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ScienceDirect Materials Today: Proceedings 2S (2015) S197 – S204

Conference MEFORM 2015, Light Metals – Forming Technologies and Further Processing

FE-simulation of galvanic corrosion susceptibility of two rivet joints verified by immersion tests M. Mandel*, L. Krüger Technische Universität Bergakademie Freiberg, Institute of Materials Engineering, Gustav-Zeuner Straße 5, 09599 Freiberg, Germany

Abstract The galvanic corrosion behaviour of an aluminium/CFRP laminate self-piercing rivet joint and an aluminium/steel blind rivet joint was analysed by finite element simulation. The corrosion potentials of the joint components were determined by electrochemical polarisation in a 5 wt.% NaCl solution, and were subsequently used as the boundary conditions in the simulation. For verification, the joints were immersed in a 5 wt.% NaCl solution for 6 weeks. The results of the simulations were fully in agreement with the results determined from the long-term immersion test. The highest corrosion rates were found at the material interface, which correlate to the highest potential gradients in the simulation. For the self-piercing rivet joint, galvanically induced pitting corrosion was found on the aluminium alloy, whereas for the blind rivet joint, no corrosion attack was observed. © 2014 The Authors. Published by Elsevier Ltd.

© 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Selection and peer-review under responsibility of the Conference Committee of Conference MEFORM 2015, Light Metals – (http://creativecommons.org/licenses/by-nc-nd/3.0/). responsibility Committee of Conference MEFORM 2015, Light Metals – Forming Selection peer-review under open access article under the CC BY-NC-ND license Formingand Technologies and Further. Thisof is the an Conference Technologies and Further (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Keywords: Electrochemical corrosion; rivet joint; potentiodynamic polarisation; simulation

* Corresponding author. Tel.: +49-(0)3731-39-3176; fax: +49-(0)3731-39-3703. E-mail address: [email protected]

1. Introduction The intensified demand for resource-efficient technologies and weight-reducing structures requires appropriate strategies in material design, treatment, and application [1-3]. With a focus on the end use and besides the mechanical requirements, high corrosion durability is now standard for long-life applications with minimized servicing costs. In light-weight structures benefitting from the Multi-Material-Design concept, galvanic corrosion can endanger the

2214-7853 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the Conference Committee of Conference MEFORM 2015, Light Metals – Forming Technologies and Further doi:10.1016/j.matpr.2015.05.010

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M. Mandel and L. Krüger / Materials Today: Proceedings 2S (2015) S197 – S204

structural integrity of the whole compound [4, 5]. Due to variations in electrochemical activity, a potential difference 'U is formed when several materials are jointly and simultaneously exposed to the same electrolyte. As a consequence, the galvanic element formed leads to accelerated dissolution of the electrochemically more ignoble component [6-9]. To ensure high corrosion durability against aggressive solutions, it is well known that corrosion protection must be considered during the design phase. One established method for the analysis of galvanic corrosion behaviour is finiteelement simulation. Several authors find good agreement between their simulations and experiments, which shows that numerical simulation is an appropriate method for the evaluation of real galvanic systems [10-14]. In this study, the galvanic corrosion behaviours of two rivet joints were analysed using the finite-element method, and the results found by simulation were compared to the corrosion behaviour in a long-term immersion test. In Fig. 1, photographs of the rivet joints are shown.

Fig 1. Photograph of the self-piercing rivet joint (a) and the blind rivet joint (b). CFRP – carbon fibre-reinforced plastic. S350GD+Z140 – hot dipcoated mild steel.

