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magnesium metal matrix composite and electrochemical response of its constituents
Electrochimica Acta 54 (2009) 1597–1606
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Corrosion behaviour of carbon fibres/magnesium metal matrix composite and electrochemical response of its constituents A. Bakkar a,c,∗ , V. Neubert a,b,∗∗ a
Institut für Materialprüfung & Werkstofftechnik Dr. Neubert GmbH, Freiberger Strasse 1, D-38678 Clausthal-Zellerfeld, Germany Zentrum für Funktionswerkstoffe gGmbH, Sachsenweg 8, D-38678 Clausthal-Zellerfeld, Germany c Metallurgical and Materials Engineering Department, Faculty of Petroleum & Mining Engineering, Suez Canal University, P.O. Box 43721, Suez, Egypt b
a r t i c l e
i n f o
Article history: Received 13 August 2008 Received in revised form 17 September 2008 Accepted 19 September 2008 Available online 14 October 2008 Keywords: Mg MMC C-fibres Corrosion Microstructure
a b s t r a c t The corrosion behaviour of carbon fibres reinforced magnesium metal matrix composite (MMC) was investigated with emphasis on the galvanic corrosion arisen between the magnesium matrix alloy and the carbon fibres. The susceptibility of carbon/magnesium interface to degradation was also ascertained. The corrosion behaviour was studied in both neutral and alkaline aqueous solutions containing different concentrations of NaCl, using electrochemical techniques, hydrogen evolution test, optical microscopy and scanning electron microscopy (SEM) coupled with EDX and WDX capabilities. Two matrix alloys were used, AS41 and AS41(0.5% Ca). The latter monolithic alloy had a significantly higher corrosion resistance marked by Ca-addition, but the presence of carbon fibres in MMCs invalidated the beneficial effect of Ca. Carbon, as an electrically conductive element, is electrochemically unstable despite its almost chemical stability in aqueous chloride environments. Therefore, the polarisation diagrams of MMCs were demonstrated in terms of the electrochemical activity of carbon fibres. This necessitated performing the electrochemical polarisation tests, individually, on both Mg MMC, monolithic magnesium matrix alloy and carbon fibres electrode. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium metal matrix composites (Mg MMCs) reinforced with carbon or graphite are one of the promising candidate materials for applications in which as high strength/weight ratio is necessary. They possess the lowest density of all engineering metallic materials and exhibit good mechanical properties and a very low coefficient of thermal expansion [1]. These unique specific properties actualised the development of C/Mg MMCs to be a precious alternative of aluminium for applications in the fields of aerospace and automotive industries. Fritze et al. [2], for instance, reported that a 30% reduction in the piston weight was achieved by its fabrication from C/Mg MMC, instead of aluminium.
∗ Corresponding author at: Institut für Materialprüfung & Werkstofftechnik (Dr Neubert GmbH), Freiberger Strasse 1, D-38678 Clausthal-Zellerfeld, Germany. Tel.: +49 5323 989 890; fax: +49 5323 989 899. ∗∗ Corresponding author at: Zentrum fuer Funktionswerkstoffe GmbH, Sachsenweg 8, D-38678 Clausthal-Zellerfeld, Germany. Tel.: +49 5323 989 890; fax: +49 5323 989 899. E-mail addresses: ashraf [email protected] (A. Bakkar), [email protected] (V. Neubert). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.09.064
However, magnesium monolithic matrix alloys suffer from a strong tendency to corrode, especially if it contained noble impurity elements (e.g. Fe, Ni and Cu) which serve as cathodic sites and accelerate the corrosion rate [3,4]. Thus, the corrosion of magnesium is highly dependent on the presence of cathodic constituent phases and thereby galvanic corrosion arose between the Mg-matrix alloy and the noble reinforcement constituent. Trzaskoma [5] reported severe degradation of the graphite fibres/magnesium composites in aqueous chloride environments. The author confirmed the important role of galvanic interaction in the aqueous corrosion rather than the rapid reaction of magnesium with the solution. Hihara and Kondepudi [6,7] investigated the galvanic corrosion behaviour of Mg MMCs based on two matrices, pure Mg and ZE41A-Mg alloy. Two types of reinforcements were used individually; SiC monofilament (MF) made of a carbon core coated by a carbon-rich SiC layer and pure SiC particles. The results showed evidence of higher galvanic corrosion rates in the presence of SiC (MF), whose electrochemical behaviour was similar to that of graphite, compared with pure SiC particles. Also, ZE41A-Mg matrix showed a better corrosion resistance than pure Mg. Hall [8] observed the evidence of galvanic corrosion for carbon fibre/magnesium MMCs in a normal laboratory atmosphere with 60% relative humidity at 20 ◦ C.
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The determined rate of penetration was about 100 m/year. The magnesium matrix contained 1 wt.% Al and Al carbides were formed at the fibre/matrix interface during squeeze casting but they had no significant effect on the atmospheric corrosion behaviour. Galvanic corrosion, in general, of MMCs based on matrices such as Al, Ti and Cu reinforced with carbon or graphite was cited as a major problem [9–16]. This is due to the high electrical conductivity of carbon. According to Pourbaix potential–pH diagram of carbon in water [17] carbon has a narrow window of stability. At highly electronegative potentials carbon is thermodynamically feasible to be reduced to methane. However, Hihara [18] reported that neither methane nor visual fibre degradation was detected when pitch-based graphite fibres (Thornel P-100) were cathodically polarised. At noble potentials carbon oxidises in aqueous solutions forming CO2 , CO and additionally some dissolved carbon species (e.g. H2 CO3 , HCO3 − and CO3 2− ). However, CO2 is believed to be the major anodic reaction [19]. Hihara and Latanision [20] reported that during anodic polarisation of Gr/Al MMC in Na2 SO4 , P-100 graphite fibres were oxidised to CO2 with crevice formation along the perimeters of fibres. It must be noted that carbon has a high chemical stability when exposed to aqueous electrolytes without application of any potential. Hihara and Latanision [21] cited that in the open circuit condition or when coupled to aluminium, graphite fibres are very inert and visual degradation is not observed. Degradation of the fibre/matrix interface compound is another possibility for the inferiority of corrosion resistance dictated by MMCs [8,12,18,21–23]. Aylor and Moran [22] found that the graphite fibres enhance the breakdown susceptibility of the passive film formed on Al in Gr/Al MMCs. They assumed that of the Al4 C3 compound formed during the fabrication process, hydrolysis in marine media and leads to a lowering in the integrity of the passive film. Based on the result that the composite does not exhibit an electropositive change in corrosion potential the authors neglected the effect of galvanic coupling between the Al and graphite. The localised corrosion at the C-fibre/Al interface was confirmed and in situ analysed by Payan et al. [23]. The present paper was focused on studying the electrochemical characterisation of C-fibres/Mg MMC with ascertaining the polarisation behaviour of the composite constituent materials, in addition to measuring the galvanic current which arises between the Mg alloys and carbon electrode. Experiments were performed in neutral and alkaline chloride-containing electrolytes. These media were selected to determine the effect of carbon incorporation on the active and passive behaviour of Mg MMC in which Mg shows passivity in alkaline environments. Hydrogen evolution measurement, as a free corrosion test, was also monitored for prolonged times. The corrosion behaviour of the 0.5% Ca-contained AS41 Mg alloy was presented, compared with that of the pure AS41 alloy, to assess whether the small Ca-addition still has the beneficial function of enhancing the corrosion resistance of the MMC as well. Microstructure analysis was undertaken using optical microscopy and scanning electron microscopy (SEM) equipped with EDX and WDX capabilities, to investigate the as-received and corroded surfaces.
