A study on the interaction between chloride ions and CO₂ towards carbon steel corrosion

Highlights

• An interaction mechanism between of Cl⁻ and CO₂ on the carbon steel corrosion exists.

• The relaxation process of water-iron species appears for the CO₂ saturated solutions.

• Cl⁻ contributes to the decrease of surface coverage of corrosion products and film breakdown.

Abstract

This article addresses the knowledge gap in the area of the interaction between Cl⁻ and CO₂ on carbon steel pipelines by comprehensively evaluating the corrosion behaviour of AISI 1020 carbon steel exposed to solutions containing Cl⁻ and/or CO₂ over time periods between 0.5 and 72 h at 60 °C. The results show Cl⁻ can reduce the surface coverage percentage of corrosion products and contributes greatly to film breakdown. The carbon steel is thus exposed to the corrosive solution containing CO₂ species, accelerating the further propagation of corrosion pits.

Introduction

Carbon steel is the most common construction material used in pipelines and plays an important role in oil and gas exploration, production and transportation [[1], [2], [3], [4]]. In contrast, its application prospects remain limited owing to the poor corrosion resistance in oil and gas environments [5,6]. Based on the Pipeline and Hazardous Materials Safety Administration (PHMSA) database, corrosion has caused ∼25 % of the natural gas transmission and gathering pipeline incidents over the last 30 years [7]. In comparison, in China, the proportion of corrosion failure is about 9 % provided by China Petroleum and Natural Gas Pipeline Company; however, in recent years, the cases of leakage accidents caused by pitting corrosion continuously increase with the aggravation of pipeline aging [8]. Instances of pipeline failure due to corrosion are associated with an inadequate understanding of the corrosive capabilities of some dissolved species, particularly carbon oxide (CO₂)-carrying species [9]. In oil and gas wells, the occurrence of CO₂ is prevalent in the production stream of natural CO₂ gas production and accompanied with produced water from reservoirs [9,10]. Carbonic acid is formed with the dissolution of CO₂ [11,12] and very corrosive to carbon steel pipelines employed in the oil and gas production sites [13]. In addition, chloride ion (Cl⁻) is a common constituent in produced water proceeding to flow into the pipelines, where its concentration varies from small quantities to high levels (0.5 − 20 wt%) [14,15]. Consequently, the inner surfaces of pipelines exposed to the CO₂-saturated saline solution, and/or the sections of pipelines which are in contact with the flowing fluids containing CO₂ and Cl⁻ can suffer notoriously high levels of corrosion, despite the use of corrosion resistance alloys [16,17]. Based on this, underpinning the understanding of the correlation between the corrosion severity of pipelines and corrosive capabilities of these dissolved species under such field conditions is of central importance to reduce the risks of failures in equipment and facility with the attendant capital and operating expenditure.

Studies on carbon steel corrosion in solutions containing Cl⁻ and/or CO₂ have been the subject of several articles [[18], [19], [20], [21], [22], [23]] and provided important contribution to understanding the corrosion mechanism. Firstly, the effect of Cl⁻ concentration on corrosion rates has attracted considerable attention. A similar result at different conditions was obtained and indicated that the injection of Cl⁻ resulted in an increase, followed by a non-linear reduction in corrosion rate beyond a certain value for systems where CO₂ partial pressure is maintained [18,19]. In comparison, Cl⁻ was reported to significantly reduce the CO₂ corrosion rate as a result of its retardation effect on the cathodic and anodic reactions at 20 °C [14]. Under these circumstances, such observations relating to the reduction in corrosion rate with increasing solution salinity can at least partly be attributed to the fact that NaCl decreases CO₂ solubility, lowering the dissolved CO₂ content. There is a consensus in the literature [20] of stating that NaCl did reduce corrosion rate when the gas partial pressure and pH are equal at room temperature by performing mass loss and electrochemical measurements in distilled water saturated with 1 bar CO₂ partial pressure, and 3.2 M NaCl solution saturated with 1 bar CO₂ partial pressure (both adjusted to pH 4). In terms of higher Cl⁻ concentrations, a decrease mechanism in relation to the corrosion rates in the CO₂-saturated solutions was the most adopted [18,[20], [21], [22]]. Considering the abovementioned issues, it can be concluded that in CO₂-bearing systems, Cl⁻ affects corrosion rates through changes in water chemistry that reduces dissolved CO₂, which reduces corrosion kinetics. In addition, the specific effects of Cl⁻ on the steel surfaces prior to the precipitation and/or corrosion product films should be further discussed; however, research in this regard is scarce.

On the other hand, the cathodic and anodic reactions were separately discussed to better set forth the corrosion mechanism of carbon steel exposed to CO₂ solutions [20,[24], [25], [26], [27], [28], [29], [30], [31], [32], [33]]. An abundance of research into the buffering effect is considered as the maximal contribution to the cathodic reaction under the service environments; however, the exact cathodic reaction with undissociated H₂CO₃ still remains open to debate, though a continuous advance in techniques was reported, involving the application of mathematical models [20,24,25], disproportionate decrease of cathodic/anodic reaction rates [26,27], and use of transient techniques et al. [20,28]. In terms of the anodic process, “catalytic mechanism” and “consecutive mechanism” (only 10 mV difference in Tafel slopes) using steady-state techniques were proposed by Heusler et al. [29] and Bockris et al. [30,31], respectively, and the mechanisms were then complemented with transient techniques since using only steady-state techniques is highly difficult to elucidate such an intricate process reported by Almedia and Mattos et al. [20,32]. In addition, the similar arguments regarding whether the adsorption of CO₂ on steel surfaces along with the use of steady-state Tafel analysis [27,33]/transient techniques [20,24,25,32] have been put forward in the recent literatures.

Despite the continuing contributions to understanding the corrosion mechanism of carbon steel in solutions containing Cl⁻ and/or CO₂, the results demonstrate some controversy and various aspects still require further clarification. Again, Cl⁻ or CO₂ was separately discussed to more accurately understand its role in carbon steel corrosion. No comparison was made with carbon steel under the specific condition containing both Cl⁻ and CO₂ and systems containing only Cl⁻ or CO₂. It is unclear that whether an interaction mechanism exists or not in the multiphase flows containing corrosive Cl⁻ and CO₂.

This article addresses the knowledge gap in the area of the interaction between Cl⁻ and CO₂ towards carbon steel corrosion by comprehensively evaluating the corrosion response of AISI-1020 carbon steel exposed to saturated CO₂ solutions containing various Cl⁻ concentrations along with solutions containing solely Cl⁻ over time periods between 0.5 and 72 h at 60 °C, and such an in-depth analysis presents a necessary complement to previous studies. A combination of surface film characteristics and additional electrochemical experiments at 25 °C were performed, and the latter is predominate in the understanding of the carbon steel corrosion prior to the precipitation. In addition, 0.05 M HCO₃⁻ was added into the testing solutions to ensure a similar surface concentration of HCO₃⁻, and its purpose is to reduce the effect of pH changes induced by salting-out, which is favorable to understand the specific effects of Cl⁻ on the steel surfaces prior to the precipitation and/or corrosion product films. Last but not least, the interaction mechanism between of Cl⁻ and CO₂ on the carbon steel corrosion will be clarified.