Effect of galvanic corrosion on durability of aluminium/steel self ...

Effect of galvanic corrosion on durability of aluminium/steel self-piercing rivet joints L. Calabrese*1, E. Proverbio1, E. Pollicino1, G. Galtieri1 and C. Borsellino2 Galvanic corrosion of steel/aluminium hybrid joints, obtained by self-piercing riveting technique, was investigated. Potentiodynamic polarisation tests, performed in 3?5 wt-% NaCl solution, evidenced the anodic and cathodic behaviour of metal constituent of the joints. Furthermore, long term aging tests were performed to evaluate the relationship among failure mechanism, joint configuration and corrosion damage in salt spray environment. The experimental results evidenced that the degradation phenomena influenced significantly performances and failure mechanisms of the joints, inducing premature failure of the joint at lower stress level at increasing aging time. Furthermore a theoretical model was proposed to forecast failure modes. Based on this model a simplified map of failure mechanisms promoted by corrosion phenomena has been drawn. Keywords: Self-piercing riveting, Corrosion, Durability, Salt spray test, Single lap shear test

Introduction In the transportation industry the demand for highly automated processes and fast implementation in the assembly is relevant. Self-piercing riveting (SPR) offers high flexibility among the joining techniques. This technique is a quite new high speed mechanical fastening technique in industrial field for localised joining of thin material sheets without a pre-drilled hole.1–3 Due to its high automation and joining speed the SPR is becoming important in automotive applications for aluminium vehicle body assembly. In the SPR technique the sheet clamping is obtained forcing a semi-tubular rivet with a punch to pierce the upper sheet of the joint and flare into the bottom sheet under the influence of the upset die. In this way a mechanical interlock between the two sheets is obtained. In particular the expansion of the rivet is due to the presence of a counter-die in the opposite side. The sheet is deformed over the die and guided inside the rivet cavity thus forcing the rivet expansion inside the sheet itself.4 The die shape also forms a button on the underside of the lower sheet without that the rivet tail pierces it. When the process is completed, the lower side of the joint reproduces the toroidal shape of the counter-die. The process cycle is shown in Fig. 1. The SPR process is increasingly used in sheet metal processing industries owing to numerous advantages. The SPR allows cost savings, high resistance and 1

Department of Industrial Chemistry and Materials Engineering, Faculty of Engineering, University of Messina, Contrada di Dio, 98166 Messina, Italy Department of Civil Engineering, Computing, Construction, Environmental and Applied Mathematics, University of Messina, Contrada di Dio, 98166 Messina, Italy

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*Corresponding author, email [email protected]

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ß 2015 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 28 December 2013; accepted 20 March 2014 DOI 10.1179/1743278214Y.0000000168

flexibility of the joined structures. Furthermore, the absence of heat avoids thermal effects on material coatings, reduces the microstructural transformations caused by traditional welding technologies and assures both high shear and pull strength. The mechanical behaviour of SPR joints, mainly by using steel rivets, can deliver a good crash-worthiness and fatigue performance in comparison with spot welded joints.5,6 All of these advantages are promising for a continuous research activity able to increase the application of SPR joints in industrial field. Several works in the literature investigated the mechanical behaviour of self-piercing riveted connections,4,7–11 evidencing the high performances compared with other types of connections. The critical analysis of the obtained test results reported in Refs. 7 and 8 has emphasised that the mechanical performances are strongly influenced by the geometrical characteristics of the joint (i.e. geometry of the rivets, distance from the edges, thickness of the plates, number of sheets). At the same time, Fu and Mallick10 and Hoang et al.11 have shown the influence of some process parameters (i.e. die pressure, pre-clamping force) on the static strength and fatigue life of selfpiercing riveted joints. Furthermore, this technology is an interesting joining alternatives to develop new products with multi-materials design.12–14 However, an aspect that limits the applicability of this type of joints is the durability behaviour in highly aggressive environmental conditions due to localised corrosion mechanisms.15,16 In fact the use of steel rivets in the aluminium frame increases the risk of corrosion phenomena due to galvanic effects, considering that the two metals have a quite different electrochemical behaviour.17 Surface irregularities or crevices could intensify the problem, leading to crevice corrosion attack,

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Effect of galvanic corrosion on aluminium/steel self-piercing rivet joints

