Electrochemical Response of Proprietary Microalloyed Steels to pH and Temperature Variations in Brine Containing 0.5% CO₂

The corrosion behavior of three new-generation microalloyed steels in CO₂ saturated brine at different pHs and temperatures was investigated using electrochemical measurements such as linear polarization resistance (LPR), Tafel polarization, and electrochemical impedance spectroscopy (EIS), and surface analyses techniques such as scanning electron microscopy coupled with energy dispersive x-ray spectroscopy (SEM/EDS) and x-ray diffraction (XRD) analysis. The microalloyed steels with ferrite-pearlite microstructures demonstrated better corrosion resistance than the specimen with bainitic structures. The analysis of the corroded surface revealed relative elemental changes of corrosion products revealing that the average ratio of Fe/O increased with an increase in pH but decreased with an increase in temperature. The electrochemical results indicated that the corrosion resistance of Steel C < Steel B < Steel A. The corrosion kinetics of the steels follow the empirical relation y = Ax^B, thus obeying the well-known Log-Log equation (Log Y = Log A + B log X) which can be used to predict long-time corrosion performance. The value of B represents the corrosion kinetics and it decreased with an increase in pH depicting corrosion deceleration but increased with temperature signifying corrosion acceleration.