Numerical Simulation of Micro-Galvanic Corrosion in Al Alloys: Effect of Geometric Factors

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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 ...

Section: Methodsmentioning

confidence: 99%

Section: Physical and Mathematical Model Descriptionmentioning

confidence: 99%

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

Wang

Yin

Jin

et al. 2017

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“…33,34 In this work, the use of chemicals-dependent electrochemical kinetics as input parameters makes the model more realistic by considering the impact of heterogenetic local environment across the active surface.…”

Section: Resultsmentioning

confidence: 99%

“…For the first time, chemical-dependent electrochemical kinetics is used as input parameters in the model to address the coupling effect of the alloy mictrostructure and its service environment quantitatively. This study, together with a previous work investigating the effect of geometric factors, 34 aim at forming a comprehensive model for the evolution of an anodic ring around an IMP, regarded as a precursor for the initiation of localized corrosion. …”

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confidence: 99%

Numerical Simulation of Micro-Galvanic Corrosion of Al Alloys: Effect of Chemical Factors

Yin

Jin

Leygraf

et al. 2017

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A finite element model for simulating the propagation of micro-galvanic corrosion of Al alloys induced by intermetallic particle was established to reveal the dynamic changes including a moving dissolution boundary, deposition of reaction products and their blocking effect. This model has previously been used to study the influence of geometrical factors such as the particle size and width of the anodic ring. In this work, we explore effects of chemical factors including pH and bulk concentration of O 2 by using chemical-dependent electrochemical kinetics as input parameters. The simulations reveal that the micro-galvanic corrosion rate is slowest at pH = 6. For pH > 6, the rise of pH increases the dissolution rate of Al and also the deposition rate of Al(OH) 3 , leading to a faster but more short localized Al dissolution. For pH < 6, the decline of pH accelerates Al dissolution and inhibits Al(OH) 3 deposition, leading to a faster and more long lasting Al dissolution. At pH ≤ 4, deposition of Al(OH) 3 becomes negligible, and localized corrosion will propagate continuously. Within the O 2 concentration range relevant for atmospheric conditions, a lower O 2 concentration in the solution leads to a slower rate of micro-galvanic corrosion. For multi-element alloys with heterogeneous microstructures, micro-galvanic couplings exist between different constituent components due to their difference in relative nobility, implying a risk for localized corrosion when exposed to a corrosive environment. In the case of aluminum (Al) alloys, depending on the composition and thermo-mechanical processing, different types of intermetallic particles (IMPs), with micron-or nanometer sizes, are formed in the microstructure.1,2 Most of the IMPs exhibit cathodic character while some type of IMPs exhibit anodic character, relative to the alloy matrix.3-7 Localized corrosion of Al alloys is commonly observed to be associated with relatively large cathodic type IMPs which contain more 'noble' elements (i.e. Cu, Fe, Mn, etc.), leading to enhanced dissolution of surrounding matrix, named 'trench formation'. [8][9][10][11] There are extensive literature reports on investigations of localized corrosion of Al alloys, providing substantial knowledge about the effect of the microstructure. [12][13][14][15][16] In contrast, despite of some experimental obervations, little is known at a fundamental level about the influence of pH on the corrosion of Al alloys. Several decades ago, Chatalov 17 and Pourbaix et al. 18 found that the corrosion rate depends logarithmically on the pH, with a minimum corrosion rate at pH close to 6. By using stereological methods, Birbilis et al. 19 observed that the mode of corrosion of AA7075-T651 alloy also depends on the pH of the solution: stochastic at acid pH, general at alkaline pH, and strongly influenced by the microstructure and electrochemistry of the intermetallics at a near neutral pH. They pointed out that the environment is of similar critical influence as the microstructure for the corrosion of Al allo...

“…13,14 IMPs play an important role in localized corrosion since their type, size, and distribution can affect electrochemical reactions, homogeneous chemical reactions as well as transport phenomena of molecular and ionic species. 8,15,16 Therefore, detailed knowledge about IMPs in the Al alloy is needed when exploring the mechanism of corrosion and its inhibition.For a number of decades, chromate-based inhibitors, pigments, and conversion coatings, have been extensively used for corrosion protection of Al alloys. 17,18 However, nowadays the use of Cr(VI) compounds has been strongly restricted due to their carcinogenicity, mutagenicity, and toxicity, 18 and some alternative schemes for corrosion protection have been reported.…”

mentioning

confidence: 99%

Corrosion Inhibition of Aluminum Alloy AA6063-T5 by Vanadates: Microstructure Characterization and Corrosion Analysis

Kharitonov

Örnek

Claesson

et al. 2018

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Corrosion inhibition of aluminum alloy AA6063-T5 by vanadates (NaVO 3 ) in 0.05 M NaCl solution has been investigated by electrochemical and weight loss measurements, and associated with microstructure and Volta potential data. X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy analyses confirmed the presence of micrometer-sized Fe-rich Al 4.01 MnSi 0.74 , Al 1.69 Mg 4 Zn 2.31 , and FeAl 3 intermetallic phases (IMPs) and nanometer-sized CuAl 2 , ZnAl 2 , and Mg 2 Si precipitates in the microstructure. Scanning Kelvin probe force microscopy measurements showed Volta potential differences of up to 600 mV between the microstructure constituents indicating a high susceptibility to micro-galvanic corrosion, with interphase boundary regions exhibiting the highest propensity to corrosion. Most IMPs had cathodic character whereas some nanometer-sized Mg-rich particles exhibited anodic nature, with large Volta potential gradients within interphase regions of large cathodic particles. Electrochemical potentiodynamic polarization measurements indicated that the vanadates provided mixed corrosion inhibition effects, mitigating both oxygen reduction, occurring on cathodic IMPs, and anodic metal dissolution reaction, occurring on anodic sites, such as Mg 2 Si and interphase boundary regions. Electrochemical measurements indicated that the sodium metavanadate inhibitor blocks active metal dissolution, giving high inhibition efficiency (>95%) during the initial exposure, whereas long-term weight loss measurements showed that the efficacy decreases after prolonged exposure. Aluminum and its alloys are attractive materials for a range of industrial applications due to cost-efficient recyclability, excellent physical and mechanical properties, such as low density, high thermal conductivity, good weldability, and high strength-to-weight ratio. [1][2][3][4] In particular, the 6xxx-series (Al-Mg-Si) alloys are widely used in aerospace, automotive, marine, and construction industries due to their relatively good corrosion resistance, formability, and low cost as compared to the 2xxx (Al-Cu) and 7xxx (Al-Zn) alloys.5-7 However, these alloys contain multiple alloying elements and their microstructure is very heterogeneous, typically consisting of a large variety of intermetallic phases (IMPs), which makes them highly susceptible to localized corrosion such as pitting or intergranular corrosion. 5,8,9 While nanometer-sized IMPs, dispersed in the Al matrix, are desirable for mechanical strength of Al alloys, micrometer-sized IMPs may induce localized corrosion due to micro-galvanic coupling between the Al matrix and the IMPs, 10-12 which may also affect the anodization process. 13,14 IMPs play an important role in localized corrosion since their type, size, and distribution can affect electrochemical reactions, homogeneous chemical reactions as well as transport phenomena of molecular and ionic species. 8,15,16 Therefore, detailed knowledge about IMPs in the Al alloy is needed when exploring the mechanism ...