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51315-5721-Effects of Intermittent DC Stray Current on Carbon Steel Under Cathodic Protection

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Product Number: 51315-5721-SG
ISBN: 5721 2015 CP
Author: Marco Ormellese
Publication Date: 2015
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Stray currents originating from DC electrical systems may cause severe corrosion damage of buried metal structures as carbon steel pipes tanks or vessels. DC interference sources include high voltage transmission lines electrical traction and power networks and cathodic protection systems.Depending on stray current source stationary or fluctuating interference may occur on steel structures even under cathodic protection condition. Nowadays international standards establish the general principles for the identification and control of DC interference mainly based on potential and voltage gradients measurements over a 24 h period. Generally in order to assess the corrosion to which any metal structure is exposed as a result of stray current the positive (anodic) potential shift of the affected structure shall be considered. As regards carbon steel under cathodic protection condition anodic DC interference is considered unacceptable if the IR-free potential is more positive than the protection potential provided by standard (-0.850 V CSE in aerobic condition). But duration and magnitude of the anodic peak should be taken into account to evaluate the actual corrosion effects of the interference on protected steel. In fact the -0.850 V CSE criterion appears to be too restrictive if short periods of anodic interference are alternated with the re-establishment of the protection condition with the beneficial increase of the pipe-to-soil alkalinity due to the cathodic current.This paper deals with the study of the effects of anodic interference on buried carbon steel structures in cathodic protection condition. Weight loss tests were carried out on cathodically protected steel specimens in soil-simulating environment in the presence of DC interference varying anodic current density (from 0.1 to 10 A/m2) and duration of the interference peaks applied for 3600 s daily overall. IR-free potential was also monitored.
Stray currents originating from DC electrical systems may cause severe corrosion damage of buried metal structures as carbon steel pipes tanks or vessels. DC interference sources include high voltage transmission lines electrical traction and power networks and cathodic protection systems.Depending on stray current source stationary or fluctuating interference may occur on steel structures even under cathodic protection condition. Nowadays international standards establish the general principles for the identification and control of DC interference mainly based on potential and voltage gradients measurements over a 24 h period. Generally in order to assess the corrosion to which any metal structure is exposed as a result of stray current the positive (anodic) potential shift of the affected structure shall be considered. As regards carbon steel under cathodic protection condition anodic DC interference is considered unacceptable if the IR-free potential is more positive than the protection potential provided by standard (-0.850 V CSE in aerobic condition). But duration and magnitude of the anodic peak should be taken into account to evaluate the actual corrosion effects of the interference on protected steel. In fact the -0.850 V CSE criterion appears to be too restrictive if short periods of anodic interference are alternated with the re-establishment of the protection condition with the beneficial increase of the pipe-to-soil alkalinity due to the cathodic current.This paper deals with the study of the effects of anodic interference on buried carbon steel structures in cathodic protection condition. Weight loss tests were carried out on cathodically protected steel specimens in soil-simulating environment in the presence of DC interference varying anodic current density (from 0.1 to 10 A/m2) and duration of the interference peaks applied for 3600 s daily overall. IR-free potential was also monitored.
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