2. Materials and methods 2.1. Materials 2.1.1. Mg alloys Two magnesium alloys were used in this study; one is the nominal AS41 Mg alloy, free of calcium and the other contained about 0.5 wt.% calcium to be labelled as AS41(0.5Ca) Mg alloy. The typical composition for the both alloys is shown in Table 1. 2.1.2. Mg MMCs The Mg alloys were reinforced with carbon short fibres (Sigrafil C-40) using the squeeze casting technique, in which the Mg alloy was forced into the carbon short fibres preforms. The equipment used was described in Ref. [24]. The carbon fibres (about 25 vol.%) were distributed quasi-isotropically in the horizontal plane. 2.1.3. C-fibres electrode Sigrafil C-40 continuous fibres, as a unidirectionally aligned tow, were wrapped and soldered by a copper sheet from one side. This soldered side was connected with a copper wire. Then, the fibres were infiltered with an epoxy resin and suspended into the electrolyte exposing a free test surface of fibres cross-sections with a total area of 0.264 mm2 . 2.2. Samples The Mg materials were provided as discs with a diameter of about 200 mm and a height of about 40 mm. Contained in these discs were the C-fibres/Mg composite blocks with dimensions of 120 mm × 120 mm × 35 mm, see Ref. [24]. Specimens were cut into small blocks 15 mm × 15 mm × 7 mm. Each specimen was successively wet ground on all faces. Those for electrochemical tests at pH 12 were welded to copper wires on one side and mounted in epoxy to be suspended in the electrolyte, exposing a free test surface of 15 mm × 15 mm. In all electrochemical tests and hydrogen evolution measurements, a fresh surface was obtained by grinding using 1200 grit SiC paper and ethanol as a lubricant and cooler. Finally, the specimens were ultrasonically cleaned with several rinses of ethanol and immediately conducted on examination. Three specimens were produced for each experiment at 25 ◦ C ± 1. 2.3. Corrosive solutions Aqueous chloride-containing solutions were prepared by dissolving analytical grade of NaCl in distilled water. Alkaline solution of pH 12 was adopted by the addition of NaOH. The solution used for hydrogen evolution measurements was buffered at pH 9.3 by addition of borax and NaOH. 2.4. Electrochemical measurements 2.4.1. Potentiodynamic polarisation Polarisation experiments were carried out using the model “Wenking LB 94L (Auto Range) Laboratory Potentiostat” and controlled by PC computer. The specimen was exposed to the NaCl solution for 5 min prior to potentiodynamic polarisation by which
Table 1 Typical chemical composition (wt.%) of both magnesium matrix alloys. Mg
Zr
Sn
Pb
Cu
Ni
Fe
Zn
Ca
Mn
Si
Al
Alloy
Bal. Bal.
<0.010 <0.010
0.0049 0.0049
0.0027 0.0027
<0.0002 <0.0002
<0.0005 <0.0005
0.0028 0.0022
0.0816 0.0852
<0.001 0.38
0.35 0.324
0.93 0.94
4.374 4.580
AS41 AS41(0.5Ca)
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time a stable potential, open circuit potential (OCP), was monitored. With reference to saturated calomel electrode (SCE), the polarisation was obtained by scanning from 500 mV more negative than the OCP at a rate of 20 mV/min. Experiments conducted in 100 ppm NaCl solution of pH 12 was carried out in a normal three-electrode cell, in which the specimen is suspended as a working electrode. On the other hand, experiments in neutral NaCl-containing solutions (pH of about 7) were performed in an Avesta cell [25]. 2.4.2. Galvanic couples In the galvanic current measurements a separate Mg alloy and a cylindrical carbon electrode with equal surface areas were coupled through a potentiostat, and by using it as a zero resistance ammeter (ZRA), the galvanic current flowing through 500 ml of electrolyte was measured. This current was monitored continuously by an X/Yrecorder. The distance between the electrodes was maintained to be 35 mm. 2.5. Hydrogen evolution test The Mg specimen was put on a glass seat in a beaker containing 1200 ml of 3% NaCl borax-buffered solution at pH 9.3. To collect the hydrogen evolved, a burette with a funnel end was placed over the specimen, which ensured the collection of all hydrogen from the specimen surface. 2.6. Corrosion morphology Corrosion morphology was characterised using optical microscopy and scanning electron microscopy. A “SX 100 Electron Probe Microanalyser” SEM equipped with EDX and WDX capabilities was used to investigate the as-received and corroded surfaces. 3. Results and discussion 3.1. Electrochemical polarisation data in alkaline media 3.1.1. Polarisation of the monolithic alloys Following the authors’ previous work [24,26–28] on the corrosion of Mg-based alloys and composites in chloride-containing alkaline solutions the break-through-potential (Ep ) is the main determining parameter for corrosion characterisation. Fig. 1 shows
Fig. 1. Potentiodynamic polarisation diagrams for monolithic AS41 and AS41(0.5% Ca) Mg alloys in 100 ppm NaCl solution; pH 12.
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that the Ca-contained alloy has a greater corrosion resistance; it had at a higher Ep value. In addition, the other corrosion tests support this result, as will be seen below in the galvanic coupling measurements and H-evolution test. The beneficial effect of Ca microadditions (up to 0.5 wt.%) was reported due to incorporation of the relatively large Ca ions in the corrosion product film, causing it to be more stable and protective [26,29]. 3.1.2. Polarisation of C/Mg metal matrix composites The polarisation data show that the electrochemical behaviours of the two composites, both based on Ca-free and Ca-contained matrixes, are similar as shown in Fig. 2. It can be clearly seen that the presence of carbon as reinforcement fibres in the Mg-based composites shifts the corrosion potential in the noble direction by more than 1000 mV, compare Figs. 1 and 2, implying the eventual galvanic coupling between Mg- and C-fibres, by virtue of their high electrical conductivity. The anodic polarisation curve can divided into three regions. The first region (I) indicates the clear passivation after the Tafel part, whereas the second region (II) is characterised by a noticeably gradual increase in the current density with a forward potential scan. Then, it is followed by a sharp increase in the current density and region (III), which may characterise the occurrence of pitting corrosion. Fig. 3 exemplifies the cyclic polarisation of C-fibres/AS41 Mg MMC, in which the potential sweep is reversed at a current density of 0.1 mA/cm2 and still lies in region (II), Fig. 2. It is seen from the figure that the current density starts to decrease on reversing the potential scan. This implies the absence of the pitting corrosion which is characterised by a continuous increase in the current density. Another point of additional significance is the occurrence of pitting corrosion occasionally, where the pitting arises in the first cycle in Fig. 3a, but appears in the second cycle in Fig. 3b. This can be interpreted as the result of the postulated reduction to methane and oxidation to CO2 of C-fibres during cathodic and anodic polarisation, respectively [17,19,20]. These cathodic and anodic reactions lead to crevice formation along the round borders of fibres, as shown in Fig. 4. Consequently, the C/Mg interface is very prone to crevice corrosion, which leads to the sudden increase in the current density at unexpected potential values within the passive range of monolithic matrix alloy, compare Figs. 1 and 3. Fig. 5 exhibits the forward and reverse potentiodynamic polarisation of C-fibres/AS41 MMC, scanned up to 1 mA/cm2 . On reversing
Fig. 2. Potentiodynamic polarisation diagrams for both C/AS41 and C/AS41(0.5% Ca) Mg MMCs in 100 ppm NaCl solution; pH 12.
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Fig. 5. Potentiodynamic polarisation diagram, reversed at current density of 1 mA/cm2 , for C/AS41 Mg MMCs in 100 ppm NaCl solution; pH 12.
Fig. 3. Examples of cyclic polarisation diagrams for C/AS41 Mg MMCs in 100 ppm NaCl solution; pH 12.
Fig. 4. Optical microscopy micrograph of C-fibres embedded in epoxy resin after anodic polarisation up to 1400 mV (vs. SCE) in 100 ppm NaCl solution; pH 12.
the potential sweep the current density continues to increase. This emphasises the occurrence of pitting corrosion in region (III), Fig. 2. Fig. 6 shows the anodic polarisation diagrams of monolithic Mg-matrix alloy, C-fibres and C/Mg MMC and a mixed-electrode diagram generated from both anodic polarisation diagrams of C-fibres and monolithic Mg-matrix alloy on the basis of the mixed-potential theory. The current density calculated for each potential of the mixed-electrode diagram = 0.75 of the current density recorded for the monolithic matrix alloy + 0.25 of the current density recorded for the C-fibres. The figure illustrates many valuable results. Firstly, the gradual increase in the current density during region II, Fig. 2, of the C/Mg MMC is due to the sharp increase in the current density for the C-fibres electrode. Secondly, the independence of the pitting potential for the MMC (Ep MMC ) on that for the matrix alloy. Thirdly, the discrepancy between the real composite diagram and the generated mixed-electrode diagram after the pitting potential for the C-fibre (Ep Fibre ), indicating that the mixedpotential theory cannot apply after (Ep Fibre ). Consequently, it can
Fig. 6. Anodic polarisation diagrams for monolithic AS41(0.5Ca) Mg-matrix alloy, C-fibre (SG 40) and C/AS41(0.5Ca) Mg MMC in 100 ppm NaCl solution; pH 12. In addition to a generated diagram on the basis of the mixed-potential theory for a composite containing 25 vol.% C-fibres in AS41(0.5Ca) Mg-matrix alloy.
