Metallurgical analysis of pimples and their influence on the properties of hot dip galvanized steel sheet

Production of galvanized steel sheets with minimum surface defects has been a major concern for automotive applications and, therefore, metallurgical assessment of defects and their effect on the properties of the coated sheet continues to be an important issue. In this paper, pimple defects in hot dip galvanized steel sheets produced in an industrial continuous process have been studied in terms of the microstructure as well as their influence on mechanical properties and corrosion behavior. Surface characteristics and microstructural features were examined by scanning electron microscopy and microanalysis. The results indicated that the major cause of pimples was the adherence of metal chips and embedded particles to the sheet surface before the formation of zinc coating. Corrosion resistance of defect samples was compared with sound galvanized specimens by salt spray and Tafel polarization tests. The time to red rust was up to 50 h shorter on pimples as compared to other regions. Evaluation of mechanical properties by tensile testing proved that pimple defects had no significant influence on strength but they caused a reduction of about 2% in ductility of the galvanized steel sheet.