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A FEM model for investigation of micro-galvanic corrosion of Al alloys and effects of deposition of corrosion products
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. Univ Sci & Technol Beijing, China.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.ORCID iD: 0000-0002-9453-1333
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.ORCID iD: 0000-0002-4431-0671
2016 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 192, p. 310-318Article in journal (Refereed) Published
Resource type
Text
Abstract [en]

A finite element method model has been elaborated aiming at a deeper insight into the influence of microstructure on micro-galvanic corrosion of Al alloys. The model considers a dynamic corroding surface and takes into account kinetic data from local electrochemical reactions, transport of O-2 and ionic species (e.g., Al3+, H+, Cl-), homogeneous reactions in the electrolyte, deposition of reaction products and its influence on both anodic and cathodic reactions. As a first step, an Al matrix with a micrometer-sized cathodic intermetallic particle exposed in 0.1 M NaCl has been considered. The simulation predicts the dynamic changes of the corroding surface, and the flow and distribution of ionic species and of O-2 in space and time. The calculated pH of the electrolyte inside and nearby the occluded corroding volume suggests the formation of insoluble Al(OH)(3) on both the cathodic and anodic areas. This results in blocking effects of anodic and cathodic reactions and in eventual termination of the micro-galvanic corrosion. The predicted deposition of corrosion product is in good agreement with in-situ atomic force microscopy measurements.

Place, publisher, year, edition, pages
Elsevier, 2016. Vol. 192, p. 310-318
Keywords [en]
FEM model, Al alloy, micro-galvanic corrosion, deposition, blocking effect
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-184970DOI: 10.1016/j.electacta.2016.01.179ISI: 000371143600036Scopus ID: 2-s2.0-84957871682OAI: oai:DiVA.org:kth-184970DiVA, id: diva2:917801
Note

QC 20160407

Available from: 2016-04-07 Created: 2016-04-07 Last updated: 2018-05-08Bibliographically approved
In thesis
1. FEM Modelling of Micro-galvanic Corrosion in Al Alloys Induced by Intermetallic Particles: Exploration of Chemical and Geometrical Effects
Open this publication in new window or tab >>FEM Modelling of Micro-galvanic Corrosion in Al Alloys Induced by Intermetallic Particles: Exploration of Chemical and Geometrical Effects
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Localized corrosion, such as pitting, crevice corrosion or galvanic corrosion, is a long-standing phenomenon that can greatly limit the life of metallic materials. For decades experimental methods have been used to try to understand the underlying physical, chemical and electrochemical processes that control localized corrosion in order to find effective protection methods against its propagation. The complexity of the phenomenon and its small geometric size have often severely restricted the basic understanding of local corrosion. In recent decades, computational methods have been developed as an alternative to the experimental methods. Compared to experimental methods, modeling and numerical simulation enable complicated systems to be systematically investigated without considering the inherent constraints of experimental methods.

    In the current Doctoral thesis, advanced calculation methodology has been used to study galvanic corrosion of an aluminum alloy with geometric resolution at micrometer level. The computational platform has been a commercial FEM-based software, COMSOL Multiphysics, which was combined with another software, Matlab. The current model system consists of a semi-spherical intermetallic particle, surrounded by a pure aluminum matrix. The aluminum surface is covered by an inert passive film, except for a ring-shaped surface around the particle itself. By assuming that the particle is electrochemically more noble than aluminum, it acts as a cathode and the surrounding aluminum ring as anode. By utilizing the FEM-based software, it has been possible to incorporate important physicochemical reactions, including the electrochemical anode and cathode reactions of the individual phases, mass transport of various chemical compounds formed during ongoing electrochemistry, homogeneous reactions in the electrolyte, as well as deposition of corrosion products consisting of Al(OH)3 along parts of the anodic area.

    What has made this study a significant step forward is that not only chemical changes but also geometrical changes have been taken into consideration in the simulation of ongoing micro-galvanic corrosion. Particularly challenging has been to mathematically master the gradual deposition of compact Al(OH)3 on an aluminum surface which gradually dissolves anodically. In the initial modeling work, the deposition of Al(OH)3 was assumed to occur only on the electrode surface, resulting in a gradual blockage of surface activity. In an even more advanced stage, the modeling has also sought to simulate the effect of a deposited porous film of Al(OH)3, formed through homogeneous reactions in the electrolyte. By taking into account inhibited diffusion and migration of chemical products that the porous film causes, its sterically inhibiting effect has for the first time been quantitatively interpreted. The porous corrosion product can most closely resemble the lid experimentally observed above local corrosion attacks, which leads to an even more diminished surface activity in electrochemical reactions compared with the deposition of only compact corrosion products on the anode surface.

    The kinetic model has resulted in a significantly deeper insight into the mechanism of micro-galvanic corrosion of the investigated system. The simulation has been shown to predict the time-dependent geometric changes of the anodically dissolved aluminum surface as well as the flow and distribution of generated chemical products. Contrary to the widely accepted perception that Al(OH)3 is not stable in the occluded acidified electrolyte environment, the calculations predict a higher local pH in the occluded electrolyte. This means that insoluble Al(OH)3 can be deposited on the electrode surface, the blocking effect of which may lead to a termination of the micro-galvanic corrosion. If the ring width is initially 0.5 μm or less, transport of OH- ions from the cathode surface to the occluded electrolyte environment is limited, leading to a local acidification within the occluded dissolving volume. At a given anodic ring width, an increased radius of the cathodic particle instead leads to an increased anodic dissolution rate by formation of a larger area for the cathode reaction. Variation of the chemical parameters in the electrolyte also shows that the simulated micro-galvanic corrosion rate of aluminum has a minimum at pH = 6. Both more acidic and more alkaline conditions result in an elevated anodic dissolution of aluminum. When pH ≤ 4, the deposition of Al(OH)3  becomes negligible, and the micro-galvanic corrosion will continue uninterrupted, completely in accordance with experimental data.

Place, publisher, year, edition, pages
Stockholm, Sweden: KTH Royal Institute of Technology, 2018
Series
TRITA-CBH-FOU ; 2018:12
Keywords
Finite Element Method (FEM), Modelling, Micro-galvanic corrosion, Al alloy, corrosion product deposition, chemical factors, steric hindrance effect
National Category
Corrosion Engineering
Research subject
Chemistry
Identifiers
urn:nbn:se:kth:diva-227315 (URN)978-91-7729-740-6 (ISBN)
Public defence
2018-06-08, Hörsal F3, Lindstedtsvägen 26, KTH campus, Stockholm, 14:00 (English)
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Supervisors
Note

QC 20180509

Available from: 2018-05-09 Created: 2018-05-08 Last updated: 2018-05-09Bibliographically approved

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