THE PARAMETERS OF ATMOSPHERIC CORROSION

Experience has shown that for a given metal or alloy, the resistance to atmospheric corrosion may differ signihcantly from one site to another. For example, the corrosion rate of galvanised steel may vary from 1 to 100 between a semi-arid zone and the atmosphere of a coastal industrial estate [1]. For a given condition, the resistance to atmospheric corrosion may also vary from one metal to another this variation can be fairly substantial. 

The resistance to atmospheric corrosion depends on several factors that are related to 

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The Parameters of Atmospheric Corrosion Table C.2.4. Solubility in water... 

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... 

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. 

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 retrospective study has been formulated with a clear paradigm of the parameters of the corrosion system. It is assumed that the significant contributors to the corrosion of metals exposed to the atmosphere are water, sea salts, sulfur oxides, nitrogen oxides, and the acidity of precipitated water. 

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. 

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 ... 

In the case of indoor corrosion, there are several parameters that can be measured to determine the corrosivity of the atmosphere. Unfortunately, there are no standards currently available that relate the corrosivity of the atmosphere to the concentrations of corrosive components, but the measurement of such components may be helpful in determining... 

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. 

Experience has shown that at a given level for all other parameters (relative humidity, degree of pollution), dusty surfaces always show more severe corrosion than clean surfaces. Often, dust is a first-order parameter for atmospheric corrosion of aluminium and can thus be more important than the quality of the atmospheric environment. 

In ISO 11844-3 [28] it is established that the combination of different parameters is what determines the corrosivity of the atmosphere. Under indoor conditions, corrosion process depends on a more complex number of parameters than outdoors however in the same way than outdoors two types of parameters are proposed ... 

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... 

Polarization of the galvanic cell. The different phenomena of polarization of the anodic and cathodic reactions (activation, diffusion, convection, etc.), should be well known as a function of the evolution and change of the properties of the interface as a function of time. The polarization behavior of the cathodic and anodic reactions on the two electrodes should be examined (see Figure 6.5). In natural atmospheres, the cathodic reaction controls frequently the attack rate. The diffusion of oxygen is an important parameter to avoid control and polarization of the corrosion by the rate of the cathodic reaction (Figure 6.12).7... 

The corrosion of metallic materials in the atmosphere has been studied extensively (1). The majority of the work in this area has been to determine the performance of materials and to evaluate mitigation techniques in environments of interest. With only a few exceptions (see for example references 2, 3), attempts have not been made in studies conducted in the United States to fully characterize the environment and to determine the relationships between components of the environment and the performance of the material of interest (see reference 4 for a recent assessment of this area). Adherent corrosion products are often characterized, but no attempts have been made, except in laboratory studies ( ), to quantitatively relate the corrosion film chemistry to environmental parameters. 

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... 

The Sandia code CONTAIN is a lumped parameter code with mechanistical models for simulating the physical and chemical conditions in the nuclear containment to predict hydrogen and steam concentration distribution as well as the consumption of H2 by respective combustion. Assuming a core meltdown accident and no vessel breach, i.e., no corrosion/concrete interaction, the code has predicted a thermally stratified containment atmosphere with relatively low temperatures in the central and lower regions which would permit steam condensation. Concerning H2 deflagration, CONTAIN predicts respective bums, if sprays are used for steam removal [56],... 

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]. 

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