Atmospheric corrosion factors

The main factor in causing filiform corrosion is the relative humidity of the atmosphere, and if this is below 65% (the critical relative humidity for the atmospheric corrosion of most metals, see Section 2.2) it will not occur. As the relative humidity increases the thickness of the filaments increases at 65-80% relative humidity they are very thin, at 80-95% relative humidity they are much wider and at approximately 95% relative humidity they broaden sufficiehtly to form blisters. 

There are many special factors controlling atmospheric bimetallic corrosion that entitle it to separate treatment. The electrolyte in atmospheric corrosion consists of a thin condensed film of moisture containing any soluble contaminants in the atmosphere such as acid fumes from industrial atmospheres and chlorides from marine atmospheres. This type of electrolyte has two characteristics which are summarised in a paper by Rosenfel d . 

It should be made clear that all the rates of rusting in the atmosphere just quoted, relate to average general penetration and take no account of pitting. Serious pitting of steel exposed to atmospheric corrosion is uncomm.on on simple test plates, but it may be necessary to allow for this in some practical cases, where local attack may be occasioned by faulty design and other factors. 

In principle, cathodic protection can be used for a variety of applications where a metal is immersed in an aqueous solution of an electrolyte, which can range from relatively pure water to soils and to dilute solutions of acids. Whether the method is applicable will depend on many factors and, in particular, economics — protection of steel immersed in a highly acid solution is theoretically feasible but too costly to be practicable. It should be emphasised that as the method is electrochemical both the structure to be protected and the anode used for protection must be in both metallic and electrolytic contact. Cathodic protection cannot therefore be applied for controlling atmospheric corrosion, since it is not feasible to immerse an anode in a thin condensed film of moisture or in droplets of rain water. 

Copson has described in considerable detail the several factors that require attention in studying atmospheric corrosion, particularly of steels. 

TOW considered as the time during which corrosion occurs is an important parameter in atmospheric corrosion of metals. It defines the possibility for atmospheric corrosion to occur based on its electrochemical nature, but corrosion rate will depend mainly in the acceleration caused by contaminants deposition and other factors. 

The four important areas of application of carbon steels are (i) atmospheric corrosion (ii) corrosion in fresh water (iii) corrosion in seawater and (iv) corrosion in soils. The atmospheric corrosion of steel is caused by major environmental factors such as (i) time of wetness as defined by ISO 9223-1992 (ii) sulfur dioxide in the atmosphere due to the combustion of fossil fuels and (iii) chloride carried by the wind from sea. The equations for corrosion rates of carbon steel by multiple regression analysis have been obtained.1... 

Other factors of importance in atmospheric corrosion of zinc are (i) the distance from the ground (ii) orientation of the samples (iii) wind or rain shielding (iv) distance to the local contaminant sources (v) wind, radiation (vi) condensation and drying rate (vii) amount of contaminants and nature of corrosion products and (viii) seasonal variation of factors also should be considered. This shows the complexity of the problem of determining the atmospheric corrosion rates to a high degree of certainty. This uncertainty is exemplified by the observed corrosion rate of 0.6-3.8 pm/yr at 26 sites in rural area in Spain.95 The corrosion rate of 8.5 pm/yr observed on the zinc coating in an under-vehicle situation is comparable to severe marine atmospheric conditions.96... 

The most desirable data are those obtained for the material of interest in the intended conditions of exposure. Such data are not readily available in the literature. Published data on atmospheric corrosion should be used with caution since atmospheric conditions are changing with time, as for example acid rain as a variable factor. Accelerated testing, including electrochemical tests, should have a good link with the natural and practical conditions. Published data should be consulted because they are generally useful. Some published data are mentioned here as examples since they are useful in selecting materials or discussion of case histories ... 

Properties Distinctive reddish color. D 8.96, mp 1083C, bp 2595C. Ductile, excellent conductor of electricity. Complexing agent, coordination numbers 2 and 4. Dissolves readily in nitric and hot concentrated sulfuric acids in hydrochloric and dilute sulfuric acids slowly, but only when exposed to the atmosphere. More resistant to atmospheric corrosion than iron, forming a green layer of hydrated basic carbonate. Readily attacked by alkalies. A necessary trace element in human diet, and a factor in plant metabolism. Essentially nontoxic in elemental form. Noncombustible, except as powder. 

The influence of mainly SO on the corrosion rate of several materials has been shown in numerous national exposure programs. During the last decades a number of empirical relations have been derived from measurements of atmospheric corrosion rates of the most important structural metals and from measurements of environmental factors. The results are usually presented in form of equations including pollution and meteorological parameters (5.). 

From the practical and economic point of view atmospheric corrosion is closely associated with centers of population. Three factors here coincide high pollution level, high density of population, which in turn means great use of materials. The rate of atmospheric corroion decreases sharply with increasing distance from the emission source. This may be illustrated by the corrosion of carbon steel as function of the distance from the stack of a polluting industry in Kvarntorp, see FIG.8 (26). 

Guttman, H. (1968) Effects of atmospheric factors on rolled zinc. Atmospheric Corrosion of Metals, ASTM STP 767, American Society for Testing and Materials, 286-308. 

Single-layer PE films containing solid VCI (G-2 and NDA) additions outdo conventional PE films by a factor of 10-15 in terms of their protective properties [72]. The corrosion rate of the steel samples insulated by PE films modified by PI and Cl and placed in a I N solution of Na2S04 (simulating a harsh condensed medium of atmospheric corrosion) is lowered by three to five times [104,118]. 

In all, the question of which process is rate limiting in atmospheric corrosion depends on many factors, including metal ion and cation transport properties through the corrosion products, and the access of oxygen through the aqueous phase. 

The relative humidity of an environment is a significant factor in corrosion of metals under atmospheric conditions. A thin layer film is formed on metals at a critical value of relative humidity depending on the nature of the metal being corroded. Dehri and Erbil [49] studied the effect of relative humidity on the atmospheric corrosion of defective polyester-coated galvanized mild steel using impedance spectroscopy. The measurements were taken at relative humidity values between 70% and 100%. The relative humidity of the atmosphere increases the corrosion rate of the underlying metal. Figure 10.11 shows the equivalent resistance circuit used to evaluate the corrosion parameters [49]. 

I.S. Cole, D.A. Paterson, Holistic model for atmospheric corrosion. Part 5—factors controlling deposition of salt aerosol on candles, plates and buildings, Corros. Eng. Sci. Tech. 39 (2004) 125—130. 

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). 

Atmosphere corrosion is the general term for all of corrosion phenomena occurring in air. The vaporized water (humid component) forms very thin water films on materials surfaces and the electrochemical reactions leading to corrosion proceed in the thin water film. There are many environmental factors existing in thin water films and they affect the corrosion mechanism. 

Atmospheric corrosion is influenced by a number of meteorological factors of which humidity is the most important. This is because the electrochemical corrosion process on metals requires moisture. 

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... 

Metallurgical factors affect metal loss and tend to corrode at a lower rate with higher alloy content. Atmospheric corrosion resistance of steel was improved by alloying with Cu, P or Cr to form passive oxide layer [50]. Studies have shown that these steels show superior corrosion resistance in particular during atmospheric exposure but not so much for immersed exposure as in seawater and close to the coastline in the presence of high chloride concentrations. Alloying elements like... 

Atmospheric corrosion of steels is very complex and is a function of interplay of various factors like electrolytes pH and potentiodynamic scan rates and so on. In order to emulate these factors to form protective rust a test plan is given in Table 4.10. 

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