THE SURFACE-TOPOGRAPHY OF SHEET STEEL GALVANIZED COATINGS

The surface topography of hot-dipped galvanized coatings has been measured for galvanizing baths of different compositions. Some of the general micro-roughness parameters and typical surface profile graphs of the galvanized sheet surface were obtained with a Talysurf 5 profilometer. Galvanized coatings with large dendritic spangles exhibit high ridges along the primary dendrite branches and large depressions at the grain boundaries between the spangles. Individual spangles have shiny and frosty segments inclined to the sheet steel surface, the difference in appearance being due to their smooth and rough surface, respectively. The surface roughness of the galvanized layer is strongly dependent on the presence of dendrites which in tum is related to the bath composition. An attempt has been made to correlate the measured surface roughness parameters with bath composition and the development of spangle structure during the solidification of the galvanized layer.