Synergistic effect of Cl− and HCO3− on the corrosion of X90 pipeline steel in an alkaline soil environment

This study systematically investigates the corrosion behavior of X90 pipeline steel in simulated alkaline soil solution (pH = 9.0) by combining electrochemical techniques and acoustic emission monitoring. The effects of chloride ions (Cl−, concentration gradient: 0, 0.03, 0.05, 0.07 mol/l) and bicarbonate ions (HCO3−, concentration gradient: 0, 0.02, 0.05, 0.10 mol/l) on corrosion characteristics were examined. Potentiodynamic polarization curves and electrochemical impedance spectroscopy tests revealed that the corrosion rate of X90 steel increases significantly with increasing Cl− concentration. For instance, the corrosion rate is 0.176 mm/a in the Cl−-free environment and rises to 0.364 mm/a when the Cl− concentration increases to 0.07 mol/l. In addition, the presence of HCO3− inhibits the corrosion process in the alkaline environment. AE signal analysis indicated that the corrosion process is dominated by the metal dissolution stage, generating two types of characteristic signal clusters: metal dissolution signals and passive film formation signals, with peak frequencies of 150 kHz and 125–150 kHz, respectively, and peak amplitudes of 5 × 10−3 V and 0.024 V. Post-corrosion microscopic observation confirmed obvious pitting corrosion on the steel surface (with diameters ∼20 μm), demonstrating that HCO3− and Cl− synergistically induce and accelerate the initiation and propagation of pitting corrosion.