QUANTIFYING THE EFFECTS OF MAJOR FACTORS AFFECTING THE EFFECTIVENESS OF CATHODIC PROTECTION OF PIPELINES

摘要

Cathodic protection (CP) is widely applied as a principal means of protecting buried steel pipeline from soil corrosion. Unfortunately in practice the potential of a buried steel pipeline could be diverted from the standard 'safe' CP level (i.e. -850 mV vs copper/copper sulphate reference electrode) due to various reasons such as stray currents, flawed CP design or faulty CP control, leading to insufficient CP (in cases of anodic potential excursions) and over-protection (in cases of excessive negative potential excursions). This problem can be further complicated when the potential fluctuates due to complex forms of external electrical interference signals that may be direct current (DC), alternating current (AC) or AC superimposed on DC in nature. Some forms of potential excursions are known to be harmful to buried steel pipelines; however currently the exact effects of potential excursions on CP efficiency and corrosion have not been sufficiently understood preliminary due to difficulties in measuring these effects. Over-protection is known to cause cathodic disbondment of pipeline coatings. Traditional methods of evaluating cathodic disbondment of pipeline coatings are based on visual inspection of pipeline conditions, and laboratory testing of cathodic disbondment resistance using standard visual inspection based methods. Unfortunately these techniques have some limitations in quantitatively and instantaneously measuring and monitoring cathodic disbondment of thick pipeline coatings. There are needs for the development of new methods that are able to perform in-situ and quantitative measurements of stray current corrosion and cathodic disbondment of pipeline coatings. This paper provides an overview of our current approaches to quantifying the influence of electrical interference signals on CP and steel corrosion using newly designed electrochemical corrosion cells; as well as to monitoring cathodic disbondment of coatings using electrochemical impedance spectroscopy. Typical results from using these new techniques for measuring stray current corrosion and for probing the cathodic disbondment of pipeline coatings have been briefly discussed.