Susceptibility to Pitting Corrosion of Nickel-Based Alloy 718 Exposed to Simulated Drilling Environments

As a result of their appropriate mechanical and non-magnetic properties, Alloy 718 (UNS N07718) has been used in directional drilling tools, particularly in applications where superior corrosion resistance is required. Drilling conditions might become severe in terms of localized corrosion when the drilling fluid contains large amounts of halide ions and elevated temperatures are involved. Even if the corrosion resistance of Alloy 718 has been studied extensively in environments commonly observed during oil and gas production, a clear understanding about the pitting corrosion resistance of this material under typical drilling conditions is still lacking. Consequently, electrochemical measurements were performed to study the susceptibility to pitting corrosion of age-hardened oil-grade Alloy 718 when exposed to chloride-containing solutions at room temperature and 150°C. Tests fluids were typically buffer solutions of pH 6 and 10 containing chloride concentrations of 17,700 ppm and 141,800 ppm. Electrochemical methods used in this study included cyclic potentiodynamic and potentiostatic polarization, as well as electrochemical impedance spectroscopy. Oil-grade Alloy 718 showed excellent pitting corrosion resistance at room temperature. The stability and protectiveness of the passive layer formed on oil-grade Alloy 718 at pH 10 is less when compared to near-neutral solutions. This could be attributed to a chromium depletion in the passive layer because of its transpassive dissolution in alkaline environments. High temperature reduced significantly the pitting corrosion resistance of the material, which becomes prone to pitting corrosion at 150°C. On the other hand, an increase in the pitting potential of Alloy 718 at 150°C was observed with increasing the pH to 10.