| Trivalent chromium conversation coating(TCCC) on hot-dip galvanizing has a lot ofadvantages compared to other chromium-free passivation，such as low cost, easy operation,technical maturity and so on, it’s one of the potential alternatives of the chromate coatingsince the toxicity of TCCC is only one percent of it; However，TCCC is porous and of lowcorrosion resistence. The surface and crack area of the basic cotatings will be covered withsealers or new compounds coatings after sealing treatment, partial self-healing ability willalso be received. Therefore, sealing treatment is the most important way that should bedeveloped, and it will greatly improve the corrosion resistance of TCCC.The formation mechanism of TCCC was systematicaliy studied by using mass gain-time,open circuit potential-time, SEM(scanning electron microscope), EDS(energy-dispersiveX-ray spectroscopy), AFM(atomic force microscope) and XPS(X-ray photoelectronspectroscopy), and the corossion mechanism was investigated by using NSS(neutral saltspray test), salt water immersion test, linear polarization and electrochemical impedancespectroscopy (EIS) technique. The influence of Na2SiO3as sealer on the micostrcuture andcorrosion resistance was studied. Conclusions are as follows:1. The optimal process parameters for the basic coating using orthogonal and singlefactor experiments are as followings: Cr2(SO4)3,40g/L, HNO3,25ml/L, H2BO3,2g/L, pH,3-4,temperature,70℃, time,1min; The increase and decrease of weight gain-time and open circuitpotential-time curves indicate that the coating will crack with time increase, and the coating isthe best when the time is inflection point60s. In summary, the formation of TCCC mainlyincludes the dissolution of zinc, the deposition of zinc ions and trivalent chromium ions anddrying coating-forming; Zinc hydroxide and oxide (Zn (OH)2and ZnO) and chromiumhydroxides and oxides (Cr (OH)3and Cr2O3) are the main components.2. The addition of sodium silicate (Na2SiO3) who exists in a physical way does not alterthe basic morphology of TCCC; the density of modified TCCC changes with the increase anddecrease of open circuit potential-time curve;60s is the best sealed time as the result of EIS,the reason may be that the rate of coating dissolution is more quickly than that of sealantdeposition through they react simultaneously when time goes too long.3. The polarization resistance of TCCC is6.05KΩ, and is61.24KΩ to the modifaiedcoating, both have a heavily improvement; The cathode and anode polarization curves bothmove to decreasing direction of the current density, but the offset degree of cathode branch issignificantly greater than the anode one to the basic coating, but just the opposite to the modified coating, which indicates that the suppress of the cathodic reaction are more effectivecompared to anode reaction to the basic coating and the opposite to the modified coating.4. Corrosion process test results show that the TCCC can be well sealed by sodiumsilicate (Na2SiO3) since the corrosion resisitence is greatly improved and the anti-corrosioncycle is delayed; The EIS show a capacitance ring in the first, but began to appear diffusionresistance and gradually move towards the low frequency end with the time, suggesting thatthe corrosion media began to penetrate into the coating, and reach substrate interface slowlyin the following time. |
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