Linking Corrosion Product Morphology to Impedance Response Across Immersion and Salt Spray Testing of Coated Steel

ABSTRACT Neutral salt spray and immersion testing are both routinely used to assess coated‐steel corrosion, yet rarely compared at the level of the corrosion product itself. This work examines whether electrochemical impedance spectroscopy differences between natural immersion, artificially aerated immersion, and neutral salt spray trace back to corrosion product architecture rather than to the test method. Bare steel and zinc‐rich and epoxy‐only scribed powder‐coated systems were characterised by EIS, imaging, and SEM. Natural immersion produced the highest impedance and a compact product; neutral salt spray produced the lowest impedance and a porous, cracked product; aerated immersion was intermediate. The same ranking was reproduced in coated systems, where the corrosion product at the defect dominated the response. Zinc rich primer coating's impedance advantage over epoxy coating was reduced under aeration, suggesting zinc's cathodic‐protection mechanism is itself oxygen‐sensitive. EIS indicators from different accelerated tests should not be assumed directly comparable.