Study On The Corrosion Behavior Of Zink-Nickel Composite Coating On Q235 Low Carbon Steel

Low-carbon steel is widely used due to its low cost and processability,but it is prone to corrosion damage.This paper aims at the preparation of trivalent chromium passivation film on the surface of low carbon steel containing zinc and nickel coating to achieve industry standard color passivation,and greatly improves the resistance to salt spray and seawater corrosion of low carbon steel through subsequent silane sealing treatment.By studying the influence of passivation process parameters and curing temperature and time of the closed treatment on the microstructure and corrosion behavior of the corresponding film layers,The relationship between coating process parameters and corrosion performance was obtained.The forming process and corrosion resistance mechanism of the sealant coating were discussed.Scanning electron microscopy(SEM)observations show that cracks occur due to surface dehydration after passivation.The depth,number,and size of cracks change with the change in passivation temperature and passivation time.Since the passivation film deactivates the hydrogen ions in the solution during the formation process,the zinc in the zinc-nickel coating on the surface of the low carbon steel consumes a higher concentration of hydroxide ions in the passivation solution,thereby forming a passivation layer.The effect of passivation treatment process on the thickness of passivation layer is not obvious,but with the increase of the temperature of passivation solution,the passivation time is prolonged,and the depth of surface cracks on the passivation layer is increased and the number is increased.Corrosion behavior studies show that with the increase of passivation p H,passivation time and passivation temperature,the corrosion resistance of the passivation layer shows a trend of increasing first and then decreasing.When the p H of the passivation solution is 1.5,the temperature is 35°C,and the passivation time is 45 s,the corrosion rate is the smallest,the polarization resistance is the highest,and the passivation layer has the highest corrosion resistance.The effect of the sealing treatment temperature on the corrosion resistance of the coating is more significant than the sealing time.When the sealing treatment temperature exceeds 175°C,the corrosion resistance of low carbon steel is greatly improved.When the curing temperature is 200°C and the curing time is 30 minutes,the corrosion current density is reduced by two orders of magnitude compared to the unsealed coating.The polarization resistance is about 43 times that of the unsealed coating,and the resistance to salt spray corrosion increases 28 times.The sealing layer effectively improves the corrosion resistance of the composite coating by filling and covering the crack formed by the passivation treatment.Because the surface of the silane-blocking layer is relatively smooth and has its own hydrophobicity,it can isolate the corrosive medium from the metal-plating layer,thereby effectively protecting the substrate and improving the corrosion performance of the composite coating.