Micro-cells beneath organic lacquers: a study using scanning Kelvin probe and scanning acoustic microscopy

The mechanism of degradation of epoxy-phenolic lacquer coated thin electro-chrome-coated mild steel (food can material) has been investigated in brine using a novel combination of experimental techniques. A scanning Kelvin probe (SKP) was used to map electrochemical potential distributions beneath the coating at several stages of exposure to 0.17 M (1% w/w) NaCl solution. An interesting sequence of events was observed, especially at defects and blisters in the lacquer. These results have been correlated with those obtained in parallel by electrochemical impedance spectroscopy (EIS), scanning acoustic microscopy (SAM) and optical microscopy. The SKP results showed coating defects to be negative (anodic) with respect to the surrounding area in the initial stages of exposure (less than or equal to 36 h). This was followed by a gradual shift in the relative potential to values of the defect and the surrounding coated metal, with the defects becoming the cathode and the corrosion products formed in the defects becoming the cathode reactant. The reduction of red rust to magnetite was confirmed using the optical microscope. SAM revealed blisters, within which new anodes were detected by SKP. (C) 2003 Elsevier Ltd. All rights reserved.