Development of anodic and cathodic blisters at a model Zn/epoxy interface studied using local electrochemical impedance

Abstract

Local electrochemical impedance spectroscopy (LEIS) and mapping (LEIM) were used in situ to follow blistering on a model epoxy-coated hot-dip galvanized steel without any initial artificial defects during alternating open circuit potential (OCP)/cathodic polarization cycles. Zones with high admittance were evident from the early stages of degradation, when no blisters were visible. Some of the zones (type I) could be distinguished on the admittance maps obtained at both open circuit and applied cathodic potentials. Other zones (type II) were not detectable by LEIM at OCP but were visible on the maps obtained under cathodic polarization. Blisters developed on both types of zones. Comparing the LEIS responses of the blistered zones with the responses of a model Zn/Cu galvanic couple, the observed behavior was attributed to the formation of a galvanic couple between the blisters, and consequently a gradual increase of anodic and cathodic activities in blisters of types I and II. LEIM is a flexible tool able to differentiate in situ between the development of anodic and cathodic zones on immersed polymer-coated metallic substrates on application of different DC biases or AC frequencies. Keywords: Local electrochemical impedance spectroscopy, Coating, Galvanic coupling, Material degradation