Abstract:

Zinc-manganese phosphate films were prepared on 16Mn steel from the phosphating solution containing lanthanum nitrate in order to improve the corrosion resistance of 16Mn steel,and the effect of mass concentration of lanthanum nitrate on the phase composition, surface morphology and corrosion resistance of phosphate films was studied.The results showed that lanthanum nitrate had almost no effect on the phase composition of phosphate films,but it could change the flatness and density of phosphate films,thus affecting the corrosion resistance.Appropriately increasing the mass concentration of lanthanum nitrate made the surface of phosphating films tend to be smooth and compact,and the corrosion resistance was gradually improved.However,when the mass concentration of lanthanum nitrate was too high,the surface of phosphating films was rough and the denseness reduced,resulting in the decrease of corrosion resistance.When the mass concentration of lanthanum nitrate was 50 mg/L,the as-prepared phosphate film had maximum charge transfer resistance,resistance value at frequency of 0.01 Hz and discoloration time of droplet,which was 5.028×10³ Ω·cm²,3.12×10³ Ω·cm² and 186 s respectively.This phosphate film exhibited better corrosion resistance than that of other phosphate films,the reason was that appropriate amount of lanthanum nitrate could accelerate the film forming rate,which was conducive to the formation of tight and compact phosphate film,and had strong ability to resist the erosion of corrosive media,thus effectively improved the corrosion resistance of 16Mn steel.

Key words: zinc-manganese phosphate film, corrosion resistance, lanthanum nitrate, electrochemical impedance spectrum, surface morphology