Today I began replacing stickers on my above ground plumbing. I decided to scrape off the old stickers (and adhesive residue) with a putty knife. After repeated scraping, the pipes started turning white (see attached picture). That is, I seem to have begun scraping off the black coating from my pipes.
Is this a sign of trouble? If so, what can I do (short of replacing the pipe)?
Is it safe to use ammonia-based cleaners (aka Windex) on these pipes?