Localized Pitting Behavior and Competitive Film Formation Mechanism of N80 Steel in High-Temperature and High-Salinity Water-Based Drilling Fluids

Driven by the extreme high temperatures (≥150 °C) and high salinities of deep wellbore environments, the corrosion failure of drill strings in water-based drilling fluids (WBDFs) is becoming increasingly severe. This study systematically investigates the corrosion behaviors and underlying mechanisms of N80 steel in simulated WBDFs under varying temperatures (80–180 °C), inorganic salts (NaCl, CaCl2, KCl), dissolved oxygen (DO) levels, and hydrodynamic conditions, utilizing weight-loss tests, SEM, and XRD. Results reveal that the corrosion kinetics are synergistically governed by temperature and salt concentration, exhibiting a non-linear “increase-then-decrease” trend with a peak at 120–150 °C. The aggressiveness of the inorganic salts ranks as NaCl > CaCl2 > KCl. Specifically, highly penetrative free Cl− in the ultra-high-concentration NaCl system induces severe interconnected localized pitting. Conversely, high-concentration Ca2+ in weakly alkaline environments couples with carbonate to form a protective inorganic scale, serving as a physical barrier. Although the “salting-out effect” of high-concentration salts effectively reduces the DO level, the synergy between residual oxygen and active Cl− in dynamic flow fields still triggers highly concealed deep under-deposit pitting. At the extreme high temperature of 180 °C, background salt ions dictate the competitive interfacial film formation: the NaCl system undergoes oxygen depolarization to generate a porous Fe3O4 layer, whereas K/Ca-containing systems promote CO32− adsorption, forming a dense and thermodynamically stable FeCO3 film. This study elucidates the fundamental mechanisms of uncontrolled pitting induced by extreme HTHP environments, providing vital theoretical guidance for the anti-corrosion design of deep-well drilling engineering.