Reactive Molecular Dynamics Modeling of Iron Passivity

Tuesday, 3 October 2017: 08:40
National Harbor 4 (Gaylord National Resort and Convention Center)
H. DorMohammad, Q. Pang, L. Árnadóttir, and O. B. Isgor (Oregon State University)
Passivity of iron in a highly alkaline environment is investigated using Reactive Force Field Molecular Dynamics (ReaxFF-MD). Simulations were performed on the Fe(110) surface that is exposed to a NaOH solution (pH=13.5) that is assumed to simulate the highly alkaline environments in concrete pore solutions. Helmholtz, Gouy-Chapman and Stern models for electrical double layer were compared in terms of their applicability in studying passivity. It was shown that only the Stern model was able to capture the expected thermodynamic passivation behavior of iron in the alkaline media. Additional ReaxFF-MD simulations with the Stern model showed that passivation process involved dissociation of NaOH to charged Na+ and OH- ions; dissociation of water into charged OH-, H3O+, O2-, and H+ species; formation of a charged layer of iron atoms at the metal-electrolyte interface; and the diffusion of oxygen into the iron bulk to form passive iron oxides. A control study was also conducted to investigate the capability of Stern model for simulating metal/electrolyte interface in a NaCl solution representing neutral conditions (pH≈7) that is expected to cause active corrosion. These simulations supported the expected corrosion behavior.