Study on the formation and corrosion mechanism of black spot defects on the surface of industrial continuously hot-dip galvanized Zn-2Al-1.5Mg coating

The present study investigates the formation mechanism of black spot defects on the surface of industrial continuously hot-dip galvanized Zn-2Al-1.5Mg coatings. Electrochemical experiments and neutral salt spray tests are conducted to study the corrosion resistance of the black spot defects and their influence on the corrosion mechanism of the coatings. The results show that the enrichment of Mg elements within the coating is the fundamental cause of the formation of this defect. The enrichment of Mg elements leads to a reduction in the size of primary Zn, an increase in the proportion of eutectic structures and the MgZn₂ within the defective coatings. Consequently, the defective coatings exhibit the higher surface hardness and, after skin pass rolling, display the different reflective properties compared to surrounding coatings. The black spots feature is attributed to the formation of oxide film containing MgO. The electrochemical test results indicate that black spot defect exhibits enhanced corrosion resistance due to the lower Volta potential difference (VPD) between each phase on its surface. Moreover, the fine and dispersed cathodes not only avoid the occurrence of extremely pH sites, but also facilitate the formation of uniform distributed corrosion products with high protective ability. In the 600-h neutral salt spray experiments, Zn-2Al-1.5Mg coatings with black spot defects exhibit enhanced corrosion resistance, which is attributed to the Mg enrichment delaying the decomposition of highly protective corrosion products.