Comparisons of corrosion behaviour for X65 and low Cr steels in high pressure CO2-saturated brine

Appropriate materials for injection pipelines and tubing for carbon dioxide geologic storage is fundamental to ensure asset integrity and save cost. This paper evaluates the corrosion behaviour of X65, 1Cr, 3Cr and 5Cr, which have the potential to be injection pipeline/tubing materials. The influence of steel Cr content on the general and localised corrosion behaviour was investigated at time periods from 6 to 192 h at 60 ⁰C and 100 bar. The evolution, morphology and chemistry of corrosion products on the surface of each material were evaluated using a combination of scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction and related to their overall corrosion protection. Results indicate that prior to the formation of protective films on the steel surfaces, the resistance of the materials to corrosion increase with increasing Cr content (Corrosion resistance: X65 < 1Cr<3Cr<5Cr). However, as corrosion products evolve, the protection afforded to the different steels significantly varies and decreases with increasing Cr content. × 65 becomes the material with the lowest general corrosion rate by the end of the 192 h experiments and 5Cr exhibits the highest corrosion rate (ranking of corrosion resistance: X65 > 1Cr>3Cr>5Cr). In terms of the corrosion products on X65, both inner amorphous and outer crystalline corrosion layers consist of FeCO3. For the Cr-containing steels, the outer layer also comprises FeCO3, but the inner layer is enriched with Cr, and is predominantly amorphous Cr(OH)3. The extent of localised corrosion (determined using surface profilometry) is noticeably less for X65 compared to the Cr-containing steels. The paper raises questions about the benefits that low Cr steels offer towards extending component design life compared to carbon steel under the test conditions considered here.