SALT SPRAY TESTS

Q. What is your opinion of the salt spray test? J.R.

 

A. This is one of those questions that has been asked for many years and still generates a lot of discussion within the metal finishing industry. The actual salt spray test and some of its many variations has been carefully defined by ASTM. The assumption is made that if an ASTM procedure is specified, the test will be run exactly as the procedure is written. This is generally true, but I have seen many instances where the procedure has been “modified” because of “testing difficulties” or other issues. While this can be a problem, the significant issue is how the salt spray test results are interpreted.
In my opinion, there are three reasons why interpretation of the results can be iffy: 

 

1. A salt spray test is not the “real” world. The actual corrosiveness of a real world situation can and often does vary from what is found inside a salt spray chamber.
2. Parts in the real world will probably see very different maintenance and cleaning procedures. This can cause identical parts to fail very differently depending on the actual environment.
3. The salt spray test, like many other tests, is an accelerated test designed to give results in a relatively short period of time.

Accelerated tests can often introduce new modes of corrosion and failure that you might not see in the real world. The alternative is to place parts in the field and wait five years to see if they hold up! Not a good idea.

I feel that salt spray testing does have merit if you are comparing two different coatings under identical salt spray conditions. If one coating significantly outperforms another, you probably are on safe ground assuming that it will probably perform better in service also. What constitutes a significant difference? I don’t think I can give a good answer to that question, because it will vary depending on the part and its application.