I recently had a discussion with a plating supplier regarding the meaning of salt spray tests with regards to the longevity of zinc plating with black chromate. At the heart of the discussion is my question: What does it mean to have a part tested to 24, 48 or 72 hours etc. of salt spray testing? B.J.
The “classic” salt spray test is the granddaddy of all corrosion tests. The idea behind this test is to expose test specimens to an atomized, fog-like, salt spray solution at a standard temperature. The specimens are inspected as the test proceeds and after so many hours the specimen will break down or corrode. The details of the procedure can be found in ASTM B117-03, http://www.astm.org. The length of time that the specimen takes to corrode is a measure of the corrosion resistance of the deposit. The procedure is designed to be an accelerated testing method. After all, we can’t wait around for months or years to determine how long a particular finish will stand up to the environment.
As with all accelerated static type tests there are drawbacks. The test is okay if the testing is done under carefully controlled conditions and the results can be compared to standards that are known to be satisfactory. When a part is tested for 24, 48 or 72 hours, unless the results are compared to standards, you really do not know much beyond the idea that the parts have survived so many hours in a salt spray environment. Many people consider the salt spray test to be a preliminary test to be followed by a more demanding test such as the CASS test (copper accelerated salt spray), ASTM B368-97.
There are many discussions of the salt spray test in metal finishing literature. A search of the PF Online web site, www.pfonline.com, came up with a number of hits. A search of the NMFRC web site, http://www.nmfrc.org, listed multiple papers on the subject.