Scanning electrochemical microscopy studies for the characterization of localized corrosion reactions at cut edges of painted galvanized steel as a function of solution pH

Coil coated steel sheets immersed in 1 mM NaCl solutions adjusted at both acidic and alkaline pH conditions were used to investigate the cut edge corrosion behaviour by scanning electrochemical microscopy. Combined amperometric/potentiometric operation revealed asymmetries in the distribution of localized anodic and cathodic activities along the cut edge related to the onset of a differential aeration mechanism. The anodic activity was initially located at the aluzinc layer coated with the thinner organic coating, whereas alkalization of the steel foil related to cathodic activity was limited by the buffering ability of the soluble metal ions. In this way, precipitation of corrosion products might block the cathodic sites, a process responsible for the eventual complete cessation of corrosion in alkaline solution for sufficiently long exposures.