How to remove black chemical stain on stainless steel?

Try soaking in 50 % bleach water.

Hi Don,

Here are some other ways to clean up stainless steel. They start with natural materials and move forward from that. One of these ideas might help. There is also a section about the chemistry of stainless steel. Wishing you the best.

https://www.thespruce.com/remove-brown-stains-from-stainless-steel-1900549

https://www.wikihow.com/Remove-Stains-from-Stainless-Steel

Hi Don: You can try pouring a little white vinegar or apple cider vinegar onto the stained surface. Let it sit for a few minutes; the gentle acid in the vinegar may lift out the stains. Rub the stained with a soft cloth, following the grain of the steel. Rinse thoroughly, and dry the surface with a clean towel. Or:

Apply a mixture of baking soda and liquid dish soap, the consistency of toothpaste, to a microfiber cloth or another soft cloth, or you can try using a magic eraser. Then rub the stain, moving back and forth in the same directions as the grain in the metal, not in a circle. After scrubbing, rinse the stainless steel surface thoroughly, and towel it dry.

That should do it. Good luck