Tetanus is caused by bacteria. It's a common misconception that this bacteria is inherently found on rusted metal but that's not true, it's found in soil. We've been taught that getting injured on a rusted piece of metal can cause tetanus because for starters, it's more likely to find rusted metal outside than inside, and secondly, it's more likely that rusted metal will injure you than non rusted metal since generally speaking people don't just leave out sharp or dangerous pieces of metal around, but rusted metal can still cut someone because of how the metal turns from smooth to jagged.

If you get cut on rusted metal at a playground for example it could very well contain tetanus, or a fence, or a sign post, or something like that, because it's outside and driven into soil. Rusted metal that's not come in contact with any possible sources of tetanus bacetria can't just have them because it's rusted.