The influence of bicarbonate ion (HCO3−) concentration and related environmental parameters such as pH and temperature on the passivation of API-X100 pipeline steel is the focus of this investigation. NS4 solution simulating groundwater trapped under disbonded coatings at regions where near-neutral stress corrosion cracking (SCC) occurs was used as a reference. Bicarbonate content in the solutions was increased steadily and the critical HCO3− concentration at which passivation becomes evident is found using potentiodynamic polarization sweeps. Multi-step dissolution is observed especially in higher pH solutions, owing to the involvement of hydroxide ions (OH−) in the adsorption of hydrous Fe(OH)2. Dynamic electrochemical impedance spectroscopy (DEIS) is used to study and compare interfacial processes in active, pre-passive, passive, and trans-passive regions. The results of the potentiodynamic and DEIS tests achieve good agreement.
The protective properties of the passive film formed at three different HCO3− concentrations (critical passivation and two higher concentrations) and temperatures (25, 50, and 75 °C) are studied. The combined effect of HCO3− and temperature resulted in a pH range from ∼6.7 to 9.3 for the nine conditions in the corresponding test matrix. Passive films were formed at a suitable anodic potential and then studied with electrochemical impedance spectroscopy (EIS), Mott-Schottky, and cyclic voltammetry (CV) tests. Evidence of adsorption and diffusion, in addition to significant resistance from the passive film, is observed and accounted for in the equivalent circuit, which achieved good correspondence when fitted with the experimental data. The protective properties of the overall passive film are enhanced with increased bicarbonate and decreased temperature. Positive slopes in the Mott-Schottky plots reveal the n-type semi-conductive behavior of the passive films in all conditions and the CV results highlight the proposed reaction sequences occurring in different potential scan regions. CV measurements also confirm the enhanced protective properties of passive films with increased bicarbonate and lower temperatures observed in the EIS analysis.
Physicochemical characterization of passive films on niobium by admittance and electrochemical impedance spectroscopy studies