Accelerated corrosion of pipeline steel and reduced cathodic protection effectiveness under direct current interference

Stray current interference is considered as a causative agent of metal corrosion. Stray current is an electric current flowing through a conductor other than the intended one. It has been investigated previously and found that stray direct and alternating currents were able to accelerate corrosion of buried pipelines. These stray currents can be categorized as either static or dynamic considering whether their direction and amplitude change with time.

Stray current that a pipeline is exposed to can be generated by outside sources including direct current rectifiers, orbital transport systems, and high voltage alternating current (AC) and direct current (DC) powerlines. When collocated with these stray current sources, pipeline is prone to suffer accelerated corrosion, therefore, leading to the so-called DC and AC corrosion.

DC corrosion of pipelines is comparable to the classic corrosion mechanism where a current flow from an anode to cathode through the soil electrolyte by ionic conductivity and from the cathode to the anode via electrons. The pipe steel oxidizes at the anode, while oxygen reduction or hydrogen evolution occurs at the cathode. Therefore, steel corrosion is severe at the anode, while there is no attack at the cathode.

DC corrosion of buried pipelines has received a fair share of attention in conjunction with the design as well as construction of high-voltage DC powerlines that are majorly situated in energy corridors where oil and gas pipelines are buried. Adequate understanding of the DC corrosion is not only helpful in understanding this phenomenon, but also important for the design of safe and reliable energy infrastructure.

Professor Frank Cheng who holds Canada research chair in pipeline engineering and his PhD student Shan Qian at the University of Calgary investigated the DC corrosion of X52 steel pipeline at varying direct current densities in an extracted soil solution by potential and pH measurements, optical microscopy, and weight loss testing. They determined the effect of DC current density on cathodic and anodic reactions. Their research work is published in journal, Construction and Building Materials.

The researchers measured the shift of cathodic protection potential under DC interference. They then analyzed the mechanistic aspects about the accelerated pipeline corrosion as well as the reduced cathodic protection effectiveness in the presence of DC interference. The effects of the DC interference on the integrity of the pipeline were finally discussed.

The authors observed that under the current testing conditions, the DC accelerated steel corrosion at the anodic zones was reversible, however its effects on the cathodic zone was irreversible owing to the strong alkalinization. In the presence of the DC interference, the applied cathodic protection on the steel pipeline could not be maintained and was shifted to the anodic and cathodic zones, respectively. In this regard, the steel in the anodic zone could not be protected by the cathodic protection from corrosion.

Qian and Cheng also observed that the solution alkalization in the cathodic zone was further enhanced by the combined effect of cathodic protection and direct current on cathodic reductive reaction. DC stray current was found to lead to cathodic and anodic interferences on buried pipelines at the same time. The anodic interferences lead to accelerated steel corrosion, which represents a critical threat to the integrity of the pipeline, while the cathodic interference causes increased coating disbondment

The work was considered as “an interesting topic with good scientific angle without leaving the engineering aspect”. Moreover, “the conclusions correlate the extensive analysis and results obtained by an original experimental set up”.