2. Experimental 2.1. Materials For both joints, an extruded sheet of the aluminium alloy EN AW-6060 including a T6 heat treatment without any passivation was used, with a thickness of 2 mm for the self-piercing rivet joint and 3 mm for the blind rivet joint. In the self-piercing rivet joint, the EN AW-6060-T6 was joined to a carbon fibre-reinforced plastic laminate (T700SC/RMI935) using an Almac®-coated self-piercing rivet with a head diameter of 7.7 mm. The mechanically applied Almac® anti-corrosion coating is based on zinc, tin and aluminium. The samples had a total length of 198 mm and an overlap of 36 mm. In the blind rivet joint, the aluminium alloy was joined to the hot-dipped steel S350GD+Z140 by a galvanic zincnickel-coated blind rivet Magna-Lok® MGLP-R8-6. The blind rivet sample had a total length of 212 mm and an overlap of 22 mm. Both joints had a width of 45 mm and were joined at the Fraunhofer Institute for Machine Tools and Forming Technology (IWU) in Dresden, Germany. By optical microscopy the crevice sizes at the overlap regions were determined to 35 μm for the self-piercing rivet joint and 40 μm for the blind rivet joint [10]. In Table 1 and 2, the chemical compositions of the aluminium alloy and the S350GD+Z140 used are given. Table 1. Chemical composition of the aluminium alloy EN AW-6060-T6 in wt% as given by DIN EN 573-3 [19]. Si

Fe

Cu

Mn

Mg

Cr

Ni

Zn

Ti

Al

0.3-0.6

0.1-0.3

0.1

0.1

0.35-0.6

0.05

-

0.15

0.1

Balance

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M. Mandel and L. Krüger / Materials Today: Proceedings 2S (2015) S197 – S204 Table 2. Chemical composition of S350GD+Z140 supplied by ThyssenKrupp in wt%. C

Si

Mn

P

S

Al

Ti

Nb

Fe

0.062

0.200

0.770

0.017

0.004

0.034

0.002

0.019

Balance

2.2. Potentiodynamic polarisation Each component of the joints was analysed separately by potentiodynamic polarisation in a 5 wt.% sodium chloride solution. To this end, the samples were washed with water, ultrasonically cleaned in acetone, and dried. Furthermore, the hot-dipped steel was investigated without the hot-dip coating to determine the corrosion potential. For this purpose, the sample was ground and polished to 1 μm. Subsequently, all samples were polarised with a potentiostat (VSP, BioLogic Science Instruments) using a three-electrode cell configuration. The corrosion potential of the blind rivet mandrel (see Fig. 1b) was determined with a capillary electrode while still in the joined condition for 1 h. For polarisation, a saturated Ag/AgCl electrode was used as the reference electrode, while a platinum electrode was applied as the counter electrode. The scan rate was 6 mV/min. The corrosion potentials were determined from the polarisation curves at the minimum of the current density. 2.3. Finite element simulation A full immersion of both joints in a 5 wt.% sodium chloride solution was defined for the simulation, with an electrolyte extension of 5 mm above the joints. The relative permittivity of the 5 wt.% sodium chloride solution is H = 68.5 [10]. The FE software ANSYS 11.0 was used for analysis of the potential distribution in the electrolyte. Both joint geometries were designed with the program SolidWorks 2009, and afterwards imported into ANSYS 11.0. Because of geometrical symmetry to the centre line and assumptions of an electrochemical steady state, only half of each joint and the corresponding electrolyte had to be modelled. For each joint component the corrosion potential ECor. determined by potentiodynamic polarisation was used as the boundary condition in the simulation. Consequently, due to electric connection by the rivets a potential difference 'U between the joint components is formed during immersion. In addition, a full penetration by the electrolyte at the crevices of the joints is assumed. For analysis of the galvanic system, the following theoretical assumptions were made: The volume of the electrolyte is constant, and the electric charge flow between the electrodes is constant and follows Ohms law. Furthermore, the corrosion potentials of the electrodes and the relative permittivity of the electrolyte are taken as constants. Hence, the galvanic system could be described mathematically by the Laplace equation (Eq. 1): ') = 0. (1) 2.4. Immersion tests The electrochemical corrosion behaviour of the joints was investigated by immersion tests in a 5 wt.% sodium chloride solution. The joints were washed with water and ethanol beforehand, and subsequently dried. They were then fully exposed to the test solution. The electrolyte had an extension of 5 mm above the joints, and was replaced daily for five days per week to ensure an electrochemically steady state. After testing was completed, the samples were purged with water and ethanol and dried again. Afterwards, the joints were observed by optical microscopy (Leica, WILD M10) and electron microscopy (TESCAN, MIRA3 XMU). For investigation of the corrosion damage at the overlaps, the rivets were drilled out mechanically.

