Stainless steel - is the stainless just a coating or is it throughout

I have a thick object made out of stainless steel, I want to cut it down in size but if my plan works, the new cut piece will be placed in an area where it will be exposed to salt water, my question is simply this:

What exactly is stainless steel? is it a coating that prevents corrosion, or is it a property mixed into the metal, meaning that wherever it is cut, it will still remain corrosion-proof?

When stainless steel is cast, the anti-corrosion elements move toward the surface, preventing corrosion. While Ashley is cor