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Fig. 7. Microstructure of C/AS41 Mg substrate, as-received: (a) optical micrograph for the horizontal plane, (b) SEM micrograph for the normal plane, (c) qualitative WDXelemental scan along the “line A” shown in micrograph (b), and (d) semilogarithmic qualitative WDX-elemental scan along the “line B” shown in micrograph (b).
be stated that the polarisation behaviour of C/Mg MMC is drawn up by the polarisation of C-fibres rather than that of matrix alloy, characterising an invariable pitting potential independent of that of the matrix alloy. Thus, the evidence of pitting in C/Mg MMC is induced by the C-fibres.
900 mV and at that potential followed by potentiostatic polarisation for 30 min, is revealed in Fig. 8. A large and deep attack is clearly seen in the area where C-fibres are agglomerated. This can be attributed to the crevice attack of C-fibres perimeters resulted
3.2. Corrosion morphology 3.2.1. Microstructure of original samples The microstructure of as-fabricated C/AS41 Mg MMC sample is depicted in Fig. 7. The illustrated phases in Fig. 7b were identified by EDX analysis. It is obvious that the present phases have a preferably inclination to precipitate close to the C-fibres. The line scan across the C-fibre/matrix interface (line A) reveals its richness in Al. Furthermore, Al is identified at the Mg2 Si/C-fibre interface (line B). It is evident that an Al–C compound is formed adjacent to the C-fibres as a result of the interfacial reaction during fabrication. This compound has been determined to be Al4 C3 or Al2 MgC2 and reported to have a thickness of few nanometers [8,30]. It is too thin to be revealed by the SEM/EDX analysis because of the coarse interaction volume diameter produced by the electron beam. 3.2.2. Microstructure of samples after polarisation measurements The microstructure of C/AS41 Mg MMC specimen, potentiodynamically polarised in 100 ppm NaCl alkaline solution up to
Fig. 8. The SEM micrograph of C/AS41 Mg MMC surface after 30 min polarisation at 900 mV in 100 ppm NaCl solution; pH 12.
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Fig. 9. (a) SEM micrograph of composite surface polarised for 3 h at 900 mV in chloride-free solution; pH 12, showing that the fibres have deeper levels compared with the matrix alloy. (b) EDX analysis of the matrix alloy pointed by “X1” in micrograph (a). (c) EDX analysis of corrosion precipitates (white) exemplified by “X2” in micrograph (a).
from their oxidation by anodic polarisation, as described above in Section 3.1.2. Fig. 9 shows the topography of C/AS41 Mg MMC which potentiostatically polarised at 900 mV for 3 h after potentiodynamic polarisation in chloride-free pH 12 solution. The C-fibres have deeper levels compared with the matrix alloy, Fig. 9a. It is believed that the C-fibres were oxidised to CO2 during anodic polarisation. It is also observed that some corrosion product precipitates (white specks), which is thought to deposit back from the electrolyte. EDX analysis of those precipitates indicates its richness in C and Al elements, compare Fig. 9a and b. 3.2.3. Microstructure of samples after free immersion The microstructure of C/AS41 Mg MMC specimen after 2 h free immersion, in absence of applied polarisation, in 100 ppm NaCl alkaline solution is shown in Fig. 10. Precursors of pitting sites are clearly seen to arise preferentially in the vicinity of, but not close to, the C-fibres. Thus, the pitting sites have no significant tendency to initiate at the C/Mg interface. This is in agreement with the results reported by Hall [8], that no visible degradation on Al4 C3 , formed at the fibre/matrix interface, was detected in the laboratory air. Fig. 10 indicates also that the corrosion propagates from the pitting sites depicting the filiform corrosion type, which has been typically reported for Mg–Al alloys [24,32,33].
3.3. Electrochemical polarisation data in neutral media Fig. 11 exemplifies the polarisation diagrams of Mg alloys and their composites in neutral aqueous solutions containing various
Fig. 10. SEM micrograph of corroded surface after 2 h free immersion, at open circuit conditions, in 100 ppm NaCl solution; pH 12.
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Fig. 11. Potentiodynamic polarisation diagrams of alloy and composite specimens in neutral 0.03, 0.3 and 1.0 wt.% NaCl solutions.
NaCl concentrations. It can be seen that the polarisation diagrams, in 0.03 wt.% (300 ppm) NaCl solution, for both Mg alloys show a pseudo-passive behaviour during the anodic polarisation up to about “−1200 mV” and here a relatively sharp increase in the current density occurs. The effect of C-fibres in abrogating this pseudo-passive behaviour is evident. On increasing the NaCl concentration, the monolithic alloy specimens exhibit no longer pseudo-passivation. However, the polarisation curves of composite specimens depict higher corrosion current densities compared with that of the monolithic alloys. Fig. 12 summarises the corrosion characteristics drawn out from the polarisation curves. At all NaCl concentrations, the composites dictate higher corrosion current densities (Icorr ) than the monolithic
alloys, Fig. 12a. This rise of Icorr values for the composite specimens increases also with the NaCl concentration, where the ratio – (Icorr )composites /(Icorr )alloys – increases from about 2.5 and 4 to about 20, at NaCl concentrations of 0.03, 0.3 and 1.0 wt.%, respectively. At 0.03 wt.% NaCl solution, the Ca-contained alloy shows lower Icorr values than the Ca-free one but this reduction in the corrosion current disappears with a further increase in the NaCl concentration. At each NaCl concentration, the Icorr values for both Ca-contained and Ca-free Mg alloy-based composites are almost the same. Regarding the variation of corrosion potential (Ecorr ) with NaCl concentration, Fig. 12b, it can be seen that at low chloride concentrations (0.03 wt.% NaCl), the Mg MMCs show more noble Ecorr values than their matrix alloys, but at higher concentrations the
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Fig. 12. Variation of corrosion current density (a) and corrosion potential (b) of alloy and composite specimens with NaCl concentration in neutral solution.
Ecorr values of MMCs change sharply towards more active values to be approximately the same as that of the monolithic matrix alloys. At low chloride concentration, the Ecorr is characteristic of the electrode surface; the Ca-contained alloy has a more noble Ecorr value than the Ca-free one, because of the beneficial effect of Ca on stabilising the partially protective film formed on Mg [26]. As to the MMCs, at low Cl concentrations the carbon fibres polarise the Mg matrix into the electropositive direction thereby recording more noble values. However, at higher Cl concentrations, the obtained Ecorr values were close to that of the Mg-matrix alloy. This result can be explained in terms of the corrosion mechanism of Mg, which involves the formation of partially protective Mg(OH)2 film in aqueous and humid atmospheres. This film is liable to reduce the electronegativity of Mg, by about 1 V, from the theoretical standard potential [31]. It is believed that at higher concentrations the chloride ions, adsorbed on the specimen surface, cause breakdown of the partially protective surface film and lead to formation of more active sites. 3.4. Galvanic current measurements With the purpose of obtaining additional evidence about the effect of C-fibres on the electrochemical behaviour of Mg-based materials, the galvanic current arisen between each of the Mgmatrix alloys and a carbon electrode was measured. Two sets of
Fig. 13. Typical galvanic current densities (a) and mixed potentials (b) of Mg-matrix alloys, coupled to a carbon electrode in 100 ppm NaCl solution, pH 12. Connection between the Mg alloy and graphite was carried out after 15 min free immersion time.
