Spray accelerated tests

Sa.lt Spray Tests. One of the older accelerated corrosion tests is the salt spray test (40). Several modifications of this imperfect test have been proposed, some of which are even specified for particular appHcations. The neutral salt spray test persists, however, especially for coatings that are anodic to the substrate and for coatings that are dissolved or attacked by neutral salt fog. For cathodic coatings, such as nickel on steel, the test becomes a porosity test, because nickel is not attacked by neutral salt fog. Production specifications that call for 1000 hours salt spray resistance are not practical for quahty acceptance tests. In these cases, the neutral salt spray does not qualify as an accelerated test, and faster results from different test methods should be sought. 

The CASS Test. In the copper-accelerated acetic acid salt spray (CASS) test (42), the positioning of the test surface is restricted to 15 2°, and the salt fog corrosivity is increased by increasing temperature and acidity, pH about 3.2, along with the addition of cupric chloride dihydrate. The CASS test is used extensively by the U.S. automobile industry for decorative nickel—chromium deposits, but is not common for other deposits or industries. Exposure cycle requirements are usually 22 hours, rarely more than 44 hours. Another corrosion test, now decreasing in use, for decorative nickel—chromium finishes is the Corrodkote test (43). This test utilizes a specific corrosive paste combined with a warm humidity cabinet test. Test cycles are usually 20 hours. 

Salt spray tests, humidity tests, and other accelerated tests, some usiag sulfur dioxide and carbon dioxide, have shown favorable results for tin—2inc ia comparison with 2iac, cadmium, and fin deposits. Chromating improves the performance. 

As of this writing the 2inc alloys are too new to have actual corrosion resistance data, except for that based on accelerated tests. Zinc—nickel usually shows better results than 2inc-cobalt in salt spray tests. The reverse is tme when the Kesternich test is used. Tin—2inc performs well in both salt spray and Kesternich tests, but appears only to equal 2inc plating and 2inc—nickel in humidity tests. 

In considering the corrosion of magnesium and its alloys it is important to examine the methods available for assessing corrosion tendencies and particularly those known as accelerated tests. Tests carried out by immersion in salt water or by spraying specimens regularly with sea-water are worthless as a means of determining the resistance of magnesium alloys under any other than the particular test conditions. Extrapolation to less corrosive conditions is not valid and even the assessment of the value of protective measures by such means is hardly possible. The reason is to be found in the fact that corrosion behaviour is directly related to the formation of insoluble... 

The most widely used accelerated tests are based on salt spray, and are covered by several Government Specifications. BS 1391 1952 (recently withdrawn) gives details of a hand-atomiser salt-spray test which employs synthetic sea-water and also of a sulphur-dioxide corrosion test. A continuous salt-spray test is described in ASTM B 117-61 and BS AU 148 Part 2(1969). Phosphate coatings are occasionally tested by continuous salt spray without a sealing oil film and are expected to withstand one or two hours spray without showing signs of rust the value of such a test in cases where sealing is normally undertaken is extremely doubtful. 

Method for copper-accelerated acetic acid-salt spray (fog) testing (CASS test)... 

There are several accelerated tests which differ in the selection of light source and cyclic exposure to varying degrees of humidity. Some accelerated tests include salt spray, heat, cold, and other weather factors. 

ASTM B368, Std. Methodfor Copper-Accelerated Acetic Acid Salt Spray (Fog) Testing (CASS Test), American Society for Testing and Materials, Philadelphia, Pa., 1990. 

Table 15.17 shows the adverse effect of stress and tropical climates on aluminum joints bonded with various adhesives. If a 30-day saltwater spray were used as an accelerated test to determine the long-term performance of these adhesives in a tropical climate, it would be very misleading. Saltwater spray had very little effect on the strength of stressed or unstressed joints with the exception of one acrylic adhesive. However, the stressed specimens in Florida almost all completely degraded. Panama was not nearly as severe an environment. These data illustrate the point that permanence or durability must be tested in the specific environment. 

Fog Boxes vs. Rain. Like many other accelerated tests, the fog box blister-resistance test shows trends rather than an exact correlation with the results observed in actual rainfall. When deionized water is used for the spray in the fog box, the test is most stringent, and blister differences can be observed in paints that do not blister when the much less stringent tap water fog is used. The tap water test correlates more closely to what is generally seen on the test fences when paint films no more than a few days old are exposed to rain. 

CCCs are not heat-tolerant [151, 152]. Exposure to elevated temperature accelerates dehydration, consolidation of the Cr(III) backbone, and losses in properties described above. Losses in CCC corrosion protection, Cr(VI) leaching, and increases in shrinkage cracking are profound when coatings are subject to temperatures in excess of 60 °C. Corrosion resistance in salt spray exposure testing and electrochemical testing can be completely lost when CCCs are heated for more than 15 min at temperatures in excess of 60 C [133]. 

For the combined cycle test, the spray testing is combined with drying and wetting. The cycle can be designed and changed flexibly. For example, the spraying will be carried out at 35 °C for several hours. Then the specimen is dried at 70 °C for a certain period and wetted at the same temperature for another certain period. The combination can simulate the weather conditions better. In such a way, the acceleration test can become closer to the real exposure test. In Japan, automobile companies usually require the related surface finishers to overcome a certain condition by the combined cycle test. 

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