Study of the delamination of organic coatings by local electrochemical impedance spectroscopy

Local electrochemical impedance mapping was used to investigate delamination phenomena at the steel/epoxy-vinyl primer interface. The delamination occurred from two types of artificial defects (cutter or machine scribing) and from ageing in a salt spray chamber. The samples were taken from the salt spray chamber after 20, 30 and 50 days of exposure. To observe delamination, the corrosion product layer was removed by a cathodic polarisation at -1.5 V/SCE for four hours. A reference sample was tested for comparison. LEIM was carried out with a Solartron device (a Solartron 1287 electrochemical interface, a Solartron 1250 frequency response analyser and a Solartron 1275) using a five-electrode configuration. The experiments were performed in a low conductivity medium (0.001 M NaCl) to optimise resolution. Mappings were performed at 5 kHz. The initiation and the propagation of the delamination of the different systems were clearly observed. The delaminated surface areas measured by visual observation after the removal of the coating are in good agreement with those determined by local electrochemical impedance mapping.