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Galvanic Corrosion Following Local Breakdown of a Scale Formed on X-65 in CO? Saturated Solutions

Picture for Galvanic Corrosion Following Local Breakdown of a Scale Formed on X-65 in CO? Saturated Solutions
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Product Number: 51315-5921-SG
ISBN: 5921 2015 CP
Author: Rafa Leiva-Garcia
Publication Date: 2015
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Galvanic corrosion following local breakdownof a scale formed on X-65 in CO2 saturated solutionsR.M. Fernández-Domene R. Leiva-García R. AkidCorrosion and Protection Centre School Of MaterialsThe University of Manchester Manchester UKProtection against corrosion of X-65 pipeline steel in sweet environments is generally provided by the formation of protective scales which result as a consequence of the precipitation of corrosion products (primarily FeCO3) on the metal surface. However when these scales are broken/damaged due to effects such as solid-particle erosion flow or mechanical stress a galvanic pair can form between the bare steel (anode) and the surrounding undamaged scale (cathode). This anode/cathode pair is the driving force in the evolution of the local corrosion damage. The aim of this work is to evaluate the magnitude of galvanic coupling that arises when the protective scale is broken and a corrosion cell is established between the defect and the surrounding scale.This study has been conducted in two stages firstly that of the formation of a protective scale on the surface of X-65 samples in CO2 saturated conditions; where the solution used to scale the surface was 0.1M NaCl solution saturated in CO2 at 80ºC at pH 7. The scale formation was monitored by means of electrochemical impedance spectroscopy (EIS) and linear polarisation resistance (LPR) measurements along with Open Circuit Potential (OCP) measurements. Additionally the scales were characterised by means of X-ray diffraction (XRD) analysis. Once the scale was formed on the sample the second stage of the experiment was conducted Here a scaled sample of X-65 steel was coupled with a fresh (non-scaled) X-65 sample in a brine solution saturated with CO2 at 60ºC. Zero resistance ammeter (ZRA) measurements were then conducted using this galvanic couple. The galvanic current and potential were registered during 24 hours. Additional measurements were then conducted using samples wherein the anode:cathode area ratio between the two samples was modified for example 1: 1:50 1:100 1:200. The changes in magnitude of the galvanic current were monitored as a function of anode:cathode area ratio.A protective scale was formed after 4 days of immersion under the experimental scaling conditions used the corrosion rate being recorded at lower than 0.1 mm/year. When the scaled samples were electrically coupled with fresh X-65 samples in the brine solution a galvanic current was established the anode of the pair being the fresh surface. This effect was enhanced when more unfavourable anode:cathode ratios were tested; to the extent that galvanic currents around 200 µA/cm2 were registered when the cathode/anode ratio was 200. It has therefore clearly been established that there is galvanic coupling between the bare metal and a scaled surface where the magnitude of the galvanic pair is found to be dependent on the anode:cathode ratio.
Galvanic corrosion following local breakdownof a scale formed on X-65 in CO2 saturated solutionsR.M. Fernández-Domene R. Leiva-García R. AkidCorrosion and Protection Centre School Of MaterialsThe University of Manchester Manchester UKProtection against corrosion of X-65 pipeline steel in sweet environments is generally provided by the formation of protective scales which result as a consequence of the precipitation of corrosion products (primarily FeCO3) on the metal surface. However when these scales are broken/damaged due to effects such as solid-particle erosion flow or mechanical stress a galvanic pair can form between the bare steel (anode) and the surrounding undamaged scale (cathode). This anode/cathode pair is the driving force in the evolution of the local corrosion damage. The aim of this work is to evaluate the magnitude of galvanic coupling that arises when the protective scale is broken and a corrosion cell is established between the defect and the surrounding scale.This study has been conducted in two stages firstly that of the formation of a protective scale on the surface of X-65 samples in CO2 saturated conditions; where the solution used to scale the surface was 0.1M NaCl solution saturated in CO2 at 80ºC at pH 7. The scale formation was monitored by means of electrochemical impedance spectroscopy (EIS) and linear polarisation resistance (LPR) measurements along with Open Circuit Potential (OCP) measurements. Additionally the scales were characterised by means of X-ray diffraction (XRD) analysis. Once the scale was formed on the sample the second stage of the experiment was conducted Here a scaled sample of X-65 steel was coupled with a fresh (non-scaled) X-65 sample in a brine solution saturated with CO2 at 60ºC. Zero resistance ammeter (ZRA) measurements were then conducted using this galvanic couple. The galvanic current and potential were registered during 24 hours. Additional measurements were then conducted using samples wherein the anode:cathode area ratio between the two samples was modified for example 1: 1:50 1:100 1:200. The changes in magnitude of the galvanic current were monitored as a function of anode:cathode area ratio.A protective scale was formed after 4 days of immersion under the experimental scaling conditions used the corrosion rate being recorded at lower than 0.1 mm/year. When the scaled samples were electrically coupled with fresh X-65 samples in the brine solution a galvanic current was established the anode of the pair being the fresh surface. This effect was enhanced when more unfavourable anode:cathode ratios were tested; to the extent that galvanic currents around 200 µA/cm2 were registered when the cathode/anode ratio was 200. It has therefore clearly been established that there is galvanic coupling between the bare metal and a scaled surface where the magnitude of the galvanic pair is found to be dependent on the anode:cathode ratio.
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