ABSTRACT

Coupons have been widely used for cathodic protection evaluation of buried metallic pipelines. Under stray direct traction current condition, however, the application of coupon techniques faces some challenges due to IR drop errors caused by the inherent stray current which cannot be interrupted. In this work, factors affecting the measurements of coupon polarization potential in the existence of stray direct traction current were investigated through field experiments. The results showed that the measured coupon polarization potential was significantly influenced by multiple parameters, such as polarization retention period, “instant-off” potential sampling delay, coupon burial depth, coupon-to-CSE interval, and the proximity of groundbeds. Based on the experimental results, practical suggestions were proposed for improving the proper use of coupons under dynamic stray direct current interference.

INTRODUCTION

It has been acknowledged1-7 that buried metallic pipelines would experience accelerated corrosion, especially pitting corrosion, in the presence of stray direct traction current. Moreover, the stray direct traction current would affect the performance of cathodic protection (CP) system and shifts the CP potential applied on pipelines to deviate from the design value8-9. The identification and evaluation of stray direct traction current interference was a research hotspot.

Nowadays, the most accepted method for identification and evaluation of stray direct traction current interference was to measure polarization potential of the interfered-pipeline by coupon technology10-13. However coupon technology faced some challenge due to the IR drop caused by the stray current which could not be controlled or interrupted during field test. Besides the measured coupon polarization potential would be affected by multiple parameters, such as polarization retention period, instant-off potential sampling delay, coupon burial depth, coupon-to-CSE interval, and the proximity of groundbeds. Unfortunately, very few literatures were available to provide practical suggestions for effective measurement of coupon polarization potential. In this work, factors that affected the measured coupon polarization potential under stray direct traction current interference were discussed and some practical suggestions were proposed.

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