51313-02457-A Proposal of AC Corrosion Mechanisms of Carbon Steel in Cathodic Protection Conditions

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Product Number: 51313-02457-SG

ISBN: 02457 2013 CP

Author: Marco Ormellese

Publication Date: 2013

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Buried pipelines used to transport hydrocarbons and dangerous fluids are provided with corrosion prevention systems consisting of an insulating coating and a cathodic protection (CP) system. CP reduces (or halts) corrosion rate so that corrosion damage cannot occur during the designed lifetime of the structure. When a metallic pipe is parallel to an AC source as the high voltage transmission lines (HVTL) AC interference can take place causing corrosion corresponding to coating defects even if the -0.850 V CSE criterion is matched. Uncertainties still exist on CP criteria in the presence of AC and on the mechanism by which AC corrosion occurs. In this paper a mechanism of AC-induced corrosion of carbon steel in CP condition is proposed. Electrochemical tests were carried out on carbon steel in soil simulating environment varying protection and interference conditions: overprotection (potential lower than -1.1 V CSE) seems to be the most dangerous condition. A two-steps corrosion mechanism is proposed. Firstly AC causes the electro-mechanical breakdown of the passive film formed on steel in CP condition (Step 1) and then corrosion occurs if the pH at the metal-to-electrolyte interface is close to 14 (Step 2). In order to investigate the AC effect on passive condition tests were carried out on passive metals as stainless steels in neutral solution in the presence of chloride ions. AC has an harmful effect on passive condition reducing the critical chlorides threshold and pitting potential and causing the film breakdown over a critical interference level. A mechanical breakdown mechanism is proposed: high alternating electric fields (in the order of 106 V/cm) within the passive film could cause the breakdown of the film itself. Electrostriction stresses are a possible explanation. After film breakdown corrosion can occur if the pH inside the crack is close to 14; this strong alkalization can be reached in overprotection condition and in the presence of AC. The model still needs to be confirmed by experimental tests.