钼酸钠含量对无铬锌铝涂层性能的影响

Impact of Sodium Molybdate Content on Properties of Chromium-free Zinc-Aluminum Coatings

研究了钼酸钠(缓蚀剂)含量变化对无铬锌铝涂层形貌、成分、孔隙率、附着力及耐蚀性能的影响。采用SEM、EDS对涂层的形貌、孔隙率和成分进行分析,采用附着力测试仪对涂层附着力进行评价,通过极化曲线、交流阻抗谱测试对涂层耐蚀性进行分析。在钼酸钠质量分数为1.3%~2.6%时,所得涂层宏观上均光滑、平整且呈现出银灰色金属光泽;微观上均完整、致密、孔隙率低,鳞片状锌铝粉层层堆叠,形成了良好的物理屏蔽作用。在钼酸钠质量分数为1.3%~2.2%时,涂层的附着力等级为ISO-1级,但更高的钼酸钠含量使涂层的附着力水平下降。随着钼酸钠含量的增加,涂层的自腐蚀电位先减小后增大,在钼酸钠含量达2.6%后,涂层的自腐蚀电位因高于Q235钢基体而无法为其提供阴极保护。涂层的自腐蚀电流在9.141×10^-6~1.176×10^-4 A/cm^2范围变化,Rf值(锌铝粉表面膜层电阻)在212.1~649.5Ω·cm 2范围变化。在钼酸钠质量分数为1.8%时,涂层的综合性能最佳,涂层的自腐蚀电流密度最小,仅9.141×10^-6 A/cm^2,Rf值最大达649.5Ω·cm^2,锌铝粉表面腐蚀产物膜层最为致密,且结合力高,耐蚀性能最好。

In this study,we sought to find out the alterations in morphology,composition,porosity,adhesion and corrosion resistance of chromium-free zinc-aluminum coatings caused by variation of sodium molybdate(inhibitor)content.Specifically,SEM and EDS was adopted to characterize the morphology,porosity and composition of the coatings.The adhesion of the coatings was evaluated by adhesion tester.The corrosion resis-tance of the coatings were analyzed based on polarization curves and electrochemical impedance spectroscopy.When 1.3wt%~2.6wt%sodium molybdate was contained,the coatings were glossy and smooth with silver-gray metallic luster in appearance.Meanwhile,the coatings were complete and compact with low porosity according to the microscopic observation,and the sheet-like zinc-aluminum powders stacked layer by la-yer,forming an effective physical shielding.When the content of sodium molybdate ranged from 1.3wt%to 2.2wt%,the adhesion of coatings leveled ISO-1.While higher sodium molybdate content would do harm to the adhesion of the coatings.As the sodium molybdate content increased,the self-corrosion potential of the coatings first dropped and then went up.When the content of sodium molybdate exceeded 2.6 wt%,the coating held a self-corrosion potential higher than Q235 steel matrix,and failed to provide cathodic protection for the matrix.Furthermore,the self-corrosion current of the coatings varied in the range of 9.141×10^-6—1.176×10^-4 A/cm^2,and the Rf value(the resistance of zinc-aluminum powders surface film layer)varied in the range of 212.1—649.5Ω·cm^2.The coatings achieved the best comprehensive performance under the sodium molybdate content of 1.8wt%.In this case,the coating exhibited the minimum self-corrosion current density of only 9.141×10^-6 A/cm^2,and the highest Rf value of 649.5Ω·cm^2.As a result,the corrosion product film on the surface of zinc-aluminum powders was the most compact,with strong adhesion and optimal corrosion resistance.