I did test steel a while back.. on a clean protected steel plate, just powder will not cause rust. BUT when I ignited the different powders, Pyrodex RS and P were the quickest to start the rusting process. One strange finding was, black powder, while it caused rust did not appear to be as aggressive a rust as some of the other powders. The best at the time I tested was Triple Seven. It took for ever for that stuff to start to rust. I used a lawn mower blade that I tested it on.