In this paper, the possibility of utilizing characteristic absorption spectra in the terahertz (THz) frequency region for the nondestructive evaluation of a corroded metal surface coated with an insulator is discussed. The infrared activity of corrosion products in response to THz waves was characterized. The THz absorption spectra of Al(OH)3 and ZnCl 2·4Zn(OH)2 powders, which are typical corrosion products in Zn-Al alloy, were measured with a frequency tunable terahertz light source based on difference frequency generation from a GaP crystal. Characteristic absorption peaks were observed in both spectra. Subsequently, THz reflection imaging was conducted on a metal surface partially covered with these corrosion products and a coating. By using the characteristic absorption peaks of the corrosion products, the spatial distribution of these products was clearly obtained. From the results, it was shown that THz waves can be applied to the non-destructive evaluation of corroded metal surfaces coated with insulators.