Waterborne Zinc And Aluminum Anti-corrosion Coating

Since 21^th century, anticorrosive industry is confront with the pressure from both environment-protection and energy-conservation. Silane-base treatments can replace the traditional treatments with remarkable advantages in relation to environmental, safety and costs issues.The Dacromet coating with high corrosion resistance technology was a popular metal anticorrosion technology for many metals. Its applications in some fields such as automobile industry were getting increasingly common because of the advantages of less pollution, excellent anticorrosion property and non-hydrogen embrittlement. Since the conventional Dacromet treatment agent contains hexavalent chromium ions, which are hazardous, adverse effects on the human body, and environmental contamination, due to the elution of chromium from protective films have been a serious concern. Furthermore, legal restrictions have been imposed in various countries on the use of hexavalent chromium even in rust inhibitors for metallic surfaces. In response to society's need for new, environmentally compatible products, many research has endeavored to develop new coating systems that are environmentally friendly. In the countries such as Germany,America,Japan and Korea ,many kinds of non-chromate zinc/Aluminium coating have developed.The objective of this thesis is to develop an environmentally benign coating,which does not utilize harmful chromium compounds and can be used for corrosion inhibition of a metal surface, comprising a slurry mixture in which coated zinc and aluminum fine powder treated with a hydrophilic coupling agent is suspended in an silane bonding solution. The optimal conditions were screened out..Since the coating hasn't any chromium ,and free of any other toxic compounds, it has friendly environmental performance. And because it composed of a large number of metal flakes, the coating is conductive. As the pre–processing of the coating doesn't have acid washing, the coating is hydrogen embrittlement free.The appearance of coating was observed and studied through microscope, SEM, and EDS. The result showed that: the coating combined with metal substrates tightly, and the thickness of coating was uniform. Corrosion potential and electrochemical impedance spectroscopy ( EIS) measurement s were used to study the corrosion electrochemical behavior of the modified coatings immersed in 3.5% NaCl solution. The results indicated that the process of action of the coating during immersion presented three stages. At the initial stage, the flaked zinc powder on the surface layer of the coating started to activate due to the attack of the electrolyte. In the middle stage, the coating exhibited electrochemical protection for the substrate. For the third stage, the barrier protection of the coating predominated. The middle stage lasted a longer period than the third stage, suggesting a strong cathodic protection and a weak barrier mechanism of corrosion prevention of the silane zinc–aluminum coating.Further more, in this thesis, we also study the corrosion properties of the coating with three different inhibiting blended in definite proportion. The results indicated that the addition of inhibitors can improve the protection performance of the coating, mitigating the consumption of the zinc and aluminium powder in the coating. Among the three inhibitors phosphomolybdate is the best, while the effect of cerium chloride is better than that of sodium molybdate.In order to put this process into industrial use, there are still many problems waiting for further research and amelioration.