The use of an acid solution to render the surface of stainless steel in a state that enhances its corrosion resistance by removing exogenous iron. This creates or restores a passive oxide layer that prevents further oxidation. The parts must be cleaned of dirt, scale, or other welding-generated compounds from the surface before passivation. This will not polish or enhance the finish provided. We use a nitric acid in water for passivation in various tanks each one meeting a variety of specification requirements. The following tests are done in our facility to verify parts have been passivated correctly: Water Immersion testing, Salt Spray testing and Copper Sulfate testing.
Copper sulfate testing is intended to determine the effectiveness of passivation. Copper sulfate identifies the appearance of free iron which is conventionally induced onto the surface of a component during fabrication with steel components. The idea behind this process is based on an oxidation-reduction reaction which induces the dissolved copper ions to envelope or plate out over the free iron particles. This test is recommended for the detection of free iron on the surface of stainless steels in the 200 and 300 series, precipitation hardened stainless steels, and ferritic 400 series stainless steels having a minimum of 16% chromium. This test is not recommended for martensitic 400 series stainless steels with less than 16% chromium because these steels will typically give a positive indication regardless of the presence or absence of anodic surface contaminants. The test solution is applied to the surface of the sample representing the lot of passivated parts, applying additional solution if needed to keep the surface wet for a period of at least 6 minutes. At the end of this period, the surface shall be carefully rinsed and dried with care taken not to disturb copper deposits if present. The test sample shall not exhibit copper deposits visible to the naked eye. A copper color on the metal surface (brown or pinkish, like a penny) indicates that surface iron was still present and is considered a test failure. If no reaction occurs it is considered a test pass.
Salt spray is another accelerated testing process used to determine the extent of corrosion resistance of a material after plating or passivation by exposing to a salt spray at an elevated temperature. Test objects are placed in an enclosed testing chamber and then is contentiously sprayed with of a salt water solution. These conditions are held based on the client's required needs.
Water immersion is an accelerated testing process used to determine the extent of the corrosion resistance of a material after being exposed to distilled water for 24 hours. This process uses a dual tank immersion system, in other words one tank above the other flowing down. The objects are placed into the top tank where it is enclosed securely, after which the distilled water flows upward into the top tank and is cycled back and forth for a 24 hour period. These parts are submerged in distilled water for one hour, and then left to dry under room temperature for another hour and repeated for 24 hours.
