Numerical Simulation of Micro-Galvanic Corrosion in Al Alloys: Steric Hindrance Effect of Corrosion Product

Abstract

An improved finite element model was established to demonstrate the steric hindrance effect of the precipitated corrosion product (Al(OH) 3 ) on micro-galvanic corrosion triggered by intermetallic particles (IMPs) in an Al-matrix. In this model, the precipitation/ dissolution of the corrosion product could occur in the whole liquid field as the result of a reversible reaction. Simulation results show that the precipitated insulating Al(OH) 3 on the electrode surface can inhibit further corrosion by reducing the conductivity of the solution and the active electrode surface area. Meanwhile, the steric hindrance effect of the precipitated Al(OH) 3 also slows down the diffusion and migration of species in the solution. Moreover, considering the porous nature of precipitated Al(OH) 3 , a porosity parameter ε was introduced to describe the degree of compactness of corrosion product, which reaches a certain minimum value ε c under a specific corrosion situation. Compared to the previous work in which a surface coverage parameter was used to describe the blocking effect of Al(OH) 3 on surface activity, the present model is more realistic in mimicking the micro-galvanic corrosion, and also useful for the simulation of the transition from metastable pit formation to pit propagation. Al alloys are widely used in many fields as structural materials because of their high strength/weight ratio. Although many Al alloys show high corrosion resistance due to spontaneous formation of a protective oxide layer on the surface, some of them are vulnerable to local corrosion when exposed to aqueous solutions containing halide ions, e.g., Cl− . 1-3 Particularly, the surface heterogeneity may provide initiation sites for localized corrosion.4,5 A precise and comprehensive understanding of the localized corrosion behavior of Al alloys is of great importance.In our series of studies, 6,7 a FEM model has been established to investigate micro-galvanic corrosion induced by cathodic intermetallic particles (IMPs), i.e., trench formation, by taking into account electrochemical kinetics of the single phases involved, homogeneous reactions occurring in the electrolyte, a moving dissolution interface, as well as the precipitation of Al(OH) 3 on the electrode surface and blocking effect of the precipitated corrosion product. However, in the model reported previously, the precipitation was assumed to occur only on the electrode surface, and the blocking effect of the precipitated corrosion product was simply described by a surface coverage parameter (θ). In that case, the precipitation within the liquid phase and multi-dimensional growth of the precipitated corrosion product were not considered, and the steric hindrance on the mass transport was neglected. Such a simplification limits the applicability of the model for simulating practical systems, since the steric hindrance effect of the precipitated corrosion product has been widely recognized to be critical for the development of pitting-like localized corrosion.For instance, Frankel et al. 8,9 ...

Modeling the effect of insoluble corrosion products on pitting corrosion kinetics of metals

“…The governing Eqs. (20)(21)(22), (26) and (31) are solved by a finite element method. The standard Galerkin 50 formulation is used to discretize the space, and the backward differentiation formula (BDF) method, 51 due to its inherent stability, is used for the time integration of the governing equations.…”

“…25 Some sharp-interface numerical models based on the ALE method have been presented, which do consider ICP formation. 17,26,27 Yin et al 27 studied the formation of corrosion products and their effects on corrosion rate and corroding surface morphology using the ALE method. In a later study, wang et al 26 extended their previous ALE-based formulation and studied the steric hindrance effect of corrosion product on corrosion kinetics.…”

“…17,26,27 Yin et al 27 studied the formation of corrosion products and their effects on corrosion rate and corroding surface morphology using the ALE method. In a later study, wang et al 26 extended their previous ALE-based formulation and studied the steric hindrance effect of corrosion product on corrosion kinetics. They found that corrosion rate significantly decreases with the formation of corrosion product on corroding surface by limiting the flow of ions through this new phase.…”

A Review of the Application of Finite Element Method (FEM) to Localized Corrosion Modeling

“…However, this modeling focused on the interaction between a single IMC and Al-matrix; future work for the interaction between multiple IMCs and Al-matrix is needed. A recent series of modeling studies performed by Yin, et al, [101][102] and Wang, et al, 103 systematically studied the deposition of Al(OH) 3 on the localized corrosion site and its blocking effect on the corrosion of the Al matrix during pit morphology development in a single-IMC (Al 3 Fe)/AA7075-matrix microgalvanic coupling. They concluded that the blocking effect of Al(OH) 3 , as well as the matrix dissolution rate, were dependent on the bulk and local solution chemistry (pH, dissolved oxygen and carbon dioxide, etc.…”

Enhanced predictive corrosion modeling with implicit corrosion products

“…On short time and length scales, microscopic models (incl. electronic and atomistic models) are often employed to compute the initiation stage, and the reaction rates and migration processes of the participating chemical species . While this approach allows to accurately model the creation and the influence of corrosion products, it is computationally expensive.…”