Mechanistic changes in cut edge corrosion induced by variation of inhibitor pigmentation in organically coated galvanised steel

Corrosion at the exposed cut edges of organically coated galvanised steel (OCS) used for construction is one of the primary failure mechanisms. The scanning vibrating electrode technique (SVET) has shown that corrosion is driven by differential aeration when materials are coated with organic over-layers of widely differing thickness and primer layers are pigmented with strontium chromate (SrCrO4). In such materials anodic activity is localised on the zinc layer proximal to the thicker organic coating with cathodic activity focused on the steel and zinc proximal to the thinner organic coating. To overcome the localisation of anodic and cathodic activity we have prepared novel OCS panels in which cathodic inhibitor primer systems based on rare earth metal cation exchanged pigments have been employed beneath the thinner organic coating and conventional strontium chromate anodic inhibitor systems beneath the thicker organic coating. The differential inhibitor loading overrides the effects of differential aeration and leads to significant passivation within the 24 h immersion period.