Electrochemical corrosion behavior of AISI 4340 steel in multi-ionic solutions

Several types of carbon and high-strength low-alloy (HSLA) steels are being considered for use in the underground reinforcement of the Yucca Mountain Nuclear Waste Repository. In this study, the electrochemical corrosion behavior and associated corrosion rates of AISI 4340 steel were conducted in three phases. In the first phase, potentiodynamic polarization and electrochemical impedance spectroscopy using three different concentrations (1X, 10X, 100X) of simulated electrolyte under aerated (oxygenated) and deaerated (nitrogenated) conditions were used to determine baseline corrosion rates from 25°C to 90°C. In the second phase, potentiodynamic polarization under deaerated conditions was used to determine the corrosion rates and passivity behavior of 4340 steel using different combinations of sodium silicate (Na2SiO 3) and sodium bicarbonate (NaHCO3), in both pure water and simulated seawater (3.5% NaCl). These experiments were carried out to examine the potential inhibiting properties of the silicate and/or bicarbonate ions on the surface of the steel. The third phase of this research study involved immersing coupons, with and without "pre-treatment" of sodium silicate, in simulated electrolyte to determine "real-world" conditions of the material. Coupons were tested under static-air, aerated and deaerated environmental conditions at 20°C, 25°C, 45°C and 75°C, to determine corrosion rates and associated behavior.;Corrosion rates using potentiodynamic polarization showed a maximum of ∼300 microm/yr at 45°C (deaerated conditions), and a maximum of ∼2200 microm/yr at 65°C (aerated conditions) in simulated electrolyte. For the silicate/bicarbonate experiments, results indicate that the steel corrodes < 70 microm/yr in solutions containing sodium silicate, but corrodes > 2,000 microm/yr in solutions containing sodium bicarbonate. Immersion corrosion rates under simulated electrolyte show that the steel corrodes at a rate > 2500 microm/yr at 75°C in aerated conditions when partially-immersed in solution, but corrodes < 0.5 microm/yr at 25°C in deaerated conditions when fully-immersed in solution. In nearly all cases, corrosion rates were higher for samples at the partially-immersed positions than at the fully-immersed or vapor-immersed positions. For the static-air conditions, rates stabilized at near-constant values of ∼15 microm/yr when the coupons were suspended above the solution, and ∼150 microm/yr for when the coupons were partially- and fully-immersed in the electrolyte. Pre-treatment of the steel coupons using sodium silicate increased the corrosion resistance by a factor of 1.2 to 4.7 when immersed in 1X YM solutions. In summary, the electrochemical corrosion rates and associated active-passive behavior of AISI 4340 steel in simulated, multi-ionic solutions will be presented.