Atmospheric corrosion tests on metals

ASTM G50, Standard Practice for Conducting Atmospheric Corrosion Tests on Metals, West Conshohoken, PA, 1992. 

ASTM G 33, Practice for Recording Data from Atmospheric Corrosion Tests of Metallic-Coated Steel Specimens G 50, Practice for Conducting Atmospheric Corrosion Tests on Metals G 84, Practice for Measurement of Time-of-Wetness on Surfaces Exposed to Wetting Conditions as in Atmospheric Corrosion Testing G 92, Practice for Characterization of Atmospheric Test Sites. 

Specific tests frequently used are (a) neutral 5 % Sodium Chloride salt spray (ASTM B 117, Test Method of Salt Spray (Fog) Testing), (b) 3.5 % Sodium Chloride by alternate immersion (ASTM G 44, Practice for Evaluating Stress Corrosion Cracking Resistance of Metals in 3.5 % Sodium Chloride Solution), and (c) exposure to various outdoor atmospheres. Guidelines for outdoor exposure are contained in ASTM G 50, Practice for Conducting Atmospheric Corrosion Tests on Metals. Generic types of atmospheres used are seacoast, industrial, urban, and rural. Sometimes specific geographical locations or local chemical conditions are important because they can produce unique results [2i],... 

The most widely used simulated service test for static atmospheric testing is described in ASTM G 50, Practice for Conducting Atmospheric Corrosion Tests on Metals. It is used to test coated sheet steels for a variety of outdoor applications. Test materials, which are in the form of flat test panels mounted in a test rack (Fig. 16), are subjected to the cyclic effects of the weather, geographical influences, and bacteriological factors that cannot be realistically duplicated in the laboratory. Test durations can last from several months up to many years. Some zinc-coated steel specimens have undergone testing for more than 30 years. 

Exposure to various outdoor atmospheres is described in ASTM G 50, Practice for Conducting Atmospheric Corrosion Tests on Metals. Atmospheres include marine (seacoast), industrial, urban, and rural types. The effects of these generic atmosphere types on the corrosion behavior of aluminum alloys are described in Chapter 8. 

Recommended practice for conducting atmospheric stress corrosion tests on metals Test method for pH of soil for use in corrosion testing... 

Atmospheric tests on 0.3% copper steel, 7 i -year exposure, from C. Larrabee, Corrosion 9,259 (1953). Atmospheric rates for zinc and copper, 10-year exposure, from Symposium on Atmospheric Exposure Tests on Non-Ferrous Metals, ASTM, 1946. Seawater data from Corrosion Handbook, H. H. Uhlig, editor, Wiley, New York, 1948. Soil data for steel are averaged for 44 soils, 12-year exposure for zinc, 12 soils, 11-year exposure for copper, 29 soils, 8-year exposure from Underground Corrosion, M. Romanoff, Circ. 579, National Bureau of Standards, Washington, D.C. 1957. 

A conventional weather station approach has often been used to monitor atmospheric variables regsuxitng atmospheric corrosion [7]. Temperature and relative humidity may be recorded continuously, and these data are used to produce average temperatures for periods of time such as days, weeks, or months. Electrical monitoring sensors have been developed to measure when wetness exists on the surface by means of detecting a potential difference between dissimilar metals [8]. A standard device is shown in ASTM G 84, Practice for Measurement of Time of Wetness on Surfaces Exposed to Wetting Conditions as in Atmospheric Corrosion Testing. The information on time of wetness is usually reduced to a percentage or fraction of time a surface is wet in a month or in a year. 

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... 

Since relative humidity plays such a key role in the corrosiveness of many environments, it is always desirable to monitor the interrelated humidity factors temperature, humidity, and dewpoint temperature. Since reliable commercial equipment is widely available, it will not be discussed further. Closely related to dewpoint is time-of-wetness (TOW), which is measured by monitoring the resistance between oppositely biased electrical conductors as a function of relative humidity. Bias can be applied through an external power source [72]. Alternatively, adjacent metal conductors can be selected to have substantially different corrosion potentials [73]. Above a critical level of relative hiunidity, the test specimen will adsorb a sufficient amoimt of moisture to produce a sharply lower resistance between conductors. The fraction of time of lowered resistance is commonly referred to as the time-of-wetness. It is one useful measure of the corrosivity of an environment. Such measurements were popular in the 1960s and 1970s. More recently, the preferred measurement, due to ease of use, is fraction of time the dewpoint is reached. A procedure for measuring time-of-wetness is contained in ASTM G 84, Standard Practice for Measurement of Time-of-Wetness on Surfaces Exposed to Wetting Conditions as in Atmospheric Corrosion Testing. 

Many applications for bar, plate, and structural steel require use of painting or other protective measures therefore, there is not a great need for corrosion testing of the bare metal. An important exception to this, however, is the use of weathering steels, such as those specified in ASTM A 242, ASTM A 588, and ASTM A 852 in atmospheric exposure. The corrosion resistance of these steels is such that they can be used in the unpainted condition for many applications, depending on the environmental conditions. For these applications, there is a need for atmospheric corrosion testing to determine the suitability of bare weathering steels in specific locations. 

The test specimens must be physically separated from each other and supported on inert racks. When supported on metal racks the test specimens must be insulated from the metal rack and from each other to prevent galvanic action. Atmospheric corrosion coupons are usually held with ceramic electrical insulators as shown in Figure 15.2. The coupons are usually mounted either at an angle of 20-30 from the horizontal or vertically (see Figure 15.3), Atmospheric corrosion tests usually last at least 1 year and very often extend to 3-5 years. 

Symposium on Atmospheric Corrosion of Non-Ferrous Metals, Amer. Soc. Test. Mat., 58th Annual Meeting, June 29 (1955). Spec. Tech. Publn. No. 175, 141-158... 

Because cast iron components are normally very heavy in section, the relatively low rates of attack associated with atmospheric corrosion do not constitute a problem and little work has been carried out on the phenomenon. A summary of some of the data available is given in Table 3.42. The most extensive work in this field was initiated by the A.S.T.M. in 1958 and some of the results produced by these studies are quoted in Table 3.43. It will be noted that there is a marked fall in corrosion rate with time for all the metals tested. 

In view of possible or probable variations in mechanical properties among different specimens of the same metal cut from different sheets or other pieces, or even from different sections of the same sheet or piece, it is necessary to pay careful attention to the initial sampling of stock to be used for control, as well as exposure, specimens. An interesting case in which several of these considerations were involved was provided by the long-time atmospheric exposure tests of non-ferrous metals carried out by Subcommittee VI of ASTM Committee B-3 on Corrosion of Non-Ferrous Metals and Alloys in which changes in tensile properties were used as one of the means of measuring the extent of corrosion. 

Recent reports [30-31] on the use of atmospheric corrosion sensors based on changes in electrical resistance showed that when there were no contaminants [29], in tests of 100-110 h., corrosion rate was zero or insignificant. These sensors can determine changes in metal thickness lower than one nanometer. However, in the presence of 0.08 ppm of S02 or 20 pg/cm2 of NaCl in the system, changes in thickness where always detected over 75% of relative humidity. Corrosion rate was determined at temperatures of 20, 30 and 40°C and the Arrhenius equation was used to calculate the activation energy of the reactions. This method is very similar to the natural conditions. 

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. 

Yu.N. Mikhailovsky, V.M. Popova and A.I. Marshakov. Field and accelerated tests of contact and volatile inhibitors of atmospheric corrosion on different metals. Protection of Metals, 2000, Vol. 36, No. 5, pp. 546-551. 

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