1 Self-piercing riveting method

a form of localised corrosion induced by electrolyte accumulation in the interstices. In joining design, the metal sheets should be also chosen in view of their electrochemical reactivity and corrosion behaviour.18 Furthermore, another aspect that influences the durability in aggressive environmental conditions is the presence of internal stresses induced by cold forming process. This could facilitate the activation and propagation of local cracks due to stress corrosion cracking, weakening furthermore the joint at long aging times.19 Howard and Sunday20 reported experimental results about corrosion degradation on SPR joints evidencing that the amount of corrosion could be reduced significantly by using a polyester coating on SPR joints. Indeed, the presence of a polymeric coating affects the corrosion mechanisms which take place on the metal joints surface. It provides a barrier action to corrosion phenomena, but, at the same time, the crevice corrosion is strongly reduced due to the sealing of crevice area. Hoang et al.11 evidenced that the mechanical strength of the aluminium riveted joints tended to stabilise just after three days of natural aging. Moroni et al.21 showed the influence of thermal cyclic aging on the performances of hybrid adhesive-mechanical joints, whereas aging influences slightly the mechanical properties of hybrid joints. Instead Furuta et al.22 investigated the effect of aggressive environmental conditions on the performances of spliced panel specimens. They evidenced that the fatigue life time of the specimens tested in a 3?5%NaCl solution were about 1/3 of those observed in the ambient conditions. In our previous work,19 long time alternate immersion tests were performed to evaluate the durability behaviour and performance of riveted joints with aluminium alloys sheets. At long aging time (60 days) the mechanical properties degraded significantly, evidencing that the corrosion phenomena influenced significantly the performance and failure mechanism of the joints.

Although the long time durability of the SPR joint in a corrosion environment is a known problem, few works focus the attention on the relationship between durability of SPR joints and electrochemical behaviour of the metal constituents.18,19 Based on these considerations the aim of this work is to evaluate the electrochemical behaviour of aluminium/ steel riveted joints focusing the attention on the galvanic coupling phenomena that occur during the degradation stages. The galvanic corrosion behaviour of joints can be evaluated by potentiodynamic polarisation tests carried out on single metal constituents.23,24 In our work, potentiodynamic polarisation tests, performed on 5 wt5% NaCl solution, evidenced the anodic and cathodic tendencies of each metal constituent of the joints. Long term aging tests were used to validate the relationship between mechanical performances, failure mechanism and corrosion degradation in salt spray environment. The experimental results evidenced that the galvanic corrosion phenomena affect significantly the mechanical stability of the metal constituents. Based on electrochemical considerations a theoretical model was proposed to predict the different failure modes observed at varying aging times. A simplified failure map of the failure mechanisms and the effect of the corrosion has been drawn.

Experimental methods Sample preparation The investigation has been carried out on single-lap riveted aluminium/steel joints. The employed materials, their thickness and either chemical and mechanical properties are reported in Table 1. The rivet is made of carbon steel and coated by an 11 mm protective layer of zinc. For all joints the upper sheet was Carbon Steel A570 and the bottom one was

Table 1 Properties of employed materials Plate material

AA6082

Carbon steel A570

Geometry Chemical composition

Thickness: 1 mm Fe50.5, Cu50.1, Si50.5, Mn50.4, Mg50.6–1.2, Cr50.25, Zn50.2, Ti50.1, Al5balance HBN(2,5/62,5/30)560 224

Thickness: 1 mm C50.3, Si50.25, Mn50.8, P50.04, Fe5balance

Hardness Yield strength/MPa

HV5170 590


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2 Single lap joint geometry (mm)

aluminium AA6082. The geometry of SPR joint is shown in Fig. 2. The joints were made by an electro-hydraulic riveting system. The duration of the process was about 2 seconds. The equipment (Textron Fastening System) was supplied by a hydraulic motor (230 V, 50–60 Hz) with an electro-hydraulic valve necessary to vary the pressure applied on the punch. The working pressure was set to 280 bar for all samples.

Salt spray test Samples were exposed to critical environmental conditions following the ASTM B 117 standard. The salt spray fog had a chemical composition of 5% NaCl solution (pH between 6?5 and 7?2). In the climatic chamber the samples were aged continuously keeping at temperature of 35uC. This last practice provides a controlled corrosive environment which has been used to obtain significant information about the corrosion resistance of the metal joint exposed at the salt spray test. Periodically, at increasing aging times, five specimens for each configuration were removed from the climatic chamber and then mechanically tested. The removed samples, clean out and dried, were preserved in a sealed plastic storage bag with silica gel desiccant to ensure no further corrosion evolution during storage.

