Significance and Use
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 2—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) within the chamber for a specified exposure time, depending upon the severity of the test environment. At the end of this time interval, the metal samples are removed and analyzed by the coulometric reduction procedure.
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 3—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 4—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 evaluation methods, such as mass gain, can be carried out with the same samples that are to be tested coulometrically. However, such procedures must precede coulometric reduction.
The conditions specified in this test method are intended primarily for tarnish films whose total nominal thickness is of the order of 102 to 103 nm (103 to 104 Å). Environmentally produced films that are much thicker than 103 nm are often poorly adherent and are more likely to undergo loosening or flaking upon placement in the electrolyte solution.
1.1 This test method covers procedures and equipment for determining the relative buildup of corrosion and tarnish films (including oxides) on metal surfaces by the constant-current coulometric technique, also known as the cathodic reduction method.
1.2 This test method is designed primarily to determine the relative quantities of tarnish films on control coupons that result from gaseous environmental tests, particularly when the latter are used for testing components or systems containing electrical contacts.
1.3 This test method may also be used to evaluate test samples that have been exposed to indoor industrial locations or other specific application environments. (See 4.6 for limitations.)
1.4 This test method has been demonstrated to be applicable particularly to copper and silver test samples (see (1)). Other metals require further study to prove their applicability within the scope of this test method.
1.5 The values stated in SI units are the preferred units. The values provided in parentheses are for information only.
1.6 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 become familiar with all hazards including those identified in the appropriate Material Safety Data Sheet (MSDS) for this product/material as provided by the manufacturer, to establish appropriate safety and health practices, and determine the applicability of regulatory limitations prior to use.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
B809 Test Method for Porosity in Metallic Coatings by Humid Sulfur Vapor (Flowers-of-Sulfur)
B810 Test Method for Calibration of Atmospheric Corrosion Test Chambers by Change in Mass of Copper Coupons
B827 Practice for Conducting Mixed Flowing Gas (MFG) Environmental Tests
D1193 Specification for Reagent Water
atmospheric corrosion; atmospheric test evaluation; cathodic reduction; control coupon evaluation; corrosion film analysis; environmental testing; monitoring environmental tests; Corrosion--metals/alloys; Coulometry--metallic materials; Cathodic reduction method; Coupon corrosion testing; Electrical contact materials; Environmental sampling/testing; Film; Industrial metals/alloys; Metals and metallic materials; Oxide analysis; Oxide metallization thickness; Reduction; Sampling ferrous alloys; Surface films; Tarnish films; Voltage;
ICS Number Code 25.220.01 (Surface treatment and coating in general)
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Citing ASTM Standards
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