Best way to keep this spot from spreading.

I was cleaning my rims and just noticed this chip on the inner lip. Rims are only 3 months old and are in perfect condition other than this one spot. I have now idea how it even happened. I’m on 19×9.5 rohana RC7s with 225/35/19s. I’m ok with the chip since it’s on the inner lip, I just don’t know the best way to seal it so the rust doesn’t spread. Thanks!