Abstract
Cathodic protection (CP), in combination with an insulating coating, is a preventative system to control corrosion of buried carbon steel pipes. The corrosion protection of coating defects is achieved by means of a cathodic polarization below the protection potential, namely −0.85 V vs. CSE (CSE, copper-copper sulfate reference electrode) for carbon steel in aerated soil. The presence of alternating current (AC) interference, induced by high-voltage power lines (HVPL) or AC-electrified railways, may represent a corrosion threat for coated carbon steel structures, although the potential protection criterion is matched. Nowadays, the protection criteria in the presence of AC, as well as AC corrosion mechanisms in CP condition, are still controversial and discussed. This paper deals with a narrative literature review, which includes selected journal articles, conference proceedings and grey literature, on the assessment, acceptable criteria and corrosion mechanism of carbon steel structures in CP condition with AC interference. The study shows that the assessment of AC corrosion likelihood should be based on the measurement of AC and DC (direct current) related parameters, namely AC voltage, AC and DC densities and potential measurements. Threshold values of the mentioned parameters are discussed. Overprotection (EIR-free < −1.2 V vs. CSE) is the most dangerous condition in the presence of AC: the combination of strong alkalization close to the coating defect due to the high CP current density and the action of AC interference provokes localized corrosion of carbon steel.
Highlights
Cathodic protection (CP), in combination with an insulating coating, is a well-known electrochemical technique that reduces the external corrosion rate of buried carbon steel pipes used to transport liquid or gas
This paper deals with a narrative literature review on alternating current (AC) corrosion assessment, protection criteria and corrosion mechanisms for buried carbon steel structures in CP condition
The AC interference level is evaluated by AC remote voltage and AC density, while the CP level is assessed by DC density and potential measurements; AC and DC densities should be measured by means of a corrosion coupon (1 cm2 area) connected to the structure in CP condition; IR-free potential is considered more accurate than ON potential, because it does not contain the ohmic drop contribution in soil; There is general agreement that the DC potential of carbon steel in CP condition increases in the presence of AC interference, a negative shift is measured at small DC density
Summary
Cathodic protection (CP), in combination with an insulating coating, is a well-known electrochemical technique that reduces (or halts) the external corrosion rate of buried carbon steel pipes used to transport liquid or gas. The induced AC voltage depends on the length of parallelism with the power line, and it is inversely proportional to the distance between the HVPL and the pipeline; the AC density, i.e., the current for unit surface, is a function of AC voltage, the coating defect dimension and soil resistivity. In the past 50 years, great effort has been made in order to propose criteria to assess AC corrosion likelihood and to understand the mechanism by which AC causes corrosion. This effort brought about the international standard ISO 18086 (Corrosion of metals and alloys—Determination of AC corrosion—Protection criteria) [4] that replaced in 2017 the EN. This paper deals with a narrative literature review, which includes a deep analysis of the AC corrosion phenomenon, in particular the assessment of AC interference, and the evaluation of AC corrosion likelihood and interference levels, the corrosion mechanism
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