experiments were applied; in one set the connection between the Mg alloy and carbon electrode was applied after 15 min of free immersion and in the other the connection was carried out immediately as of immersion. Mg is stable in alkaline solutions and has a passive protective film of Mg(OH)2 [31,33]. This film is expected to be enriched with immersion time under open circuit conditions. Thereby, it resists pitting for long or short time even after connection with a carbon electrode. However, as a result of the connection with carbon, Mg electrode exerts an anodic reaction which comprises dissolution of Mg2+ ions and provision of free electrons to the carbon electrode to preserve the electrical balance [24,32,33]. Taking into account that the Mg(OH)2 film breaks down in the presence of chloride ions, the permanent anodic reaction leads to the breakdown of hydroxide film and formation of active sites resulting pitting corrosion, and hence the galvanic current increases sharply as shown in Fig. 13a. The effect of small Ca-addition is evident on the stabilisation of the hydroxide film formed. The Ca-contained alloy sustained monitoring low galvanic corrosion current (∼1 A/cm2 ) without pitting for longer times. Additional valuable results are provided by recording the mixed potential against time, Fig. 13b. The variation in the potential is in the same accordance with that of the current density. Higher galvanic corrosion current densities were obtained with immediate connection on exposure of the specimen surface to the solution, Fig. 14. This can be explained that the kinetics of the anodic
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Fig. 14. Galvanic current densities between Mg alloys and carbon electrode in 100 ppm NaCl solution, pH 12. Connection between the Mg alloy and carbon was carried out immediately on immersion.
dissolution of Mg2+ ions, polarised by the carbon electrode, are faster than that of the formation of protective Mg(OH)2 film. The galvanic corrosion current densities in neutral Cl-containing solution were also measured, Fig. 15. It is clear that the Cacontained alloy maintained lower galvanic current densities than the Ca-free one, irrespective of whether the connection was immediately or after 15 min of immersion. However, each of the alloys exhibits higher current densities in immediate connection conditions compared with that resulted from connection after free immersion. The beneficial effect of Ca in reducing the galvanic current is attributed to the incorporation of Ca atoms in the corrosion product film, causing it to be more stable and protective. A more detailed interpretation of the effect of Ca-addition was reported in another study by the authors [26]. In a comparison observation between the galvanic current densities arisen in immediate and in delayed connection conditions for both neutral and alkaline solutions, it can be stated the following: (1) in the neutral solution, the free immersion for 15 min enables the formation of a corrosion products film, and further corrosion is determined by the successive diffusion of Mg ions through that film, thus it will be a diffusion-depending process; (2) in alkaline solu-
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Fig. 16. Hydrogen evolution of Mg alloy and composite specimens for 50 h immersion in buffered 3% NaCl solution; pH 9.3.
tion, the free immersion leads to formation of the passive protective film of Mg(OH)2 , and further arising corrosion follows the pitting mechanism; and (3) the beneficial effect of Ca in the neutral solution hinders the advanced corrosion through the corrosion product film. In the alkaline solution the Ca affects to stabilise the hydroxide passive film and to delay its breakdown. However, as soon the passive film breaks down the Ca-effect seems to be neglected. 3.5. Hydrogen evolution test data The hydrogen evolution test was applied for prolonged times to compare between Mg alloys and their composites under free immersion conditions. The fact, that the gravimetric tests on the composite materials are limited because the fibre-reinforced MMCs develop deep pits which entrap corrosion products and corrosion solution [34], gives an additional importance to the H2 -evolution test as an effective comparative method. Fig. 16 shows the monitored H2 evolution for investigated Mg materials after 50 h. It is clear that the composite specimens record higher H2 -evolution rates than their monolithic matrix alloys. Additionally, the corrosion rate of the Ca-contained Mg alloy is significantly lower than that of the Ca-free one. However, this corrosion inhibiting effect by Ca-addition is rendered invalid with the presence of C-fibres in MMCs, where the fibre-reinforced composite based on Ca-contained Mg alloy has H2 -evolution values close to that based on Ca-free Mg alloy. In other words, the two composites monitor equal corrosion rates to be about 1.5 times of the Ca-free monolithic alloy and three times of the Ca-contained one. The galvanic effect by C-fibres can be represented as in the conditions of galvanic coupling with immediate connection, Fig. 14. Rapid dissolution of Mg2+ ions, induced by the galvanic coupling with C-fibres, does not allow the formation of protective Mg(OH)2 film. Hence, the effect of Ca is invalidated, where its influence is stabilising of the Mg(OH)2 film. 4. Conclusions The following conclusions have emerged from a detailed experimental study on the corrosion behaviour of C/Mg MMC:
Fig. 15. Galvanic current densities between Mg alloys and carbon electrode in neutral 1000 ppm NaCl solution.
(1) In alkaline solutions, where Mg is passive, the corrosion potential (Ecorr ) of C/Mg MMCs is more noble than their monolithic matrix alloys, implying the eventual galvanic effect of C-fibres by virtue of their high electrical conductivity.
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(2) Applying electrochemical polarisation on C/Mg MMCs creates crevice attack at perimeters of C-fibres and hence leads to crevice corrosion at C/Mg interface. As a result, the MMC shows high corrosion rates, regardless the corrosion resistance of the Mg-matrix alloy. (3) In free immersion conditions, pitting corrosion is the dominating type. The pitting sites have no significant tendency to initiate at the C/Mg interface (4) The galvanic coupling with C-fibres leads to severe corrosion of Mg-matrix composites. (5) The C-fibres invalidate the virtual effect of alloying elements on the corrosion resistance. (6) The C/Mg interface compounds seem to have no significant influence on the corrosion behaviour of MMC. (7) The H2 -evolution test is an appropriate comparative method for corrosion rates of Mg alloys and composites. The two C/Mg MMCs studied monitor similar corrosion rates to be about 1.5 times of the lower corrosion resistance Mg-matrix alloy and three times of the higher corrosion resistance one. References [1] W.A. Ferrando, J. Mater. Eng. 11 (1998) 299. [2] V. Fritze, H. Berek, K.U. Kainer, S. Mielke, B. Wielage, Fibre reinforced magnesium for automotive applications, in: B.L. Mordike, K.U. Kainer (Eds.), Conference Proceedings: Magnesium Alloys and their Applications, Wolfsburg, Germany, 1998, p. 635. [3] J.D. Hanawalt, C.E. Nelson, J.A. Peloubet, Trans. AIME 147 (1942) 273. [4] E. Ghali, W. Dietzel, K.U. Kainer, J. Mater. Eng. Perform. 13 (2004) 7. [5] P.P. Trzaskoma, Corrosion 42 (1986) 609. [6] L.H. Hihara, P.K. Kondepudi, Corros. Sci. 34 (1993) 1761. [7] L.H. Hihara, P.K. Kondepudi, Corros. Sci. 36 (1994) 1585. [8] I.W. Hall, Scripta Metall. 21 (1987) 1717. [9] M. Saxena, O.P. Modi, A.H. Yegneswaran, P.K. Rohatgi, Corros. Sci. 27 (1987) 249.
