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51315-5753-Cathodic Protection Criteria in The Presence of AC Interference

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Product Number: 51315-5753-SG
ISBN: 5753 2015 CP
Author: Andrea Brenna
Publication Date: 2015
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Carbon steel pipes used to transport hydrocarbons and dangerous fluids are provided with corrosion prevention systems namely an insulating coating such as polyethylene or polypropylene type and a cathodic protection (CP) system which reduces (or halts) corrosion rate below 10 mm/y which is the maximum accepted corrosion according to standards.The presence of AC interference may cause serious corrosion damages on metallic structures even under CP condition. When a metallic pipe is parallel to an AC interference source as high voltage transmission lines or AC traction systems AC interference can take place by a conduction or an induction mechanism causing corrosion corresponding to coating defects even if the -0.850 V CSE criterion is matched.Field failures and laboratory investigations have confirmed that at industrial AC frequencies (50 or 60 Hz) corrosion can occur even if CP is correctly applied and that AC corrosion is less than that caused by the equivalent direct current. Nevertheless uncertainties still exist on protection potential interval and on protection current density to reach acceptable protection if AC interference is present.Moreover doubts are revealed about the electrical and/or electrochemical parameters to consider in order to prevent and monitor AC-induced corrosion.In order to assess cathodic protection criteria in the presence of AC interference weight loss tests on cathodically protected carbon steel specimens were performed in soil-simulated conditions in the presence of AC densities ranging from 10 to about 500 A/m2. Samples were cathodically polarised in the presence of AC interference for four months: protection potential protection current density and AC density were periodically monitored. On the basis of obtained results a corrosion risk map is proposed: overprotection (potential more negative than -1.1 V CSE) seems to be the most dangerous condition in the presence of AC.

Carbon steel pipes used to transport hydrocarbons and dangerous fluids are provided with corrosion prevention systems namely an insulating coating such as polyethylene or polypropylene type and a cathodic protection (CP) system which reduces (or halts) corrosion rate below 10 mm/y which is the maximum accepted corrosion according to standards.The presence of AC interference may cause serious corrosion damages on metallic structures even under CP condition. When a metallic pipe is parallel to an AC interference source as high voltage transmission lines or AC traction systems AC interference can take place by a conduction or an induction mechanism causing corrosion corresponding to coating defects even if the -0.850 V CSE criterion is matched.Field failures and laboratory investigations have confirmed that at industrial AC frequencies (50 or 60 Hz) corrosion can occur even if CP is correctly applied and that AC corrosion is less than that caused by the equivalent direct current. Nevertheless uncertainties still exist on protection potential interval and on protection current density to reach acceptable protection if AC interference is present.Moreover doubts are revealed about the electrical and/or electrochemical parameters to consider in order to prevent and monitor AC-induced corrosion.In order to assess cathodic protection criteria in the presence of AC interference weight loss tests on cathodically protected carbon steel specimens were performed in soil-simulated conditions in the presence of AC densities ranging from 10 to about 500 A/m2. Samples were cathodically polarised in the presence of AC interference for four months: protection potential protection current density and AC density were periodically monitored. On the basis of obtained results a corrosion risk map is proposed: overprotection (potential more negative than -1.1 V CSE) seems to be the most dangerous condition in the presence of AC.

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