Degradation is revealed for cross-arms used in an electric transmission tower on the Pacific coast of Japan by evaluating both paint degradation and steel corrosion. The angled steels exhibit various levels of degradation according to their face orientation and the installation height. The corrosion states are provided as maps with a special resolution of sub-mm order, using the terahertz wave nondestructive measurement. Rust fluid appears to transfuse along the steel/coating interface and induces underfilm corrosion on the nearest faces. The skyward face does not show clear corrosion, indicating the washing effect of rainfall. Electrochemical impedance shows that paint degradation is severe in faces that get a direct sea breeze, whereas the opposite faces show good integrity. The degradation also becomes more severe with increasing installation height. The two analytical techniques show that underfilm corrosion shows rapid progress when the electrical receptivity of the coating is on the order of 10⁷ Ω·cm².