I use stainless tanks for all my cleaning and boiling when rust blacking but have a mild steel tank for caustic after getting some coloured sheen on a couple of parts which I'm told was down to a galvanitic reaction. It was greeny blue or dark red tint which showed in certain light . Quiet odd really, I don't understand how it happened but the plain steel tank cured it . I had my tanks made in a local fabrication shop, works out pretty cheap, plus you can decide on the depth and width.

Rust never sleeps !