Influence of Remote Cathodes on Corrosion Mechanism at Exposed Cut Edges in Organically Coated Galvanized Steels

The scanning vibrating electrode technique (SVET) was used to study the influence that accelerating corrosion, by attaching remote stainless steel cathodes, has on the corrosion mechanism occurring at exposed cut edges of 0.7-mm gauge organically coated galvanized steels (OCS). Galvanized steel samples were coated with organic coating layers (200 μm polyvinyl chloride [PVC] on one side and between 5 μm and 36 μm polyester on the other) to produce model cut edges with varying degrees of coating asymmetry. Under free corrosion conditions in 5% aqueous sodium chloride (NaCl) in the absence of polarization, such materials exhibited an asymmetric corrosion profile, which likely was caused by the establishment of differential aeration. Anodic activity was localized proximal to the thicker PVC coating with cathodic activity on the steel and zinc proximal to the thinner polyester coating. SVET testing and electrical impedance spectroscopic (EIS) measurements showed that there was no activity occurring via electrolytic transport through the organic coating, with the exception of the thinnest (5 μm polyester) coatings, which had measurable pore resistance. Attaching a 4-cm² remote stainless steel cathode to the 20-mm exposed cut edge caused the anodic currents emanating from the cut edge to increase fiftyfold, accompanied by a dramatic change in observed mechanism. The location of anodic and cathodic activity on the cut edge was altered significantly with both zinc layers acting as focal anodes. Removal of the remote cathode returned the corrosion current to a similar level seen in the unpolarized conditions, but the original mechanism never was recovered, and both zinc layers remained anodic. Cyclic wet and dry testing using a paint-undercutting accelerated test (PUCAT) apparatus was used to demonstrate the influence that this mechanistic change had on the degree of PVC coating delamination away from the exposed cut edge.