Corrosion behaviour of three types of feather, frosty and shiny sectors in the hot-galvanized zinc layer on steel has been studied in 1, 0.1, 0.05M KOH solutions by using potentiodynamic polarization and AC impedance measurement techniques, as compared with that of bulk pure zinc. Ptentiodynamic polarization experiments were carried out in the applied potential range from $-2.0V_{SCE}$(cathodic range) to $1.0V_{SCE}$(anodic range) with a scanning rate of $5 mVs^{-1}$. AC impedance measurements were conducted by superimposing AC voltage of 5mV amplitude in the frequency range of $10^{-2}$ to $10^5 Hz$ on constant DC potentials of -1.4V to $-0.9V_{SCE}$. The first current peak was observed in the potential range of -1.33 to $-1.2V_{SCE}$ for the layer specimens including the sector as well as bulk the pure zinc specimen, while the second current peak resulting due to the oxidation of adsorbed hydrogen was detected at $-0.7V_{SCE}$ only from the sector specimens. The feather, frosty and shiny sector specimens increased in the height of the second current peaks in that order. The difference in the height of second current peak among the three sector specimens is discussed in terms of the competitive contributions of surface roughness, composition and preferred orientation of sector to the hydrogen oxidation. From the impedance spectra, the pure zinc specimen showed Warburg type impedance due to the zinc dissolution into the electrolyte in low frequency ranges at the applied anodic potential of $-1.0V_{SCE}$. On the other hand the specimens of sectors demonstrated negative polarization resistance due rather to the formation of passivating film. The negative polarization resistance decreased in initial stage, and then increased in later stage. It is suggested that the decreased negative polarization resistance in the initial stage is caused by hydrogen adsorbed on surface of hot-galvanized zinc layer, being oxidized and simultaneously impeding the formation of passivating film. However, since in the later stage hydrogen oxidation into the electrolyte has been completed and hence scarcely affected the formation of well-aged film the negative polarization resistance increased. The impedance change of the pure zinc and coating specimens is discussed as related to passivation behaviour of the specimens.