Single lap shear test Shear testing of single-lap joints was performed, by means of a universal testing machine (Zwick-Roell Z250) equipped with a 50 kN load cell and a cross-head rate of 1 mm min–1 (displacement control test). In order to define an adequate number of samples, the statistically procedure of design of experiments was applied. In particular, five replicas were carried out with varying aging condition. As a consequence of this preliminary study 40 samples were made.

The effect of the corrosion evolution on the joint failure mechanisms were investigated using a stereomicroscope Zeiss Stemi 2000-C in the range 2?56– 12?56 magnification conditions.

Electrochemical characterisation The electrochemical behaviour of the rivet and the sheets (aluminium and carbon steel alloys) was evaluated by means potentiodynamic polarisation test. These tests were carried out using a PAR Versastat 4 potentiostat. The used cell was a conventional three electrode set-up with the metal sample as working electrode, a platinum rod as counter electrode and a saturated KCl Ag/AgCl as reference electrode. The measurements were performed at room temperature in 3?5% NaCl electrolytic solution open to air. The exposed area within testing solution was about 0?5 cm2 and the samples were immersed in the corrosive media for 15 min before the polarisation test. The anodic and cathodic polarisation curves were recorded using a sweep potential test in the range ¡2000 mV respect open circuit potential including a reverse scan. The scan rate was 1 mV s–1. With the purpose to investigate their corrosion sensitivity bimetallic areas in the SPR joint were electrochemically tested under the same previous condition with a total area of 2?50 cm2. In Fig. 3 a schematic drawing of the electrochemical tested surfaces of the SPR joint is reported. Two testing configuration were performed on SPR joint. A first test (testing area 1) was performed on the overlapping area between aluminium and steel sheets. A second one (testing area 2) on the overlapping area between rivet head and carbon steel sheet. The electrochemical cells were sealed by using a silicone o-ring. The ratios between aluminium/steel sheet areas and rivet head/steel sheet areas are about 1 : 1. Three potentiodynamic polarisation tests were repeated for each sample investigated in this work. All

3 Scheme of electrochemically tested areas of SPR joint

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4 Evolution of failure load versus aging time for SPR joints

potentials were referred to the saturated KCl Ag/AgCl electrode.

Results and discussion Lap shear strength Figure 4 shows the failure load versus aging time trends of the single lap shear tests for the SPR joints. The resistance of the SPR joints decreases at increasing aging time in salt spray environment. Three stages can be identified: (i) at first the resistance of the joint is quite stable. In this phase the joint resistance is due to both the friction resistance, resulting from the contact pressure between the aluminium and steel sheets induced by forced mechanical joining, and the shear resistance offered by the rivet. At the sheets interfaces of overlapping area and around the rivet an initial deposit of salts was observed, favouring a better contact between the sheets through which the joint maintains a stable resistance. At the same time, the dissolution of aluminium sheet begins to wear from the bottom sheet. A bearing failure mechanism on the thinner sheet was observed (picture reported in Fig. 4, stage I). The bearing load, FB, is given by the product between the bearing stress, sB, and the cross-section area of the hole, given by FB5sBds, where d is the diameter of the hole and s is the thickness of the sheet. Therefore, the thin thickness of the lower aluminium sheet induces a very low bearing load. The corrosion phenomena are not yet significant enough to induce heavy effects on the mechanical strength of the joint, although sometimes the tail pull-out failure mechanism was observed (ii) in the range of 3–6 weeks of aging in salt spray chamber, the resistance of the joint progressively decreases. In this phase more competing failure mechanisms were observed (bearing, tail pull-out, net-tension and shear out). The reduction of the SPR joint performances could be considered as a consequence of the electrochemical activity of metal constituents and the subsequent formation of corrosion products. Oxides and hydroxides of Al, e.g. Al2O3, Al(OH)3, formed at long aging