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M. Saxena, B.K. Prasad, T.K. Dan, J. Mater. Sci. 27 (1992) 4805. L.H. Hihara, R.M. Latanision, Corrosion 48 (1992) 546. B. Wielage, A. Dorner, Comp. Sci. Technol. 59 (1999) 1239. D.J. Blackwood, A.W.C. Chua, K.H.W. Seah, R. Thampuran, S.H. Teoh, Corros. Sci. 42 (2000) 481. R.R. Zahran, I.H.M. Ibrahim, G.H. Sedahmed, Mater. Lett. 28 (1996) 237. J.E. Orth, H.G. Wheat, Appl. Comp. Mater. 4 (1997) 305. P.K. Rohatgi, D. Nath, J.K. Kim, A.N. Agrawal, Corros. Sci. 42 (2000) 1553. M. Pourbaix, N. Zoubov, J.V. Muylder, Atlas D’ Équilibres Électrochimiques, Gauthier-Villars&Cie , Paris, 1963, p. 453. L.H. Hihara, Metal matrix composites, in: R. Baboian (Ed.), Corrosion Tests and Standards, ASTM; MNL 20, Philadelphia, PA, 1995, p. 531. K. Kinoshita, Carbon, Electrochemical and Physicochemical Properties, Wiley, New York, 1988, p. 316, 328. L.H. Hihara, R.M. Latanision, Corrosion 47 (1991) 335. L.H. Hihara, R.M. Latanision, Int. Mater. Rev. 39 (6) (1994) 245. D.M. Aylor, P.J. Moran, J. Electrochem. Soc. 132 (1985) 1277. S. Payan, Y.L. Petitcorps, J.M. Olive, H. Saadaoui, Composites A 32 (2001) 585. A. Bakkar, V. Neubert, Corros. Sci. 49 (2007) 1110. R. Qvarfort, Corros. Sci. 28 (1988) 135. V. Neubert, A. Bakkar, Effect of small Ca-additions on the corrosion resistance of AS41-Mg alloy, in: K.U. Kainer (Ed.), Proceedings of the 6th International Conference on Magnesium Alloys and Their Applications, Wiley-VCH Verlag, Weinheim, 2004, p. 638. V. Neubert, I. Stulikova, B. Smola, A. Bakkar, B.L. Mordike, Kovove MaterialyMetal. Mater. 42 (2004) 31. A. Bakkar, V. Neubert, Corros. Sci. 47 (2005) 1211. H. Alves, U. Koester, Improved corrosion and oxidation resistance of AM and AZ Mg alloys by Ca and RE additions, in: K.U. Kainer (Ed.), Proceedings of the Conference on Magnesium Alloys and Their Applications, DGM, Munich, Germany, 2000, p. 339. B.L. Mordike, P. Lukac, Surf. Interface Anal. 31 (2001) 682. M.M. Aveddesian, H. Baker (Eds.), Magnesium and Magnesium Alloys, ASM Specially Handbook, ASM International, Materials Park, OH, USA, 1999, p. 194. E. Ghali, Mater. Sci. Forum 350–351 (2000) 261. E. Ghali, Magnesium and magnesium alloys, in: R.W. Revie (Ed.), Uhlig’s Corrosion Handbook, 2nd edition, Wiley, New York, 2000, p. 793. S. Coleman, The corrosion of metal matrix composites, Ph.D. Thesis, School of Materials Science, University of Bath, 1991.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Corrosion behaviour of carbon fibres/magnesium metal matrix composite and electrochemical response of its constituents A. Bakkar a,c,∗ , V. Neubert a,b,∗∗ a
Institut für Materialprüfung & Werkstofftechnik Dr. Neubert GmbH, Freiberger Strasse 1, D-38678 Clausthal-Zellerfeld, Germany Zentrum für Funktionswerkstoffe gGmbH, Sachsenweg 8, D-38678 Clausthal-Zellerfeld, Germany c Metallurgical and Materials Engineering Department, Faculty of Petroleum & Mining Engineering, Suez Canal University, P.O. Box 43721, Suez, Egypt b
a r t i c l e
i n f o
Article history: Received 13 August 2008 Received in revised form 17 September 2008 Accepted 19 September 2008 Available online 14 October 2008 Keywords: Mg MMC C-fibres Corrosion Microstructure
a b s t r a c t The corrosion behaviour of carbon fibres reinforced magnesium metal matrix composite (MMC) was investigated with emphasis on the galvanic corrosion arisen between the magnesium matrix alloy and the carbon fibres. The susceptibility of carbon/magnesium interface to degradation was also ascertained. The corrosion behaviour was studied in both neutral and alkaline aqueous solutions containing different concentrations of NaCl, using electrochemical techniques, hydrogen evolution test, optical microscopy and scanning electron microscopy (SEM) coupled with EDX and WDX capabilities. Two matrix alloys were used, AS41 and AS41(0.5% Ca). The latter monolithic alloy had a significantly higher corrosion resistance marked by Ca-addition, but the presence of carbon fibres in MMCs invalidated the beneficial effect of Ca. Carbon, as an electrically conductive element, is electrochemically unstable despite its almost chemical stability in aqueous chloride environments. Therefore, the polarisation diagrams of MMCs were demonstrated in terms of the electrochemical activity of carbon fibres. This necessitated performing the electrochemical polarisation tests, individually, on both Mg MMC, monolithic magnesium matrix alloy and carbon fibres electrode. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium metal matrix composites (Mg MMCs) reinforced with carbon or graphite are one of the promising candidate materials for applications in which as high strength/weight ratio is necessary. They possess the lowest density of all engineering metallic materials and exhibit good mechanical properties and a very low coefficient of thermal expansion [1]. These unique specific properties actualised the development of C/Mg MMCs to be a precious alternative of aluminium for applications in the fields of aerospace and automotive industries. Fritze et al. [2], for instance, reported that a 30% reduction in the piston weight was achieved by its fabrication from C/Mg MMC, instead of aluminium.
∗ Corresponding author at: Institut für Materialprüfung & Werkstofftechnik (Dr Neubert GmbH), Freiberger Strasse 1, D-38678 Clausthal-Zellerfeld, Germany. Tel.: +49 5323 989 890; fax: +49 5323 989 899. ∗∗ Corresponding author at: Zentrum fuer Funktionswerkstoffe GmbH, Sachsenweg 8, D-38678 Clausthal-Zellerfeld, Germany. Tel.: +49 5323 989 890; fax: +49 5323 989 899. E-mail addresses: ashraf [email protected] (A. Bakkar), [email protected] (V. Neubert). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.09.064
However, magnesium monolithic matrix alloys suffer from a strong tendency to corrode, especially if it contained noble impurity elements (e.g. Fe, Ni and Cu) which serve as cathodic sites and accelerate the corrosion rate [3,4]. Thus, the corrosion of magnesium is highly dependent on the presence of cathodic constituent phases and thereby galvanic corrosion arose between the Mg-matrix alloy and the noble reinforcement constituent. Trzaskoma [5] reported severe degradation of the graphite fibres/magnesium composites in aqueous chloride environments. The author confirmed the important role of galvanic interaction in the aqueous corrosion rather than the rapid reaction of magnesium with the solution. Hihara and Kondepudi [6,7] investigated the galvanic corrosion behaviour of Mg MMCs based on two matrices, pure Mg and ZE41A-Mg alloy. Two types of reinforcements were used individually; SiC monofilament (MF) made of a carbon core coated by a carbon-rich SiC layer and pure SiC particles. The results showed evidence of higher galvanic corrosion rates in the presence of SiC (MF), whose electrochemical behaviour was similar to that of graphite, compared with pure SiC particles. Also, ZE41A-Mg matrix showed a better corrosion resistance than pure Mg. Hall [8] observed the evidence of galvanic corrosion for carbon fibre/magnesium MMCs in a normal laboratory atmosphere with 60% relative humidity at 20 ◦ C.
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The determined rate of penetration was about 100 m/year. The magnesium matrix contained 1 wt.% Al and Al carbides were formed at the fibre/matrix interface during squeeze casting but they had no significant effect on the atmospheric corrosion behaviour. Galvanic corrosion, in general, of MMCs based on matrices such as Al, Ti and Cu reinforced with carbon or graphite was cited as a major problem [9–16]. This is due to the high electrical conductivity of carbon. According to Pourbaix potential–pH diagram of carbon in water [17] carbon has a narrow window of stability. At highly electronegative potentials carbon is thermodynamically feasible to be reduced to methane. However, Hihara [18] reported that neither methane nor visual fibre degradation was detected when pitch-based graphite fibres (Thornel P-100) were cathodically polarised. At noble potentials carbon oxidises in aqueous solutions forming CO2 , CO and additionally some dissolved carbon species (e.g. H2 CO3 , HCO3 − and CO3 2− ). However, CO2 is believed to be the major anodic reaction [19]. Hihara and Latanision [20] reported that during anodic polarisation of Gr/Al MMC in Na2 SO4 , P-100 graphite fibres were oxidised to CO2 with crevice formation along the perimeters of fibres. It must be noted that carbon has a high chemical stability when exposed to aqueous electrolytes without application of any potential. Hihara and Latanision [21] cited that in the open circuit condition or when coupled to aluminium, graphite fibres are very inert and visual degradation is not observed. Degradation of the fibre/matrix interface compound is another possibility for the inferiority of corrosion resistance dictated by MMCs [8,12,18,21–23]. Aylor and Moran [22] found that the graphite fibres enhance the breakdown susceptibility of the passive film formed on Al in Gr/Al MMCs. They assumed that of the Al4 C3 compound formed during the fabrication process, hydrolysis in marine media and leads to a lowering in the integrity of the passive film. Based on the result that the composite does not exhibit an electropositive change in corrosion potential the authors neglected the effect of galvanic coupling between the Al and graphite. The localised corrosion at the C-fibre/Al interface was confirmed and in situ analysed by Payan et al. [23]. The present paper was focused on studying the electrochemical characterisation of C-fibres/Mg MMC with ascertaining the polarisation behaviour of the composite constituent materials, in addition to measuring the galvanic current which arises between the Mg alloys and carbon electrode. Experiments were performed in neutral and alkaline chloride-containing electrolytes. These media were selected to determine the effect of carbon incorporation on the active and passive behaviour of Mg MMC in which Mg shows passivity in alkaline environments. Hydrogen evolution measurement, as a free corrosion test, was also monitored for prolonged times. The corrosion behaviour of the 0.5% Ca-contained AS41 Mg alloy was presented, compared with that of the pure AS41 alloy, to assess whether the small Ca-addition still has the beneficial function of enhancing the corrosion resistance of the MMC as well. Microstructure analysis was undertaken using optical microscopy and scanning electron microscopy (SEM) equipped with EDX and WDX capabilities, to investigate the as-received and corroded surfaces.
