Photoelectrochemical investigation of corrosion using scanning electrochemical techniques

Abstract

The scanning vibrating electrode technique (SVET) has been used to investigate the influence of pigment catalysed photodegradation on the mechanisms of corrosion occurring at exposed metallic cut edges of organic coated galvanised steel materials. Galvanised sheet steel samples (0.7 mm thick with 20 μm zinc galvanising applied both sides) were coated with symmetric thickness polyester resin layers pigmented with photoactive titanium dioxide. Samples were then irradiated on one side only with UVA light (365 nm at 2×10 17 photons s −1 for 24 h), producing asymmetric photodegradation of the pigmented polymer layers. Prior to photodegradation the zinc layers exposed at an edge produced by cutting through the coated sheet behave anodically and the exposed steel acts cathodically when immersed in aqueous aerated 5% NaCl. As immersion time increases there is small amount of through coating anodic activity which leads to the anodic/cathodic current balance in the plane of scan over the cut edge falling to 70% after 12 h immersion. Subsequent to photodegradation, local coating thinning is evident in the UV irradiated coating facilitating greater oxygen transport to the metal surface in this region. The resulting differential aeration cell initially focuses anodic activity on the zinc surface proximal to the non-irradiated coating. Cathodic activity proximal to the photodegraded polymer layer results in its chemical degradation through base catalysed hydrolysis of the ester function. The resulting breakdown of coating integrity in the irradiated coating leads to both anodic and cathodic activity occurring through the photodegraded coating and a subsequent loss in overall SVET current balance in the plane of scan above the cut edge.