Effect of Calcium Oxide on Black Phosphating Bath for Steel-iron

The effect of calcium oxide (CaO) on phosphating was studied to obtain feasible black phosphating bath of Zn-Ca system.The components of the phosphating bath were analyzed by X-ray fluorescence (XRF).The structure and growth rate of the phosphating film had been studied,and polarization curves of Q235 steel in the phosphating bath had been tested.Results showed that the concentration of Ca2+ in the phosphating bath changed dramatically with increasing of CaO,The diameter of the crystals decreased obviously and the film becames compact.The content of Ca and the growth rate of the film increased with prolong of phosphating time.When 0.50 mol/L CaO was added,the black phosphating coating with 6.32% Ca and 14.62% Zn was gained,the diameters of crystals were 3~5 μm,the weight of coating was 2.3 mg/cm2,and the polarization capability of the phosphating bath becames lower.