Research Of Non-Chromium Passivation On Hot Dip Galvanized Sheet

With Cr⁶⁺ for chromate having the toxicity, disadvantage in environmental protecttion, its application at domestic and international suffer the restrict already and gradually, according to present condition. Now, there is no non-chromium passivation craft which can substitute chromium passivation. So the need of looking for a kind of replacing the green craft is very urgent affair.The research compared the passivation result with different system. It is discovered that the phytic acid and sodium in system is nearer the result of chromate than other systems.The influence of assistant agents on passivation films was studied specially. The optimum composition of the passivation solution was gained by orthogonal test as follow:phytic acid10g/L,sodium30g/L, H₂O₂40ml/L,H₂SO₄10ml/L, H₃PO₄8ml/L, HNO₄5ml/L,A4g/L.The best process conditions are:treating time 20-30s,temperature 30-50℃,dry temperature 50-60℃,dry time 10min,stop air time:20-30s,pH 2-4.And found that phytic acid, sodium silicate system of passive film generated by the silane treatment, the corrosion resistance close to the level of chromate.Further study on corrosion resistance and corrosion mechanism was carried on. Zinc samples treated by phytic acid, sodium silicate and DB-560 silane were being soaked by 3.5% NaCl to experiment for 7 days and not be seen white spot appearing. Polarization curve and EIS on different passive film and galvanized film were investigated.The electrochemical parameters of electrochemistry were gained by computer fitting. Using the parameters of E_corr,I_corr,P_po and R_d,the corrosion resistance of passive film and chromate film was compared and analysised. The results showed that: the corrosion current density of zinc samples treated by phytic acid, sodium silicate and DB-560 silane is smallest, the resistance Rd is the blank sample's 120 times, and it have the best corrosion resistance.The corrosion current tested by Tafel test and the polarization resistance by EIS method indicated the different corrosion mechanism to substrate and higher corrosion resistance; explained the influence of different system on the content of films, corrosion resistance,corrosion mechanism and illuminate the function of assistant component with scan electron microscope and surface energy spectrum.