have some stainless steel that is rusted, I thought stainless steel couldn't rust?

Stainless steel can develop rust. It is much less likely to rust but it can still rust. Leave a wet cast iron skillet in a stainless steel sink and you'll see a rusted ring in your sink. The stainless steel didn't rust but the pan stained the surface and it will not be easy to remove.

It is also possible for stainless steel to corrode (rust). There are over 150 grades of stainless steel and some are more prone to corrosion than others. Generally, the higher the chromium content, the less likely the steel will rust. However, over time, rust can and will develop on stainless steel. To prevent stainless steel from rusting, you need to passivate it. Passivating stainless steel is a process that allows stainless steel to retain its corrosion resistance.

above information provide by Shenzhen oem metal stamping fabrication