Advanced Corrosion Modelling: An Atomistic Approach to Describe the Corrosion Inhibition Process
Creators: name: Castillo Robles, Jose
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 490105
subject: Dynamical systems in applications
classificationCode: 401605
subject: Functional materials

description: Toxic inorganic compounds, usually used as corrosion inhibitors within paint films, are being replaced by more environmentally friendly compounds in coatings across aerospace, automotive, and infrastructure industries. However, the corrosion inhibition mechanism behind these compounds must be better understood. Molecular modelling can provide insights by analyzing the metal-inhibitor-water interface, but a realistic representation of the electrochemical processes is crucial. In this thesis, six critical aspects must be considered to realistically describe the corrosion inhibition process in modelling: the electronic properties of isolated inhibitors, the interaction of the inhibitor with the surface, the surface model, the effect of the anodic and cathodic zones on the surface, the solvent effects, and the electrodes’ potential effects. While the first three aspects are usually considered in the literature, the latter three and some more complex surface models are not. Here, two main ones will be the focus: the applied electrode potential and solvent effects.

The system chosen to showcase these two aspects is the adsorption of 2-mercaptobenzimidazole (MBI) on Cu(111) surfaces. First, molecular modelling based on Density Functional Theory (DFT) combined with the Non-Equilibrium Green’s Functions (NEGF) formalism was used to address the effect of the applied electrode potential in the MBI adsorption under different coverage and voltages. Both coverage and voltage play crucial roles in the adsorption of the inhibitor. The coverage determines the extent of the surface area protected from corrosion by the inhibitor. On the other hand, the voltage is essential to describe the cathodic and anodic zones within the material. It induces the system’s polarization, potentially influencing the inhibitor’s orientation towards adsorption and its charge distribution. Our results show that the system becomes more stable as the coverage of molecules increases, forming a self-assembled monolayer (SAM). The electronic structure, characterized through the analysis of the charge density difference and density of states, is sensitive to the applied voltage, inducing a charge redistribution in the system, where the MBI is acting as a metal and accumulates the induced charge at the molecule’s edge.

The final step addressed the solvent effects in the MBI adsorption, where the methodology combining the NEGF formalism and the Quantum Mechanics/Molecular Mechanics (QM/MM) method was used in a (5×6) Cu surface slab under varying coverage conditions: bare surface, a low-coverage with one MBI adsorbed, and a high-coverage case with six MBI adsorbed, and under different applied voltages (-1, 0 and +1 V). The dynamics explicitly included water molecules, with their quantity adjusted to match the density of liquid water under standard pressure and temperature conditions (1 g/cm³). Classical molecular dynamics were performed for the isothermal-isobaric ensemble and canonical ensemble to calibrate the system’s pressure and temperature. The reference temperature and pressure were 300 K and 1 bar, respectively.

After the classical molecular dynamics calibration, the QM/MM-NEGF molecular dynamics were performed on the systems under varying coverage and voltages from the equilibrated geometries. Structural descriptors such as the water’s density profiles, water dipole moment and angular distributions were evaluated for the different concentrations and applied voltages. The electronic structure was analyzed through the evolution of the net Hirshfeld charges along the dynamics.

The results suggest that a stable SAM is formed in the high-coverage case. It prevails along the dynamics and acts as a physical barrier between the water molecules and the surface, preventing their interaction with the metallic surface. The results in the bare surface and with one MBI adsorbed showed a water layer directly in contact with the Cu surface, contrary to the case with a SAM adsorbed, where the first water layer is located at the end of the SAM structure. The results also indicated an induced polarization in the system due to the applied voltage on the system, which is reflected in the electronic structure and the structural descriptions evaluated. In this sense, the net Hirshfeld charges of the left and right Cu surface change according to the voltage, besides the charges in the MBI. The dipole moments and angular distribution revealed the water molecules’ polarization, primarily with the ones located at the inhibitor-surface interface. The water molecules aligned according to the voltage sign, and in the case of positive voltages, their hydrogen atoms pointed out towards the left Cu surface. Negative potentials induced the opposite effect. Even at 0 V, the simulations with the SAM adsorption showed a polarization, which is evident in the angular distributions and indicates the outstanding charge redistribution caused by the SAM upon adsorption on the surface.

Finally, a collaboration with experimental partners in RMIT University was performed to investigate the adsorption of 2-mercaptobenzothiazole (2-MBT) and 2-aminobenzothiazole (2-ABT) on galvanized steel, using a Zn(0001) model surface. Two concentrations at the surface were investigated: a low (1 molecule/nm²) and high (3 molecules/nm²) concentration cases, identifying the most stable adsorption modes. The findings aligned with experimental data using X-ray Photoelectron Spectroscopy (XPS), which revealed the bonding between 2-ABT and the Zn through the endo S and the 2-MBT through the endo N and exo S. Electronic structure analysis revealed the inhibitor’s charge density donation to the Zn surface. The experimental data also highlighted the strong interaction between 2-MBT and the surface, potentially forming complexes with Zn²⁺ ions.

Therefore, this thesis highlighted the importance of the six critical aspects of corrosion inhibition modelling and demonstrated that the proposed QM/MM-NEGF approach significantly advances modelling electrochemistry and can be successfully applied in corrosion inhibition studies. From these critical aspects, this thesis focused on two fundamental ones: the solvent and electrode potential effects and how the inclusion of these effects allows us to understand the role of the inhibitor in the protection of the materials, in this case by the formation of stable films with the applied voltages, and the charge redistribution at the inhibitor structure.

The results from this QM/MM-NEGF approach differentiate from the usual corrosion inhibition modelling, which lacks a precise description of the atomistic process occurring at the metallic-inhibitor-aqueous interface. Instead, the QM/MM-NEGF approach provides an accurate and predictive way to simulate fundamental interactions that lead to corrosion inhibition properties. Furthermore, the results from this approach can potentially be used to feed artificial intelligence (AI) methods and multiscale models, which can bridge the gap between nanoscale corrosion inhibition modelling and the continuum scale of corrosion inhibition processes. This could enhance our understanding of corrosion inhibition and lead to more effective solutions.