times (showing their typical white coloration) induce an increase in volume in the overlapped joining area. When the layer of oxides grows, it influences significantly the failure. In particular the presence of brittle thick oxide and oxyhydroxide layer in the overlapping area and on the rivet tail reduces the interlocking force between the sheets. This compromises the resistance of the joint favouring mainly the unbuttoning of the rivet from the lower aluminium sheet (tail pull-out), as confirmed on the central picture reported in Fig. 4 at medium aging times. In the tail pull-out failure mechanism the joint resistance is sustained only by the shear behaviour of the rivet. In the overlapping area the rivet head penetrates in the upper sheet and then it pulls out from the deformed lower sheet (iii) at long aging times (over 6 weeks), the aging treatment reduces significantly the performances of the joint favouring the occurrence of premature failure mechanisms induced by localised corrosion phenomena. The mechanical characteristics of the joints are compromised for the advanced degradation state of the lower aluminium sheet. Consequently, with the relevant corrosion deterioration of the aluminium plate, the final failure occurs prematurely at very low stress values due to a net-tension propagation of manufacturing cracks located in the rivet tail or shear out failure on the edge aluminium sheet (picture reported in Fig. 4, stage III). This mechanism could be classified as cleavage failure of the mechanical joint.25 Indeed, the presence of circular or radial cracks on the rivet button favours the activation of shear out or/and net tension failure mechanisms.19,26

Electrochemical characterisation Figure 5 gives the ascending and descending potentiodynamic polarisation curves for the rivet and the aluminium 6061 and steel A570 sheets tested separately. The zinc coated rivet (Fig. 5a) and the aluminium sheet (Fig. 5b) evidenced an open circuit potential (OCP) quite similar, 21?310 V(Ag/AgCl) and 21?214 V/(AgAgCl) respectively (the corrosion potential, OCP, were determined at the minimum of current density). Instead the steel sheet evidenced a more noble potential of about 800 mV (the observed OPC of A570 steel was 20?398 V(Ag/AgCl)). This implies that significant galvanic coupling phenomena are possible in this type of mixed joints. In particular the rivet and the aluminium sheet are the anodic areas and the steel sheet is the cathodic area. The anodic dissolution could become a crucial point in the durability in severe environmental conditions due to the large-scaled area cathode/anode ratio and the presence of interstices between the overlapping region, that speed up the phenomenon. The curve of the rivet (Fig. 5a) shows, on the ascending curve, a progressive increase in the current density at increasing potential up to 20?400 V(Ag/ AgCl) with a maximum current at about 25?0 mA cm–2. This trend can be related with the dissolution of the superficial zinc layer (section 1 in Fig. 5a). Afterwards, increasing furthermore the potential, a reduction in the current density was observed, reaching a local minimum of about 14?5 mA cm–2 at 20?050 V(Ag/AgCl), followed by a further increase (section 2 in Fig. 5a). This local


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5 Potentiodyamic polarisation curve of a rivet, b Al 6082 sheet and c steel A570 sheet

deflection of the current density versus potential trend can be related with the full dissolution of the zinc layer and the activation of dissolution of the steel substrate that is characterised by better electrochemical performances.18 Consequently the second monotone increase of the curve (section 3 in Fig. 5a) can be related only with the dissolution of the steel substrate. At 1?500 V(Ag/AgCl) the descending trend of the polarisation curve begins. The reverse scan shows a quite different behaviour compared with the ascendant one. On section 3 reported in Fig. 5a, the differences between the two scans are not significant. Afterwards, due to a further reduction of the potential (section 2 in Fig. 5a) the sample evidenced a deep drop on current density, reaching a minimum at 20?546 V(Ag/ AgCl). This potential is the OCP of the carbon steel substrate, confirming that the zinc was totally dissolved and the rivet remained uncoated after the ascending phase of the test. The ascending scan of aluminium sheet evidence a corrosion potential of 21?214 V(Ag/AgCl). At first,

