Corrosion of New-Generation Steel in Outer Oil Pipeline Environments

New-generation high-strength pipeline steels are susceptible to corrosion attacks at their outer surfaces, from bicarbonate and carbonate in the hydrated soils in which pipelines are buried. The nature of the corrosion attacks requires implementing modified electrochemical methods by which the corrosion reactions can be studied comprehensively. From that, the findings can be linked to the alloying elements of the new pipeline materials, to modify them from both the strength and corrosion resistance perspectives. This paper presents an electrochemical study on the corrosion of API-X100 steel, in aerated bicarbonate-carbonate solutions, with cyclic voltammetry. At a fast 10 mV/s scan, over ten cycles, the passive films appeared as transparent layers, regardless of bicarbonate and carbonate concentrations. The surface controlled the reactions more than the passive films. The passive films showed evidence of forming gradually with increased cycling. Carbonate competed with bicarbonate during dissolution, decreasing its rate and making passivation faster. Bicarbonate in carbonate solutions catalyzed dissolution and disrupted passivation. The passive films with increased bicarbonate concentrations had higher transpassivation potentials.