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Under the Microscope – Random Breakages – Stainless Steel Circlips

circlips

This Knowledge Hub post details how we solved a problem with random breakages of steel circlips.

Some time ago, we came across a baffling situation involving random breakage of stainless steel circlips. The circlips we supplied were for use as links in chain mail gloves, used, not in suits of armour, but in the butchery trade, to protect workers’ hands from knife cuts.

Our customer had developed an accelerated test protocol, designed to replicate the cleaning processes that the gloves would be subjected to over many years of use. The problem was that he was finding that the circlips were breaking for no apparent reason. The circlips were not being subjected to any significant stress, and neither were any aggressive chemicals being used in the cleaning process.

Having witnessed the cleaning procedure and examined some broken parts under a microscope, we were at a loss to conclude the failure mechanism. Further discussion with our industry research organisation, eventually highlighted what was going on.

The cleaning process, although only using water, involved blasting the gloves under very high pressure. Whilst it is often assumed that stainless steel is an inherently corrosion resistant material, this is not actually the case; it contains something like 70% iron and it derives its corrosion resistance from a passive surface film which forms naturally over time. Without this layer, it will readily corrode.

It transpired that the high-pressure water jet was removing the passive film, rendering the circlips liable to corrosion. Spots of rust would form before the passive film was recreated. During the next cleaning session, the passive film was again blasted away, together with the rust, leaving corrosion pits. These pits naturally held moisture which again prompted further growths of corrosion, only for this to be removed during the next cleaning cycle. Eventually, a sufficient proportion on the wire section was eaten away to prompt the wire to snap under the pressure of the jet.

The mystery was therefore solved. The fact that failures didn’t occur during actual use confirmed that it was the accelerated nature of the test procedure that was causing the problem; if the circlips were allowed time to regrow their passive surfaces, there was no problem.

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