Corrosion Mechanism of Cathodic Protected Transmission Pipeline Steel with Damaged Enamel Coatings

Internal coatings have been largely used for corrosion protection of pipeline. One of the concerns with internal coating is mechanical damage threat since the flowing solids will generate abrasive forces and impact on the internal coating. Although the coating cannot prevent the exposed substrate steel from corrosion at the damaged sites, cathodic prevention will continue to exert protection effect on the pipeline. In this study, corrosion mechanism of pipeline steel coated with two types of enamel under cathodic protection were investigated in 3.5 wt.% NaCl solution with electrochemical impedance spectroscopy (EIS) tests. Experiment results show that although cathodic protection cannot improve the corrosion resistance, all the damaged coating under cathodic protection do not have corrosion products generated on the damaged sites. Scanning electron microscopy (SEM) images indicate that damaged enamel coatings still have bonding coating left on the substrate steel.