Effects of Rare-Earth Metal on Microstructure and Corrosion Resistance of Arc-Sprayed Zn-Al-Mg Coating

Cored wires and high velocity arc spraying technique (HVAS) were applied to produce Zn-Al-Mg and Zn-Al-Mg-Re alloy coatings on low carbon steel substrates. And the effects of rare-earth metal on microstructure and corrosion resistance of the Zn-Al-Mg coating were investigated. The microstructures and mechanical properties were studied by SEM, EDS and XRD. The coatings show a typical aspect of layered thermal sprayed material structure. SEM results revealed that the addition of small amount of REM to the cored wires would result in a fine grained structure in the coating layer together with a dense microstructure, which is the reason for the adhesion strength enhancement and the porosity reducing of the coating. And the electrochemical corrosion mechanisms of the coatings were discussed. Chemical analysis of the coating indicated the composition to be Zn-16.5Al-5.9Mg-4.6O-RE (wt%). The phases of the coatings are Zn, Al₅Mg₁₁Zn₄, MgZn₂ and Al₃Mg₂ mainly, together with oxide ZnO, ZnAl₂O₄, and MgAl₂O₄. The electrochemical corrosion behaviors of Zn-Al-Mg-RE coating were investigated in 5%NaCl solution comparing with Zn-Al-Mg coating. Electrochemical measurements in the forms of potential-time and potentiodynamic polarization tests showed that such two coatings behaved excellent electrochemical corrosion resistance in salt solution, and the Zn-Al-Mg-RE coating was much more stable. Electrochemical impedance spectroscopy (EIS) results revealed that small amount of rare-earth metal can not promote to form the passive film but it could enhance the surface property of the coating extraordinarily, which will has a great effect on the corrosion behaviors of the coating. Keywords: Zn-Al-Mg-RE coating; high velocity arc spraying; cored wires; potentiodynamic polarization; electrochemical impedance spectroscopy