2. Materials and methods 2.1. Materials 2.1.1. Mg alloys Two magnesium alloys were used in this study; one is the nominal AS41 Mg alloy, free of calcium and the other contained about 0.5 wt.% calcium to be labelled as AS41(0.5Ca) Mg alloy. The typical composition for the both alloys is shown in Table 1. 2.1.2. Mg MMCs The Mg alloys were reinforced with carbon short fibres (Sigrafil C-40) using the squeeze casting technique, in which the Mg alloy was forced into the carbon short fibres preforms. The equipment used was described in Ref. [24]. The carbon fibres (about 25 vol.%) were distributed quasi-isotropically in the horizontal plane. 2.1.3. C-fibres electrode Sigrafil C-40 continuous fibres, as a unidirectionally aligned tow, were wrapped and soldered by a copper sheet from one side. This soldered side was connected with a copper wire. Then, the fibres were infiltered with an epoxy resin and suspended into the electrolyte exposing a free test surface of fibres cross-sections with a total area of 0.264 mm2 . 2.2. Samples The Mg materials were provided as discs with a diameter of about 200 mm and a height of about 40 mm. Contained in these discs were the C-fibres/Mg composite blocks with dimensions of 120 mm × 120 mm × 35 mm, see Ref. [24]. Specimens were cut into small blocks 15 mm × 15 mm × 7 mm. Each specimen was successively wet ground on all faces. Those for electrochemical tests at pH 12 were welded to copper wires on one side and mounted in epoxy to be suspended in the electrolyte, exposing a free test surface of 15 mm × 15 mm. In all electrochemical tests and hydrogen evolution measurements, a fresh surface was obtained by grinding using 1200 grit SiC paper and ethanol as a lubricant and cooler. Finally, the specimens were ultrasonically cleaned with several rinses of ethanol and immediately conducted on examination. Three specimens were produced for each experiment at 25 ◦ C ± 1. 2.3. Corrosive solutions Aqueous chloride-containing solutions were prepared by dissolving analytical grade of NaCl in distilled water. Alkaline solution of pH 12 was adopted by the addition of NaOH. The solution used for hydrogen evolution measurements was buffered at pH 9.3 by addition of borax and NaOH. 2.4. Electrochemical measurements 2.4.1. Potentiodynamic polarisation Polarisation experiments were carried out using the model “Wenking LB 94L (Auto Range) Laboratory Potentiostat” and controlled by PC computer. The specimen was exposed to the NaCl solution for 5 min prior to potentiodynamic polarisation by which
Table 1 Typical chemical composition (wt.%) of both magnesium matrix alloys. Mg
Zr
Sn
Pb
Cu
Ni
Fe
Zn
Ca
Mn
Si
Al
Alloy
Bal. Bal.
<0.010 <0.010
0.0049 0.0049
0.0027 0.0027
<0.0002 <0.0002
<0.0005 <0.0005
0.0028 0.0022
0.0816 0.0852
<0.001 0.38
0.35 0.324
0.93 0.94
4.374 4.580
AS41 AS41(0.5Ca)
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time a stable potential, open circuit potential (OCP), was monitored. With reference to saturated calomel electrode (SCE), the polarisation was obtained by scanning from 500 mV more negative than the OCP at a rate of 20 mV/min. Experiments conducted in 100 ppm NaCl solution of pH 12 was carried out in a normal three-electrode cell, in which the specimen is suspended as a working electrode. On the other hand, experiments in neutral NaCl-containing solutions (pH of about 7) were performed in an Avesta cell [25]. 2.4.2. Galvanic couples In the galvanic current measurements a separate Mg alloy and a cylindrical carbon electrode with equal surface areas were coupled through a potentiostat, and by using it as a zero resistance ammeter (ZRA), the galvanic current flowing through 500 ml of electrolyte was measured. This current was monitored continuously by an X/Yrecorder. The distance between the electrodes was maintained to be 35 mm. 2.5. Hydrogen evolution test The Mg specimen was put on a glass seat in a beaker containing 1200 ml of 3% NaCl borax-buffered solution at pH 9.3. To collect the hydrogen evolved, a burette with a funnel end was placed over the specimen, which ensured the collection of all hydrogen from the specimen surface. 2.6. Corrosion morphology Corrosion morphology was characterised using optical microscopy and scanning electron microscopy. A “SX 100 Electron Probe Microanalyser” SEM equipped with EDX and WDX capabilities was used to investigate the as-received and corroded surfaces. 3. Results and discussion 3.1. Electrochemical polarisation data in alkaline media 3.1.1. Polarisation of the monolithic alloys Following the authors’ previous work [24,26–28] on the corrosion of Mg-based alloys and composites in chloride-containing alkaline solutions the break-through-potential (Ep ) is the main determining parameter for corrosion characterisation. Fig. 1 shows
Fig. 1. Potentiodynamic polarisation diagrams for monolithic AS41 and AS41(0.5% Ca) Mg alloys in 100 ppm NaCl solution; pH 12.
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that the Ca-contained alloy has a greater corrosion resistance; it had at a higher Ep value. In addition, the other corrosion tests support this result, as will be seen below in the galvanic coupling measurements and H-evolution test. The beneficial effect of Ca microadditions (up to 0.5 wt.%) was reported due to incorporation of the relatively large Ca ions in the corrosion product film, causing it to be more stable and protective [26,29]. 3.1.2. Polarisation of C/Mg metal matrix composites The polarisation data show that the electrochemical behaviours of the two composites, both based on Ca-free and Ca-contained matrixes, are similar as shown in Fig. 2. It can be clearly seen that the presence of carbon as reinforcement fibres in the Mg-based composites shifts the corrosion potential in the noble direction by more than 1000 mV, compare Figs. 1 and 2, implying the eventual galvanic coupling between Mg- and C-fibres, by virtue of their high electrical conductivity. The anodic polarisation curve can divided into three regions. The first region (I) indicates the clear passivation after the Tafel part, whereas the second region (II) is characterised by a noticeably gradual increase in the current density with a forward potential scan. Then, it is followed by a sharp increase in the current density and region (III), which may characterise the occurrence of pitting corrosion. Fig. 3 exemplifies the cyclic polarisation of C-fibres/AS41 Mg MMC, in which the potential sweep is reversed at a current density of 0.1 mA/cm2 and still lies in region (II), Fig. 2. It is seen from the figure that the current density starts to decrease on reversing the potential scan. This implies the absence of the pitting corrosion which is characterised by a continuous increase in the current density. Another point of additional significance is the occurrence of pitting corrosion occasionally, where the pitting arises in the first cycle in Fig. 3a, but appears in the second cycle in Fig. 3b. This can be interpreted as the result of the postulated reduction to methane and oxidation to CO2 of C-fibres during cathodic and anodic polarisation, respectively [17,19,20]. These cathodic and anodic reactions lead to crevice formation along the round borders of fibres, as shown in Fig. 4. Consequently, the C/Mg interface is very prone to crevice corrosion, which leads to the sudden increase in the current density at unexpected potential values within the passive range of monolithic matrix alloy, compare Figs. 1 and 3. Fig. 5 exhibits the forward and reverse potentiodynamic polarisation of C-fibres/AS41 MMC, scanned up to 1 mA/cm2 . On reversing
Fig. 2. Potentiodynamic polarisation diagrams for both C/AS41 and C/AS41(0.5% Ca) Mg MMCs in 100 ppm NaCl solution; pH 12.
