Compare the Corrosion Resistance of 3%Cr Steel with API X65 Pipeline Steel in CO2 Corrosion Environment

Low alloy steels containing 1~5wt% Cr have been developed as tubing and casing due to their CO₂ corrosion resistance and cheaper cost, while poor understanding about the weldability degradation induced by Cr addition impedes their application as pipeline steel in the last decade. In recent years, an improved steel containing 3%Cr has been developed as a candidate of pipeline steel due to its good performance of both weldability and corrosion resistance. This paper made a compare about CO₂ corrosion behaviors between this 3%Cr pipeline steel and commercial API X65. High-pressure high-temperature corrosion experiments were carried out in autoclaves in order to simulate the typical corrosion environment of oil and gas pipelines. The effects of temperature and CO₂ partial pressure on corrosion behaviors of both materials were investigated. With temperature and CO₂ partial pressure increasing, the corrosion resistance of 3%Cr steel was superior to X65 both of a remarkable lower corrosion rate and the resistance to localized corrosion in rigorous corrosion environments. The analysis of composition and microstructure morphology reveals that the corrosion product scale of 3%Cr containing steel was composed of FeCO₃, Cr(OH)₃ and Cr₂O₃. An amorphous scale without grain boundaries and pores was formed on the surface of 3%Cr containing steel due to the Cr element enrichment, which offered considerable resistance to corrosive medium and resulted in the low corrosion rate and free of localized corrosion.