Corrosion Behavior and Characteristics of 3Cr Steel in Coexisting H₂S- and CO₂-Containing Solutions

The corrosion behavior and characteristics of 3Cr steel in coexisting H₂S- and CO₂-containing solution at temperature from 25 to 65 °C were investigated by potentiodynamic polarization sweep, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) with energy-dispersive x-ray (EDX). The EIS data fitted by ZsimpWin software were analyzed and discussed. The experimental results indicated that corrosion potential (E_corr) became more negative and that corrosion current density (i_corr) decreased as the Na₂S·9H₂O concentration (i.e., H₂S/HS⁻) increased from 0 to 1.6%, with the obvious passivation characteristics occurring in anodic polarization branch curve in 1.6% Na₂S·9H₂O case. Warburg impedance characteristics of diffusion process in low-frequency region following a depressed capacitive semicircle in the intermediate frequency region were observed with the addition of certain Na₂S·9H₂O concentration or temperature, with the phase angle peaks for H₂S-containing cases in the intermediate frequency region shifting to low-frequency direction and raising toward −90° significantly. Severe corrosion pits or cavities or river-liking platform could be observed after removing the corrosion products. The contents of S element in the “first deposition” black corrosion product layer were higher than those in the “second deposition” gray corrosion product crystals. All result displayed the inhibition effect with the addition of Na₂S·9H₂O. In addition, one four-stage corrosion model involving H₂S and CO₂ corrosion was proposed.