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Fig. 5. Potentiodynamic polarisation diagram, reversed at current density of 1 mA/cm2 , for C/AS41 Mg MMCs in 100 ppm NaCl solution; pH 12.
Fig. 3. Examples of cyclic polarisation diagrams for C/AS41 Mg MMCs in 100 ppm NaCl solution; pH 12.
Fig. 4. Optical microscopy micrograph of C-fibres embedded in epoxy resin after anodic polarisation up to 1400 mV (vs. SCE) in 100 ppm NaCl solution; pH 12.
the potential sweep the current density continues to increase. This emphasises the occurrence of pitting corrosion in region (III), Fig. 2. Fig. 6 shows the anodic polarisation diagrams of monolithic Mg-matrix alloy, C-fibres and C/Mg MMC and a mixed-electrode diagram generated from both anodic polarisation diagrams of C-fibres and monolithic Mg-matrix alloy on the basis of the mixed-potential theory. The current density calculated for each potential of the mixed-electrode diagram = 0.75 of the current density recorded for the monolithic matrix alloy + 0.25 of the current density recorded for the C-fibres. The figure illustrates many valuable results. Firstly, the gradual increase in the current density during region II, Fig. 2, of the C/Mg MMC is due to the sharp increase in the current density for the C-fibres electrode. Secondly, the independence of the pitting potential for the MMC (Ep MMC ) on that for the matrix alloy. Thirdly, the discrepancy between the real composite diagram and the generated mixed-electrode diagram after the pitting potential for the C-fibre (Ep Fibre ), indicating that the mixedpotential theory cannot apply after (Ep Fibre ). Consequently, it can
Fig. 6. Anodic polarisation diagrams for monolithic AS41(0.5Ca) Mg-matrix alloy, C-fibre (SG 40) and C/AS41(0.5Ca) Mg MMC in 100 ppm NaCl solution; pH 12. In addition to a generated diagram on the basis of the mixed-potential theory for a composite containing 25 vol.% C-fibres in AS41(0.5Ca) Mg-matrix alloy.
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Fig. 7. Microstructure of C/AS41 Mg substrate, as-received: (a) optical micrograph for the horizontal plane, (b) SEM micrograph for the normal plane, (c) qualitative WDXelemental scan along the “line A” shown in micrograph (b), and (d) semilogarithmic qualitative WDX-elemental scan along the “line B” shown in micrograph (b).
be stated that the polarisation behaviour of C/Mg MMC is drawn up by the polarisation of C-fibres rather than that of matrix alloy, characterising an invariable pitting potential independent of that of the matrix alloy. Thus, the evidence of pitting in C/Mg MMC is induced by the C-fibres.
900 mV and at that potential followed by potentiostatic polarisation for 30 min, is revealed in Fig. 8. A large and deep attack is clearly seen in the area where C-fibres are agglomerated. This can be attributed to the crevice attack of C-fibres perimeters resulted
3.2. Corrosion morphology 3.2.1. Microstructure of original samples The microstructure of as-fabricated C/AS41 Mg MMC sample is depicted in Fig. 7. The illustrated phases in Fig. 7b were identified by EDX analysis. It is obvious that the present phases have a preferably inclination to precipitate close to the C-fibres. The line scan across the C-fibre/matrix interface (line A) reveals its richness in Al. Furthermore, Al is identified at the Mg2 Si/C-fibre interface (line B). It is evident that an Al–C compound is formed adjacent to the C-fibres as a result of the interfacial reaction during fabrication. This compound has been determined to be Al4 C3 or Al2 MgC2 and reported to have a thickness of few nanometers [8,30]. It is too thin to be revealed by the SEM/EDX analysis because of the coarse interaction volume diameter produced by the electron beam. 3.2.2. Microstructure of samples after polarisation measurements The microstructure of C/AS41 Mg MMC specimen, potentiodynamically polarised in 100 ppm NaCl alkaline solution up to
Fig. 8. The SEM micrograph of C/AS41 Mg MMC surface after 30 min polarisation at 900 mV in 100 ppm NaCl solution; pH 12.
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Fig. 9. (a) SEM micrograph of composite surface polarised for 3 h at 900 mV in chloride-free solution; pH 12, showing that the fibres have deeper levels compared with the matrix alloy. (b) EDX analysis of the matrix alloy pointed by “X1” in micrograph (a). (c) EDX analysis of corrosion precipitates (white) exemplified by “X2” in micrograph (a).
from their oxidation by anodic polarisation, as described above in Section 3.1.2. Fig. 9 shows the topography of C/AS41 Mg MMC which potentiostatically polarised at 900 mV for 3 h after potentiodynamic polarisation in chloride-free pH 12 solution. The C-fibres have deeper levels compared with the matrix alloy, Fig. 9a. It is believed that the C-fibres were oxidised to CO2 during anodic polarisation. It is also observed that some corrosion product precipitates (white specks), which is thought to deposit back from the electrolyte. EDX analysis of those precipitates indicates its richness in C and Al elements, compare Fig. 9a and b. 3.2.3. Microstructure of samples after free immersion The microstructure of C/AS41 Mg MMC specimen after 2 h free immersion, in absence of applied polarisation, in 100 ppm NaCl alkaline solution is shown in Fig. 10. Precursors of pitting sites are clearly seen to arise preferentially in the vicinity of, but not close to, the C-fibres. Thus, the pitting sites have no significant tendency to initiate at the C/Mg interface. This is in agreement with the results reported by Hall [8], that no visible degradation on Al4 C3 , formed at the fibre/matrix interface, was detected in the laboratory air. Fig. 10 indicates also that the corrosion propagates from the pitting sites depicting the filiform corrosion type, which has been typically reported for Mg–Al alloys [24,32,33].
3.3. Electrochemical polarisation data in neutral media Fig. 11 exemplifies the polarisation diagrams of Mg alloys and their composites in neutral aqueous solutions containing various
Fig. 10. SEM micrograph of corroded surface after 2 h free immersion, at open circuit conditions, in 100 ppm NaCl solution; pH 12.
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Fig. 11. Potentiodynamic polarisation diagrams of alloy and composite specimens in neutral 0.03, 0.3 and 1.0 wt.% NaCl solutions.
NaCl concentrations. It can be seen that the polarisation diagrams, in 0.03 wt.% (300 ppm) NaCl solution, for both Mg alloys show a pseudo-passive behaviour during the anodic polarisation up to about “−1200 mV” and here a relatively sharp increase in the current density occurs. The effect of C-fibres in abrogating this pseudo-passive behaviour is evident. On increasing the NaCl concentration, the monolithic alloy specimens exhibit no longer pseudo-passivation. However, the polarisation curves of composite specimens depict higher corrosion current densities compared with that of the monolithic alloys. Fig. 12 summarises the corrosion characteristics drawn out from the polarisation curves. At all NaCl concentrations, the composites dictate higher corrosion current densities (Icorr ) than the monolithic
alloys, Fig. 12a. This rise of Icorr values for the composite specimens increases also with the NaCl concentration, where the ratio – (Icorr )composites /(Icorr )alloys – increases from about 2.5 and 4 to about 20, at NaCl concentrations of 0.03, 0.3 and 1.0 wt.%, respectively. At 0.03 wt.% NaCl solution, the Ca-contained alloy shows lower Icorr values than the Ca-free one but this reduction in the corrosion current disappears with a further increase in the NaCl concentration. At each NaCl concentration, the Icorr values for both Ca-contained and Ca-free Mg alloy-based composites are almost the same. Regarding the variation of corrosion potential (Ecorr ) with NaCl concentration, Fig. 12b, it can be seen that at low chloride concentrations (0.03 wt.% NaCl), the Mg MMCs show more noble Ecorr values than their matrix alloys, but at higher concentrations the
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Fig. 12. Variation of corrosion current density (a) and corrosion potential (b) of alloy and composite specimens with NaCl concentration in neutral solution.
