Effect of Impurity SO₂ on Corrosion and Stress Corrosion Cracking of X65 Steel in Supercritical CO₂ Streams

ABSTRACT

Pipeline transportation is a critical section in the context of carbon capture and storage (CCS) systems. Transported supercritical CO₂ (sc-CO₂) usually contains corrosive impurities, particular SO₂, that can lead to exacerbated corrosion damage on pipeline integrity. However, there is still no clear image to describe how the impurities affect corrosion due to very limited database and some controversial results. More importantly, there is a serious concern on the stress corrosion cracking (SCC) susceptibility of the pipeline because of the presence of corrosive agents and high operating pressure of sc-CO₂ mixtures. Surprisingly, few works have been done on this issue. In this paper, the effects of SO₂ on corrosion and SCC susceptibility of X65 steel were investigated in sc-CO₂ environment containing impurities SO₂, O₂ and H₂O. The results show that the addition of only 100 ppm SO₂ into the sc-CO₂ system significantly increases corrosion rate, consistent with some previous results. Under such corrosive condition, however, SCC crack is not found in the steel specimen even heavily strained up to 0.6%. This finding opens a new window on the development of corrosion control strategy of sc-CO₂ pipelines. Corresponding corrosion mechanisms in the media are also discussed and proposed.

1. INTRODUCTION

With the growing demands on power and heat supply, increasing emission of CO₂ from fossil fuel combustion is recognized as the main contributor to the global climate change. Carbon capture and storage (CCS) technology is the most promising solution that can keep the core values of fossil fuel power plants while significantly reducing CO₂ emission. The successful deployment of CCS depends on a safe, reliable and cost-effective CO₂ transportation from flue gas of fossil fuel power plant to specific storage or utilization locations. To achieve the targets, the transported CO₂ is compressed into supercritical state, and conventional carbon steels are used for the construction of the sc-CO₂ transportation pipes.

Although the carbon steels are strong enough to meet the mechanical requirements for the safe and reliable pipe operation, their resistance to corrosion attack remains a challenge. Pure sc-CO₂ has no threaten to the pipes from corrosion point of view. However, the transported sc-CO₂ always contains different amounts of impurities, such H₂O, H₂, Ar, O₂, H₂S, SO_x, NO_x, depending on the sources and capture technologies applied.^{1, 2} These impurities could corrode the steels, resulting in severe damage as well as cracking. The thorough removal of the impurities from captured CO₂ stream is unacceptable due to the relatively high cost. The practical way is to control their concentration within certain ranges.³ However, no consensus has been achieved on the maximum allowable concentrations of impurities due to the lack of abundant experimental data and field experience.