Study on Initial Corrosion Behavior of Zn-Al-Mg Coating and Pure Zinc Coating in Typical Atmospheric Environment
The work aims to improve the understanding of corrosion behavior of Zn-Al-Mg coated steel in different atmospheric environments,and to study the composition and corrosion rate of corrosion products of Zn-Al-Mg coated steel in China area under different climatic conditions within 1 year.The structures of the GI coating and the ZM coating were studied by SEM,GDS and XRD,and the corrosion tests of the GI coating and the ZM coating in Turpan,Jiangjin,Qingdao and Wanning,were compared.There were no peaks of Mg and Al elements in the GI coating,but there were obvious Mg and Al elements in the ZM coating,which were evenly distributed in the coating.The surface of GI plate was a pure zinc coating,and there were a lot of binary eutectic phases composed of Zn and MgZn2 on ZM surfaces.The corrosion rate of Wanning GI was 4.56 times that of ZM,Jiangjin GI was 3.76 times that of ZM,Qingdao GI was 2.84 times that of ZM,and Turpan GI was ZM.The traditional electrochemical test adopted the solution system,which was equivalent to the corrosion of galvanized sheet in solution.The metal corrosion rate basically did not change with the thickness of the liquid film,and the diffusion of oxygen in it was slow,which became a control link of the test.In this paper,the flowing electrolyte solution was used for the test to increase the diffusion coefficient of oxygen in the electrolyte.For the GI coating,the self-corrosion current density in Jiangjin and Wanning was low,and for the ZM coating,the self-corrosion current density in Wanning,Hainan was the lowest.The atmospheric corrosion rate of zinc was mainly affected by relative humidity,chloride ion content,SO2 content,etc.,and the corrosion rate of zinc was mainly affected by the composition of corrosion products.The protection of corrosion products depended on its chemical composition,conductivity,adhesion,compactness,solubility,thickness,morphology and hydrophilicity.Different rust layers had different inhibition effects on cathodic oxygen reduction.Zn5(CO3)2(OH)6 had a good inhibition effect on oxygen reduction.The overall current density of the ZM coating was smaller than that of the GI coating,which was consistent with the atmospheric corrosion law.ZM material had obvious advantages in Wanning area with high salt and humidity,but less advantages in Turpan area with relatively mild corrosion environment.This was because different environments would affect the structure and properties of the corrosion products,and the structure and properties of the products would affect their corrosion resistance.The high chlorine environment had great damage to the protective corrosion of GI Zn5(CO3)2(OH)6,and it was easy to transform the corrosion products into loose and conductive ZnO in the high alkaline environment.In the high chlorine environment,Mg2+ ions which could react with OH were released due to anodic dissolution of MgZn2 in the coating to form magnesium hydroxide(Mg(OH)2).Replacing zinc oxide with Mg(OH)2 on ZM surface was considered to reduce cathodic oxygen reduction reaction(ORR),buffer the increase of pH value at cathode position,stabilize basic zinc carbonate,and significantly improve corrosion resistance.The ZM coating material can form stable corrosion products and reduce cathodic oxygen reduction rate in a high-salt and high-humidity environment,and has a broad application prospect in a typical atmospheric environment.
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