We manufacture packaging machinery for the food industry. Our machine bodies are generally fabricated from 4 mm 304 2B finish stainless steel. After fabrication, grinding and sanding the bodies are glass bead blasted. Our problem is that some machines in certain environments (i.e., chlorinated water in the same room, cleaning and sanitising agents) the machine bodies tend to discolour. A rust like brown stain appears on the stainless. It also appears on some 304 shafting and any case hardened and ground linear bearing slide material discolours and becomes pitted.
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If the glass beads have previously been used on steel you cannot use them on stainless or you will have this problem.
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Stainless does not mean immune to corrosion. Any environment with chlorides can results in corrosion of type 304 SS. The previous comment about abrasives is right on. Contaminated grinding media can also cause a problem.
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I suspect that your problem is a combination of the environment and the surface condition resulting from the bead blasting. Microscopically, the bead blasted surface consists of many small crevices. If water with chlorides gets into these crevices, the potential for crevice corrosion is great. Ground surfaces are also prone to these crevice corrosion sites. Once the corrosion starts in these crevices they will be prone to continued corrosion unless the surface finish is improved. Case hardening diminishes the corrosion resistance in general and may not require crevices to initiate pitting corrosion.
