Effect of temperature on CO₂ corrosion production scales on 3%Cr pipeline steel

Economic low-chromium steels show superior resistance to CO₂ corrosion.The corrosion performance of 3%Cr pipeline steel was studied with a high-temperature and high-pressure autoclave.In a CO₂ environment,X-ray diffraction(XRD),scanning electron microscopy(SEM) and energy-dispersive spectrometry(EDS) were used to analyze the morphologies and characteristics of corrosion production scales on 3%Cr steel.The formation of the scales at different temperatures was also discussed.The results show that when the CO₂ partial pressure is 0.8 MPa and the flow velocity is 1.0m·s^-1 within the temperature range of 40 to 140℃,the average corrosion rate first increases and then decreases with increasing temperature,and its maximum can be obtained at about 100℃.For all the test conditions no localized corrosion was observed.There is a two-layer structure in the scales.The inner layer,which is mainly composed of Cr compounds and amorphous FeCO₃,is denser and more stable because of Cr enrichment.The Cr content of the inner layer increases with increasing temperature and the mass ratio of Cr to Fe can reach 8/5.The outer layer is formed from the precipitation of crystalline FeCO₃.