Before any part can leave the workshop, it has to prove itself in corrosion testing, which simulates the environmental influences it is exposed to in real life. We explain the main tests and what you can expect from them.
The Neutral Salt Spray Test
In Neutral Salt Spray (NSS) testing a part is exposed in a test chamber which is sprayed with a fine salt solution. As corrosion appears on the surface of the test item this is recorded and the hours between start of test and appearance of corrosion are measured and reported. For Zinc Nickel coatings 500-1,000 hours or more are typical for most applications and for most specifications the automotive industry demands the upper end of this range. For Zinc Nickel Plating, visit SWMF.
The CASS Test
The CASS (copper-accelerated acetic acid salt spray) test is another type of test used to assess corrosion. This test is much faster than the neutral salt spray test and is typically used where parts are required to be tested within a short space of time or where the relevant specification requires the test to be carried out using CASS.
Reading White and Red Rust
White corrosion forms as a white zinc oxide layer on the coating surface and can be acceptable for a certain amount of time. Red rust forms as iron oxide, which penetrates through the coating from the base metal. This type of corrosion must not be visible within the agreed time frame.
Adhesion Testing
Cross-hatch adhesion tests are carried out to test the adhesion of the coating to the base metal. The test involves applying adhesive tape to a scribed grid in the coating and then pulling off the tape to check for any signs of damage. The coating must not exfoliate off the base metal.
Zinc nickel plating itself is tested for properties such as adhesion and surface finish in accordance with. For testing of complete parts, the following standards are typical: