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An investigation on the Corrosion Behavior of Al (Mg-TiFe-SiC) Matrix Composite in Acidic and Chloride Media
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ScienceDirect Materials Today: T Proceedinngs 18 (2019) 382 27–3834
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IICMPC-201 19
An A invesstigation on the Corrosio C on Behav vior of Al A (Mg-T TiFe-SiC C) Matrixx C Composi ite in Accidic and d Chlorid de Mediaa Sam muel Olukaayode Akin nwamidea*, Serge Mu udinga Lem mikaa, Feyik ikayo Adam msc, a b O Ojo Jeremiaah Akinrib bide , Bolannle Tolulop pe Abe , Peter Apataa Olubambiia a
Centre for Trribocorrosion and d Nanoengineerinng, University of Johannesburg, J Jo ohannesburg 20006 South Africa b Tshwa ane University off Technology, Preetoria 0001, South h Africa c American Univversity of Nigeria a, Yola, Nigeria
Absttract This study was connducted to invvestigate the co orrosion of aluuminium matrix x composites reinforced withh ferrotitanium and silicon carbiide. As-cast purre aluminium and a aluminium matrix m composiites were produ uced by stir casting. A fixed voolume of ferrottitanium and varyiing proportionss (0.5, 2, 5, and 7 wt. %) of siliicon carbide weere used for thee fabrication of the composites . Chronopotenttiometry and poten ntiodynamic poolarization meassurements weree used to study tthe corrosion behaviour of the fabricated com mposites. Pits with w irregular shapees and sizes weere observed onn the corroded substrates. s Therre were similarrities between th he polarization curves of the cast c samples in su ulphuric acid (H H2SO4) and soodium chloride (NaCl) solutioons. Compositees with silicon n carbide reinfoorcements exhiibited better corro osion resistancee with lower corrrosion potentiaal values.
© 2019 Elsevier Ltd. A All rights reserveed. Selecction and peer-revview under responnsibility of the 9th h International C Conference of Matterials Processing g and Characterizzation, ICMPC-20 019 Keyw words: Aluminium m matrix composiite; Ferrotitanium m; Silicon carbide ; Potentiodynamiic polarization, Chronopotentiomeetry.
1. In ntroduction Corrrosion which is a form off degradation has created m many problem ms in various fields f where m metallic strucctures are utiliised. Alum minium usagee has become extensive in applications a w where propertties such as lig ght weight, duurability and a high strengtth to weig ght ratio are eessential [1, 2]. Improvemeent in propertiies of aluminiium paved thee way for devvelopment of aluminium a matrix com mposites (AMC C), with goodd chemical, physical p and mechanical properties p [3, 4]. AMC caan be fabricaated using sim mple meth hods (such as casting) and at a low cost in comparisoon with fabrication of otherr metallic matterials (titaniu um, copper) which w are more m complexx and expensivve. Studies haave demonstraated the suscep ptibility of AM MC to corrosiion which can n be a direct reesult from m any of the foollowing. (a) interfacial i phaase formation between matrrix and reinfo orcements (b) galvanic corro osion occurrinng at the interface i of thhe matrix and reinforcementt (c) alteratioon of microstru ucture resultin ng from contaamination duriing the processsing of AMC A [5-7]. Adddition of reinnforcement sho ould howeverr influence corrrosion behaviiour of aluminnium matrix composites.
*Corrresponding authoor. Tel,: +277851990655 E-ma ail address: akinw [email protected] 2214--7853 © 2019 Elssevier Ltd. All rigghts reserved. Selecction and peer-revview under responnsibility of the 9th h International C Conference of Matterials Processing g and Characterizzation, ICMPC-20 019
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Investigations by various researchers assumed the corrosion behaviour of AMC to be unpredictable, as various differences were observed in results obtained for different AMC systems [8-10]. There was an increase or decrease in current densities of aluminium matrix composites reinforced with different types of nanoceramics [11]. Addition of ferrotitanium to reinforcing materials in the fabrication of AMC can enhance its corrosion and wear resistance in several engineering application [12]. Corrosion behaviour of aluminium reinforced with varying proportions of silicon carbide was studied by Zakaria[13], observations showed improvement in the corrosion resistance of aluminium reinforced with silicon carbide in contrast to the pure aluminium matrix. Further observation revealed an increment in corrosion resistance of reinforced composites with increase in volume fraction of silicon carbide. Kenneth et al[14] investigated the corrosion behaviour of thermal cycled aluminium composite with rice husk and silicon carbide as reinforcements. This gave an improved corrosion of reinforced composite in chloride media. Excellent corrosion resistance of aluminium 7075 reinforced with silicon carbide and titanium carbide was also observed by Sambathkumar[15]. Several studies have shown improvement in corrosion resistance of aluminium matrix composites, which confirms the fact that aluminium reinforced with silicon carbide can be a suitable material for engineering applications where excellent corrosion resistant is required [16, 17]. Nevertheless, little or no literature is available on reinforcement of aluminium with silicon carbide and ferrotitanium hence, this work purpose of this present study. Experimental procedure 2.1 Materials An Ingot of aluminium 5083 series (99.9% purity) was used as the matrix, this was obtained from Insibi company in South Africa. Composition of the aluminium matrix is shown in Table 1. Ferrotitanium (Ti-Fe) and silicon carbide (SiC) were used as major reinforcements. 2.2 Methodology An aluminium ingot was placed in graphite crucible and heated until molten in an electric furnace. The temperature of the melt was maintained between 650-670℃. Magnesium weighed at 30 g was added to the melt of aluminium to ensure excellent wettability between the matrix and the reinforcement. Ferrotitanium powder also weighed at 30 g mass was preheated with different masses of silicon carbide in a muffle furnace at temperatures ranging between 850-890℃ prior addition to the melt of aluminium and magnesium. Silicon carbide was added as reinforcement in varying amounts of 0.5%, 2%, 5% and 7 wt.%. Prior to the addition of silicon carbide, thorough mixing was done for a period of 15 mins to ensure homogeneous dispersion of the reinforcement in the aluminium matrix. Casting of silicon carbide reinforced composite was carried out in a die preheated to a temperature of 700 0C for 2 h to ensure fluidity of the melt. For comparison, reference alloys of pure aluminium and aluminium with magnesium and ferrotitanium were cast using the same method but no silicon carbide was added. Samples for microstructural analysis were cross-sectioned from as-cast composites, then cold mounted in epoxy resin. The grinding of sample surfaces was carried out using 320 – 1200 grits of silicon emery paper and polished to a 1 µm using diamond suspension. Samples were washed with acetone and dried in air. Corrosion testing of the aluminium composites was done with chronopotentiometry and potentiodynamic polarisation techniques in corrosive media of 0.05 M NaCl and 0.3 M H2SO4 solutions with a Versa studio potentiostat equipped with three electrode corrosion cells: working, saturated silver/silver chloride (Ag/AgCl) reference and a graphite counter electrode. Working electrodes were prepared by attaching an insulating wire to the other side of a polished sample with an aluminium tape, before cold mounting in a resin. Prior to mounting, samples used as the working electrode were polished and ground until a mirror-like surface was achieved using an automatic polisher with different sizes of diamond paste suspensions. Open circuit potential (OCP) was measured for 2 h, while potentiodynamic polarisation tests were carried out at a scan rate of 1 mV/s starting from 0.25 - 1.5 V. The electrolyte was replaced after each scan. Results and Discussion 3.1 Microstructural analysis Microstructures of as-cast pure aluminium and composites are shown in Figures 1. Formation of pores and acicular structures are visible in Figure 1a. Dendritic structures which are snowflake like crystals were formed from solid aluminium during liquid to solid transformation[18]. Formed dendrites were observed to disappear upon the addition of silicon carbide and ferrotitanium reinforcements. Even distribution of reinforcement particles in the matrix of aluminium as observed in Figures 1b. and Figure 1c. can improve the mechanical properties of composites[19].
