Characterization and control of a new kind of black spot defects on surface of hot-dip galvanized low-aluminum Zn-1.5Al-1.1 Mg coatings

Abstract

Surface defects can seriously affect the appearance and performance of Zn-Al-Mg coatings. In this work, a new kind of black spot defects on the surface of hot-dip galvanized low-aluminum Zn-1.5Al-1.1 Mg coatings were investigated. Results revealed that the microstructures of normal and black spot regions were both composed of primary Zn, Zn/MgZn2 binary eutectic (BE) and Zn/MgZn2/Al ternary eutectic (TE). This kind of black spots with a diameter of 1 μm were caused by the difference in light reflection with normal region after generating local pseudo-eutectic structures. When the solidification cooling rate was controlled to 18 °C/s, the black spot defects were completely eliminated. Furthermore, the corrosion current density was reduced to 1.2 μA/cm2 and the coating resistance increased to 2427 Ω·cm2 in 3.5 wt% NaCl solution.