Why does my old car always rust in the most difficult to get to places?

Honest Joe,Basingstoke

Well, you could say this is Sod’s Law, Joe but it’s really because of crevice corrosion.

Crevice corrosion, like pitting corrosion described in the answer to this question, is a form of localised attack. It occurs due to a differential aeration effect, i.e. due to a lower dissolved oxygen content in the water within a crevice or under a deposit compared to the dissolved oxygen content in the bulk environment. This makes the metal within the crevice anodic compared to the rest of the metal surface and hence it corrodes preferentially. Usually, this takes the form of pitting attack within the crevice.

Because a crevice can be seen as a pre-existing pit, crevice corrosion occurs more readily than pitting on a bare, flat surface in the same environment. For this reason, crevice corrosion on stainless steels or aluminium occurs at a lower chloride content than is required for pitting corrosion. It is always a good idea to try to design out crevices for any metallic system, which will come into contact with water. If this is not possible, then more attention should be paid to the environmental conditions likely to be experienced in service.

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