| To reduce the occurrence of defects on both galvanized and galvannealed sheet steels in industrial continuous galvanizing line, the morphology and formation mechanism of surface defects have been investigated by optical microscopy, scanning electron microscopy and roughness profiler. It is obvious that both removal of oxides on steel surface and cleanness of the continuous galvanizing line are essential to produce defect-free galvanized and galvannealed coatings. In galvannealed steel, the existence of thin oxides at the interface between coating and substrate causes the inhibition of the Fe-Zn reaction and, thus, results in the under-alloying; some parts of coatings are still remained as pure-Zn. On the other hand, in galvanized steel, Mn-containing oxides account for the poor wettability of molten Zn and the formation of dross-like defect. Some of the steel fragments and/or fines observed on the surface of galvannealed coating are considered to be an origin of the spot defects. It was also concluded that the defects generally considered as dross are mostly not the case. |
|
