Not to trump Mike, as I actually respect his opinion, but galvanic corrosion should be a minor consideration to someone that does not let their car get or stay wet all the time. Some do, but the average owner does not.
Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte.
Salt water is a good electrolyte so if you drive your car in the ocean you are at a particular risk.
As has been pointed out before, hardware store Type A pointed tapping screws are readily available at local hardware stores if you don't want to pay $9.99 for Summit's free shipping.
I have the steel rivet nuts in my car and SS hex head machine screws. I wouldn't use aluminum rivet nuts on anything.
Then again, my side sills are non-metallic and the 3/16" thick aluminum splitters makes for a continuous flat washer.
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While you have the sills off and you're worried about corrosion, you should check the condition of your body seam sealer where the cats are. I repaired mine and then repainted black.
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