The salt spray test is an accelerated corrosion test conducted on materials and products to see how well they handle damage inflicted by salt. The material to be tested is usually placed into a chamber and a solution of sodium chloride is sprayed onto its surface. The test can help researchers and product designers develop paints, coatings, or film that is more resistant to salt damage. Salt damage is very rare for most types of products, however; the salt spray test is generally unreliable since it may not take into consideration many of the natural variables that cause corrosion.
A salt spray test is also known as the salt fog test. A sample is usually placed in a temperature controlled container and a 5% sodium chloride solution is dispersed onto it. The sample is wet throughout the test and the temperature is kept constant. The test duration can be anywhere between 24 hours and 1,000 hours. At regular intervals, the samples are rotated to allow even coating of the salt solution.
The result of the test is rusting of a surface. The number of hours until the first sign of rust is noticed is recorded. Other methods include recording the number of hours until 5% of the surface is rusted. The criteria differs among laboratories.
One of the best way to prevent rusting on steel is to coat the steel with zinc or aluminum. Galvanized steel is coated with zinc and generally lasts about 10 hours in the salt spray test. The best performing steel is typically coated with aluminum and zinc. It lasts about 50 hours before 5% of the surface is covered with rust.
This coating is important for applications that are exposed to the weather, such as roofing material. It is recommended that roofing material be coated with zinc before the paint layer is applied. This will reduce corrosion associated with rain or surf splash.
The salt spray test doesn't take into account the exposure to ultraviolet rays, which are primarily responsible for degrading painted materials. Another questionable aspect of the test is the fact that the sample is exposed to wet conditions continuously. This may not be the case for most products when they are actually used. The test also produces discrepancies between identical samples during testing. For example, one sample may take 5 hours for rust to form, while an identical sample may take 10 hours.