Those cowl vents are Chinese made stainless, most likely will have some carbon in it causing the rust. The trim ring is chrome plated bronze/brass so there could be a bit of galvanic action going on. I have installed a few of these to replace those
cheap plastic Nicro vents on other boats. Hood/Pompanette sells these and a few others, so if it's really a bother, you can always get new ones.