Hoppe's No. 9 is used on a Model 642 I've had since 1998 with no detrimental effects to the surfaces.

I've long wondered just how damaging Hoppe's No. 9 really was to nickel. I once had an ancient .32-20 Hand Ejector that had a rough looking nickel finish. I'd always heard that Hoppe's No. 9 would cause nickel to flake and come off. I decided I'd just strip the remainder of the bad looking nickel off and thought to try Hoppe's No. 9 on it. I bathed the revolver in Hoppe's and left it overnight for many weeks. I left parts of that revolver soaking in Hoppe's No. 9 literally for months. Nothing happened. This would have been in the early 1980s.