Improved AC/DC/AC Method for Evaluating the Corrosion Resistance of Polymer Coatings with Defects

Direct to metal (DTM) coatings are prone to physical defects because of their single-layer structure, which can lead to rapid corrosion failure. This study addresses the challenge of rapidly evaluating corrosion resistance in defective DTM systems by optimizing an AC/DC/AC electrochemical protocol. This study proposes an improved “Cathodic DC” method (potentiostatic polarization at −1.5 V for 150 s per cycle) and systematically compares its outcomes with natural immersion and a salt spray test. The results demonstrate that the Cathodic DC condition accelerates coating failure by 80% while aligning well with natural immersion in three key aspects: (1) variations of electrochemical parameters (|Z|0.01 Hz and Rct), (2) corrosion product composition (FeOOH identified via Raman spectroscopy), and (3) the morphology. Validation across four DTM coatings revealed consistent corrosion resistance rankings with salt spray tests, confirming the method’s reliability. Consequently, this method shows promise for rapidly evaluating the corrosion resistance of coatings with defects.