Shot blasting is used primarily to improve a surface finish, whether that improvement means removing rust or preparing a part for coating. Shot peening is used primarily to remove residual stresses from a part, therefore strengthening it, and enhancing the shelf life.
Parts can be blasted with glass, ceramic, organic material, plastic and steel media. All the different media’s have their pros and cons. However, if the process or specification requires steel shot to blast or peen the stainless steel, oxidization/rusting will occur afterwards on the stainless steel part.
You would think that stainless steel is exactly that – stainless. What makes stainless steel stainless? In a word, chrome. Stainless steel is an alloy of iron with a minimum of 10.5 percent chromium. Chromium produces a thin layer of oxide on the surface of the steel—the “passive” layer—that prevents surface corrosion. When shot blasting/peening with a steel media, the carbon steel media used embeds contaminants into the stainless steel. Unprotected carbon steel rusts when exposed to air and moisture. This iron oxide film (rust) is “active” and accelerates corrosion by making it easier for more iron oxide to form.
One way to eliminate this issue, is to blast/peen the stainless parts with stainless steel media. By doing this, no contaminants are embedded into the stainless part, and thus, no rusting. However, only stainless steel parts can be blasted in stainless steel media. Should you blast a carbon steel part in stainless steel media, you will contaminate your media, which will in turn contaminate future stainless parts. Stainless steel media is very expensive in comparison to carbon steel media, so most mass finishers do not have machines loaded with stainless steel media, and if they do, the price reflects this as they are limited in the jobs they can run in that machine.
Another method to solve this issue is passivation. In stainless steel, passivation means removing the free iron from the surface of the metal using an acid solution to prevent rust. When the surface iron is removed, the other components of the alloy (primarily chromium, often nickel as well) are left behind as a surface layer over the underlying steel. Upon exposure to air, these elements react with oxygen to form an oxide layer that protects the rest of the steel from corrosion. solutions, bleach or salt (oceanic environments) all will contribute to the need for passivation of the stainless steel.
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