Predictive Multiphysics Modeling of Corrosion Behavior and Passive Layer Formation in Hydroxyapatite‐Coated Titanium Implants

The corrosion protection provided by hydroxyapatite (HAP)–ionic liquid (IL) composite coatings on Ti‐6Al‐4V titanium implants is governed by coupled transport–reaction mechanisms controlling passive layer formation and electrochemical behavior. In this study, a physics‐based multiphysics model is developed to predict the spatiotemporal evolution of a calcium–phosphate (Ca–P) passive layer on HAP–IL coated Ti‐6Al‐4V substrates in chloride‐containing simulated body fluid. The model integrates multi‐species ionic diffusion, Butler–Volmer electrochemical kinetics, and precipitation‐driven Ca–P layer growth within a unified framework. An Arbitrary Lagrangian–Eulerian (ALE) formulation is employed to accurately track the moving boundary associated with passive layer thickening during long‐term simulations. Model predictions are validated against short‐term electrochemical impedance spectroscopy (EIS) data, showing good agreement with a deviation of approximately 15%. Results demonstrate a significant reduction in chloride ion flux (up to 90%) due to barrier formation and interfacial effects within the coating. Sensitivity analysis confirms the robustness of passive layer thickness predictions. Overall, the findings provide mechanistic insight into corrosion inhibition and establish a predictive framework for optimizing HAP–IL coatings on Ti‐6Al‐4V titanium implants.