Analysis of Black Line Defects in Hot-dip Al-Zn Sheets

Abstract

In order to identify the causes of black line defects in thin specification hot-dip Al-Zn sheets, a ZEISS Merlin Compact field emission scanning electron microscope (SEM) and an attached OXFORD X-max N type energy spectrometer were utilized to analyze the surface and cross-section of black line defects in hot-dip Al-Zn steel plates, as well as the microstructure observation and chemical composition of the steel substrate surface after corrosion and removal of the coating. Analysis was also conducted on the steel substrate surface after the coating had been removed by corrosion. Results showed that the black line defects on the upper surface of the hot-dip Al-Zn steel sheet were caused by a thicker passive film due to scratches on the steel substrate. On the lower surface of the hot-dip Al-Zn sheet, the black line defects were attributed to a thicker passive film resulting from concave micro-deformations of the lower surface coating. Scratches on the surface and depressions of the coating of the Al-Zn steel sheet were found to cause different reflections of light from the rest of the surrounding area, forming a visually perceived black line defect.

Keywords

• hot-dip al-zn sheet; the black line; defect; passive film