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David C. Silverman |
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Overview of the Experimental Technique The sequential immersion test combines a measurement of absorption and desorption with a measurement of change in hardness to provide a reasonable, first pass screening of chemical compatibility and permeation properties of nonmetallic materials. The test is most suited to providing a first pass examination of compatibility of elastomers and possibly some thermoplastic candidates with an environment. The test is probably best suited for first pass screening of elastomer lining and o-ring candidates. This test may not be applicable to first pass screening of FRP/GRP systems because of the complex nature of these materials. If the results are consistent with the material being incompatible with the environment, then that candidate is very likely not compatible and need not be considered further. If the results are consistent with marginal or acceptable behavior, then additional confirming tests are needed that include conditions, especially mechanical, that more closely simulate those that are expected. A large number of application specific tests exist (see ASTM Standards for examples).In the laboratory, the nonmetallic sample is suspended in the test environment and then is suspended in air in a forced air oven at the same temperature. The fluid in the soak portion of the test should contain all of the components expected in the field as well as duplicate such variables as temperature, pressure, and flow past the surface. As discussed in the section Sorption and Diffusion Influence on Sample Size and Test Duration the samples should be thin so as to force one dimensional diffusion. The sample is removed periodically during exposure to each of these environments, patted dry, and weighed. The change in weight is calculated. After an appropriate time period, often between 30 and 60 days, the sample is hung in a drying oven at the same temperature. The section on "Sorption and Diffusion Influence on Sample Size and Test Duration" discusses a possible approach for estimating the appropriate soaking and drying time periods. This two-part procedure is used to establish the mass change profile during both the soaking cycle and the drying cycle. The rate of mass change is dictated by the adsorption of environmental components onto the surface, any equilibrium established between the material adsorbed on the surface and in the bulk fluid, any equilibrium established between the material adsorbed and the air during drying, the rate of diffusion of species into and out of the test specimen, and possible binding or other interaction between components in the environment that migrated into the material and the material itself. One of the strengths of SEQEXPERT is to enable predictions to be made without the sample necessarily reaching the steady state uptake or loss. Typical time intervals between weight measurements for both the soak and dry portions are 1, 3, 7, 10, 20, 30... days. Adhering reasonably closely to the first two time intervals is important because a significant change usually occurs during that initial period. This rapid change can help define the curvature which is used as one of the inputs to SEQEXPERT. The total test time is dictated by the rates at which the environmental components can migrate into and out of the material. The test duration for either portion can be changed as the results are visually examined or even analyzed by SEQEXPERT. Typical total test times are 30 to 60 days for the soak cycle and 15 to 45 days for the dry cycle with the longer test times tending to be needed for thicker samples or slower migration processes in the nonmetallic materials. Slower adsorption or diffusion processes tend to occur with more promising candidates. The ability to plot the data as mass or weight fraction increase or decrease versus time and then compare specimens by information in that plot is predicated on all samples in the testing having similar dimensions. The change in hardness of the sample before and after exposure is also measured and provides an indication of the mechanical stability of the exposed material. More information can be found in D. C. Silverman, "Artificial Neural Network Predictions of Degradation of Nonmetallic Lining Materials from Laboratory Tests", Corrosion, Vol. 50, p. 411, 1994 1  (514k)
and in "Chemical Resistance of Polymeric Materials by Periodic Evaluation", NACE Standard
TM0196-96, NACE International, 1996.The specimens for this test should be as thin as possible so that the sample thickness is much less than the length and width. That geometry ensures that diffusion is virtually one dimensional. This figure (450k).
Showing acceptable behavior in this chemical compatibility test is a necessary but not a sufficient condition for general use of the non-metallic material. Mechanical functionality in the environment also depends on the end use application. The change in hardness determined in this test provides one gross view of the effect of the environment on the mechanical attributes of the nonmetallic material. For example whereas use as a gasket or o-ring requires good compressibility characteristics, use as a lining requires retention of good adhesion properties along with minimal degradation in mechanical strength. Subjecting the exposed sample of a good candidate as indicated by the sequential immersion test to an appropriate mechanical test that more closely resembles its ultimate use is often advisable to make the final decision about acceptability. Previous Page: Introduction-What is SEQEXPERT? Next Page: Using SEQEXPERT-a step-by-step procedure 1 © NACE International publication and year shown in citation above. All rights reserved. Displayed with permission from NACE International, Houston, TX (http://www.nace.org). Published in Corrosion, in the month and year shown in the citation above. |
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David C. Silverman, Ph.D. - Primary Consultant |
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