The alloy metals react chemically with the oxygen in the air, forming a thin layer that protects the iron from corrosion. When stainless steel is processed, the protective surface layer is penetrated. This is seen as dark tarnishing around welded seams, cutting edges, and burnt grinding dust. The pickling process is used to remove harmful discoloration – both the oxides and the underlying de-chromed layer. After this, the protective layer can be restored.