Corrosion Behavior Of Typical Fasteners In Simulation Marine Environment

On the ship and in marine engineering, a lot of steel fasteners are used for connecting. Corrosion is one of the important reasons for failure of fastener occurred in the marine environment. Typically, surface treatment is required to improve the corrosion resistance of fasteners. Galvanized passivation coating, with its simple process, thermostability, anticorrosion performance, density and aesthetic et al, is extensively developed and applied in the marine environment. In addition to the features of zinc-aluminum coating, such as barrier protection, chromate passivating and sacrificial anode protection, epoxy rich-aluminum/sintered zinc-aluminum coating (referred to the composite coating, the same hereinafter), in terms of corrosion resistance and coating compactness, presents characteristics suitable for marine application.Corrosion of steel fasteners used in the marine environment is closely related with the temperature, salinity and other environmental factors. In this paper the corrosion influence facters in marine environment, surface treatment of the fasteners and the study methods of coating failure process were introduced. Galvanized passivation coating and composite coating were seletced for protective treatment of45#steel fasteners. Their corrosion behavior in simulated marine environment was studied in the laboratory.Surface morphology and basic properties of two kinds of coatings were studied by scanning electron microscopy(SEM), thickness test and the adhesion strength test. The thickness and adhesion of galvanized passivation coating were15～20μm and0～1grade. The thickness and adhesion of composite coating were16～20μm and1～2grade. The adhesion strength of the composite coating was slighterly lower than that of the galvanized passivation coating. The corrosion behavior of two kinds of coatings was studied by corrosion potential and electrochemical impedance spectroscopy. The two kinds of coatings were immered in lwt%,3wt%,5wt%,7wt%NaCl solutions and20℃,35℃,50℃seawater. What’s more, the neutral salt spray test was used to simulate the corrosion behavior of the two kinds of coatings in marine atmosphere environment.The two kinds of coatings could efficiently protect the steel substrate both in NaCl solutions with different content and seawater.Give priority to the influence of Cl" concentration. The corrosion processes of galvanized passivation coating showed three stages in NaCl solutions:the dissolution of passivation film, the sacrifice anode protection of galvanized coating and the corrosion of steel substrate. In NaCl solutions with different concentration, the corrosion processes of galvanized passivation coatings had the same discipline. The corrosion processes of composite coatings also showed three stages:the damage of epoxy rich-aluminum layer and the corrosion of Al, electrochemical protection of zinc-aluminum coating, the barrier protection of corrosion products. In the scope of NaCl concentration that had been studied, the corrosion processes of composite coatings showed the same trend.In the initial stages, the corrosion resistance of two kinds of coatings first decreased and then increased with Cl-concentration increasing. Two kinds of coatings showed the poorest corrosion resistance in5%NaCl solutions. Comparative study in5%NaCl solutions showed that the reaction resistance of the composite coating was242kΩ·cm2. The reaction resistance of the galvanized passivation coating was3.53kΩ·cm2.The results in seawater showed that the corrosion processes of galvanized passivation coatings and composite coatings were the same as that in NaCl solutions. Due to large changes of seawater temperature in the practical application, the corrosion behavior of two kinds of coatings in20℃、35℃、50℃seawater was studied.With the temperature increasing, the coating resistance of the galvanized passivation coating decreased from1.05kΩ·cm2to0.59kΩ·cm2. The coating resistance of the composite coating decreased from15.8kΩ·cm2to10.3kΩ·cm2. Comparing the corrosion resistance of two coatings in50℃seawater, the results showed that the reaction resistance of composite coating and galvanized passivation coating was95.9kΩ·cm2and24.1kΩ·cm2in the initial immersion. However, the corrosion resistance of composite coating decreased more obvious than galvanized passivation coating at the last stage。The neutral salt spray test, which was used to simulate marine atmosphere environment, showed that the two coatings both had good protective properties, the corrosion resistance of composite coating was significantly better than galvanized passivation coating. Times of appearing red rust of composite coating and galvanized passivation coating were1780h and420h, respectively.