The poison seeps through the Pokemon's rocky body due to cracks and whatnot. This is unlike the Steel type, which lets the poison bounce or slide off the Pokemon's body harmlessly.
But acids and such (such as the move Acid) should make steel compounds degenerate
This isn't necessarily true and I keep having to say this in topics.
If you put 98% sulfuric acid into a carbon steel pipe it will NOT DO ANYTHING TO IT. Steel is very resistant to many pure acids. --- "As I was walking up the stairs, I met a man who wasn't there, he wasn't there again today, i wish I wish he'd go away."