On the Corrosion Mechanism of CO₂ Transport Pipeline Steel Caused by Condensate: Synergistic Effects of NO₂ and SO₂

To study the effects of condensed acid liquid, hereafter referred to as condensate, on the CO₂ transport pipeline steels, gas mixtures containing a varying concentration of H₂O, O₂, NO₂, and SO₂, were proposed and resulted in the condensate containing H₂SO₄ and HNO₃ with the pH ranging from 0.5 to 2.5. By exposing the pipeline steel to the synthetic condensate with different concentration of acidic components, the corrosion kinetic is significantly changed. Reaction kinetic was studied using electrochemical methods coupled with water analysis and compared with surface analysis (scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffractometry (XRD)) of corroded coupons. The results showed that, although the condensation of NO₂ in the form of HNO₃ causes faster general corrosion rate, it is the condensation of SO₂ in the form of H₂SO₄ or the combination of SO₂ and NO₂ that may cause much more severe problems in the form of localized and pitting corrosions. The resulting corrosion forms were depended on the chemical nature of acids and their concentration at the same investigated pH. The effects of changing CO₂ flow rate and renewing condensate on pitting corrosion were further studied.