Corrosion testing simulated service tests

Hi) In very critical situations, test for corrosion under simulated service conditions. All these are sensible precautions, whether molybdenum disulphide is used or... 

Electrochemical tests provide a means to understand the corrosion process, simulate service conditions, or accelerate evaluation of a material [27]. ASTM G 3, Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing ASTM G 5, Standard Reference Test Method for Making Potentiostatic and Potentiodynamic Polarization Measurements and ASTM G 61, Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys provide background in some of these techniques. 

Materials evaluation should be based only on actual data obtained at conditions as close as possible to intended operating environments. Prediction of a material s performance is most accurate when standard corrosion testing is done in the actual service environment. Often it is extremely difficult in laboratory testing to expose a material to all of the impurities that the apparatus actually will contact. In addition, not all operating characteristics are readily simulated in laboratory testing. Nevertheless, there are standard laboratory practices that enable engineering estimates of the corrosion resistance of materials to be evaluated. 

A convenient size for a circular coupon is 3 8 cm dia., a thickness of 0 - 32 cm and a central hole of 1 1 cm. Although inherent in the philosophy of corrosion testing, the use of coupons with surfaces that simulate those in service has been found to be unsatisfactory owing to irreproducibility, and the standard procedure normally adopted is to abrade down to 120-grit. ASTM Method G4 1984 gives details of preparation of specimens, evaluation of replicate exposures and the application of statistical methods. 

D 2570 1985 Method for simulated service corrosion testing of engine coolants... 

Various corrosion tests often provide dissimilar results, and disagreement with practical experience is also common. This is mostly due to the dissimilar conditions of convection. It is therefore necessary either to simulate as closely as possible the actual conditions of service, or to test the material over a wide range of conditions and thus to extract information on all the parameters involved. 

Corrosion tests have inevitable limitations in their capacities to mimic actual service conditions of equipment. Standard, ambient pressure, immersion test procedures, with intermittent fluid refreshment, are available for both metallic and nonmetallic materials, but are limited to the ambient pressure boiling point of the fluid, and provide limited scope to simulate the effects of stress, geometry, heat transfer, and fluid flow. Such fesf procedures can be conducted at plant pressures and temperatures in autoclaves, and can be upgraded to focus on specific factors such as fluid flow and heat transfer. Even so a laboratory test, however elaborate, is a poor substitute for a test in the plant itself. 

Depending upon the expected mechanism of corrosion (i.e., general or localized pitting, crevice, cracking or gctivanic attack), different tests can be designed to detect the susceptibility to attack. Table 1 summarizes some of the more common forms of corrosion and the appropriate laboratory tests used to evaluate the susceptibility of a matericti to a specific mode of attack [/]. Many of these tests are accelerated rather than simulated service (e.g., an accelerated test used to detect susceptibility to a form of corrosion), and are routinely performed to provide information for use in ... 

D 2570 Test Method for Simulated Service Corrosion Testing of Engine Coolants... 

Most metak and alloys are thermodynamically unstable and will therefore attempt to revert back to their native stable state (e.g., red rust or ferric oxide). The rate at which this reversion (corrosion) occurs varies with the pyarticular alloy and the environment to which it is exposed. The purpose of the laboratory corrosion test is to best simulate these conditions, determine corrosion rates, and predict the various modes of attack. Although each data point is important, the results can be misinterpreted and lead to the wrong conclusion if they are not properiy evaluated. Therefore, a laboratory corrosion test can be most useful only if the test results can be compared with actual plant or process service experience. 

Autoclave corrosion tests are a convenient means for laboratory simulation of many service environments for the purpose of evaluating the corrosion resistance of materials and for determining the effects of metallurgical, processing, and environmental variables on corrosion processes. The reason for such tests is to more closely recreate the high temperatures and pressures commonly occurring in commercial or industrial processes. 

In other systems, similar effects of dissolved gaseous species can be important and may require the use of HT/HP corrosion test procedures to give accurate simulation of service environments. Examples of such conditions are those that contain carbon dioxide, sulfur dioxide, and NOx, which can determine the pH of the aqueous phase and affect the severity of corrosion reactions. 

In many situations, actual service environment samples are retrieved and brought to the laboratory to provide the most representative media in which to conduct such tests. In these cases, sampling techniques play a major role in determining the successful usage of simulated enviromnent corrosion tests. 

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... 

The uniform corrosion test methodologies outlined below are intended to provide the user with an overview to the process. They are centered on tests in aqueous applications, but can also be adopted to monitor uniform corrosion concerns in an atmospheric exposure. Uniform corrosion tests are only useful if the test results demonstrate that the corrosion was indeed uniform. The tests described below discuss the general procedures and areas of concern. The test results will be valid to the extent that the test simulates the potential material in-service application. 

In 1930, Huey [4] described another simulated service test used at the Du Pont Co. since 1927 to quantitatively detect variations in the performance of iron-12 to 18 % chromium alloys intended for service in nitric acid plants. For rapid results, he selected a concentrated solution of 65 % nitric acid, which is near the constant boiling concentration of 68.5 %. Five 48-h periods, each with fresh acid solution, were proposed. Fresh solutions are needed to minimize the effect of corrosion products as described below. The weight... 

It was soon found that among the variables affecting corrosion rates were certain heat treatments that made not only the ferritic stainless steels subject to intergranular attack, but also the austenitic, 18Cr-8Ni alloys. From this simulated service test in boiling 65 % nitric acid, there evolved Practice C of ASTM A 262. Its large-scale use by one... 

Testing should be properly planned as discussed in Planning and Design of Tests. Tests should be relevant and correlate with actual field use. Accelerated tests often provide useful information, but degradation processes in such tests may not reflect the actual corrosion mechanisms by which the materials naturally deteriorate. Testing may require laboratory, field, simulated service evaluations, or a combination of the tests. Specific test methods are discussed in the following sections. 

Immersion Corrosion—Accelerated Tests To simulate service of components that will be immersed in fluids, ASTM G 31 should be followed. This standard includes many precautions relating to how laboratory testing environments may differ from actual service. Issues such as immersion solution composition, temperature, aeration, velocity, and volume must be addressed thoroughly before the design of the test can be considered complete. For tests involving corrosion in water, two ASTM D 2688 and ASTM D 2776 should be referenced, which determine corrosivity by weight loss and electrical methods, respectively. 

The performance characteristic to be determined, e.g., mass loss, thickness loss, or strength loss, must be specified before selecting and performing the corrosion test. The environment must be specified, and a decision on whether or not to test in that specific environment (i.e., an in-plant or in-service test) or in a simulated environment (i.e., in a laboratory test) must be made. Any deviations from the actual... 

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. 

The preferred procedure for corrosion testing tantalum and niobium materials, as stated previously, is to test by exposure to actual service conditions. However, in situations when this is not possible and it is necessary to obtain corrosion test data under simulated service conditions in the laboratory, guidance can be obtained from the following ASTM standards contained in the ASTM Annual Book of Standards Volume 03.02 ... 

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