If I understand correctly you must be referring to 'Sensitization' of Stainless Steel 'cause this term is used specically for this material.
When Stainless Steels solidify from molten condition, the last traces containing relatively more carbon content deposit along the grain boundaries of the material. When this S.S. is welded certain regions a little away from the weld joint are heated to temp. of 500-800 deg. C. At this temp. range the carbon from the grain boundaries combines with Cromium adjacent to the boundaries to form Chromium Carbide. In doing so, there is depletion of Chromium in the adjacent areas of the grain boundaries. This causes the S.S. to loose corrosion resistance in these areas as the chromium content goes well below the min % required. This is called 'Sensitization'.
Passivation is the process of creating a strong layer of oxide to give corrosion resistance on the surface of a metal such as Carbon Steel or Stainless Steel by cirulating or applying suitable chemicals.
Sensedization
Re: sensedization
