Passivation has two meanings. One is, as stated above, the process of acid cleaning which removes foreign matter, especially iron, which hinders the formation of a sound passive film. The other more fundamental meaning relates to the formation of the passive film itself, which on stainless steel forms spontaneously in air or water. It consists of several atomic layers of oxygen and hydrogen bonded to the chromium-rich surface in such a way that ionic conductivity through the layer is negligible, hence precluding dissolution of the alloy.
The only problem with the first meaning is that it leads some people to believe that the acid wash process creates the passive film rather than simply removing contamination which hinders its formation; a small but important distinction.
Incidentally, a acid wash can also pickle. This process is the removal by dissolution of surface layers of the alloy. After oxidation the surface can have lower levels of chromium than the bulk composition and if this layer isn't removed then the lower chromium surface produces a passive film which is "weaker" (actually thicker, but less effective in preventing corrosion). Some passivation is done with strong enough acid than some pickling occurs, which is why some people think passivation creates the corrosion resistance. After welding when scale forms and is removed abrasively, pickling must be done to remove the chromium-depleted surface. At the same time oxide particles are also removed, so two important things are done in one process. As if this isn't confusing enough, the weld's corrosion resistance is still inferior because there can remain some microscopic chromium-depleted regions around manganese sulfide inclusions in the weld zone. Electropolishing or chemical polishing can remove these pit-initiating zones and restore full corrosion resistance.
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