Stainless steel is specified for a number of reasons as the material used for the manufacture of components. There are times when stainless is used for its esthetic properties, other times it is specified as the component is being used for the food, pharmaceutical or beverage industry. Whatever the reason, the final step in the process is stainless passivation.
During the manufacturing process, most of the machines used for rolling, machining and forming the stainless component are made of steel. Although they cannot be seen, iron molecules are detached from the machines and deposited on the stainless component. As soon as the component is introduced into air, these microscopic particles begin to corrode or rust. Stainless passivation eliminates these iron atoms and returns the stainless to a condition where it can resist corrosion.
Sulfides are also a potential problem. The sulfides are the result of sulfur being added to the stainless at the mill. The sulfur is used to improve the machineability by improving the ability for the alloy to form chips, which break away easier from the steel cutting tool. Unless the stainless is properly passivated upon completion of fabrication, the sulfides can form sites, which allow for localized attacks upon the integrity of the stainless surface. Stainless passivation removes all these sulfides that have become resident on the surface.
Passivation must be done on clean parts:
The passivation process will fail if the parts are not clean before being immersed in the acid bath. It is often surmised that the acid will do the cleaning, but it will not. The grease, and other organic and inorganic contaminates will turn into gas bubbles in the presence of the acid, adhere to the surface and stop the process from being effective.
Testing the passivated component:
The passivation process is to introduce the stainless component into a bath of nitric acid solution. The part is left in the bath for a certain amount of time, and the bath is at a certain temperature. The variables are based upon a host of circumstances such as the complexity, size and alloy type of the stainless. Once the passivation process has been completed, the surface must be tested to insure success.
The question that needs to be answered is, “did the stainless passivation remove the free iron and provide the optimum corrosion resistance.” The tests that are used must be selected based on the grade of stainless being evaluated. A test that is overly hard will fail even the best of material, conversely, a test that is too easy will allow inferior parts to pass.
The tests are conducted in cabinets that can maintain 100% humidity for 24 hours at 95 degrees F. When the test piece is removed, if it is free from any evidence of rust, it can be considered as a pass.
Stainless passivation can be accomplished two ways; by putting the part in an acid bath or subjecting it to electropolishing. Electropolishing done at New England Electropolishing is the superior method.
