Effect of temperature on CO2 corrosion of carbon steel

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Received: 17 December 2008        Revised: 14 February 2009        Accepted: 16 February 2009        Published online in Wiley Interscience: 18 March 2009

(www.interscience.wiley.com) DOI 10.1002/sia.3057

Effect of temperature on CO2 corrosion of carbon steel

Z. F. Yin,a,b∗ Y. R. Feng,b W. Z. Zhao,a Z. Q. Baib and G. F. Linb

This work investigates the effect of temperature on the corrosion product layer of carbon steel exposed to a CO2-containing solution. The measurement techniques, such as scanning electron microscopy with energy dispersive spectrometry, X-ray diffraction, and X-ray photoelectron spectroscopy, were used to systematically characterize the morphology and composition of the corrosion product layer. The corrosion rates were calculated by weight loss method. The corrosion mechanisms as a function of temperature are studied and discussed. The results showed that temperature is an important factor in the corrosion rate of carbon steel. Copyright Ⓧc  2009 John Wiley & Sons, Ltd.

Keywords: CO2 corrosion; carbon steel; corrosion product layer; XPS; solution chemistry[pic 3]

Introduction

Dissolved CO2 in aqueous solutions can lead toserious corrosion on the steel pipelines or equipments used in the oil/gas production or transport, which can even result in major safety accident.[1,2] The understanding of CO2 corrosion mechanisms under the effects of many mechanical and environmental factors, such as flow, temperature, pressure, oil– water ratio, pH, solution chemistry, and corrosion product layer, has been of great concern in corrosion field.[1,3 – 6] Carbon steels are often used as materials in oil-producing and -gathering facilities; however, they are very susceptible to corrosion in CO2 environments, leading to severe damage.

It is known that CO2 hydration is a slow chemical reaction and can be the rate-determining step for the overall corrosion rate. Only a small fraction of carbonic acid is dissociated in two steps to form hydrogen, bicarbonate, and carbonate ions. The anodic reaction is iron dissolution and the cathodic reactions include one or more of the following: hydrogen ion reduction, carbonate acid reduction (perhaps bicarbonate ion reduction), and water reduction (generally occurring at a pH value above 6).[3,7]

Temperature is one of the most important influencing factors in CO2 corrosion environments. In general, temperature plays an important effect on the formation rate of corrosion product layer. At low temperatures (below 60 ◦C), no protective layer is formed on the steel surface because of the high solubility of the iron carbonate layer. The corrosion product layers are generally porous and loose, leading to increased corrosion rates. As temperature increases, the corrosion product layers become more compact, dense, and adherent to the steel surface, resulting in the formation

of more protective precipitates on the steel surface. Therefore, the corrosionrategenerallydecreaseswhenthetemperatureisaround or above 80 ◦C.[8– 12]

The effect of protective corrosion layers related with different kinds of variables in CO2 environments has been discussed. The main corrosion product of carbon steel is iron carbonate (FeCO3), which precipitates on the steel surface when the FeCO3 level is high enough close to the steel surfaces.[8,13] Depending on the type of the corrosion product layers present and the experimental conditions under which these layers can be formed, the corrosion

product layers can offer some protection against corrosion by reducing mass transfer of the reactants and products between bulk solution and steel surface.[14] If the products are adherent and dense, they can cover the steel surfaces, leading to a good protection. However, localized corrosion can also occur when the product layers break down locally and cannot be repaired by nucleation and growth. The kinetics of FeCO3 precipitation seems to be a controlling factor for the protection of corrosion product layers. At higher temperatures, the FeCO3 solubility is decreased and the precipitation is much faster.[12,15] In addition, protective product layers can be formed more easily and fast at higher pH values in CO2 environments.[11,15,16] Many protective mechanisms of corrosion product layers are published in the past decades; for instance, Nesic et al.[11] thought that the product layers act as a physical barrier that can restrict the diffusion transfer of aggressive species and prevent further steel dissolution.

The difficulty in defining and describing corrosion mechanisms involving many variables in CO2 environments leads to complexity in understanding the morphologies of corrosion product layers, solution chemistry, and corrosion prediction models. The purpose of the present work is to investigate the effect of temperature on corrosion product layers of carbon steel exposed to CO2- containing solution by using SEM, energy dispersive spectrometry (EDS), X-ray diffraction (XRD), and XPS measurement techniques. The corrosion mechanisms are also studied as a function of temperature and discussed.

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∗ Correspondence to: Z. F. Yin, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China.

E-mail: yinzhifu919@sohu.com

1. School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China

1. The Key Laboratory for Mechanical and Environmental Behavior of Tubular Goods, Tubular Goods Research Center, CNPC, Xi’an 710065, People’s Republic of China

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Surf. Interface Anal. 2009, 41, 517– 523        Copyright Ⓧc  2009 John Wiley & Sons, Ltd.

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Figure 1. Morphologies of corrosion product layers formed at various temperatures: (a) 50 ◦C, (b) 70 ◦C, (c) 100 ◦C, (d) 130 ◦C, (e) 150 ◦C, and (f) 180 ◦C. Magnification 1000×.

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