Effect of rare earth on the coating-forming and mechanism of phosphatization

It was ascertained that when a RE element was added in bath, the sample was improved on the anti-corrosion power of the coating because of the increasing of covering rate of formless crystal Zn2Fe(PO4)2·4H2O (marked P) crystals and the ratio of P/(P+H) (H was the mark of Zn3(PO4)2 crystal) in the coating, combination of which with components parsing by EDS indicated that the sequence of contribution elements P and Zn to erosion resistance of coatings was P>Zn. And the correlative mechanism was discussed, which made it clear that owing to the particularity of the outer-shell electron structure and larger ionic radius, RE was so easy to be polarized and metamorphosed itself that it adsorbed lightly on the basic body to pose gels. They efficiently reduced the activation energy which was required for formation of a new solid phase of phosphates and made it also possible to engender effectively active nucleation regions of cathode and anode under low temperature phosphating condition, which was propitious to formation, densification and uniformization of the phosphate crystal nucleus and growth of the crystallite and coating buildup. Thus it could be seen that REN played the role of surface regulator and accelerant, which speeded up the phosphating, as well as bids amount of porosity of the coating fall to improve the corrosion resistance of the coating.