Study On Failure Behavior And Corrosion Resistance Comparison Of Hot-dip Prepainted Sheet

Hot-dip prepainted sheet is coated with organic layer on hot-dip sheet, combinating both of their advantages. It is widely used in construction, household appliances, transportation and etc. Some failure problems will inevitably exist during service of prepainted sheet, which can not only cause serious economic loss, but also damage the safety of engineering projects. Therefore, it’s urge to study the failure behavior and mechanism of prepainted sheet. Al-Zn alloy coating and pure zinc coating are the two most common metal coatings for prepainted sheet. This article studied the failure behavior of zinc coating prepainted sheet produced by the cooperative enerprise, and compared the corrosion resistance of zinc coating prepainted sheet and Al-Zn Alloy coating prepainted sheet, and the corrosion resistance of anti-fingerprint and passive film on hot-dip Al-Zn sheet. The detailed contents are as following:(1)Stereomicroscope, metallomicroscope, leakage detection, scanning electron microscopy(SEM), energy spectrum analysis(EDS)and corrosion weightlessness experiment were used to study the failure behavior of zinc coating prepainted sheet. The results showed that the corrosion cause included thin zinc coating, leakage of organic coating and irregularity of substrate, etc. In addition, we put forward solving measures: firstly, selecting the surface smooth cold-rolled sheet. Secondly, ensuring the surface of galvanized sheet is clean before painting, no contamination from grease and particle impurity. Lastly, the thickness of zinc layer should not be less than 70g/m2 in order to ensure the corrosion resistance of prepainted sheet.(2) Metallographic observation, cyclic corrosion test, electrochemical impedance spectroscopy(EIS), scanning electron microscopy(SEM) and energy dispersive spectrum(EDS) were used to study the corrosion resistance of zinc coating prepainted sheet and Al-Zn alloy coating prepainted sheet which have the same thickness of metal coating(20um). The results showed that prepainted sheet of Al-Zn alloy coating has better corrosion resistance than that of pure zinc coating due to the unique two-phase structure of Al-rich and Zn-rich. In corrosion test at 250 cycles(a total of 2000h), serious red rust appeared on the prepainted sheets of zinc coating, and no obvious corrosion on the prepainted sheets of Al-Zn alloy coating. In the EIS test, the impedance value of prepainted sheets of pure zinc coating began a sharp decline after 2 days immersion, while the impedance value of Al-Zn alloy coating prepainted sheets began a sharp decline after 15 days immersion.(3) Immersion test, polarization curve, electrochemical impedance spectroscopy(EIS), scanning electron microscopy(SEM) and energy spectrum analysis(EDS) were used to study the corrosion resistance of anti-fingerprint and passive film on hot-dip Al-Zn sheet. The results showed that anti-fingerprint sample has better corrosion resistance than passive one due to the compact structure on the surface. In 3.5wt.% NaCl solution, the corrosion rate of anti-fingerprint sample is 0.553 time of passive one. In polarization curve test, the anode and cathode reaction of anti-fingerprint sample are inhibited in varying degrees, and the corrosion current density is 0.041 time of passive one. In EIS test, impedance value of anti-fingerprint sample is one magnitude higher than that of passive one.