Cathodic Protection Affected by Anodic Transients: Critical Duration and Amplitude

Tan, Mike Yongjun (Institute for Frontier Materials) | Huo, Ying (Institute for Frontier Materials)

OnePetro 

ABSTRACT

Anodic transient is a form of stray current induced potential excursion that is believed to affect buried pipeline cathodic protection (CP) systems by shifting pipeline potential away from safe CP potential levels. However the actual effect of anodic transients on pipeline corrosion has not been sufficiently quantified due to the lack of direct and reliable evidences. In this work a novel research methodology has been employed to quantify the dynamic effects of anodic transients on CP and corrosion by means of an electrochemically integrated multi-electrode array, often referred to as the wire beam electrode (WBE). It is shown that anodic transients do not necessarily cause pipeline corrosion, as long as their amplitude and duration are below critical values. These critical values are termed the critical anodic transient duration and amplitude, and are explained as the incubation period and the potential required for passivity to break down locally.

INTRODUCTION

Cathodic protection (CP) is a widely applied technique for protecting buried steel pipelines from corrosion. In order to ensure the effectiveness of CP, various industry standards and criteria have been developed with the most commonly used criteria being the CP potential of −850 mV vs copper/copper sulphate reference electrode (CSE) [1,2]. Under this potential level, a steel pipeline buried in pH neutral soils is considered to be ‘safely’ protected although some doubts about the applicability of this standard ‘safe’ CP potential to more complex environments still exist [3-5]. In practice, however, the potential of a buried steel pipeline could be diverted from the designed CP level due to various reasons such as flawed CP design and control, stray currents and other forms of electrical interference signals [6-9], leading to insufficient CP (in cases of anodic potential excursions) and over-protection (in cases of excessive negative potential excursions). These interference signals 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 the exact effects of potential excursions on CP efficiency and corrosion have not been sufficiently understood. Although attempts have been made over the past decades [6-10], prior work in this area has been predominantly empirical that are insufficient for achieving a fundamental understanding of the phenomenon. Due to this lack of understanding, currently there is no unified consensus on how ‘big’ such CP excursion (either in magnitude, frequency or duration) needs to be in order to cause major corrosion problems. This uncertainty has caused significant difficulties in selecting suitable parameters in industry standards.