Understanding Salt Spray Tests

If you spend enough time with screws, you are going to come across salt spray test data. We found ourselves asking what the heck is the difference between 100 hours and 1,000 hours and what exactly is a Salt Spray Test measuring.

Black oxide coated #8 x 3/4” Pan Head Deep Thread Screws
Black oxide coated #8 x 3/4” Pan Head Deep Thread Screws

A Salt Spray Test (aka Salt Fog test) is a standardized test method used to test the durability of protective coatings. The value of the test is that it allows you to compare the relative corrosion resistance of various coatings in conditions that mimic a corrosive environment, but in an accelerated or compressed time frame. So, for example, a ruspert coating that lasts for 1,000 hours in a severe environment is more durable than a black oxide coating that only lasts for 24 to 96 hours.

Salt Spray Testing is typically performed in accordance with the ASTM B117 testing procedure. Samples are placed in a closed chamber and exposed to a dense metal fog which simulates severely corrosive conditions. The tests typically last from 8 to 3,000 hours and employ a 5% sodium chloride solution with a pH between 6.5 and 7.2. There are a variety of other corrosion tests, but the salt spray test is very standard, even though there is some disagreement about how closely it recreates real life conditions. For example, many people question how appropriate the test is when the application does not involve exposure to salt chemicals.

Some examples of relative salt spray test results are as follows:

  • Ruspert Coating (200 & 300) – Samples are free from red rust after 1000 hrs.
  • Black Oxide – Samples are known to show corrosion after 24 – 96 hrs
  • Blue Climaseal – Samples show less than 10% red rust after 720 hr
  • Stainless Steel (304 & 316) – Results have varied depending on the finish. Most samples show very little surface corrosion after 1000 hrs

Although it is an efficient way to check corrosion, the slat spray test is not 100% accurate as it does not completely mimic the natural corrosion process which is very complicated. Regardless, it is widely required by most companies before the use of most coated metals.