Thank you all. They're about 3yrs old. We clean them and all, but over time it just happened. Im not sure what type of metal they're made of. Im still trying to search that.
As for the suggested cleaning methods, i have tried baking soda, soaking in Dawn and an All Purpose Cleaner that I use for everything because it's so effective. Nothing has worked. I will try oven cleaner next to see how that fairs.