The effects of corrosion product deposit on the subsequent anodic and cathodic reactions of X-70 steel in a near-neutral pH solution were investigated by localized electrochemical impedance spectroscopy (LEIS), scanning vibrating micro-electrode (SVME) and macroscopic EIS measurements as well as surface analysis technique. The test solution used was NS4 solution, which has been widely used to simulate the dilute electrolyte trapped between coating and pipeline, with the chemical composition (g/l): 0.483 NaHCO3, 0.122 KCl, 0.181 CaCl2.2H2O and 0.131 MgSO4.7H2O. The solution was purged with 5% CO2 (balanced with N2 gas). It is found that the deposit layer formed on the steel surface is porous, non-compact in nature. The presence of a corrosion product layer would enhance adsorption, but significantly inhibit absorption and permeation of hydrogen atoms into steel. It is due to the porous structure of the deposit that generates a spatial separation of cathodic and anodic reaction sites, resulting in an increased effective surface area for hydrogen adsorption and, simultaneously, a ''blocking'' effect on hydrogen absorption and permeation. The deposit enhances greatly anodic dissolution of the steel, which is attributed to the adsorption of the intermediate species and the resultant ''self-catalytic'' mechanism for corrosion of the steel in near-neutral pH solution. In the presence of corrosion product deposit on the pipeline steel surface, pipeline corrosion, especially pitting corrosion, is expected to be enhanced. Stress corrosion cracks could initiate from the corrosion pits that form under deposit. However, deposit does not contribute to hydrogen permeation, although the hydrogen evolution is enhanced.
Effects of corrosion product deposit on the subsequent cathodic and anodic reactions of X-70 steel in near-neutral pH solution
Einfluss der Korrosionsproduktablagerung auf die anschließenden anodischen und kathodischen Reaktionen von Stahl X-70 in pH-neutraler Lösung
Corrosion Science ; 50 , 11 ; 3116-3122
2008
7 Seiten, 7 Bilder, 1 Tabelle, 26 Quellen
Article (Journal)
English
Spannungsrisskorrosion , Lochkorrosion , Rohrstahl , Einflussgröße , Korrosionsprodukt , anodische Reaktion , kathodische Reaktion , wässrige Lösung , Simulationsversuch , Rohrleitungskorrosion , Wasserstoff , Absorption , Adsorption , Korrosionsmechanismus , Polarisationsmessung , Mikroelektrode , elektrochemische Impedanzspektrometrie
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