Chiang Mai Journal of Science

Post-treatment  by  sodium  silicate  after  zinc  phosphating  on  cold-rolled  steel  was  studied. The  steel  substrates  were  firstly  phosphate  in  a  zinc-phosphating  bath  containing  ZnO, NaF, H₃PO₄  and  Ni(NO₃)₂·6H₂O  under  controlled  conditions. Post-treatment  by  sodium  silicate  was  consequently  performed  by  varying  the sodium  silicate  concentration  and  immersing  time  in  the  range  of  33.3-100 wt%  and 1-10 minutes, respectively. The  coating  layers  before  and  after  post-treatment  were  characterized  by  X-ray  diffractometry  and  scanning  electron microscopy-energy  dispersive  X-ray  microanalysis. The  phosphated  layer  was  formed  to  be mainly  amorphous  sodium  silicate. The  optimum  post-treatment  condition  was using  the  33.3wt%  sodium  silicate  solution  at 5 minutes  immersing  time, by  which  an  increase  in corrosion  resistance  can  be adequately  obtained  after  post-treatment.