A recently developed Zn-Al hot-dip coating on steel has a higher corrosion resistance than that of the Zn hot-dip coating. For estimating the corrosion rate for the hot-dip coating on steel, the polarization resistance (Rp
) was measured using a coulostatic method. The cross-section of the Zn-Al hot-dip coating was analyzed by X-ray and EPMA. The results showed a tendency in which the Al concentration increased from the surface of the coating toward the base steel side. Based on the Rp
values of five kinds of Zn-Al coatings hot-dip coated in a bath with 5, 10, 15, 20 and 30% Al, the coating obtained in the 15% Al bath showed the highest Rp
value. In other words, this indicated that the corrosion resistance of the coating from the 15% Al bath was superior. Electrochemical impedance spectroscopy (EIS) with a frequency response analyzer (FRA) was used to obtain further information about the interface of the Zn-Al hot-dip coating and a 5% NaCl aqueous solution of pH 8. The measurement results showed two capacitive semicircles on the high-frequency side and a diffusion curve on the low-frequency side, which are considered to be formed by a typical Randles
-type equivalent circuit. This results the fact that the corrosion reaction is rate limited by both the charge and material transfer processes.