Impedance Models and Water Transport Behavior of Steel Pipe Coated with Cathodically Polarized Enamel

Cathodic protection (CP) integrated with a protective barrier (e.g., thick organic coating) has been widely applied for the corrosion protection of steel pipelines. However, CP may make the pipe-coating interface become a more alkaline environment, potentially leading to cathodic delamination of the coating. It is, therefore, essential to evaluate the compatibility of new coating with CP in steel pipeline applications. Porcelain enamel, as an inorganic coating material, is widely used in chemical reactors, heat exchangers, or food-processing vessels due to its excellent engineering properties such as high abrasion, corrosion, and heat resistances. In this study, enamel-coated steel pipe samples with various levels of cathodic protection were tested to investigate their impedance models by electrochemical impedance spectroscopy (EIS). One electrical equivalent circuit (EEC) models were used to fit the EIS data of coated samples without CP, while two EEC models were employed for samples with CP. Coating capacitance was used to investigate the water absorption characteristic in coating. Through scanning electron microscopy (SEM), no corrosion products and delamination of enamel coating were found at the interface of steel substrate and enamel coating due to its chemical bond with metal substrates.