Inspite of significant advancements in continuous galvanizing technology and the resulting improvements in surface quality of hot dip galvanized and galvannealed coatings, producing entirely blemishfree coatings still poses a challenging task. Thus, surface defects are encountered intermittently in all continuous galvanizing and galvannealing lines. A thorough and in-depth understanding into the genesis and origin of these defects helps in identifying and employing prompt remedial countermeasures to alleviate their occurrences. The paper reviews the commonly confronted defects in continuously galvanized and galvannealed sheet products along with their tell-tale characteristics as revealed by metallographic techniques and the means of eliminating them.

Although almost every tiny defect in a galvanized coating may look like a dross particle, dross entrapment actually accounts for only a small portion of the defects found in hot dip galvanized coatings. In fact, the average size of intermetallic particles suspended in a bath is normally less than 15 μm. Such small top-dross particles are often found buried in the coating underneath patches of oxide skin picked up mostly on the edges of the coated strip and are, as such, not visible and distinguishable. As a matter of fact, most defects occur because of a rough or mechanically damaged substrate surface, insufficient cleaning of the substrate, poor bath chemistry management and line equipment maintenance. Thus, galvanizers producing coated products must improve quality of the incoming steel strip, the line equipment, the in-line heat treatment regime and the coating bath in order to meet the stringent quality requirements for exposed automotive body panel applications. For lines producing both galvannealed as well as galvanized products, the additional challenges are to improve bath chemistry control in galvannealing campaigns to minimize bottom-dross formation and to develop a galvannealing to galvanizing transition procedure to either fully clear up or minimize the disturbance to the accumulated bottom dross.