Effect of the Acid Environment on the Electrochemical Behaviour of 1045 Steel Reinforced Cement

Abstract

External corrosion is one of the most common causes of oilwell casing failure. Hostile environments can be due to acidizing treatments. Although it is common to add corrosion inhibitors and oxygen scavengers in acidic solutions to control external casing corrosion, their real efficiency is unknown yet. Therefore, it is important to establish how aggressive to steel are the different hostile environments to help decide which acidic systems can be used. A comparative evaluation of the corrosion of steel immersed in hardened cement slurries submitted to commonly acidizing agents is suggested. The performance of Special Class Portland Cement Slurries reinforced with polished SAE 1045 steel was evaluated by electrochemical measurements as a function of time. Open circuit potential, polarization curves and electrochemical impedance spectroscopy were studied. 15.0 wt% HCl, 12.0 wt% HCl + 3.0 wt% HF (regular mud acid), 10.0 wt% HAc + 1.5 wt% HF and a simulated hardened cement slurry pore solution were used as electrolytes. The most aggressive acid solution to plain Portland hardened cement slurries was the regular mud acid. 10.0 wt% HAc + 1.5 wt% HF electrolytes were the least aggressive ones, showing open circuit potentials around +250 mV compared to -130 mV of the simulated hardened cement slurry pore solution after the first 24 hours of immersion. This behavior was observed during two months at least. Similar corrosion rates were shown between both electrolytes, around 0.01 μA.cm-2. Total impedance values, insipient arcs and large polarization resistance capacitive arcs on the Nyquist plots, indicating passivity process, confirmed the behaviour of the system in the 10.0 wt% HAc + 1.5 wt% HF electrolyte.