Atmospheric corrosion benefits

The atmospheric corrosion data in Table 4.34 (and also Table 13.8) is related to historic environments. Current use in the industrial areas listed with acidic pollution would show much lower corrosion rates as the corrosion of zinc in the atmosphere is essentially related to the SOj content (and the time of wetness) and in many countries the sulphurous pollution has been greatly reduced in the past 20 years. Zinc also benefits from rainwater washing to remove corrosive poultices thus, although initial corrosion rates are usually not very different on upper and lower surfaces, the latter tend —with time—to become encrusted with corrosion products and deposits and these are not always protective. 

Equipment at high or low temperatures is insulated to conserve energy, to keep process conditions from fluctuating with ambient conditions, and to protect personnel who have occasion to approach the equipment. A measure of protection of the equipment metal against atmospheric corrosion also may be a benefit. Application of insulation is a skilled trade. Its cost runs to 8-9% of purchased equipment cost. 

Chromium is a metal that readily forms an oxide that is transparent and happens to be extremely resistant to further degradation. As a further benefit to alloying with steel, it is less noble than iron and thus tends to form its oxide first. Increasing the chromium content in steel gradually above about 2% improves mild atmospheric corrosion resistance steadily up to a level of about 12% where corrosion is essentially arrested. For exposure to mild, wet environments the addition of about 11% chromium is sufficient to prevent "rusting" of steel components, hence the term stainless. 

The atmospheric pollution prevailing in special industrial or laboratory locations may induce more severe corrosion, e.g. the vapours from concentrated hydrochloric or acetic acid will etch tin, and moist sulphur dioxide will produce a sulphide tarnish, as will hydrogen sulphide at temperatures above about 100°C. The halogens attack tin readily. The commonly used volatile corrosion inhibitors are without adverse action although the benefit derived from their use is doubtful. 

The results from lengthy exposure periods are of course more relevant for practical purposes e.g. for classification of the corrosivity of atmosphere on a given location or for cost-benefit analysis. In this case the corrosion rate may be assessed from yearly mean values of the concentration of pollutants and from the time-of-wetness class estimated from meteorological measurements. 

Because of the relatively high resistivity of atmospherically exposed concrete substructures, most anodes utilize impressed current to achieve the necessary driving voltages to supply the current required for corrosion control. However, an exception to this is the use of sacrificial zinc anodes for CP of coastal bridges in Florida, which have a relatively low concrete resistance. However, studies continue to examine the use of sacrificial anodes because of the benefit of its low maintenance compared to impressed-current CP systems. Two of these studies are the following ... 

The presence of atmospheric SO3 is therefore detrimental to the MgO inhibition of V2O5 corrosion (Hancock 1982). The injection of dispersed magnesia additive (Blauenstein 1977) yielded many benefits on a 300-MW oil-fired boiler (1) improved combustion as indicated by the ability... 

The service temperature is normally duplicated exactly in laboratory simulations. Unlike assware experiments, HT/HP corrosion tests can be conducted at temperatures above the atmospheric boiling f)oint of the test environment. This is an important benefit of HT/HP testing which normally results in better simulation of actual service conditions. However, problems may arise when the corroding system is not isothermal as in the case under heat transfer conditions commonly found in many types of industrial process equipment such as heat exchangers and dynamic process vessels. Temperature can also be used to accelerate reactions in the laboratory. This can have its drawbacks which... 

Usually, there are always some minor leakages of air in industrial processes. Another source of oxygen may be the organic binders, which decompose at relatively low temperatures and might become the constituent of a nitrogen atmosphere. It is quite common for silicon oxynitride to appear in the final composition. Later we will discuss the benefits of silicon oxynitride in corrosion and oxidation resistance. 

This technology offers the benefits of a flux for successful o9xide removal, working at atmospheric pressure while avoiding the disadvantages of postbraze treatments and corrosion susceptibility. The nonhygroscopic and noncorrosive potassium fluoroaluminates are used as fluxes [76]. 

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