Atmospheric corrosion classification

PACER LIME algorithm for atmospheric corrosion classification... 

Knotkova, D. (1993). Atmospheric corrosivity classification. Results of the international testing programme ISOCORRAG. Paper presented to International Corrosion Conf, Houston, TX, typescript, 16 pp. 

The nature and rate of atmospheric corrosive attack are dependent on the composition and properties of the thin-film surface electrolyte. Time of wetness and the type and concentration of gaseous and particulate pollutants in the atmosphere largely affect these in turn. The classification of atmospheric corrosivity is important for specifying suitable materials and corrosion protection measures at the design stage and for asset maintenance management to ensure adequate service life. Two fundamental approaches to classifying atmospheric corrosivity have been followed, as shown in Fig. 2.7. These two approaches to environmental classification can be used in a complementary manner to derive relationships between atmospheric corrosion rates and the dominant atmospheric variables. Ultimately, the value of atmospheric corrosivity classifications is enhanced if they are linked to estimates of actual corrosion rates of different metals or alloys. 

The classification given in Table 1.2 is based on the various forms that corrosion may take, but the terminology used in describing corrosion phenomena frequently places emphasis on the environment or cause of attack rather than the form of attack. Thus the broad classification of corrosion reactions into wet or dry is now generally accepted, and the nature of the process is frequently made more specific by the use of an adjective that indicates type or environment, e.g. concentration—cell corrosion, crevice corrosion, bimetallic corrosion and atmospheric corrosion. 

Metals and alloys removal of corrosion products from corrosion test specimens Metals and alloys atmospheric corrosion testing general requirements for held tests Corrosion of metals and alloys. Classification of corrosivity of atmospheres Corrosion of metals and alloys. Guiding values for the corrosivity categories of atmospheres... 

ISO CORRAG [23], the International Organization for Standardization (ISO) has implemented a classification system for evaluating atmospheric corrosivity and on the basis of variables that are fairly easy to obtain [24, 25]. This ISO classification has found several applications, for example, to predict the long-term corrosion behavior in different environments and to evaluate the effect of protective coatings. It contains two principally different approaches of assessing the corrosivity of any outdoor atmospheric environment. The first is based on exposure of standard specimens of steel, copper, zinc, and aluminum for one year whereby the corrosion effect is measured through mass loss measurements. One of five measured corrosivity classes... 

The International Standard Organization classification of atmospheric corrosion... 

The evaluation and classification of atmospheric metal corrosivity help develop corrosion protection strategies and optimize material service Hfe. Two fundamental approaches are used to estimate the relationships between atmospheric corrosion rates of metals and the atmospheric variables such as pollutants types and their concentration in the atmosphere, the temperature, and the time of wetness. 

The International Standard Organization (ISO) developed a corrosivity classification system verified through exposure that has been carried worldwide. The ISO classification system is based on the assumptions that only the time of wetness and the concentration of pollutants in the atmosphere, SO2 and chlorides, control the corrosion rates of metals. Table 10.1 shows the Hst of ISO standards related to atmospheric corrosion of metals [39]. 

The International Organization for Standardization Technical Committee on Corrosion of Metals and Alloys, ISO/TC156, has developed a systematic approach to chissifi-cation of atmospheric corrosivity [13], This approach is based on two different methods. The first method involves exposure of standard coupons of steel, zinc, copper, or aluminum to the atmosphere at the site where the classification is to be made. The selection of which metal is to be used will depend on the purpose of the test. Panels are... 

This system of atmospheric classification is now being revised to create a new approach based on dose-response functions for steel, copper, and zinc. Because the corrosion of aluminum occurs by a pitting or localized mechanism, the traditional approach of using mass loss to determine severity of attack is often misleading. Atmospheric corrosion problems with aluminum alloys are most frequently a result of metallurgical conditions rather than environmental conditions, and the behavior of aluminum may be excluded in the upcoming revision of the ISO 9223-6 documents. 

Although the ISO approach to classihcation of the atmosphere was envisioned to cover both indoor and outdoor atmospheres, the classification range on indoor atmospheres was too broad to be useful for many applications. The corrosion concerns for indoor atmospheres covered three different categories. One major area of concern was storage of metallic parts and compionents in warehouses. The concern... 

The corrosivity of indoor atmospheres must be determined by standard panel exposures. Currently ISO/TC 156 is developing standards on indoor corrosivity classification. These standards have not yet been adopted. 

Corrosion of Metals and Alloys Classification of the Corrosivity of Atmospheres Corrosion of Metals and Alloys Guiding Values for the Corrosivity Categories of Atmospheres Corrosion of Metals and Alloys—Aggressivity of Atmosphere-Methods of Measurement of Pollution Data... 

Considering the above, test sites should be selected to provide general corrosion performance in one or more of these atmospheric classifications. ASTM Committee G01.04 on Atmospheric Corrosion operates sites that include all three, and testing can be done in these providing the data accumulated become part of the public record. In addition, there are several private, government, and commercial testing facilities already established where exposure space can be rented. 

The rural, industrial, and marine atmospheres described earlier are very broad classifications, and do not adequately define specific exposure conditions. Other categories have been proposed, such as desert, tropical, urban, semiindustrial, and industrial-marine, but even these are not satisfactory for predictive purposes [9]. In the mid-1970s. Technical Committee 156 on Corrosion of Metals and Alloys of the International Organization for Standardization (ISO) identified atmospheric corrosion as a priority area for study. From that, a classification system was developed, and described by four standards (Table 3). The system is based on quantitative values for TOW, SO2, and Cl deposition rates, and/or mass loss of steel, aluminum, zinc, and copper samples exposed for at least one yetir. The TOW can be determined from meteorological data or measured by devices such as described in ASTM G 84, the ionic species deposition rates by ASTM G 92 and either of the Cl methods described earlier, and the mass losses determined by the procedures discussed earlier in this section. The levels of ionic species, TOW, and mass losses are placed in categories as to... 

Aluminum finds wide applications in what would be classified as outdoor atmospheres. Corrosion of aluminum is self-limiting in all outdoor classifications whether it be seacoast, urban, industrial, or rural, and the rate of corrosion becomes very slow within 5 years except for alloys and tempers that are subject to exfoliation. 

While it is generally important to rank macro-level environments according to a normalized corrosivity classification, specific information about atmospheric corrosivity and corrosion rates is often required on the micro level. For example, a corrosion risk assessment may be required for a military aircraft operating out of a specific air base environment. One such requirement resulted in a report of the... 

Procedure and limitations. The ISO corrosivity classification from atmospheric parameters is based on the simplifying assumption that the time of wetness (TOW) and the levels of corrosive impurities determine the corrosivity. Only two types of corrosive impurities are considered, namely, sulfur dioxide and chloride. Practical definitions for all the variables involved in calculating an ISO corrosivity index follow. 

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