S.O.Akinwamid de et al./ Materialls Today: Proceeedings 18 (2019) 3827–3834 3
(a)
38299
(b) Ti-F Fe Pores
50 µm
50 µm m
(c)
SiC
50 0 µm Fig 1. Opptical micrograph hs of (a) Pure alum minium (b) Al +M Mg + TiFe (c) Al + Mg+ TiFe+ 7 w wt% SiC.
measurementss 2 and 3 show ws the open ciircuit potentiaal (OCP) in 0.05 M NaCl and a 0.3 M H22SO4 media reespectively. From F P values for puure aluminium m were observved to fluctuatte within the first f 4000s, beefore stabilisin ng at a potential of lar trend was observed in the ferrotitannium reinforcced compositee. OCP of coomposites witth silicon carrbide ere observed to stabilize att lower potenttials in compaarison to the stabilizing s pot otential of pure aluminium. The OCP values during d first 40 000s indicatess the formation n and dissoluttion of passivve layers on th he cast samplees in . This could be b as a result of o pores and ccracks which serve as path hs through whhich the solutio on of NaCl ennters yer. This was also a observed in a study byy Obadele et al a [20]. the sample with w highest weight w fractioon of silicon carbide attain ned the higheest potential at -0.507 V. The of silicon carrbide particless can provide resistance to corrosive attack during cheemical dissolu ution of SO42- ions d[21]. The ferrrotitanium reeinforced com mposite stabiliised at a potential of -0.553 V. This caan be ascribeed to ion by the formation f of a dense passsive layer on n the titanium m present in the ferrotitaanium at ambbient 23]. Stability of o OCP in co omposites withh 5 and 7 wt.% of silicon carbide reinfo forcements weere observed after e due to re-paassivation of composites in tthe corrosive media[24]
Fig g. 2. Open circuit potential of samp ples in 0.05 M Na aCl
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Fig. 3. Open circuitt potential of sam mples in 0.3 M H2SO S 4
3.4 Potentiodynam P mic polarization measuremeent Corrrosion potentiials and curreent density vaalues obtainedd from Tafel fits f of the pollarisation curvves are given n in Tables 2. and Tablle2. Corrosionn studies in 0..05 M NaCl and a 0.3 M H2S SO4 media sho ows the stabillisation of purre aluminium at a potentiall of 0.83 V and -0.555 V respectivvely, while alu uminium mattrix compositte (AMC) witth the highestt weight perccentage of sillicon carbide was stabiilized at a higgher potentiall of -0.68 V. From Table 2, a decrease was observeed in corrosio on potentials were w obseerved to decreease with incrreasing conten nt of silicon caarbide. This can c be ascribeed to the dom mination of thee cathodic proocess relattive to the annodic processs. Corrosion resistance off aluminium matrix m compo osites is alsoo dependent on o the changge in conccentration of cchloride. A sim milar observaation was madde with pure aluminium haaving the highhest corrosion potential. Sillicon carbide prevents ccorrosion of AMC A by inhibiiting actions oof SO42- and Cl C - ions presen nt in the corrossive media[21 1]. Figu ures 4. and Figure 5. show ws polarizatio on curves forr aluminium and a reinforced d compositess in the as-caast condition. The cathodic range w was observed to t have equal current denssities for all th he samples in n Figure 4. Fuurther observaation showed that com mposites with silicon carbide, ferrotitan nium and maggnesium havee no passive range with aan anodic diissolution in pure alum minium. From m Figure 5, thee polarization scans of all ssamples exhib bited similar anodic a and cat athodic polarissation curves with pseu udo-passive beehaviour in 0.3 0 M H2SO4 solution[20]. Improved co orrosion resistance of the aaluminium maatrix compositte in corro osive media ccan be attributeed to the even n dispersion off silicon carbiide and ferrotitanium reinfoorcements with hin the aluminnium matrrix, as these reeinforcementss inhibit sites where pittingg is likely to be b initiated[25 5] . Migrationn of oxygen an nions in sulphhuric acid solution cann increase the growth rate of aluminium m oxide films, thereby redu ucing the ratee at which hy ydrogen evoluution occu urs in this sollution. The anodic a reactio on of composiites in the solution can theerefore be redduced by the reaction betw ween silicon and oxygeen[21]. The coorrosive action of chloride ions in 0.05 M NaCl coulld be reducedd by presence of silicon carrbide and ferrotitanium m. Presence off reinforcemen nts can result in chemical dissolution d off chloride ionss. It can therrefore be assuumed that the corrosive action of chlooride ions in 0.05 0 M NaCl w was reduced by b the presence of silicon caarbide and ferrrotitanium. Table 1. Tafel polarissation results of pure aluminium annd aluminium maatrix composites in i 0.05 M NaCl
Al Al+ +Mg+TiFe Al+ +Mg+TiFe+0..5%SiC Al+ +Mg+TiFe+2% %SiC Al+ +Mg+TiFe+5% %SiC Al+ +Mg+TiFe+7% %SiC
Ecorr (V) vs (Ag/AgCl) -0.83 -0.74 -0.71 -0.74 -0.69 -0.68
Icorr (µA A) 3.07 0.21 0.39 1.25 0.85 0.59
Table 2. Tafel polarisaation results of pu ure aluminium andd aluminum matrrix composites in 0.3 M H2SO4
All Al+ +Mg+TiFe Al+ +Mg+TiFe+0..5%SiC Al+ +Mg+TiFe+2% %SiC Al+ +Mg+TiFe+5% %SiC Al+ +Mg+TiFe+7% %SiC
Ecorr (V) vs (Ag/AgCl) -0.57 -0.55 -0.53 -0.50 -0.48 -0.47
Icorr (µA A) 81.44 67.62 131.03 162.38 53.35 69.03
S.O.Akinwamid de et al./ Materialls Today: Proceeedings 18 (2019) 3827–3834 3
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Figg. 4. Potentiod dynamic polariization for sam mples in 0.05 M NaCl soluttion
Fig. 5. Poten ntiodynamic ppolarization fo or samples in 0.3 0 M H2SO4 3.5 Optical O microscopy analysiis of substrates Figure 6.. shows typicaal optical micrrographs of thhe substrates after a the corrosion tests. Corrrosion pits arre dominant inn the solid d solution of α-Al and surrroundings off silicon carbiide and ferrottitanium partiicles. Cracks observed in Figure 6a cann be attrib buted to localized corrosionn by pitting in n aggressive chhloride enviro onment[26]. Microstructura M al studies of th he sample surffaces reveealed pits of irrregular shapees and sizes were formed foor both as-castt pure aluminiium (unreinfor orced matrix) and a the reinfoorced com mposites. Corrrosion by pittiing was evideent in fabricatted aluminium m matrix comp posites when they were in the passive state. s Thiss can be as a rresult of disrupptions in the passive p film laayer formed due d to ions preesent in the agggressive enviironments[27, 28]. How wever, the size of pits in the t compositees decreased w with increasin ng addition of o silicon carbbide and a fix xed proportioon of ferro otitanium. Som me reports haave been given n on the decreease of corrossion resistancee in fabricatedd aluminium matrix m composites due to the formaation of Al4C3 phase[29, 30]. The addittion of ferrottitanium particles can decrrease the corrrosion rate off the com mposites througgh the formatiion of a third intermetallic pphase in the composites, c th hereby decreassing the rate at a which the Al A 4C3 phasse is formed.
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(b)
(a)
Pit
50 µm
50 µm (d)
(c)
Al4C3
50 µm (e)
50 µm (f)
50 µm
50 µm
Figure 6. Optical micrograph of corroded as-cast aluminium and reinforced aluminium matrix in 0.3 M Sulpuric acid for (a) Pure aluminium (b) Al+Mg+TiFe (c) Al+Mg+TiFe+0.5%SiC (d) Al+Mg+TiFe+2%SiC (e) Al+Mg+TiFe+5%SiC (f) Al+Mg+TiFe+7%SiC
3.5 Scanning Electron Microscopy analysis of corrosion substrates Figure 7 (a-c)shows the scanning electron microscopy (SEM) micrographs of the as-cast pure aluminium and composites after immersion in 0.05 M sodium chloride. Evidence of pit formation was visible across the surfaces of pure aluminium and composites in the corrosive medium. Formation of irregular shaped pits was observed to be more pronounced as they spread across the surface of pure aluminium. Pits formation in composites were observed around silicon carbide particles, which indicates an interruption of pits growth on the surface of the composites. SEM images also shows that pits formed around silicon carbide particles is an indicates of galvanic coupling occurring at the aluminium- silicon carbide and titanium (from ferrotitanium) interface [31]. These interfaces serve as preferential sites for initiation of pitting corrosion as a result of easy breakdown of the formed passive films at these sites. The presence of more aluminium-silicon carbide-titanium interface across the surface of the composites makes distribution of pits more uniform, thereby decreasing the depth of pits formed. Result from energy-dispersive-Xray (EDX) also confirms the presence of various elements present in alloyed aluminium and composites after corrosion.