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increasing the potential, the current increases progressively until to reach a plateau (section 4 in Fig. 5b). This behaviour could be related with a passivation region where, during the initial stages of corrosion phenomena, the aluminium surface is electrochemically covered by an oxide layer (Al2O3). The passivation current is about 461023 mA cm–2. At higher potentials, due also to the presence of chloride ions in the electrolyte solutions, the passive layer becomes unstable promoting a new dissolution stage at higher potential. The breakdown potential, identified by the significant increases in the current density, was observed at about 20?590 V(Ag/ AgCl). In this stage stable pit growth is expected. During the reverse scan a relevant difference, comparing the ascending and descending trend, can be identified just in the section 4 of the polarisation curve. The aluminium sheet, after that the surface corrosion phenomena occurred, not evidenced a passive section but it shows an active electrochemical behaviour. The corrosion potential increased of about 350 mV (the OCP is 20?853 V(Ag/AgCl)). The normal and reverse polarisation curves obtained for steel sheet are quite similar. The sample exhibits an active behaviour, the OCP is in the range 20?400 V(Ag/ AgCl) and 20?540 V(Ag/AgCl) respectively in ascending and descending scans. Analysing the polarisation curves and comparing the OCP of each joint constituent (summarised in Table 2) some considerations about the evolution of galvanic induced corrosion of the mixed joints can be deduced. The steel sheet has a corrosion potential higher than aluminium sheet and the zinc coated rivet. The steel sheet is the cathode in the galvanic coupling, instead the aluminium and rivet could act as anodic regions. Consequently the relationship between steel/aluminium and steel/rivet coupling can be considered during aging in aggressive environmental conditions. In Fig. 6 the cross-section of the joint is drawn, evidencing the potential degradation areas (circular lines) due to galvanic corrosion. Three critical galvanic coupling areas were identified: (i) GC1 area is related with the galvanic coupling induced by the overlapping of aluminium and steel sheets (ii) GC2 is related with the galvanic coupling between the rivet head and the upper steel sheet (iii) GC3 is due to a galvanic coupling of threeelements: rivet tail, aluminium and steel sheets. The corrosion mechanisms that take place on the SPR joints are influenced by the presence of polymer coatings. In this case the interstitial regions are much less critical due to sealing action offered by polymer coating. Consequently the degradation phenomena, induced by crevice corrosion, are strongly limited enhancing the service life of the joint. GC1

Figure 7 shows the potentiodynamic polarisation curve of the aluminium/steel joint. The tested area is 1?5 cm2. Table 2 Comparison of OCP of ascending and descending polarisation curves for individually tested materials Material

OCP ascending/ V(Ag/AgCl)

OCP descending/ V(Ag/AgCl)

Rivet Al 6082 Carbon steel A570

21.310 21.214 20.398

20.546 20.853 20.540

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6 Scheme of critical degradation area due to galvanic corrosion on hybrid SPR joint

The mixed corrosion potential, in ascending and descending scan, is quite similar with that observed for only Al 6082 alloy. In the galvanic system constituted by aluminium/ carbon steel sheets, the Al6082 is more negative than the carbon steel component in this chloride solution, indicating that the carbon steel is the cathode that induces an accelerated dissolution of the aluminium, instead having an anodic behaviour. From the point of view of durability in chloride environmental conditions, GC1 is a risky area because the anodic dissolution is localised only on the aluminium foil. As shown in Fig. 4, after 7 weeks of aging, the degradation of the lower aluminium sheet is sufficiently advanced to lead to mechanical instability of the joint at very low load values.

of the zinc layer that overcomes the underlying steel plate. Consequently it is expected that at long aging times, the difference in electrochemical potential between the rivet and the steel sheet will becomes negligible, reducing the risk of galvanic corrosion in this area. GC3

Figure 8 shows the potentiodynamic polarisation curve for the rivet/steel galvanic coupling. The tested area is about 1?5 cm2. The evolution of current density at increasing potential is compatible with the trend observed for the rivet constituent in Fig. 5a. In descending scan the potential has a significant increase as a consequence of the large dissolution of the zinc coating in the rivet. In fact the reverse mixed potential is 20?639 V(Ag/AgCl) near the observed OCP of reverse scan for the rivet and steel sheet (about 20?540 V(Ag/AgCl)). The unfavourable geometrical constitution of a small anode (rivet) and a large cathode (carbon steel sheet) enforces the dissolution of the anodic component.17 This galvanic corrosion, although characterised by an high current density in the first stage of the zinc dissolution, then becomes less relevant due to a progressive dissolution

