Atmospheric corrosion pollutants deposition

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. 

The significance of the measured properties of residual fuel oil is dependent to a large extent on the ultimate uses of the fuel oil. Such uses include steam generation for various processes, as well as electrical power generation and propulsion. Corrosion, ash deposition, atmospheric pollution, and product contamination are side effects of the use of residual fuel oil, and in particular cases, properties such as vanadium, sodium, and sulfur contents may be significant. 

Atmospheric Atmospheric corrosion due to the combined effects of rain and the deposition of salt and other pollutants will affect most equipment. Corrosion occurs while the metal surface is wet, and is strongly influenced by the composition of deposits (such as sulfates from industrial atmospheres and chlorides from marine atmospheres). External corrosion of steel and stainless steel process equipment beneath thermal insulation and fireproofing is of particular concern. 

NO in combination with SOp has a synergistic corrosion effect especially indoors on electrical contact materials, copper and steel. The influence of acid precipitation may differ for different metals and depends also on the pollution level. The atmospheric corrosion of metals due to acid deposition is in most regions mainly a local problem restricted to areas close to the pollution source. 

The great differences of the corrosion rate in restricted geografical areas have also been demonstrated by construction of corrosion maps for cities or whole countries. The corrosion map of zinc for UK (28), the corrosion map of several metals for the Sarpsborg/Fredrikstad area in Norway ( ) and the corrosion map of steel for Madrid (30) may serve as examples. The very strong local variations of atmospheric corrosion of metals implies also the major role of dry deposition of pollutants. The wet deposition does not by far exhibit such strong variations as the corrosion rate. 

The atmospheric corrosion of metals caused by acid deposition is mainly a local problem restricted to areas close to the pollution source. 

In a separate study, the effects of NaCl and SO2 air pollutants on the corrosion of zinc were investigated by Qu et at. [68]. Influence of NaCl deposition and SO2 on atmospheric corrosion of zinc at 90% RH and 25 °C is shown in Fig. 10.20 [68]. The corrosion rate decreases with time due to the large amounts of deposit buildup onto the zinc surface. NaCl increases the initial corrosion of zinc in air in the presence and absence of SO2. Presence of only SO2 slowly increases the initial corrosion rate. The synergistic corrosion effect was observed in the presence of both contaminants. 

Atmospheric corrosion is electrochemical corrosion in a system that consists of a metallic material, corrosion products and possibly other deposits, a surface layer of water (often more or less polluted), and the atmosphere. The general cathodic reaction is reduction of oxygen, which diffuses through the surface layer of water and deposits. As shown in Section 6.2.5, the thickness of the water film may have a large effect, but it is more familiar to relate atmospheric corrosion to other parameters. The main factors usually determining the accumulated corrosion effect are time of wetness, composition of surface electrolyte, and temperature. Figure 8.1 shows the result of corrosion under conditions implying frequent condensation of moisture in a relatively clean environment (humid, warm air in contact with cold metal). 

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... 

The relative humidity is the most critical parameter for atmospheric corrosion, because it determines whether condensation can take place. At the metal surface, condensed water forms an electrolyte with the salts deposited from pollutants and thus permits electrochemical reactions to take place. In principle, condensation of water occurs when the relative humidity reaches 100%. However, in practice it takes place often at lower values of relative humidity ... 

Precipitation in the form of rain has a dual effect on the atmospheric corrosion of metals. On the one hand, it affects corrosion by forming a phase layer of moisture on the material surface and by adding corrosion stimulators in the form of and SO4 . On the other hand, it tends to wash away pollutants deposited on the surface during the preceding dry period. Whereas the first two processes promote corrosion, the third tends to decrease corrosion. The significance of the two latter processes is dependent on the ratio of dry and wet deposition of pollutants. 

The corrosion on the skyward side of metal plates in a strongly polluted atmosphere will be substantially lower than that on the downward side. In a strongly polluted atmosphere, where dry deposition is considerably greater than wet deposition of sulfur pollutants, the washing effect of rain will predominate. In areas that are not heavily polluted the situation will be reversed. The corrosive action of the rain is more important consequently, the skyward side of metal plates will have a higher corrosion rate. 

Wet atmospheric corrosion results from repeated wet and dry cycles, the presence of pollutants, and the formation of an aqueous layer in which the atmospheric pollutants dissolve. The wet cycles result from dew, fog, rain, or snow. In many cases the dew, fog, rain, or snow may already contain the dissolved corrodent, which then deposits on the surface. 

Depending on the conditions, rain can either increase or decrease the effects of atmospheric corrosion. Corrosive action is caused by rain when a phase layer of moisture is formed on the metal surface. Rain creates thicker layers of electrolyte on the surface than dew. The corrosive activity is increased when the rain washes corrosive promoters such as H and SO from the air (acid rain). Rain has the ability to decrease corrosive action on the surface of the metal as a result of washing away the pollutants deposited during the preceding dry spell. 

The primary cause of atmospheric corrosion is the dry deposition of sulfur dioxide on metallic surfaces. This type of corrosion is usually confined to areas having a large population, many structures, and severe pollution. Therefore, the atmospheric corrosion caused by sulfur pollutants is usually restricted to an area close to the source. 

Sulfur dioxide is a primary pollutant leading to the atmospheric corrosion of zinc. It controls the corrosion rate when the relative humidity is in the area of 70% or above. Sulfur oxides and other air pollutants are deposited on zinc surfaces either by dry or wet deposition. Regardless of the method of deposition, the sulfur dioxide deposited on the zinc surface forms sulfur-ous or other strong acids, which react with the protective zinc oxide, hydroxide, or basic carbonate film to form zinc sulfate. The film of protective corrosion products is destroyed by the acids, which reforms from the underlying metal, causing continuous corrosion by an amount equivalent to the film dissolved, hence to the amount of sulfur dioxide absorbed. Corrosion rates increase even further when the relative humidity exceeds 85%. 

In the presence of atmospheric acidifying pollutants, such as SO2, the anode reaction is facilitated and, consequently, the total corrosion rate as well. Upon deposition of SO2, interaction with the aqueous phase proceeds with the following reactions ... 

Extensive outdoor exposure programs have provided evidence that corrosion rates can be interpreted in terms of deposition of primarily SO2 and CE and time of wetness. The history of indoor exposure programs is much shorter. Atmospheric corrosion indoors is, in general, influenced by more constant humidity conditions and lower levels of SO2 and CE, from which follows an increased relative importance of other corrosion stimulants including organic gaseous species and particulate pollutants. 

The same applies to mineral oils and greases that are based on cmde oil. Certain greases that are deposited on outdoor cables in aluminium during their fabrication provide effective protection against possible atmospheric corrosion due to moisture and pollution for a very long time (several decades). 

In general for atmospheric corrosion, dusts and solid precipitates are hygroscopic and attract moisture from air. Salts can cause high conductivity, and carbon particles can lead to a large number of small galvanic elements since they act as efficient cathodes after deposition on the surface. The most significant pollutant is SO, which forms H SO with water. Water that is present as humidity bonds in molecular form to even the cleanest and well-characterized metal surfaces. ... 

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