Salt Spray Weathering

Salt spray testing under ASTM B117 conditions subjects the test samples to conditions that are actually more corrosive than usual “real world” exposure. This is because the test uses sodium chloride in de-ionized water and usually lacks the moderating effects of other dissolved salts such as those containing calcium and magnesium, which tend to be somewhat protective.

is the most popular form of testing for protective coatings. These tests have been used for more than 90 years as accelerated tests in order to determine the degree of protection afforded by both inorganic and organic coatings on a metallic substrate. The most widely used salt spray (fog) tests are described below.

The neutral salt spray (fog) test (ASTM B 117): is perhaps the most commonly used salt spray test in existence for testing inorganic and organic coatings, in particular where such tests are used for material or product specifications. The duration of the test can range from 8 to over 3000 hours, depending on the product. A 5% sodium chloride solution containing not more than 200 parts per million (ppm) total solids and with a pH range of 6.5 to 7.2 is used. The temperature of the salt spray chamber is controlled to maintain 35 + 1.1 or – 1.7Þ C (95 + 2 or -3Þ F) within the exposure zone of the closed chamber.

The acetic acid salt spray (fog) test (ASTM G 85, Annex A1): is also used for testing inorganic and organic coatings, but is particularly applicable to the study or testing of decorative chromium plating and cadmium plating on steel or zinc die castings, as well as for the evaluation of product quality. This test can be as short as 16 hours, although it normally ranges from 144 to 250 hours or more. Similar to the neutral salt spray test, a 5% sodium chloride solution is used, but the solution is adjusted to a pH range of 3.1 to 3.3 by the addition of acetic acid. The temperature of the salt spray chamber is controlled to the same temperature range as for neutral salt spray.

The copper accelerated acetic acid salt spray (fog), or CASS test (ASTM B 368): is primarily used for the rapid testing of chromium plating on steel and zinc die castings. It is also useful in the testing of anodized, chromated, or phosphated aluminum. The duration of this test ranges from 6 to 720 hours. A 5% sodium chloride solution is used, with one gram of copper (II) chloride dihydrate added to each 3.8 liters of salt solution. The solution is then adjusted to a pH range of 3.1 to 3.3 by adding acetic acid. The temperature of the salt spray chamber is controlled to the same temperature range as for neutral salt spray and for acetic acid salt spray.