Study on Cathodic Protection Behavior of X80 Steel under Simulation Disbonded Coating in Near-Neutral Solution

Abstract

Oil and gas pipelines use coating and cathodic protection methods to prevent corrosion, and a micro-corrosive environment will be formed where the coating is damaged and peeled off, causing pipeline steel corrosion accidents. The corrosion behavior of X80 steel under the disbonded coating was studied based on the microelectrode technique, and the local open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and the linear polarization impedance of X80 steel were measured in NS4 solution under the disbonded coating with different cathodic protection potentials. Results showed that when no cathodic protection was applied, the potential at the opening was the highest, the charge transfer resistance and linear polarization impedance were the smallest, and the corrosion rate was the fastest. When the cathodic protection potential of -870 mV was applied, the potential at the opening was the lowest and was not corroded, and the charge transfer resistance and linear polarization impedance were high in the middle of the gap and were not corroded, the bottom of the gap was not subjected to cathodic protection, and the corrosion rate was the fastest. When the cathodic protection potential of -1 000 mV was applied, as the distance from the opening increased, the potential moved forward, the charge transfer resistance and linear polarization resistance increased, and the gap was over-protected state. When the cathodic protection potential was insufficient, the corrosion behavior of the specimen from the opening to the bottom of the gap was different. Besides, increasing the cathodic protection potential could prevent corrosion in the gap, but too high cathodic protection potential caused the gap in an over-protection state.

Keywords

• x80 steel|disbonded coating|ns4 solution|corrosion|cathodic protection