Effect of Surface Segregation during Annealing and Surface Oxide Film on Phosphate Coating of Cold Rolled Steel Sheet

Abstract

The effects of the segregation of some elements and the transmutation of surface oxide film induced during box annealing on the phosphating of cold rolled steel sheet have been studied through the measurement of spontaneous potential in the KH₂PO₄/NaOH buffer solution of pH 7 and the surface analysis by IMMA, ESCA and F. X..
The first stage of the phosphate reaction is the dissolution of oxide layer at the steel surface. The dissolution rate of oxide film depends largely on the bonding state of oxygen in the surface (whether Fe-O bond or Fe-OH bond).
The layer having Fe-OH bond decreases the rate of phosphate nucleation.
The second stage is the growth of phosphate film. It is considerably affected by the enrichment of Mn and P at the steel surface that occurs during annealing. These elements are identified as the oxide state and have influence on the dissolution of iron, because of constituting the local cell with matrix iron. The Mn enrichment accelerates iron dissolution and so enhances the growth of phosphate film. But the P enrichment behaves oppositely.