The present trend in environmental testing of materials with electrically conductive surfaces is to produce, under accelerated laboratory conditions, corrosion and film-forming reactions that are similar to those that cause failures in service environments. In many of these procedures the parts under test are exposed for days or weeks to controlled quantities of both water vapor and pollutant gases, which may be present in extremely dilute concentrations.
Note 28212;Descriptions of such tests can be found in Practice B 827
Many of these environmental test methods require monitoring of the conditions within the chamber during the test in order to confirm that the intended environmentally related reactions are actually taking place. The most common type of monitor consists of copper, silver, or other metallic coupons that are placed within the test chamber and that react with the corrosive environment in much the same way as the significant surfaces of the parts under test.
In practice, a minimum number of control coupons are placed in each specified location (see Test Method B 810
Other corrosion film evaluation techniques for metallic coupons are also available. The most common of these is mass gain, which is nondestructive to the surface films, but is limited to the determination of the total amount of additional mass acquired by the metal as a result of the environmental attack.
Note 38212;Detailed instructions for conducting such weighings, as well as coupon cleaning and surface preparation procedures, are included as part of Test Method B 810
Note 48212;Some surface analytical techniques (such as X-ray methods) can provide nondestructive identification of some compounds in the films, but such methods, for example, X-ray diffraction, can miss amorphous compounds and compounds present in quantities less than 5 % of the tarnish film volume.
With the coulometric technique, it is possible to resolve the complex total film into a number of individual components (Fig. 1) so that comparisons can be made. This resolving power provides a fingerprint capability for identifying significant deviations from intended test conditions, and a comparison of the corrosive characteristics of different environmental chambers and of different test runs within the same chamber.
The coulometric reduction procedure can also be used in test development and in the evaluation of test samples that have been exposed at industrial or other application environments (6). However, for outdoor exposures, some constraints may have to be put on the amount and type of corrosion products allowed, particularly those involving moisture condensation and the possible loss of films due to flaking (also see 4.9 and 8.3.2).
In laboratory environmental testing, the coulometric-reduction procedure is of greatest utility after repeated characterizations of a given corrosive environment have been made to establish a characteristic reduction curve for that environment. These multiple runs should come from both the use of multiple specimens within a given test exposure as well as from several consecutive test runs with the same test conditions.
The coulometric-reduction procedure is destructive in that the tarnish films are transformed during the electrochemical reduction process. Nondestructive ev.........
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