Maintenance 6· Mold!! Let's kill it!
What you are looking at is truely nickel plating over a base material. Whether it's brass or some other material may depend on the quality of the fixture. Stainless cannot be plated onto some other material. Only base materials can be plated onto other metals. Stainless is an alloy, so you can't plate it onto something, however, you could plate nickel onto stainless. Brushed stainless, brushed chrome and brushed nickel are all distinctive finishes and can be told apart from each other, although stainless steel alloys with high nickel content are more difficult to distinguish. Most nickel plated finishes that I've seen on household fixtures are clear coated with a polyurethane to help prevent spotting and for wear resistance. The mechanical guts in faucets are ususally the same regardless of finish, so I don't know why one finish would be more prone to leak problems over another.