Most of the soap pump lid adapters sold elsewhere are made of tinplated steel, which rusts quickly and easily. These are made of 430 grade stainless steel, which is resistant to corrosion and rust. However, it is not as resistant as 304 stainless steel. Once we sell through these we will be switching to 304 stainless steel lids.
If you do see corrosion or rust, clean it off with barkeepers friend, magic eraser, or a similar product. Water with iron content and certain soaps encourage corrosion.
If you do not also purchase a soap pump, you are getting a regular mouth Mason jar lid with a 27mm hole in the middle.