Development of X65 Pipeline Steel for Supercritical CO₂ Transport Environment

A new X65 pipeline steel is designed with low C and low Mn，and via micro-alloying，and its mechanical properties and corrosion behavior in supercritical CO₂ environment are also studied in comparison with the ordinary X65 pipeline steel. The results show that the microstructure of the two steels are dominated by polygonal ferrite and acicular ferrite，and the grain size of the newly-designed steel is smaller than that of the ordinary one；the impact energy of the two steels at -80 ℃ are over 350 J，which meets the low temperature toughness as for the supercritical CO₂ transport environment； the mechanical properties of the two steels are just slightly different；however the corrosion resistances of the two steels are remarkably different under the water saturation and supercritical CO₂ environment conditions featuring 35 ℃，14.5 MPa and 200 r/min；the corrosion thinning rate of the X65 steel and the X65-1 steel are 3.16 mm/a and 2.34 mm/a respectively；and the corrosion resistance of the newly-designed steel is 1.35 times higher than that of the ordinary X65 steel；the Cr element mainly exists as Cr₂O₃ and Cr （OH）₃，which prevents corrosive ions from entering the protective matrix so as to improves its corrosion resistance.