Sometimes with years of use your pressure cooker/canner can become discolored. This discoloration on the inside of an aluminum pressure cooker is harmless. But if it bothers you, there is a way to get rid of it.
What you will need:
Vinegar or Cream of Tartar
Boil a mixture of either 1 tablespoon of vinegar or 2 tablespoons cream of tarter in a quart of water (in your discolored cooker).
Or you can scrub out your pressure cooker/canner with an aluminum cleaner.