Atmospheric corrosion parameters

Time of wetness (TOW), considered as the time during which the corrosion process occurs, is an important parameter to study the atmospheric corrosion of metals. According to ISO-9223 standard, TOW is approximately the time when relative humidity exceeds 80% and temperature is higher than 0°C. No upper limit for temperature is established. In tropical climates, when temperature reaches values over 25°C, evaporation of water plays an important role and the possibility to establish an upper limit respecting temperature should be analyzed. The concept of TOW assumes the presence on the metallic surface of a water layer however, there are recent reports about the formation of water microdrops during the initial periods of atmospheric corrosion, showing that the idea of the presence of thin uniform water layers is not completely in agreement with the real situation in some cases (particularly indoor exposures). 

Taking into account the electrochemical nature of the atmospheric corrosion process it is absolutely necessary to use the concept of Time of Wetness (TOW). It is a concept commonly used in atmospheric corrosion of metallic materials and refers to the time when the metal is sufficiently wet for corrosion reaction to occur, that is, when an electrolyte is present in the metallic surface. Under the particular characteristics of atmospheric corrosion there are time periods where corrosion could not occur due to the absence of an electrolyte in the metallic surface. The lowest outdoor TOW values are observed in the desert regions, as also in the Antarctic and Arctic regions. Atmospheric corrosion rates of metals at these climatic conditions are also very low and in the case of cold regions, the increase of temperature leads to the increase of TOW and corrosion rate [11], In principle, TOW is a parameter that depends upon both the climatic conditions and in the characteristics of the metallic surface. 

The cleaning effect of rain is important in the corrosion process. In many cases this variable is significant, as well as the inclusion of time of rain as an independent variable in explaining the influence of different parameters in atmospheric corrosion rate of basic metals. 

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. 

Time of wetness (TOW), considered as the time during which the corrosion process occurs, is an important parameter to study the atmospheric corrosion of metals. According to... 

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.). 

The results of the experiments suggest that the amount of soluble Zn corrosion product formed can be estimated if the deposition of the precursor gas phase species can be determined. However, the question of what controls the formation of the protective layer has not been discussed. In the next section, a model for the atmospheric corrosion of galvanized steel is formulated in which both the role of deposition and the parameters that control both the formation of soluble and insoluble products are addressed. For the remaining discussion, the term insoluble... 

Conclusion. Although much is not known, and quantitative data is scarce, we conclude that a model of the adsorbed aqueous phase provides a perspective on observations made in many different aspects of atmospheric corrosion. The parameter which we have used as a rational gauge of this data in the thickness of the water adsorbed on the oxyhydroxide surface. 

From previous sections, it is evident that many parameters can influence the atmospheric corrosion of a given metal. In order to perform more systematic studies of their influence, laboratory tests are frequently used in which only a few selected parameters are varied. By applying laboratory tests, individual parameters can be studied and varied under controlled conditions. Comparisons can also be made of the corrosion resistance in a given atmospheric environment of different metals under well-defined conditions. This section discusses... 

A laboratory test must be designed and performed so that the most important parameters from an atmospheric corrosion perspective are controlled. Reproducibility and the ability to mimic the atmospheric corrosion in different ambient environments are other important criteria of a laboratory test. Parameters to consider in accelerated tests are sample preparations prior to exposure, relative humidity, temperature, exposure time, corrodents, and corrodent delivery rate [1]. 

Corrosion in natural outdoor or indoor environments is complex because of the influence of many different parameters. Yet, it is possible to grasp, at least qualitatively, the behavior of atmospheric corrosion through consideration of concepts and... 

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]. 

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). 

The parameters that determine time of wetness and composition of surface electrolyte have been surveyed by Kucera and Mattson [8.1]. They present also a thorough description of the mechanism, with thermodynamic and kinetic aspects of corrosion on various materials. For instance, they consider potential-pH diagrams as a useful thermodynamic basis for understanding atmospheric corrosion. 

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 ... 

There are several other aspects of atmospheric characterization which, if measured, can provide pertinent information related to the performance of materials. Probably the most important and universal is the determination of the TOW, described earlier. ASTM G 84, Practice for Measurement of Time-of-Wetness on Surfaces Exposed to Wetting Conditions as in Atmospheric Corrosion Testing, describes instrumentation and techniques for measuring TOW. This parameter can most easily (and adequately) be estimated from weather bureau records. It is generally accepted that metal atmospheric test specimens are wet when the temperature is above freezing and the relative humidity is above 80 %. There are indications that corrosion can occur under arctic conditions down to 0°F if... 

The airside of the radiator, including the fins and the outside of the tubes, is exposed to the road climate. The atmospheric corrosion of radiators is influenced hy the following parameters [28] the time of wetness as corrosion of practical importance generally will take place only when the metal surface is. covered with a moisture film the t3fp>es and contents of air pollutants from urban and industrial sources splashing from the road. 

In this work, the role of NO2 in the atmospheric corrosion of zinc was analyzed from a detailed characterization of corrosion products. Laboratory tests with exposru e parameters close to the conditions observed in real atmospheres were performed, with the aim of simulating close-to-reality corrosion mechanisms. Low-pollutant concentrations and shortterm exposmes were carried out. For these reasons, XPS were used for the analysis of very thin corrosion layers formed. 

The atmospheric corrosion rate is influenced by many parameters, one of the more important being the formation and protective ability of the corrosion... 

A useful parameter is the dry deposition velocity, which is defined as the ratio of deposition rate or surface flux per time unit of any gaseous compound and the concentration of the same compound in the atmosphere [46]. The concept of dry deposition velocity of SO2 and its relevance to atmospheric corrosion rates is well established [47]. By examining data based on both field and laboratory exposures, it can be concluded that the factors controlling dry deposition fall into aerodynamic processes and surface processes. Aerodynamic processes are connected with the actual depletion of the gaseous constituent (e.g., SO2) in the atmospheric region next to the aqueous phase and the ability of the system to mix new SO2 into this region. This ability depends on, for instance, the actual wind speed, type of wind flow, and shape of sample. Surface processes, on the other hand, are connected with the ability of the aqueous layer to accommodate SO2. This ability increases with the thickness of the aqueous layer and, hence, with the relative humidity, the pH of the solution (as discussed earlier), and the alkalinity of the solid surface. 

The atmospheric corrosion rate is influenced by many parameters, one of the most important being the formation and protective ability of the corrosion products formed. The composition of the corrosion products depends on the participating dissolved metal ions and the access to anions dissolved in the aqueous layer. The eventual thickening of the film of corrosion products can be described in a sequence of consecutive steps—dissolution-coordination-reprecipitation—where the dissolution step is acid dependent, coordination is based on the HSAB principle, and reprecipitation depends on the activities of the species involved. 

Because of the many parameters capable of influencing atmospheric corrosion rates, exposure programs have placed increased emphasis on standardized test procedures. The International Organization for Standardization (ISO) has formulated several corrosion testing standards [85-88] and also implemented a... 

Direct measurement of atmospheric corrosion and corrosivity. Atmospheric corrosion damage has to be assessed by direct measurement if no preexisting correlation between atmospheric corrosion rates and atmospheric parameters is available. Such a correlation and even data on basic atmospheric parameters rarely exist for specific microenvironments, necessitating direct measurement of the atmospheric corrosivity and corrosion rates. 

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