Photoelectrochemical investigation of corrosion using scanning electrochemical techniques

The scanning vibrating electrode technique (SVET) has been used to investig ate the influence of pigment catalysed photodegradation on the mechanisms o f corrosion occurring at exposed metallic cut edges of organic coated galva nised steel materials. Galvanised sheet steel samples (0.7 mm thick with 20 mu m zinc galvanising applied both sides) were coated with symmetric thick ness polyester resin layers pigmented with photoactive titanium dioxide. Sa mples were then irradiated on one side only with UVA light (365 nm at 2 x 1 0(17) photons s(-1) for 24 h), producing asymmetric photodegradation of the pigmented polymer layers. Prior to photodegradation the zinc layers expose d 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 coatin g 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. Subse quent to photodegradation, local coating thinning is evident in the UV irra diated coating facilitating greater oxygen transport to the metal surface i n this region. The resulting differential aeration cell initially focuses a nodic activity on the zinc surface proximal to the non-irradiated coating. Cathodic activity proximal to the photodegraded polymer layer results in it s chemical degradation through base catalysed hydrolysis of the eater funct ion. The resulting breakdown of coating integrity in the irradiated coating leads to bath anodic and cathodic activity occurring through the photodegr aded coating and a subsequent loss in overall SVET current balance in the p lane of scan above the cut edge. (C) 2000 Elsevier Science Ltd. All rights reserved.