USING LOCAL ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY TO EXAMINE COATING FAILURE

Local electrochemical impedance spectroscopy (LEIS) was used to locate and examine the electrochemical properties of artificial and natural defects in a heat-cured, urea-formaldehyde modified epoxy coating. This coating is used to protect the interior of tin-coated mild steel cans sealed using resistance welding. Samples containing weld coating, nonweld coating, and an artificial defect were immersed in a 0.10 M sodium chloride (NaCl) solution buffered to pH 7.2 with boric acid/sodium borate. LEIS maps demonstrated that the coating failed preferentially over the weld seam at a rate greater than that at the artificial defect. Failure was detected by LEIS prior to visual observation of coating failure. Comparable electrochemical impedance spectroscopy (EIS) data generated after each LEIS experiment could be modeled by the conventional two-time constant porous penetration model. LEIS results demonstrated that there were three time constants in this system (coating, defect, and weld seam), indicating that a method for determining local impedance properties is preferable.