Volume 57

Corrosion Mechanism of Cr-containing Steel in Water-Saturated Supercritical CO2 with SO2 Impurity Wei Zhang, Chunxia Zhang, Zehui Zhao, Pei Wang, Yongyang Zhao, Hongxin Ding, Yong Xiang

Abstract

In CO2 energy storage systems, the working fluid CO2 contains impurities (H2O, SOx) from capture processes, leading to severe equipment corrosion issues that threatens the system stability. In this study, we investigated the corrosion behavior of N80, 3Cr, 9Cr and 13Cr steels in water-saturated supercritical CO2 (s-CO2) with 60 ppmv SO2 at 120°C and 20 MPa to simulate the operating conditions of CO2 energy storage applications. Multi-scale test methods were employed to systematically reveal the corrosion mechanism of the materials. Uniform corrosion rates were determined using weight-loss measurements, the surface morphology of corrosion products was characterized through scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray dif-fractometry (XRD), and the characteristics of pitting corrosion was measured by laser confocal microscopy. The results showed that for uniform corrosion, the uniform corrosion rates were measured as 0.170 mm/a for N80 steel and 0.167 mm/a for 3Cr steel, indicating nearly identical corrosion behavior. In comparison, the rates significantly decreased to 0.079 mm/a for 9Cr steel and 0.015 mm/a for 13Cr steel, demonstrating the corrosion resistance enhancement with increasing Cr content. For localized corrosion, all Cr-containing steels except 13Cr exhibited a significant tendency for pitting corrosion. Specifically, for steels with Cr content below 13%wt, the maximum pitting rate demonstrated a positive correlation with Cr content. N80 steel displayed the lowest pitting factor, attributable to its highest uniform corrosion rate. For the corrosion product scales, pits and cracks were observed on the surfaces of all tested steels. XRD analysis revealed that the corrosion product films on both N80 and high-Cr (3Cr/9Cr/13Cr) steels primarily contained FeCO3, with potential presence of FeSO4·4H2O or FeSO3·3H2O. Cross-sectional EDS line scan analysis demonstrated that apart from differences in corrosion product film thickness, the structural distinction between N80 steel and high-Cr steels stemmed from a thin Cr-rich layer formed at the surface, where Cr content differences were identified. This study indicates that Cr-containing steels pose a corrosion risk under water-saturated supercritical CO2 environment with SO2 impurity.

Keywords CCUS, supercritical CO2, SO2, uniform corrosion, localized corrosion, Cr steel