I have a rusted knife with a delrin handle and I was wondering if it would damage the handle if I were to soak it in vinegar (to remove the rust).
Because the acetal polymer Delrin is end-capped, there are no free acetal groups that might undergo acid cleavage. Vinegar is a dilute (~5%) sollution of acetic acid in water, and even if it were concentrated acetic acid, it's a pretty weak acid. My guess is that you'd be safe in doing this, however, I doubt that acetic acid is going to remove the rust, and might actually encourage further rusting in the areas that you've cleaned.
I'd recommend soaking it in a solution of oxalic acid, which has long been used for rust removal. It actually reduces the oxide to a more soluble form and I've used it for years for cleaning rust stains off of minerals in my collection. It's also a very weak acid, but it is toxic if ingested, unlike acetic acid. Don't keep you knife in there any longer than you need to a store it in a dry place to prevent further rusting.
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