10298 Electrochemical Properties of Pipeline Steel X-65 in Dilute HC03-/CO32-Solutions

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Product Number: 51300-10298-SG

ISBN: 10298 2010 CP

Author: Li Yan, Robert Grant Worthingham and Fraser King

Publication Date: 2010

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In this study, the electrochemical behaviour of X65 pipeline steel was investigated in three dilute HCO₃^- solutions, i.e., 10 mM, 20 mM, and 50 mM, purged with 5% CO₂/N₂ or 100% CO₂ to achieve the typical pH of the trapped water under a disbonded shielding coating, i.e., pH 5.5 to 7.5. The measurements of corrosion potential (E_corr) showed that the free corrosion process of X65 carbon steel in dilute HCO₃^- solutions involves Fe – 2e^- - Fe²⁺ as the anodic reaction coupled with H⁺ + e^- - ½H₂ as the cathodic reaction. The investigation of the anodic polarization behaviour of X65 carbon steel in dilution HCO₃^- solutions purged with N₂ + 5% CO₂ confirmed that the anodic reaction, Fe – 2e^- - Fe²⁺, was under mixed diffusion-electrochemical control. The anodic Tafel slope displayed a value of ~60 mV/decade in the 10mM HCO₃^- solution. As the HCO₃^- concentration was increased from 10 mM to 50 mM, the anodic Tafel slope constantly increased from ~ 60 to ~ 90 mV/decade. Cathodic polarization curves displayed a current-peak region at the low cathodic polarization range, -0.95 V to -0.75 V, and a typical Tafel region at potentials beyond -0.95 V. The current-peak within the low cathodic polarization range could be due to the catalyzing effect of the partially oxidized surface on the hydrogen evolution reaction (HER), and this catalyzing effect was enhanced by increasing HCO₃^- concentration or/and pH. A high cathodic Tafel slope, i.e, ~ 200 mV/decade, was observed for the HER at E < -0.95 V.

Keywords: Open circuit potential (Ecorr), rotation rate, mass-transport, anodic polarization, cathodic polarization, external corrosion, near-neutral pH SCC, disbondment, trapped water, shielding coating