The salt spray test (or salt frog spry test) is the most popular global test to be performed on stainless steels. And some clarifications are needed.
Salt spray test. Some clarifications.
One of the main things to take in consideration is that of the ASTM B117 “Standard Practice for Operating Salt Spray (Fog) Apparatus” specification. In fact, this is not to be considered as the only existing. For example, the international standard ISO 9227 is similar to the ASTM B117 and it covering the same topic. The standards of ASTM B117 doesn’t define the samples that will be tested. Therefore, they may come from a semi-finished product or may be finished components or parts of these.
But just what is the test? Salt spray testing is an accelerated corrosion test that produces a corrosive attack to coated samples in order to evaluate (mostly comparatively) the suitability of the coating for use as a protective finish. The apparatus for testing consists of a closed testing cabinet/chamber, where a salt water (5% NaCl) solution is atomized by means of spray nozzle using pressurized air. (in a chamber temperature of +35C)
Test results. How to interpret them?
Interpreting the results of a salt spray test campaign with stainless steel is extremely confusing. In fact, it is the customer (on the basis of own needs) that define the evaluation criterion of the results, not the specification B117. Looking at the samples, we will be able to establish the following condition during the test.
After so many hours the customer may decide to finish the exposure, to evaluate the status of the samples and determine the success of the test.
What to expect from the results?
Therefore, this test is purely “qualitative”. So, we shouldn’t expect to define a fixed number of hours that characterize the corrosion resistance of a particular type of stainless steel. In addition, the salt spray test cannot become a discriminating factor to determine the quality of this material. What are the results of these tests? Unfortunately, it isn’t possible to determine how many hours in salt spray must resist a specific type of stainless steel.
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