Stainless steel consists of iron with added alloy metals. The most common are:
The alloying metal react chemically with the oxygen in the air, forming a thin layer that protects the iron from corrosion.
When stainless steel is machined, the protective surface layer is penetrated, this is seen as dark tarnish around weld seams, cutting edges and burnt sanding dust.
The pickling process must remove harmful tarnishes - both the oxides and the underlying chromium-plated layer, after which the protective layer can be restored.
The pickling can be done either as dip pickling or as spray pickling.