Studies On The Deterioration Process Of Sintered Zinc-aluminum Coating And Composite Coating

Sintered zinc-aluminum coating, which developed rapidly since it has been originated, is widely used for steel fastener against corrosion. It provides corrosion protection with features such as barrier protection, chromate passivating and sacrificial protection. In marine corrosion environment, the coating anticorrosion is effected by its weak waterproof and inevitable defect during application.In this paper the preparation process, the formation mechanism, the corrosion resistance mechanism of sintered zinc-aluminum coating and the study methods of coating failure process are introduced. The samples of sintered zinc-aluminum coating on steel substrate are prepared with artificial defect in different width. The anticorrosion performances of perfect and defected coatings are characterized by neutral salt spray test, dip immersion and wet-dry cyclic tests in seawater. The results show that the anticorrosion performance is few affected when the defect is less than 1mm, but the coating exhibits bad anticorrosion performance when the defect is larger than 3mm.The deterioration processes of the sintered zinc-aluminum coating are studied by polarization curves, electrochemical impedance spectroscopy and scanning vibrating electrode techniques. The results show that passivation range of defected coating is shorter than that of the perfect. The corrosion current increases and the polarized resistance decreases while the defect width increases. Both of the perfect coating and the defected one are mainly exhibit cathodic protection lasting a long period. The electrochemistry corrosion easily reacts with the defect width increase. The results of SVET show that the coating represents self-healing function for the defect.Composite coating was prepared by adhereed the epoxy Al-rich coating on sintered zinc-aluminum coating. The composite coating is more compact than the sintered zinc-aluminum coating by scanning electron microscope (SEM). The composite coating thickness is only 5μm thicker than that of the sintered zinc-aluminum coating. The composite coating adhesion, tested by cross cut test, is little higher than that of the sintered zinc-aluminum coating. The anticorrosion performances of composite coating are much better than that of the sintered zinc-aluminum coating. The composite coating can endure more than 2880 hours during salt spray test, more than 120 days during dip immersion test and more than 182 wet-dry cycles during wet-dry cyclic test. The potential variety of the composite coating during dip immersion and wet-dry cyclic tests is more stable than that of the sintered zinc-aluminum coating. The epoxy Al-rich coating can decrease the corrosion current and increase the polarized resistance of the coating. A typical Warburg impedance character progressively appears at the initial stage; Al flakes of the epoxy Al-rich coating exhibit cathodic protection with the immersion time.The samples of composite coating are prepared with artificial defect about 0.1 mm and 1 mm. The anticorrosion performances of the defected coating are characterized by neutral salt spray test, dip immersion and wet-dry cyclic test in seawater. The results show that the anticorrosion performance of the 0.1 mm defected coating is much higher than that of the 1 mm defected coating. The defected coating obviously exhibits passivation with the increase of potential. The steel is protected by the coating which provides cathodic protection.The anticorrosion tests results of the defected coating show that composite coating exhibits more sensitive to defect than the sintered zinc-aluminum coating. The composite coating exhibits well anticorrosion while the defect is very minute. The anticorrosion of the composite coating decrease rapidly with the increase of defect width.In conclusion, the sintered zinc-aluminum coating can exhibit high anticorrosion performance, while the defect confined certain range. Those provide foundation for the application of sintered zinc-aluminum coating in marine environment. The organic top coating technique affords new method for the application of sintered zinc-aluminum coating in seawater. It should notice that composite coating is more sensitive to defect than that of sintered zinc-aluminum coating.