Most sanitary vessel manufacturers are very careful about segregating stainless steel from carbon steel and iron to avoid free iron contamination on the surface. It is a fact that every piece of commercially available stainless steel will have been worked over with carbon steel equipment at some point prior to purchasing by the vessel manufacturer. For improved corrosion resistance, the free iron left on the surface must be removed. Passivation is a process whereby the free iron is removed and a beneficial oxidation layer or film is formed on the surface of the metal. Passivation is a combination of both a cleaning process and the forming of a protective “passive” layer. For passivation to take place, it is important that the surface of the metal is absolutely clean and free of all grease, oil, and any other contamination that may inhibit the passivation process. Passivation is performed by subjecting the metal surface to an acid that will clean the metal and form the passive layer. The exact temperature, time, concentration and type of acid vary from manufacturer to manufacturer. A typical passivation procedure is to circulate a 20-50% nitric acid solution at between 120 and 160 deg F for between ½ to 2 hours. Other acids such as citric and Phosphoric have been used for passivation treatments, but the best results are obtained from Nitric.
There are several tests to determine if the passivation has been successful. A common test is the Copper Sulfate test, in which the surface is soaked for 6 minutes with a Copper Sulfate solution, then rinsed and examined. Any free iron on the surface will show up as a copper or pink color. Salt spray and saline tests are sometimes used as a test for passivation, though not recommended by several vessel manufacturers.