Effect of additives on the properties of phosphate conversion coating on electrogalvanized steel sheet

• Ni2+ and Mn2+ cations in the solution increase nucleation sites of phosphate grain.
• Ni2+ and Mn2+ cations in the solution reduced the phosphate coating porosity.
• Phosphate conversion coating with fewer pores shows better corrosion resistance.
• Ni2+ in the solution is first reduced to Ni, which accelerates Zn dissolution.
• Mn2+ in the solution enhances nucleation of ZnMn2(PO4)2·4H2O.

Ni2+ and Mn2+ ions have been added separately to the phosphate solution to minimize the porosity of the phosphate conversion coating on electrogalvanized steels. Results showed that the Ni2+ or Mn2+ in the solution reduced the grain size and porosity of the phosphate coating; thereby, the corrosion resistance was enhanced. However, Ni2+ and Mn2+ played a different role in the coating formation. Ni2+ in the solution was reduced by Zn to form Ni, which enhanced the dissolution of Zn to promote the nucleation of hopeite grains. Mn2+ in the solution facilitated the nucleation by increasing the impingement of reacting species.