Preparation Of Sherardized-zn-al Composite Coating And Research On Its Resistant Corrosion

Fasteners are a type of mechanical parts widely used for fastening and connecting.With the development and utilization of the sea resource and scale, numerousinfrastructures have been constructed, which results in the increasing demand forfasteners. However, steel fasteners are easily corroded in marine environment. Varioussurface treatment technologies and coatings of fasteners were introduced in this paper. Asherardized-zinc-aluminum composite coating was designed and its preparationtechniques and corrosion resistance were studied, then it is expected to lengthen lifetimeof fasteners and safeguard the safe use of marine infrastructures.In this paper, the heating temperature, holding time and amount of activator forpreparation of sherardized coating were optimized. Sherardized-zinc-aluminumcomposite coating prepared was on sherardized coating. Morphology, composition andcrystal structure of the coatings were analyzed or observed by scanning electronmicroscopy (SEM), energy-dispersive X-ray analysis (EDS), X-ray diffraction (XRD) andX-ray fluorescence (XRF). The performances of composite coating such as adhesion andcorrosion resistance were investigated by cross cut test and neutral salt spray test. Opencircuit potential (OCP), polarization curve and electrochemical impedance spectroscopy(EIS) were used to investigate anticorrosion mechanism of coatings in corrosive medium.The main conclusions are as follows:(1) The optimal conditions for preparation of sherardized coating is380℃ofheating temperature,1.5hours of holding time and2%of activator content. XRD resultsindicated that sherardized coating is composed of Fe-Zn alloy, ZnO and Zn.(2) The salt fog spray time of sherardized coating is120hours. Electrochemical properties of sherardized coating indicate it corroded more slowly in seawater than in3.5%NaCl solution. The coating soaked in seawater after60days could still protect thesubstrate.(3) Post-treatment is used on sherardized coating by pickling method. Compositecoating forms on it. Composite coating has good metal luster appearance and its thicknessis25μm. Adhesion test is Gt1level by cross cut test.(4) Electrochemical properties of composite coatings show that the coatings gothrough destruction of the protective layer, active dissolution of metal powder andgeneration and deposition of corrosion products when they are soaked in corrosivemedium. OCP of composite coating is-0.74volt soaked in3.5%NaCl solution after60days, which shows that composite coating is more protective than sherardized coating andzinc-aluminum coating. Neutral salt spray test time of composite coating is2200hourswhile rust spots appear. It is because of synergistic effect between the coatings.