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3. Results and discussion 3.1 Potentiodynamic polarisation Fig. 2 gives the potentiodynamic polarisation curves of the components used in the joints. In the self-piercing rivet joint, the CFRP was electrochemically nobler than both the Almac®-coated rivet and the aluminium alloy. In the blind rivet joint, the base material of the S350GD+Z140 and the aluminium alloy were electrochemically nobler than the ZnNi-coated rivet and the hot-dipped S350GD+Z140. The corrosion potentials observed are summarised in Table 3.

Fig. 2. Potentiodynamic polarisation curves of the components of the self-piercing rivet joint (a) and of the blind rivet joint (b). 1 – Almac®-coated self-piercing rivet; 2 – EN AW-6060-T6; 3 – CFRP; 4 – S350GD+Z140; 5 – ZnNi-coated blind rivet; 6 – EN AW-6060-T6; 7 – S350GD+Z140, polished. Table 3. Corrosion potentials of the components determined by potentiodynamic polarisation in 5 wt.% NaCl solution.

Self-piercing rivet joint

Blind rivet joint

Nr.

Material

ECor (V/Ag/AgCl)

1

Self-piercing rivet

-0.98

2

EN AW-6060-T6

-0.75

3

CFRP

-0.08

4

S350GD+Z140

-1.02

5

Blind rivet

-0.87

6

EN AW-6060-T6

-0.74

7

S350GD+Z140, pol.

-0.60

8

Rivet mandrel

-0.50

3.2 FEM simulation Fig. 3 presents the meshed electrolyte, while Fig. 4 shows the calculated potential distribution above the rivet joints. The potential distribution revealed the highest potential gradients for all material interfaces and for both joints. In the self-piercing rivet joint, the Almac®-coated rivet and the aluminium alloy were anodically polarised by the CFRP. Consequently, the highest corrosion rates were expected at the whole rivet surface because of the inappropriate cathode/anode area ratio. In contrast, the potential distribution along the aluminium alloy surface was more inhomogeneous. Due to the parallel electrode configuration and the small crevice size at the overlap, the aluminium alloy exhibited the highest corrosion rates. At the end of the overlap and with increasing distance, the potential gradient decreased. Therefore, the corrosion attack at the aluminium alloy had to decline with increasing distance between the CFRP and the aluminium alloy. In the blind rivet joint, in contrast, the noblest electrode (the rivet mandrel) had the smallest size, and the


cathode/anode area ratio was more favourable. Furthermore, the hot dip of the S350GD+Z140 was electrochemically more ignoble than the other components in the joint. In the test solution, as a consequence, the hot dip protected all components cathodically. Because the highest potential gradients were at the material interfaces, it was expected that the hot dip would first be corrosively attacked at the cutting edges of the S350GD+Z140 sheet, at the overlap of the joint, and at the joint location close to the blind rivet head.

Fig. 3. Meshed electrolyte above the self-piercing (a) and the blind rivet joint (c). Side views of the joint locations are presented in detail (b) for the self-piercing rivet joint and (d) for the blind rivet joint. The numbers correspond to the corrosion potentials (see Table 3) and were used as boundary conditions.

Fig. 4. Calculated potential distribution in the electrolyte for the self-piercing rivet joint (a) and the blind rivet joint (b).

3.3 Immersion test Fig. 5 presents a photograph of the self-piercing rivet joint after 6 weeks of immersion in 5 wt.% NaCl solution.

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The photograph reveals a homogeneous corrosion attack at the rivet, such that the Almac® anticorrosion layer was fully dissolved by the time the test had finished. In the micrographs Fig. 5(b) and Fig. 5(d), pitting corrosion was found close to the overlap. The pitting was observed at a maximum distance of approximately 8 mm from the overlap. Furthermore, it was found that the pit density decreased with increasing distance to the overlap. This behaviour indicated that the anodic polarisation induced by the CFRP had a limiting influence on the distance effect. In Fig. 5(c), a homogeneous corrosion attack at the aluminium alloy surface along the whole overlap occurred when the CFRP was removed. The uniformity of the corrosion damage is a result of the full penetration by the electrolyte, the high potential difference, and the parallel electrode configuration at the overlap.

