The difference between pickling and passivation?

Though pickling do improve corossion resistance but never to the same degree as passivation itself. The chart on the right is self-explanatory. Pickling is the removal of any high temperature scale and any adjacent low chromium layer of metal from the surface of stainless steel by chemical means. While, Passivation is the treatment of the surface of stainless steels, often with acid solutions (or pastes), to remove contaminants and promote the formation of the passive film on a freshly created surface (e.g. through grinding, machining or mechanical damage). After pickling, passivation should be done to obtain maximum corrosion resistance.