate. Contamination may occur in the presence of unwanted steam volatile materials (such as silica volatiles), minerals carried over with BW, oil and process materials infiltrating the steam-water circuit, or by the results of corrosion processes. 

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