Published: Jan 1957
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The salt-spray test was originally-developed some 40-odd years ago (1) for the purpose of quickly evaluating the resistance of plated coatings to attack by marine atmospheres. Through the years an ever increasing use has been made of this test for the purpose of controlling quality of electroplated articles. Today the salt-spray test is employed not only to evaluate the quality of all types of electroplated coatings, both of the cathodic and of the anodic type, but is also frequently used for testing completely assembled equipment. Many specifications contain requirements based on performance in the salt spray, compliance with which is rigidly enforced. The endpoint requirements will vary, depending on the type of coating tested or on the combination of dissimilar metals used. In the case of electropositive coatings, the endpoint is usually specified in terms of a permissible number of corrosion spots per unit area that will develop in a given time. Electronegative coatings, such as zinc and cadmium on steel, are evaluated by their ability to prevent rust formation. Notwithstanding its widespread use, the salt-spray test has frequently been subject to criticism for being unrealistic and misused (2, 3, 4). Its role as a forecaster of service performance has been challenged, and failure to correlate salt-spray test results with performance to actual atmospheric environments have been reported in the literature (5, 6, 7). Inability to obtain reproducible results, both within any one test box as well as amongst several boxes, has also been unofficially reported by many users of this test. Although its shortcomings are thus generally recognized the salt-spray test continues to form a part of many finish specifications, both in industry and government agencies. The lack of a more suitable replacement is the most frequent reason given for the retention of the salt-spray test as a standard test. The American Society for Testing Materials has standardized a salt-spray testing procedure with its Method B 117. The uniformity of conditions within the test chamber operated in accordance with this test procedure is controlled primarily by three physical parameters; rate of salt-fog precipitation, concentration of collected salt solution, and ambient temperature. Rather wide limits are allowed for the precipitation rate. The concentration of collected solution and the temperature are kept within much narrower limits. In addition, the chemical purity of the salt and the pH of the salt solution are also controlled. No provisions are made for calibrating the salt-spray test chamber against some standard. Government specifications on the salt spray are in the main similar to those covered by the ASTM method.
Member of Technical Staff, Bell Telephone Laboratories, Inc., Murray Hill, N. J.