Atmospheric exposure tests environment

Media considerations. SCC tests can be divided into those conducted in natural environments, such as atmospheric exposure tests and seawater immersion tests, and those which are conducted under laboratory conditions or other fabricating operations. The principal disadvantage of atmospheric exposure tests is the comparatively long time required for their completion however, they are reliable since they can reflect the projected use. There is a standard practice for evaluating stress-corrosion cracking resistance of metals and alloys by alternate immersion in a solution of NaCl 3.5%, pH 6.5. For spray testing, ASTM B-117, 2003 states the relevant conditions for conducting the test. (ASTM G44)4... 

Laboratory corrosion tests for exfoliation corrosion susceptibility are a necessary tool for research and quality control engineers however, the validity of such accelerated tests depends on their relationship to realistic service conditions and their sensitivity to various degrees of susceptibility. The tests must be discriminating and yet not so severe as to be unrealistic. For the majority of engineered structures, exposure to outdoor atmospheres provides a baseline that is representative of many service conditions, except for structures that are subjected to unusual chemical environments. Experience has shown that seacoast conditions are more corrosive to aluminum alloys than inland urban and industrial conditions (see Fig. 2), and seacoast atmospheric exposure tests have been particularly useful for the validation of accelerated exfoliation tests [9]. 

Sprowls, D. O., Sitmmerson, T. J., and Loftin, F. E., Exfoliation Corrosion Testing of 7075 and 7178 Aluminum AUoys-Interim Report on Atmospheric Exposure Tests, Corrosion in Natural Environments, ASTM STP 558, ASTM International, West Conshohocken, PA, 1974, pp. 99-113. 

An extensive study of the corrosion of metals in tropical environments has been carried out by Southwell, etal . Tests have included atmospheric exposure, and exposure in sea-water under mean tide and fully immersed conditions for a range of ferrous and non-ferrous metals and alloys. 

Abstract The environmental distribution of radionuclides, released from nuclear facilities and other sources, and the principles of the emergency countermeasures for radiation protection of the public and workers are discussed in this chapter. The concentration levels of radionuclides in various aquatic and terrestrial environments and the exposure levels of the population due to the various sources of radiation (natural and artificial radionuclides, cosmic radiation, diagnostic medical examinations, atmospheric nuclear tests, etc.) are presented. 

Section HI covers Types of Tests (H. Hack, Section Editor) includirrg laboratory-accelerated tests, field tests, and service tests. The chapters in this section provide basic principles, describe test techniques and specific considerations such as specimen preparation, test duration and acceleration factors, and cite pertinent standards. Chapters included under laboratory tests are electrochemical, cabinet, immersion, high temperature, and high pressure. Field Tests chapters include atmospheric exposure, seawater, fresh water, and soil. Under service tests are industrial applications and high temperature environments. 

Another variation of the immersion test is the cyclic test procedure where a test specimen is immersed for a period of time in the test environment, then removed and dried (either air diy or use of heat lamps), then re-immersed to continue the cycle. Normally hundreds of these cycles are completed during the course of the test program. In ASTM G 60, test specimens are dipped in a test solution before being exposed to atmospheres varying in relative humidity. The intent of the test is to develop a layered corrosion product similar to that found on sheltered outdoor exposure test specimens. These tests can be either manually performed or conducted in apparatus equipped for automatic cycling. 

When products or materials are exposed to the natural environment, they are considered field or service tests. Included in this category are atmospheric exposure, mobil test racks, fleet tests, and field surveys (Table 8). 

D-1828. Practice for Atmospheric Exposure of Adhesive-Bonded Joints and Structures. Whereas the three preceding standards utilize controlled laboratory conditions, this standard (a) defines classes of natural exposure and (b) outlines a weathering test practice for measuring strength loss as a function of time in those exposures. Several, somewhat extreme, natural environments and their associated geographic locations are mentioned. 

Accelerated tests do not precisely predict long-term corrosion behavior however, answers are needed quickly in the development of new materials. For this reason, accelerated tests are used to screen candidate alloys before conducting atmospheric exposures or other field tests. They also are used for production control of exfdiation-resistant heat treatments. Selection of the most appropriate test procedure(s) depends on the type of alloy to be tested, the anticipated service environment, and the purpose of the test (Ref 7). 

The relative susceptibHity of several commercial aHoys is presented in Table 8. The index used is a relative rating based on integrating performance in various environments. These environments include the harsh condition of exposure to moist ammonia, Hght-to-moderate industrial atmospheres, marine atmosphere, and an accelerated test in Mattsson s solution. The latter testing is described in ASTM G30 and G37 (35,36) and is intended to simulate industrial atmospheres. The index is linear. A rating of 1000 relates to the most susceptible and zero designates immunity to stress corrosion. 

The reproducibility of test results between labs using the neutral salt spray tests has not been consistent, but the repeatability, within one lab, is better, and the test has value in comparing variations in coating systems. Correlation of hours of exposure in the salt spray test to actual performance of the plated part in service, even in marine atmospheres, is not consistent and usually avoided. A classic example is that cadmium deposits outlast zinc deposits on steel in salt spray tests and clean marine atmospheres, yet zinc outlasts cadmium when exposed to real, industrial atmospheres, because of the presence of sulfur-bearing corrodents in industrial environments. An important variable in salt spray testing is the position of the surface to be tested. Whereas the surface of test panels is specified to be 15—30° from the vertical (40), when salt spray testing chromated zinc-plated specimens, this range has appeared excessive (41). 

Tests by Clark for the Corrosion Sub-committee of the American Welding Society were carried out at severe marine and industrial sites. After four years, the greatest protection to steel was given by sprayed aluminium coatings combined with aluminium vinyl paint in the following environments id) sea air, ib) sea-water immersion, (c) alternate sea-water immersion and exposure to air (tidal conditions) and id) industrial atmospheres contaminated with sulphur compounds. 

The precautions generally applicable to the preparation, exposure, cleaning and assessment of metal test specimens in tests in other environments will also apply in the case of field tests in the soil, but there will be additional precautions because of the nature of this environment. Whereas in the case of aqueous, particularly sea-water, and atmospheric environments the physical and chemical characteristics will be reasonably constant over distances covering individual test sites, this will not necessarily be the case in soils, which will almost inevitably be of a less homogeneous nature. The principal factors responsible for the corrosive nature of soils are the presence of bacteria, the chemistry (pH and salt content), the redox potential, electrical resistance, stray currents and the formation of concentration cells. Several of these factors are interrelated. 

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