Ecorr values of MMCs change sharply towards more active values to be approximately the same as that of the monolithic matrix alloys. At low chloride concentration, the Ecorr is characteristic of the electrode surface; the Ca-contained alloy has a more noble Ecorr value than the Ca-free one, because of the beneficial effect of Ca on stabilising the partially protective film formed on Mg [26]. As to the MMCs, at low Cl concentrations the carbon fibres polarise the Mg matrix into the electropositive direction thereby recording more noble values. However, at higher Cl concentrations, the obtained Ecorr values were close to that of the Mg-matrix alloy. This result can be explained in terms of the corrosion mechanism of Mg, which involves the formation of partially protective Mg(OH)2 film in aqueous and humid atmospheres. This film is liable to reduce the electronegativity of Mg, by about 1 V, from the theoretical standard potential [31]. It is believed that at higher concentrations the chloride ions, adsorbed on the specimen surface, cause breakdown of the partially protective surface film and lead to formation of more active sites. 3.4. Galvanic current measurements With the purpose of obtaining additional evidence about the effect of C-fibres on the electrochemical behaviour of Mg-based materials, the galvanic current arisen between each of the Mgmatrix alloys and a carbon electrode was measured. Two sets of
Fig. 13. Typical galvanic current densities (a) and mixed potentials (b) of Mg-matrix alloys, coupled to a carbon electrode in 100 ppm NaCl solution, pH 12. Connection between the Mg alloy and graphite was carried out after 15 min free immersion time.
experiments were applied; in one set the connection between the Mg alloy and carbon electrode was applied after 15 min of free immersion and in the other the connection was carried out immediately as of immersion. Mg is stable in alkaline solutions and has a passive protective film of Mg(OH)2 [31,33]. This film is expected to be enriched with immersion time under open circuit conditions. Thereby, it resists pitting for long or short time even after connection with a carbon electrode. However, as a result of the connection with carbon, Mg electrode exerts an anodic reaction which comprises dissolution of Mg2+ ions and provision of free electrons to the carbon electrode to preserve the electrical balance [24,32,33]. Taking into account that the Mg(OH)2 film breaks down in the presence of chloride ions, the permanent anodic reaction leads to the breakdown of hydroxide film and formation of active sites resulting pitting corrosion, and hence the galvanic current increases sharply as shown in Fig. 13a. The effect of small Ca-addition is evident on the stabilisation of the hydroxide film formed. The Ca-contained alloy sustained monitoring low galvanic corrosion current (∼1 A/cm2 ) without pitting for longer times. Additional valuable results are provided by recording the mixed potential against time, Fig. 13b. The variation in the potential is in the same accordance with that of the current density. Higher galvanic corrosion current densities were obtained with immediate connection on exposure of the specimen surface to the solution, Fig. 14. This can be explained that the kinetics of the anodic
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Fig. 14. Galvanic current densities between Mg alloys and carbon electrode in 100 ppm NaCl solution, pH 12. Connection between the Mg alloy and carbon was carried out immediately on immersion.
dissolution of Mg2+ ions, polarised by the carbon electrode, are faster than that of the formation of protective Mg(OH)2 film. The galvanic corrosion current densities in neutral Cl-containing solution were also measured, Fig. 15. It is clear that the Cacontained alloy maintained lower galvanic current densities than the Ca-free one, irrespective of whether the connection was immediately or after 15 min of immersion. However, each of the alloys exhibits higher current densities in immediate connection conditions compared with that resulted from connection after free immersion. The beneficial effect of Ca in reducing the galvanic current is attributed to the incorporation of Ca atoms in the corrosion product film, causing it to be more stable and protective. A more detailed interpretation of the effect of Ca-addition was reported in another study by the authors [26]. In a comparison observation between the galvanic current densities arisen in immediate and in delayed connection conditions for both neutral and alkaline solutions, it can be stated the following: (1) in the neutral solution, the free immersion for 15 min enables the formation of a corrosion products film, and further corrosion is determined by the successive diffusion of Mg ions through that film, thus it will be a diffusion-depending process; (2) in alkaline solu-
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Fig. 16. Hydrogen evolution of Mg alloy and composite specimens for 50 h immersion in buffered 3% NaCl solution; pH 9.3.
tion, the free immersion leads to formation of the passive protective film of Mg(OH)2 , and further arising corrosion follows the pitting mechanism; and (3) the beneficial effect of Ca in the neutral solution hinders the advanced corrosion through the corrosion product film. In the alkaline solution the Ca affects to stabilise the hydroxide passive film and to delay its breakdown. However, as soon the passive film breaks down the Ca-effect seems to be neglected. 3.5. Hydrogen evolution test data The hydrogen evolution test was applied for prolonged times to compare between Mg alloys and their composites under free immersion conditions. The fact, that the gravimetric tests on the composite materials are limited because the fibre-reinforced MMCs develop deep pits which entrap corrosion products and corrosion solution [34], gives an additional importance to the H2 -evolution test as an effective comparative method. Fig. 16 shows the monitored H2 evolution for investigated Mg materials after 50 h. It is clear that the composite specimens record higher H2 -evolution rates than their monolithic matrix alloys. Additionally, the corrosion rate of the Ca-contained Mg alloy is significantly lower than that of the Ca-free one. However, this corrosion inhibiting effect by Ca-addition is rendered invalid with the presence of C-fibres in MMCs, where the fibre-reinforced composite based on Ca-contained Mg alloy has H2 -evolution values close to that based on Ca-free Mg alloy. In other words, the two composites monitor equal corrosion rates to be about 1.5 times of the Ca-free monolithic alloy and three times of the Ca-contained one. The galvanic effect by C-fibres can be represented as in the conditions of galvanic coupling with immediate connection, Fig. 14. Rapid dissolution of Mg2+ ions, induced by the galvanic coupling with C-fibres, does not allow the formation of protective Mg(OH)2 film. Hence, the effect of Ca is invalidated, where its influence is stabilising of the Mg(OH)2 film. 4. Conclusions The following conclusions have emerged from a detailed experimental study on the corrosion behaviour of C/Mg MMC:
Fig. 15. Galvanic current densities between Mg alloys and carbon electrode in neutral 1000 ppm NaCl solution.
(1) In alkaline solutions, where Mg is passive, the corrosion potential (Ecorr ) of C/Mg MMCs is more noble than their monolithic matrix alloys, implying the eventual galvanic effect of C-fibres by virtue of their high electrical conductivity.
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(2) Applying electrochemical polarisation on C/Mg MMCs creates crevice attack at perimeters of C-fibres and hence leads to crevice corrosion at C/Mg interface. As a result, the MMC shows high corrosion rates, regardless the corrosion resistance of the Mg-matrix alloy. (3) In free immersion conditions, pitting corrosion is the dominating type. The pitting sites have no significant tendency to initiate at the C/Mg interface (4) The galvanic coupling with C-fibres leads to severe corrosion of Mg-matrix composites. (5) The C-fibres invalidate the virtual effect of alloying elements on the corrosion resistance. (6) The C/Mg interface compounds seem to have no significant influence on the corrosion behaviour of MMC. (7) The H2 -evolution test is an appropriate comparative method for corrosion rates of Mg alloys and composites. The two C/Mg MMCs studied monitor similar corrosion rates to be about 1.5 times of the lower corrosion resistance Mg-matrix alloy and three times of the higher corrosion resistance one. References [1] W.A. Ferrando, J. Mater. Eng. 11 (1998) 299. [2] V. Fritze, H. Berek, K.U. Kainer, S. Mielke, B. Wielage, Fibre reinforced magnesium for automotive applications, in: B.L. Mordike, K.U. Kainer (Eds.), Conference Proceedings: Magnesium Alloys and their Applications, Wolfsburg, Germany, 1998, p. 635. [3] J.D. Hanawalt, C.E. Nelson, J.A. Peloubet, Trans. AIME 147 (1942) 273. [4] E. Ghali, W. Dietzel, K.U. Kainer, J. Mater. Eng. Perform. 13 (2004) 7. [5] P.P. Trzaskoma, Corrosion 42 (1986) 609. [6] L.H. Hihara, P.K. Kondepudi, Corros. Sci. 34 (1993) 1761. [7] L.H. Hihara, P.K. Kondepudi, Corros. Sci. 36 (1994) 1585. [8] I.W. Hall, Scripta Metall. 21 (1987) 1717. [9] M. Saxena, O.P. Modi, A.H. Yegneswaran, P.K. Rohatgi, Corros. Sci. 27 (1987) 249.
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