Abstract
Zn-Al-Mg hot-dip-galvanized steel sheets exhibit some specific surface phenomena that have not been observed on GI coatings and have only been published to a limited extent. One visually very striking optical appearance has been discussed in the literature under the names "freckles" and "dark spots." However, only a rough formation mechanism has been described to date. Brisberger et al. found foreign particles in the center of the defect that could be considered to be a nucleation seed for a freckle. However, the influence of these occurrences on processing properties has not yet been described. For more detailed information on the formation and cause of freckles, metallographic examinations and laboratory trials were carried out in a hot-dip galvanizing simulator to gain a fundamental understanding of the formation mechanism. Topography and processing properties, such as phosphatability, paint appearance, forming properties, and corrosion resistance, were assessed by various methods. Freckles exhibited locally altered crystallization due to a foreign particle, which has an impact on the Zn-Al-Mg coating itself as well as its complex microstructure. New findings on the formation mechanism were obtained from investigations with our hot-dip simulator, which showed possibilities for controlling these surface phenomena in an industrial environment.