ABSTRACT

AC corrosion experiments were carried out in two kinds of typical soil simulated environments with different contents of calcium ion (Ca2+), magnesium ion (Mg2+) and sodium ion (Na+) to study the effects of environmental and cathodic protection parameters on AC corrosion rates. Based on the weight loss experiments results and corrosion morphology analyses in two different environments, as well as under different CP polarized potentials and AC current densities, the difference of AC corrosion behavior in different environments and CP levels were analyzed. Besides, the effective CP polarized potentials under different AC current densities in two environments were explored.

INTRODUCTION

In the recent twenty years, AC corrosion problem has drawn wide attention in the international corrosion field.1-22 Many AC corrosion cases indicated that AC corrosion could occur when protection criterion was met, which means the industry accepted CP criterion was not suitable to decrease the corrosion rate to negligible level under AC interference. However, how to evaluate the corrosion risk and how to choose CP parameters to protect pipeline from corrosion under AC interference have not reached an agreement currently. One paper noted that the “conventional” -0.85 V (referenced to copper/saturated copper sulfate reference electrode, abbreviated as CSE) CP criterion was not adequate in the presence of AC interference. Based on weight loss and electrochemical measurements, the paper concluded that AC corrosion rate could be decreased to negligible level at potentials more negative than -1.15 V (CSE).1 Carpentiers et al. also suggested that the off-potential of the pipeline should be more negative than -1.15V (CSE). 2 Fu et al. concluded that if AC current density was less than 30 A/m2, CP polarized potential should not be positive than -0.95 V (CSE), and if AC current density was more than 500 A/m2, CP polarized potential should not be more negative than -1.25 V (CSE). 3 Xu et al. indicated that when AC current density was less than 400 A/m2, AC corrosion could be mitigated well if CP polarized potential was more negative than -1.07 V (CSE). 4 Based on the results of field measurement, Nielsen proposed that “the CP level has a dramatic influence on the AC corrosion process” and that “excessive” CP can exacerbate AC attack and thus should be avoided. 5 The paper reported that corrosion rates gradually increased to as high as 10 mm/y at direct current (DC) about 10 A/m2 and off-potentials of approximately -1.10 V (CSE) (measured by electrical resistance probes over a two-week period). From the above researches, it could be seen that the published papers regarding to the appropriate CP levels in the presence of AC interference are seemingly contradictory.

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