Atmospheric Corrosion Tests

For many years, different individuals and organizations have been putting specimens out in the atmosphere in all kinds of tests and shapes. There has been an attempt to standardize some of these tests, as well as the method of reporting the results. When initiating corrosion tests in the atmosphere, these standardized procedures should be consulted and used whenever possible. 

Flat panels exposed boldly to the environment on special exposure racks are a common method for testing materials and protective coatings (Fig. 9.39). Various other specimen configurations have been used, including stressed U-bend or C-ring specimens for stress corrosion cracking (SCC) studies, and crevice assemblies to verify the susceptibility of specific configurations (Fig. 9.40). 

In addition to the atmospheric corrosivity conditions at the test site the following factors are important for the design and interpretation of atmospheric corrosion tests  

In most exposure tests, enough specimens are used so that removals may be made after predetermined periods of 1 to 20 years. Very short-term tests usually can be misleading in that the condition 

Some tests are continued imtil failure, as in SCC testing. However, in tests of protective coatings, as well as in others, periodic measurements of typical properties such as degree of undercutting are more adequate (Fig. 9.43). 

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Table 4.11 Atmospheric corrosion tests on copper and copper alloys...

Practice for making and using U-bend stress corrosion test specimens Recommended practice for laboratory immersion corrosion testing of metals Method for vibratory cavitation erosion test Practice for recording data from atmospheric corrosion tests of metallic-coated steel specimens... 

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... 

Results of outdoor and indoor corrosion rate and corrosion aggressivity in tropical corrosion test stations of Cuba and Mexico are reported. The results mainly concern to natural atmospheric corrosion tests obtained in the western side of the Isle of Cuba and in the Campeche State located at the Yucatan Peninsula in Mexico. The two regions are located in the tropical climate and receive the influences of the waters of the Atlantic Ocean and the Mexican Gulf. Data processed in this paper correspond to atmospheric corrosion tests carried out during a long period of time, about the last 20 years in Cuba and the last 10 years at Campeche up to the present. 

All the atmospheric corrosion tests discussed thus far should be used as guides, and suitable tests should be devised and conducted in the environment of interest to assess the corrosivity of the environment. 

Some useful and pertinent information on atmospheric corrosion test site plans, hardware and other relevant information in the literature25 30 may be used in planning atmospheric corrosion testing program. 

ASTM G84, Standard Practice for Measurement of Time-of-Wetness on Surfaces Exposed to Wetting Conditions as in Atmospheric Corrosion Testing, West Conshohoken, PA, 1993. 

H.H. Lawson, Atmosphere Corrosion Test Methods, NACE International, Houston, TX, 1995. 

ASTM G33, Standard Practice for Recording Data from Atmospheric Corrosion Tests of Metallic-coated Steel Specimens, West Conshohoken, PA, 1988. 

The long-term atmospheric corrosion tests conducted in Australia, Brazil, Canada, Czechoslovakia, Finland, Germany, Japan, Latin America, New Zealand, Norway, Russia, Scandinavia, Singapore, Spain, Sweden, Taiwan, UK and USA along with the number of... 

Outdoor Tests. One standard protocol for atmospheric corrosion testing is to mount small rectangular plates (1.6 x 2.4 cm) on a test rack at about 30 from horizontal, about 3 feet off the ground, usually facing south (Figure 4). The test plates are held off the rack by porcelain insulators, and although strictly speaking a new boundary layer should form on each plate, there may be some positional differences in corrosion rate due to turbulence created by the plates first encountered by the wind flow. 

However, considering the atmospheric boundary layer profiles as discussed above, the local velocity at the rack may be considerably lower, especially for test sites in either forested or urban areas. This result emphasizes the need to measure local wind speeds at atmospheric corrosion test sites, at the test rack height. Turbulence intensity measurements might be useful as well. 

ISO 8565 1992. Metals and alloys - Atmospheric Corrosion Testing - General Requirements for Field Tests. Geneva, ISO Copyright Office, 1992. 

In atmospheric corrosion testing, it is customary to perform tests in special climate chambers in addition to field tests. The tests are used for comparison but are also valuable for determining the behavior of anticorrosive films and coatings. The conditions used to obtain the appropriate atmospheres, constant or alternating condensed water climates, with and without the presence of such additional substances as sulfur dioxide and salt spray, and at various pH values, are specified in the standards (DIN 50 018 1978 DIN 50 021 1975 ASTM B 117-85 1985 ASTM G 87-84 1984 ASTM G 91-86 1986 ASTM G 85-85 1985). 

Atmospheric corrosion testing is important to the suppliers of metals and to the engineers and architects who use metals under atmospheric conditions. Reviews have been prepared summarizing the atmospheric corrosion standards and testing procedures of the American Society for Testing and Materials [27-30]. 

Horton et al. [55] observed that when steels containing Cu and Ni are exposed in industrial and marine atmospheres, the Cu and Ni appear in the mst layers both in the loose outer and adherent inner mst on skyward and ground ward surfaces. Also it was shown by chemical analysis that Ni, Cu, Cr and Mn from weathering steel appear in the mst layer and provides protection. Presence of chlorides in the atmosphere accelerates corrosion of steels leading to the formation of basic Fe ", Fe chlorides and jS FeOOH. Townsend et al. [56] conducted 8-year atmospheric corrosion tests on weathering steel in mral, industrial and marine environments with different heated conditions and indicated that heat treatments have no effect on the corrosion resistance/performance of weathering steels. 

Atmospheric corrosion tests were continued for years with number of panels, and to have traceability, each uncoated panel was marked by hole with respect to reference plate made as per standard [3] as shown in Fig 2.5. The reference plate made of Aluminium has identical size of test panels and a hole is made in left hand corner marked as R, which stands for reference mark. Just below this hole, nine holes are made vertically engraving numbers 1-9 and similarly, in the right hand side, again nine holes are made vertically giving numbers 10-90 from top to bottom. At the top of the panel, five holes are made horizontally to represent hundreds. 

ASTM 033 2004 Standard practice for recording data from atmospheric corrosion tests of metallic-coated steel specimens... 

One-year test results on zinc and cadmium are in Table 2.22. The results of long-term atmospheric corrosion tests on zinc and cadmium coatings (Fig. 2.35) are compared with those of accelerated corrosion tests (Fig. 2.36) on the same coatings in three cabinets warm and humid, SO2 gas, and sea mist (Strekalov and Berukshtis, 1965). 

Townsend, H. E., Borzillo, A. R., and Barber, W. D. (1988). Performance of Al-Zn alloy coated sheet after 32 years of atmospheric corrosion testing, 2nd Int. Conf. on Zinc-Coated Steel Sheet, pp. SAl. 1-11. 

The less dense exterior part of the rust layer contains mostly the phases y-FeOOH and a-FeOOH. The latter has the higher thermodynamic stability of the two but forms more slowly. For this reason, accelerated atmospheric corrosion tests yield lepidocrocite (y-FeOOH) primarily. 

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