Analysis on the causes of spot scratches on the surface of cold rolled hot dip galvanized sheet

The causes of point like scratch defects on the surface of two types of hot dip galvanized sheets were analyzed using macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis methods. The results show that there was a zinc iron alloy phase in the first type of point scratch substrate, and the cause of defects was related to zinc ash. Zinc ash flowed downward inside the coating under the action of an air knife, causing point scratch defects. Foreign objects containing Cr elements were found in the second type of point scratch substrate, and the cause of the defect was related to the cleaning section electrode plate. The presence of scratches and foreign objects caused local surface leakage, forming point scratch defects.