Significance and Use
Erosion Environments—This test method may be used for evaluating the erosion resistance of materials for service environments where solid surfaces are subjected to repeated impacts by liquid drops or jets. Occasionally, liquid impact tests have also been used to evaluate materials exposed to a cavitating liquid environment. The test method is not intended nor applicable for evaluating or predicting the resistance of materials against erosion due to solid particle impingement, due to “impingement corrosion” in bubbly flows, due to liquids or slurries “washing” over a surface, or due to continuous high-velocity liquid jets aimed at a surface. For background on various forms of erosion and erosion tests, see Refs (1) through (7). Ref (6) is an excellent comprehensive treatise.
Discussion of Erosion Resistance—Liquid impingement erosion and cavitation erosion are, broadly speaking, similar processes and the relative resistance of materials to them is similar. In both, the damage is associated with repeated, small-scale, high-intensity pressure pulses acting on the solid surface. The precise failure mechanisms in the solid have been shown to differ depending on the material, and on the detailed nature, scale, and intensity of the fluid-solid interactions (Note 1). Thus, “erosion resistance” should not be regarded as one precisely-definable property of a material, but rather as a complex of properties whose relative importance may differ depending on the variables just mentioned. (It has not yet been possible to successfully correlate erosion resistance with any independently measurable material property.) For these reasons, the consistency between relative erosion resistance as measured in different facilities or under different conditions is not very good. Differences between two materials of say 20 % or less are probably not significant: another test might well show them ranked in reverse order. For bulk materials such as metals and structural plastics, the range of erosion resistances is much greater than that of typical strength properties: On a normalized scale on which Type 316 stainless steel is given a value of unity, the most resistant materials (some Stellites and tool steels) may have values greater than 10, and the least resistant (soft aluminum, some plastics) values less than 0.1 (see Refs (7) and (8)).
Note 1—On failure mechanisms in particular, see in Ref (6) under “The Mechanics of Liquid Impact” by W. F. Adler, “Erosion of Solid Surfaces by the Impact of Liquid Drops” by J. H. Brunton and M. C. Rochester, and “Cavitation Erosion” by C. M. Preece.
Significance of the Variation of Erosion Rate with Time:
The rate of erosion due to liquid impact or cavitation is not constant with time, but exhibits one of several “erosion rate-time patterns” discussed more fully in 10.3.3. The most common pattern consists of an “incubation period” during which material loss is slight or absent, followed by an acceleration of erosion rate to a maximum value, in turn followed by a declining erosion rate which may or may not tend to a “terminal” steady-state rate. The significance of the various stages in this history can differ according to the intended service applications of the materials being tested. In almost no case, however, are significant results obtained by simply testing all materials for the same length of time and comparing their cumulative mass loss.
The “incubation period” may be the most significant test result for window materials, coatings, and other applications for which the useful service life is terminated by initial surface damage even though mass loss is slight.
For bulk materials, this test method provides for determination of the “nominal incubation period” as well as the “maximum erosion rate,” and material ratings based on each. Empirical relationships are given in Annex A2 by which the nominal incubation period and the maximum erosion rate can then be estimated for any liquid impingement conditions in which the principal impingement variables are known. It must be emphasized, however, that because of the previously described variation of erosion rate with exposure time, the above-mentioned parameters do not suffice to predict erosion for long exposure durations. Extrapolation based on the maximum erosion rate could overestimate the absolute magnitude of long-term cumulative erosion by a factor exceeding an order of magnitude. In addition, it could incorrectly predict the relative difference between long-term results for different materials.
Because of these considerations, some experimenters concerned with long-life components may wish to base material ratings not on the maximum erosion rate, but on the lower “terminal erosion rate” if such is exhibited in the tests. This can be done while still following this test method in many respects, but it should be recognized that the terminal erosion rate is probably more strongly affected by secondary variables such as test specimen shape, “repetitive” versus “distributed” impact conditions, drop size distributions, and so forth, than is the maximum erosion rate. Thus, between-laboratories variability may be even poorer for results based on terminal erosion rate, and the test time required will be much greater.
This test method is applicable for impact velocities ranging roughly from 60 m/s to 600 m/s; it should not be assumed that results obtained in that range are valid at much higher or lower velocities. At very low impact velocities, corrosion effects become increasingly important. At very high velocities the material removal processes can change markedly, and specimen temperature may also become a significant factor; testing should then be done at the velocities corresponding to the service environment.
Related Test Methods—Since the resistances of materials to liquid impingement erosion and to cavitation erosion have been considered related properties, cavitation erosion Test Methods G32 and G134 may be considered as alternative tests to this test method for some applications. For metals, the relative results from Test Method G32 or G134 should be similar but not necessarily identical to those from a liquid impact test (see 5.2). Either Test Method G32 or G134 may be less expensive than an impingement test, and provides for standardized specimens and test conditions, but may not match the characteristics of the impingement environment to be simulated. The advantages of a liquid impingement test are that droplet or jet sizes and impact velocities can be selected and it can simulate more closely a specific liquid impingement environment. A well-designed liquid impingement test is to be preferred for elastomers, coatings, and brittle materials, for which size effects may be quite important.
Scope
1.1 This test method covers tests in which solid specimens are eroded or otherwise damaged by repeated discrete impacts of liquid drops or jets. Among the collateral forms of damage considered are degradation of optical properties of window materials, and penetration, separation, or destruction of coatings. The objective of the tests may be to determine the resistance to erosion or other damage of the materials or coatings under test, or to investigate the damage mechanisms and the effect of test variables. Because of the specialized nature of these tests and the desire in many cases to simulate to some degree the expected service environment, the specification of a standard apparatus is not deemed practicable. This test method gives guidance in setting up a test, and specifies test and analysis procedures and reporting requirements that can be followed even with quite widely differing materials, test facilities, and test conditions. It also provides a standardized scale of erosion resistance numbers applicable to metals and other structural materials. It serves, to some degree, as a tutorial on liquid impingement erosion.
1.2 The values stated in SI units are to be regarded as standard. The inch-pound units in parentheses are provided for information.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
