ABSTRACT

Soil is a complex environment where various forms of localized corrosion could occur on buried metal structures such as pipelines. Protective coatings and cathodic protection (CP) are widely applied as principal means of protecting buried steel pipeline from soil corrosion, unfortunately coating defects and disbondment could create some complex environmental conditions for localized corrosion to initiate and propagate. Stray currents could also cause dynamic and rapid localized corrosion on buried pipelines. This paper summarizes typical results and first-hand experiences from recent work aimed at developing corrosion probes based on an electrochemically integrated multi-electrode arrays for monitoring and understanding complex forms of localized corrosion on buried pipelines. Experimental and field testing results have demonstrated the capability of the probes to be used in laboratory research and field conditions. Evidence found on localized corrosion under disbonded coatings and stray currents illustrate some of the unique advantages of the electrode array method for visualizing and understanding localized corrosion of buried steels. It also briefly discusses needs for future work to enhance the reliability of corrosion probes as a structural health monitoring tool for early detection and diagnosis of corrosion, for providing industrial system ‘health’ alarm, for forecasting maintenance requirements, and for generating data for integrated and automated corrosion management systems.

INTRODUCTION

Buried steel pipelines are extensively used for transporting gas, oil, water and other fluids. Due to the low corrosion resistance of steels, the external surface of underground pipelines is usually protected by organic coatings and cathodic protection (CP). Unfortunately, coatings are not perfect and they usually contain defects of varying nature and size including holidays, cracks, and scratches due to manufacturing flaws, construction damage, exposing the pipe's external metal surface directly to the soil environment. CP works as a backup to prevent corrosion by polarizing the metal to an electrochemical potential, usually −850mVCSE (copper/copper sulphate electrode) that is generally accepted as safe protection potential.1 However, in practice, the effectiveness of CP could be further influenced by factors such as stray current2–5 and cathodic disbondment6–10, resulting in localized corrosion of external pipeline. Over the past eight years, extensive research has been carried out under the Energy Pipeline Cooperative Research Centre research program. An overview of some findings related to the monitoring and understanding localized corrosion on buried steel pipelines is presented.

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