Predictions of Surface Electrochemistry of Saturated and Alkaline NH₄Cl Solutions Interacting with Fe(110) from Ab Initio Calculations

Ammonium chloride (NH₄Cl) precipitation can present deleterious effects on refinery surfaces when it combines with condensed water vapor to produce highly concentrated chloride and ammoniacal solutions. Herein, we have utilized density functional theory methods to compute the adsorption energies for various NH_x, OH_x, Cl, and H species on the lowest energy surface presented by iron. Adsorption energies are combined with thermodynamic analysis to develop phase diagrams for the various species that may dominate the surface adsorption coverage. Our results indicate that N, O, Cl, and H each possess regions of predominance on the surface Pourbaix diagram under conditions where a saturated NH₄Cl solution is present. While N typically does not interfere with O adsorption except at very high anodic potentials, and is unlikely to depassivate any protective oxide films, Cl can compete with O surface stability, providing a competitive mechanism for hindering repassivation and/or accelerating the rate of metal dissolution.