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4.2 - Passivation of Stainless Steel Stainless steel parts, especially those used in Aerospace, Military, and Food Industries, are most often “passivated” before being used. Why? Parts made of stainless steel are being formed or machined with hardened steel (iron) tools. Microscopically some of the iron adheres to the surface of the stainless steel parts, and when these parts are put into use, the iron will rust. Therefore, the stainless steel parts will appear to be showing orange discoloration or rusting... and this defeats the purpose of making parts out of stainless steel in the first place! To solve this problem, the stainless steel must be “passivated” or perhaps a better word is “cleaned” of any metallic impurities on its surface. Strong acids such as Nitric (sometimes Chromic) are used to clean the surface. The most common specifications describing the “passivation” process are ASTM-A-380 and MIL-STD- 753. There should be no dimensional change and no color change when stainless steel parts are passivated, except that oil, dirt, and other impurities are removed. So, how do you know when your parts have been passivated? The specifications previously mentioned describe tests which must be done to verify parts have indeed been passivated. Processing in the proper solutions for 30 to 60 minutes is no guarantee of success, unless one of these tests is actually performed to verify the results (note: the part tested must again be passivated because the testing procedure itself is a contaminant). If you are having parts passivated, be sure to have your vendor describe his test process and certify the results. Any additional paperwork costs will be offset by your “peace of mind” as you avoid customer complaints of, “rust on stainless steel parts?” Sincerely, |
