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A Mechanistic Study On The Effect Of Salt Concentration On Uniform Corrosion Rate Of Pipeline Steel In Acidic Aqueous Environments

Product Number: 51321-16788-SG
Author: Fazlollah Madani Sani; Bruce Brown; Srdjan Nesic
Publication Date: 2021
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CO2/H2S corrosion in oil and gas fields is often associated with acidity and salinity of fluids. However, the role of salinity is usually ignored in corrosion studies and corrosion prediction models. As part of a major project to model uniform CO2/H2S corrosion of carbon steel oil and gas pipelines in high salinity environments, the research reported herein investigated uniform CO2 corrosion in high salinity acidic media. Experiments were carried out using a rotating disk electrode (RDE) in CO2-saturated aqueous solutions at 10oC and a constant pH of 3.0, whilst varying NaCl concentration from 1 wt.% to 20 wt.%. The RDE setup was used because mass transfer of species to the surface in this setup is well understood. A low temperature was chosen to better distinguish between charge transfer and mass transfer-controlled reactions underlying the corrosion process. Surface analysis indicated that the corrosion process was uniform, and no corrosion layer formed on the steel surface. Both linear polarization resistance (LPR) and potentiodynamic polarization (PD) techniques showed a general decrease in the uniform corrosion rate with increasing salt concentration. The analysis of the PD sweeps showed that salt concentration did not influence the mechanism of anodic and cathodic reactions and the decrease in the corrosion rate was due to deceleration in the rate of both anodic and cathodic reactions. Modeling results showed that salt concentration mostly affected the rate of mass transfer of H+ ion to the surface by changing H+ diffusion coefficient and H+ activity coefficient.

CO2/H2S corrosion in oil and gas fields is often associated with acidity and salinity of fluids. However, the role of salinity is usually ignored in corrosion studies and corrosion prediction models. As part of a major project to model uniform CO2/H2S corrosion of carbon steel oil and gas pipelines in high salinity environments, the research reported herein investigated uniform CO2 corrosion in high salinity acidic media. Experiments were carried out using a rotating disk electrode (RDE) in CO2-saturated aqueous solutions at 10oC and a constant pH of 3.0, whilst varying NaCl concentration from 1 wt.% to 20 wt.%. The RDE setup was used because mass transfer of species to the surface in this setup is well understood. A low temperature was chosen to better distinguish between charge transfer and mass transfer-controlled reactions underlying the corrosion process. Surface analysis indicated that the corrosion process was uniform, and no corrosion layer formed on the steel surface. Both linear polarization resistance (LPR) and potentiodynamic polarization (PD) techniques showed a general decrease in the uniform corrosion rate with increasing salt concentration. The analysis of the PD sweeps showed that salt concentration did not influence the mechanism of anodic and cathodic reactions and the decrease in the corrosion rate was due to deceleration in the rate of both anodic and cathodic reactions. Modeling results showed that salt concentration mostly affected the rate of mass transfer of H+ ion to the surface by changing H+ diffusion coefficient and H+ activity coefficient.

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