Mechanistic changes in cut-edge corrosion induced by variation of organic coating porosity

Abstract

The scanning vibrating electrode technique (SVET) has been used to investigate the mechanism of corrosion occurring at exposed metallic cut edges of galvanised steel materials coated with identical thickness organic polymeric layers. Organically coated galvanised steel samples have been prepared by application of a 30 μm polyester coating to both sides of a pre-treated hot dip zinc (99.85%) aluminium (0.15%) galvanised steel substrate. In separate samples the coatings were applied to both sides simultaneously and then cured or applied sequentially to each side curing after each application. Electrical impedance spectroscopy (EIS) was used to probe the porosity of the resulting organic coatings. This revealed that simultaneous coating application and curing resulted in both polymer layers showing EIS pore resistance of 0.6 MΩ whereas sequential coating resulted in one side showing a pore resistance of 0.6 MΩ and the other showing a capacitive EIS response with no measurable porosity. When the metallic cut edges of organically coated steel materials, coated with symmetric thicknesses of organic layers of identical pore resistance, are exposed to 5% NaCl the SVET iso-current contour maps show that both zinc layers behaving as local anodes. In addition, there is significant sample passivation in the 24 h exposure period. In the samples coated with materials of identical thicknesses but different porosity, the SVET iso-current contour maps show anodic activity focussed beneath the organic coating with the capacitive EIS signature. The greater porosity of the coating on the opposite side of the exposed cut edge enables oxygen transport to the delaminating surface and this forces anodic activity onto the other zinc layer.