The effect of surface roughness and humidity on the corrosion behavior of X70 pipeline steel in super-critical CO2 environments with SO2, H2O, and O-2 was investigated. Scanning electron microscopy, 3D surface profilometry, energy-dispersive X-ray spectroscopy, X-ray diffraction, and 3D measuring laser microscopy were used to analyze the corrosion behavior. Weight loss analysis showed that the time-averaged corrosion rate varied from 0.03 mm/y to 1.78 mm/y depending on the initial humidity and surface roughness. Initial surface roughness exerts no influence on corrosion rates under relative humidity of <55% and >88%. However, at relative humidity between 60% and 75%, a rougher surface leads to high corrosion rate of 0.8 mm/y. Corrosion rate increases as humidity increases. The safe limit of humidity is estimated to be 2450 ppm under 10 MPa and 50 degrees C. The O-2 environment enhanced the corrosion behavior, and high O-2 concentration induced localized corrosion, such as pitting-and mesa attack. (C) 2015 Elsevier B.V. All rights reserved.