Atmospheric environment service test

The performance of protective coatings can be determined through accelerated laboratory tests but these are not the true representation of actual service conditions. Hence field tests are carried out to get reliable performance data on uncoated and coated steels. The protective systems and their corresponding corrosion data are available in the relevant overseas codes for reference. These data are not applicable in Indian specific context as atmospheric corrosion is location specific. Atmospheric corrosion is the frontier research area where limited work has been carried out as it comprises three phases (sohd/atmospheiic/liquid environment). Therefore it is significant to study the rust on MS and WS as well as coated steels in a given atmospheric environments. This is important for the... 

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. 

Service tests characterize the performance of materials in specific applications and should mimic the environment of the actual application as much as possible. This includes simulating the chemical composition of the atmosphere, the temperature and thermal cycle profiles, stress states, fatigue conditions, and design. These tests may be accomplished by exposing test racks to actual service conditions or through the use of prototype apparatus designed and constructed to duplicate the end-use application. Additionally, laboratory service tests designed to evaluate the effect of one or more critical aspects of the exposure conditions may be employed. Service test data enable one to determine failure mechanisms, component or material life, or to screen materials for an application. 

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). 

Alternative atmospheric cabinet simulation tests are available, including ASTM G 87, Practice for Conducting Moist SO2 Tests and ASTM G 85, Practice for Modified Salt Spray Testing. Service in environments where humidity or moisture varies may be simulated by cyclic humidity or alternate immersion tests. See ASTM G 60, Test Method for Conducting Cyclic Humidity Tests and ASTM G 44, Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5 % Sodium Chloride Solution. 

Action of Vacuum on Spacecraft Materials. For service beyond the atmosphere, the vacuum environment allows materials to evaporate or decompose under the action of various forces encountered (1,18,19). These forces include the photons from the sun, charged particles from solar wind, and dust. The action of space environment on materials and spacecraft can be simulated by a source—sink relationship in a vacuum environment. Thus, for example, the lifetime of a solar panel in space operation may be tested (see Photovoltaic cells). 

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). 

Field tests (tests in real environments), in which replicate test samples of metals or alloys —referred to as test coupons or specimens—are exposed to the actual environmental conditions expected in service, e.g. the atmosphere, the ground, the sea, etc. 

Care in designing and conducting the test in no way reduces the need for discrimination on the part of the person using the test data in the selection of a coating for a particular purpose. Test environments must reflect the deteriorating influences of the service for which they are applicable. A coating system cannot reliably be selected for service in a chemical plant on the basis of performance determined in a rural atmosphere. 

Service (plant) and field testing, including exposure of coupons, test specimens or components in process environments of industrial plants as well as in special natural fields (in seawater, atmospheres, soils etc.)... 

Due to the wide range of condition for corrosion occurring, there are many tests to evaluate corrosion. These tests include the study of atmospheric corrosion, corrosion during episodic wet/dry conditions, corrosion under fully immersed conditions, corrosion in soil, corrosion in aqueous solutions and corrosion in non-aqueous solutions and in molten salts. Moreover, it is important to consider the in-service environment and the objectives of the tests being performed. 

Aqueous solutions were used to simulate specific conditions to carry out electrochemical tests on both field exposed and fresh panels. The test electrolytes are SAEJ 2334 solution (0.25 % NaHCOs -I- 0.5 % NaCl -I- 0.1 % CaCb, pH 9.1) to simulate the atmospheric conditions in the laboratory at 25 3 °C, neutral salt solution (3.5 % NaCl, pH 6.7) and weakly alkaline solution (0.1 M Na2S04 -I-0.1 N NaCl, pH 8.5) to get chloride and sulphate ions in the environment for carrying out tests. Electrolyte used in SAEJ 2334 test was used to determine corrosion performance for coating system as this solution shows a high degree of correlation with field service conditions [5]. 

The service temperature is normally duplicated exactly in laboratory simulations. Unlike assware experiments, HT/HP corrosion tests can be conducted at temperatures above the atmospheric boiling f)oint of the test environment. This is an important benefit of HT/HP testing which normally results in better simulation of actual service conditions. However, problems may arise when the corroding system is not isothermal as in the case under heat transfer conditions commonly found in many types of industrial process equipment such as heat exchangers and dynamic process vessels. Temperature can also be used to accelerate reactions in the laboratory. This can have its drawbacks which... 

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]. 

The test method uses an ammonia atmosphere to simulate service conditions under which stress corrosion cracking may occur. This test method is suitable only for products fabricated from copper alloys that are known to be susceptible to stress corrosion cracking in ammonia vapor atmospheres. It is intended to create an enviromnental condition of reproducible severity, but it is well known that the critical step in the cracking mechanism is the development of an environment in the condensate film that occurs on the test specimen, which is rich in complex copper ions. 

Tests to determine the aggressivity of environment are used in design of new lines to determine the need for additional corrosion protection of the lines to resist particularly polluted atmosphere, aggressive soil, or water. Coatings and cathodic protection are frequently added to safeguard the lines and to match the service life of other components. 

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). 

Fla 20 rrelation of accelerated test media with service environment (industrial atmosphere). Combined data for five lots of rolled plate of aluminum alloy7039-T64 (4.0Zn-2.8Mg4).3Miv0.23C[). Tests in 3.5% sodium chloride were similar to ASTM G 44, except salt solution was made with commercial grade sodium chloride and New Kensington tap water. Source Ref 30... 

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