Effect of a DC Stray Current on the Corrosion of X80 Pipeline Steel and the Cathodic Disbondment Behavior of the Protective 3PE Coating in 3.5% NaCl Solution

Abstract

The influence of a direct current (DC) stray current on the corrosion and cathodic disbondment of X80 steel coated with a polyethylene (3PE) coating in 3.5% NaCl solution was studied by immersion experiments, electrochemical tests, three-dimensional microscopy, and a surface analysis. The results showed that the potential of the X80 steel sample shifts under the direct current. After 100A/m 2 DC interference was applied, the potential of the sample in the anode region positively shifted from -0.68 to -0.43 V. At the same time, the sample in the cathode region negatively shifted to -1.45 V. Under the DC anode action, the X80 steel corrosion exhibited no passivation and followed Faraday's law of electrolysis, in which the corrosion rate is proportional to the current density. Three-dimensional (3D) digital microscopy showed that, as the DC current increased, the depth of the corrosion pit also increased (gradually), indicating a higher corrosion degree. The sample in the cathode region only underwent a hydrogen evolution reaction, which caused cathodic disbondment of the coating. The stray current had a critical current density for the coating disbonding: the coating delamination area reached its maximum and then remained unchanged.Coatings 2019, 9, 29 2 of 12 of stray current corrosion on buried metal pipelines in a subway and discussed potential measures to minimize and eliminate the influences of stray current corrosion. Zhu et al. [10] proposed an effective stray current drainage method based on the mechanism of stray current interference to minimize corrosion. Currently, scientists have a good agreement on the mechanism by which DC stray currents cause corrosion, i.e., that it is caused by an electrochemical reaction and the amount of corrosion is proportional to the stray current's density, which follows Faraday's law of electrolysis [11][12][13].The main focus of the research efforts is whether or not the buried pipelines experience DC interference and the resulting corrosive damage. However, cathodic disbondment of the coating due to DC stray currents in the pipeline area is less studied. When a DC current passes through the damaged portion of the coating of buried long-distance pipelines, an anode region forms that causes electrolytic corrosion of the metal pipeline, resulting in a corrosion piercing [14,15]. In contrast, DC passing through the cathode region does not cause stray current corrosion of the pipeline because of the protection by the cathode current. However, if the potential of the cathode area is too negative, a cathodic hydrogen evolution reaction (HER) and an oxygen reduction reaction (ORR) occur on the pipeline's surface, causing cathodic disbondment of the coating [16][17][18]. Nielsen et al. [19], after field investigations and tests, determined that the surface of buried steel pipelines under cathodic protection (CP) generates large amounts of OH − that raise the localized pH around the defect areas, which eventually leads to a pH increase in the majority of the area around the buried p...

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