AC-INDUCED CORROSION AND INTERACTIONS WITH CATHODIC PROTECTION

For many years, the tribal knowledge within the corrosion community was that alternating currents (AC) had little or no detrimental effects on the corrosion of metal structures. More recently, it has been recognized that AC-induced corrosion can have major deleterious effects. An exemplar of the problem is buried pipelines that are co-located with high-voltage transmission lines for electrical distribution, although other applications are likely to apply where there is a combination of alternating currents and cathodic protection. The controlling parameters are not fully defined, and their relationship to corrosion rates is not well established. For Induced AC plus DC for cathodic protection, the trends are to exacerbate the problem with greater magnitude of induced current density, fewer and smaller coating defects (holidays) and higher levels of cathodic protection. The objective of this work is to examine the effects of AC-induced corrosion and interactions with cathodic protection under controlled laboratory conditions. The findings will better inform mitigation methods, monitoring techniques, risk assessment and integrity management.

INTRODUCTION

Induced alternating current (AC) degradation has become more widely recognized as a threat to the integrity of underground structures, e.g. pipelines co-located with high-voltage transmission lines, ACpowered rail transit systems, and structures where there are stray AC currents. High induced AC 2 voltages on the pipelines lead to AC current discharge at coating defects, which can cause severe corrosion even when the cathodic protection criteria is deemed to be satisfied. However, the mechanisms of AC corrosion are still not completely understood, empirical results for factors affecting AC corrosion are in the early stages of development. Mitigation methods are available but technical basis and validation can be improved.

The objective of this work is to examine the effects of Alternating Current (AC) induced corrosion, AC/DC effects, and interactions with cathodic protection under controlled laboratory conditions. A primary focus is to determine the effects of surface films and corrosion products on metals on the modulation of the AC/DC currents. In particular, the work includes analyses of metal-oxide-metal (MOM) junctions which can have semiconductor properties and nonlinear effects on the currents. Further, the effects of local environment changes, such as pH, on the AC induced corrosion are examined. The relationships among AC/DC polarization, surface films, the aqueous environment and corrosion are to be determined. The initial work is in the context of buried pipelines that are co-located with high-voltage electrical transmission lines, however, the enhanced understanding of these effects are relevant to a broader range of applications.

This paper presents a description of the AC Induced corrosion risk for buried pipelines, an overview of some relevant corrosion principles, and description of the laboratory program to examine this corrosion degradation.

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