Influence of heat treatment on the corrosion behaviour of stainless steels exposed to CO_2-saturated saline aquifer water

摘要

When operating a saline aquifer Carbon Capture and Storage (CCS) site certain property requirements of pipe steels need to be fulfilled to withstand the distinct corrosive environment (heat, pressure, salinity of the aquifer, CO_2-partial pressure). This can be achieved by applying different heat treatments. However, heat treatment also results in different microstructures with different arrangements of atoms and phase precipitation, which are likely to affect the corrosion behaviour. To predict the reliability concerning the injection process of compressed emission gases (with high CO_2 content) the influences need to be investigated. In this study two different injection pipe steels with 13% chromium and 0.46% carbon (1.4034, X46Cr13) as well as 0.20% carbon (1.4021, X20Cr13) are investigated. Prior to long term corro-sion experiments samples of both steels were heat treated by comparable routines. With 60°C, 100 bar and an artificial brine the experimental conditions are close to those present at a CCS-site in the Northern German Basin. In this condition the CO_2 will be present in its supercritical state. Additionally, experiments at ambient pressure and constant CO_2-flow rate are carried out to investigate the influence of pressure. In general higher carbon content exhibits better corrosion resistance with respect to corrosion rate, amount of pits and maximum intrusion depth in the case of local corrosion. At 100 bar hardened steels with martensitic microstructure have the lowest corrosion rates with an average of 0.003 mm/a after 2000 h exposure time. However, tempering the hardened steel again leads to a deprived corrosion resistance. At ambient pressure normalized steels show the lowest corrosion rates, indicating that the corrosion is driven by grain boundaries.The corrosion resistance scales with low austenitizing temperature and short austnitizing time. For example, tempering for 30 minutes prior to exposure to the distinct CCS-environment exhibits a corrosion rate of about 0.01 mm/a, whereas tempering for 90 minutes results in 0.025 mm/a. Furthermore, higher austenitizing temperatures (755 °C) lead to enhanced pit corrosion and higher maximum intrusion depths (0.55 mm after 4000 h exposure). The influence of tempering is the key factor, which leads to the formation of carbide and cementite precipitations. Therefore the relation between toughness, stiffness and corrosion resistance needs to be considered carefully.