ABSTRACT
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.
INTRODUCTION
Internal pipeline lining is widely accepted and used to minimize the pipeline internal corrosion. Concrete, rubber, and plastics are commonly used materials for internal pipeline linings. However, concrete lining can reduce the inner diameter of pipelines, and its roughness may consume high energy during operation. Rubber lining is not oil, flame and abrasion resistant, and it is susceptible to temperature change.1 Cathodic protection (CP), acting as a complementary corrosion protection technique, is often combined with epoxy coating to protect metallic pipelines. However, CP can make the cathodic surface strongly alkaline and generate hydrogen at exposed metal surfaces. Alkalization is the predominant reason for cathodic disbonding of epoxy lining, which proceeds mainly through hydrolysis of the interfacial bonds that attach the coating to the substrate.2-3
Porcelain enamel, as an inorganic material, is chemically bonded to substrate metals by fusing glass frits at a temperature of 750°C~850 °C. It can not only be finished with a smooth and aesthetical surface, but also provide good chemical stability and corrosion resistance, as well as excellent resistance to abrasion and thermal shocks in extreme erosion environment.4 Unlike epoxy coating, enamel coating has no under-film corrosion when locally breached due to its chemical bond with metal substrates.5 The corrosion resistances of enamel coating applied to the steel pipes in 3.5 wt. % NaCl solution were evaluated and compared with those of epoxy-coated pipes in our previous study.6-7 The test results showed that the enamel-coated samples outperformed the epoxy-coated samples.
