Case note: 440C bearings pitting on a washdown line

The line ran a standard washdown cycle: 60 °C water with a chlorinated alkaline sanitiser, followed by a rinse. The bearings were 440C stainless with contact seals, chosen precisely because the duty was wet. They were being replaced every eight weeks with rust-coloured pits on the raceways.

The assumption was a bad batch. Three batches from two suppliers later, the pattern had not changed — which is the point at which the material, not the supplier, becomes the suspect.

440C is martensitic: it hardens because of its carbon, and the chromium that provides corrosion resistance is partly consumed forming chromium carbides. Around each carbide there is a narrow zone depleted of chromium, and those zones are where chloride attacks. Fresh water and food acids do not break the passive layer; hot chlorinated sanitiser does.

The pitting we saw was characteristic: isolated pits in the loaded zone and under the seal lip, not the uniform staining of general rust. That pattern is a chloride signature.

1. Low-load positions — guide rollers, tensioners — moved to sealed 316 bearings, which tolerate the chlorinated sanitiser and do not need much load capacity.
2. Loaded positions stayed on 440C but with a contact seal and an NSF H1 grease with a water-resistant thickener, so the sanitiser was kept away from the raceway rather than resisted by it.
3. The washdown procedure was changed to a clean-water rinse before the sanitiser, which reduced the chloride concentration the bearings actually saw.