Fig. 5. (a) Photographs of the self-piercing rivet joint after the immersion test. (b) Micrograph of joint at the CFRP/EN AW-6060-T6 interface. (c) Photograph of the aluminium alloy after removal of the CFRP. (d) SEM micrograph of a pit close to the joint overlap.

Fig. 6 presents the photographs of the blind rivet joint after test completion.

Fig. 6. (a) Photograph of the blind rivet joint after 6 weeks of immersion in a 5 wt.% sodium chloride solution. (b) Micrograph of the S350GD+Z140/EN AW-6060-T6 interface, and, (c) photograph of the EN AW-6060-T6 after removal of the S350GD+Z140.

The photograph reveals a predominantly homogeneous corrosion attack at the hot dip of the S350GD+Z140 and a


uniform discoloration of the aluminium alloy. In contrast, the rivet exhibited metallic brightness and no corrosion attack. Furthermore, at the end of the overlap, no pitting corrosion was found on the surface of the aluminium alloy (see Fig. 5(b)). These effects are attributed to the cathodic protection of the S350GD+Z140 by the hot dip. It is noteworthy to mention that the main task of the hot dip was the cathodic protection of the base material of the S350GD+Z140. Due to the full immersion of the sample into the electrolyte, this effect was extended to all of the electrochemically more noble components of the joint. Moreover, after removal of the S350GD+Z140, the surface of the EN AW-6060-T6 did not exhibit any indication of corrosion attack. The appearance of the aluminium alloy at the overlap was comparable to the initial state (see Fig. 1(b)). Consequently, it was assumed that during the accelerated dissolution of the hot dip, the crevice of the joint was barred by voluminous corrosion products at the sheet edges. This effect impeded any penetration of the electrolyte into the overlap of the joint, and no corrosion reaction was initiated. The observations made during the immersion tests of both rivet joints are fully in agreement with the results found in the FEM analysis. The heaviest corrosion attacks at the joints correlate well to the highest calculated potential gradients in the FEM analysis. 4. Conclusion In this study, the electrochemical corrosion behaviour of two rivet joints produced according to a light-weight design concept was analysed by FEM simulation and verified by an immersion test. The first joint was a self-piercing rivet joint and consisted of a sheet of the aluminium alloy EN AW-6060-T6 and a CFRP laminate joined by an Almac®-coated self-piercing rivet. The second joint was a blind rivet joint, and consisted of an extruded sheet of the aluminium alloy EN AW-6060-T6, a sheet of a hot-dipped steel, and a zinc/nickel-coated blind rivet. For finite element analysis, the corrosion potentials of the joint components were determined by potentiodynamic polarisation and used as the boundary conditions in the simulations. An evaluation of the simulation results was carried out by immersion tests in a 5 wt.% sodium chloride solution. In the simulations, the highest potential gradients were determined at the material interfaces in both joints. It was concluded from these results that the intensity of the potential gradients correlated to the locations at which the most extreme corrosion attacks occurred. The results of the immersion test confirmed the results found by simulation. In the self-piercing rivet joint, the CFRP led to the accelerated and homogeneous dissolution of the Almac® coating of the self-piercing rivet. On the aluminium alloy, a homogeneous corrosion attack was found at the overlap of the joint. This kind of corrosion attack is related to the electrode configuration, the high potential difference between the electrodes, and the full penetration of the electrolyte into the crevice of the joint. Furthermore, pitting corrosion at the end of the overlap was found at a maximum distance of 8 mm. Moreover, it was found that the pit density decreased when the distance to the CFRP laminate increased. No corrosion damage was found for the blind rivet joint at either the aluminium alloy or at the rivet head. The hotdip of the steel sheet was the electrochemically most ignoble component in the system, and led to the cathodic protection of all joint components. Furthermore, due to the increased formation of corrosion products, the crevice of the joint was barred and no electrolyte penetration occurred. In conclusion, the results produced by FEM simulation correlated well with the corrosion behaviour observed in the immersion tests, and the prediction of the real corrosion behaviour by means of FEM analysis is therefore deemed possible. Acknowledgement The authors gratefully acknowledge the German Research Foundation (DFG) and Allianz Industrie und Forschung (AIF) for their financial support of this work. References [1] [2] [3]

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