It is much more complex to assess the galvanic coupling phenomena in correspondence of the rivet tail. The infiltration of the electrolyte solution in the overlapping area between the aluminium and carbon steel sheets can occur by capillary forces at the crevice27 producing a new electrolyte cell at the rivet/sheets interface. The penetration of the solution in the interstices is favoured by the initial corrosion phenomena that take place in GC1 area. The corrosion products favour the enhancement of the hydrophilic behaviour of the area favouring the permeation of water within the crevice between the sheets. The cross-sections of the joints at increasing aging times are reported in Fig. 9. At initial stages of corrosion the main failure mechanism, occurred during the single lap shear test, is bearing of rivet on the lower aluminium sheet. In Fig. 9a, the presence of oxide multilayers between the sheets and in the rivet interstices was observed (points A in Fig. 9). In this phase, the corrosion phenomena are not significant and consequently do not influence the rupture mechanism of the joint. At medium aging times an increase in corrosion products within the joint (points B and C in Fig. 9) is observed. The corrosion product interlayer in the rivet tail reduces the rivet/sheet interlocking favouring the unbuttoning of the rivet from the lower sheet. At high aging times (the picture on the right of Fig. 9) the thinning of the aluminium sheet is relevant (points D in Fig. 9). The corrosion products in the rivet tail induce

7 Potentiodynamic polarisation curve of aluminium/ Carbon steel galvanic coupling in 3?5% NaCl solution

8 Potentiodynamic polarisation curve of rivet/steel galvanic coupling in 3?5% NaCl solution

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9 Cross-sections of joints at different aging times

stress concentration in this region, widening the cracks originated during riveting. Therefore such defects led to the joint failure by net tension and/or shear out mechanism, as described in Fig. 4. Based on these observations we can deduce that the galvanic corrosion in GC3 area plays an important rule at medium and long aging times, reducing the sheets/ rivet interlocking and favouring the propagation of longitudinal or transversal crack in the lower aluminium button, when the aluminium sheet at long aging times has become thinner.

Failure map The results obtained in this experimental campaign evidenced that the aging time influences the failure strength and the fracture mode of the SPR joint. In particular the progressive deterioration of the joint, as previously verified, due to the galvanic corrosion phenomena, induces a transition from bearing to tail pull-out and finally shear-out (or/and net-tension) fracture mode. From a theoretical point of view, in a specific geometric condition, a particular fracture mechanism takes place when the apparent resistance is lower than the other competing fracture mechanisms. The force, required to generate bearing on the thinner sheet, can be defined by following expression FB ~sYC (aCS sdown ) d

(1)

where sYC and sdown are respectively the ultimate compressive strength and the thickness of the metal sheet. d is the diameter of the rivet button. aCS is a timedependent coefficient related with the progressive reduction of the thickness, induced by the slight corrosion dissolution of the aluminium plate during aging time. The interpretation of fracture mechanisms inherent to the phenomenon of rivet pull-out is much more complex. The geometry of the rivet and its threedimensional nature increase the difficulty in getting an overall system of governing equations that predict the self-riveting joint strength in the pull-out condition. Based on Porcaro et al.’ work,28 a simplified approach can be used to identify the rivet tail pull-out (FTP) by using the following expression FTP ~aCTP aTP sYT ½d(aCS sdown )3 1=2

(2)

where sYT is the ultimate tensile strength of the metal plate. aTP is a coefficient estimable by experimental results. Instead aCTP is a new time-dependent corrosion parameter that justifies the relevant reduction of the tail pull-out resistance, FTP of the self-riveting joint.

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The corrosion phenomena, during aging time, cannot significantly influence the aCS parameter, whereas the metal dissolution is not such as to reduce heavily the thickness of the aluminium sheet. Instead a corrosion contribute respect to pull-out mechanism can be considered. The formation of metal oxide, due to crevice corrosion, and the crack propagation due to stress corrosion cracking on the rivet tail could reduce the mechanical interaction of the joint and modify significantly the rivet-sheet interface. These effects could be evaluated by the coefficient aCTP that decreases significantly at medium aging times (aCTP< 0?85 at 3 weeks), confirming the reduction of the tail pull-out resistance (FTP) of the self-riveting joint. The force required to generate shear-out failure of the less resistant sheet can be defined by following expression FSO ~sYS (aCS sdown )(aCSO h)

(3)

where sYS is ultimate shear stress. h is the distance of the rivet from the sheet edge and aCSO is a corrosion timedependent coefficient related with the progressive reduction of this distance induced by the galvanic corrosion of the anodic aluminium sheet during salt spray test. The images reported in Fig. 4 evidence that such distance is high at low aging times, but it reduces significantly with increasing aging time. Consequently aCSO