S.O.Akinwamide et al./ Materials Today: Proceedings 18 (2019) 3827–3834
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(a)
(b)
(c)
Fig. 7. SEM and EDX micrographs of (a) Pure aluminium (b) Al+Mg+TiFe+0.5%SiC (c) Al+Mg+TiFe+7%SiC Conclusion This research investigated the corrosion properties of aluminium and aluminium matrix composites reinforced with ferrotitanium and silicon carbide in 0.05 M NaCl and 0.3 M H2SO4. Corrosion resistance was greatly improved in aluminium composite with a combination of ferrotitanium and silicon carbide as reinforcement in sodium chloride and sulphuric acid media. The shift in potential towards the cathodic region observed from the polarization curves is due to the inhibition of corrosion provided by the reinforcement additions. The SEM micrographs of samples immersed in 0.3 M H2SO4 confirmed the formation of pits across the surface of pure aluminium. Inhibition to formed and propagated pits were also observed on the surface of composites. The lowest corrosion rate was observed in the test conducted in 0.3 M H2SO4 solution for sample with 7 wt.% silicon carbide reinforcement. Acknowledgements The authors wish to thank the National Research Foundation for funding this research and the Global excellence stature (GES) of University of Johannesburg South Africa for funding the doctoral studies of Samuel Olukayode Akinwamide. References [1] B. Obadele, Z. H. Masuku, and P. Olubambi, Powder Technol. 230 (2012) 169-182. [2] J. Hashim, L. Looney, and M. Hashmi, J. Mater. Process. Technol. 92 (1999) 1-7. [3] T. Christy, N. Murugan, and S. Kumar, J. Mineral. Mater. Charact. Eng. 9 (2010) 57. [4] K. Alaneme, Int. J. Mech. Mater. Eng. 7 (2012) 96-100. [5] D. Berkeley, H. Sallam, and H. Nayeb-Hashemi, Corros. sci 40 (1998) 141-153.
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[6] S. Winkler and H. Flower, Corros. sci. 46 (2004) 903-915. [7] K. A. El-Aziz, D. Saber, and H. E.-D. M. Sallam, J. Bio. Tribo Corros. 1 (2015) 5. [8] B. Bobić, S. Mitrović, M. Babić, and I. Bobić, Tribol. Ind. 32 (2010) 3-11. [9] A. J. Dolata, M. Dyzia, and W. Walke, Solid State Phenom. 191 (2012) 81-87. [10] K. K. Alaneme, T. M. Adewale, and P. A. Olubambi, J. Mater. Res. Technol., 3 (2014) 9-16. [11] J. A. Picas, A. Forn, E. Rupérez, M. T. Baile, and E. Martín, Plasma Processes and Polym. 4 (2007) 579-583. [12] S. Weber and H. Berns, Materialwiss. Werkstofftech. 38 (2007) 205-211. [13] H. Zakaria, Ain Shams Eng. J. 5(2014) 831-838. [14] K. K. Alaneme, J. O. Ekperusi, and S. R. Oke, J. King Saud Univer. Eng. Sci 30 (2018) 391-397. [15] M. Sambathkumar, P. Navaneethakrishnan, K. Ponappa, and K. Sasikumar, Lat. Am. J. Solid Struct. 14 (2017) 243-255. [16] K. Alaneme, B. Ademilua, and M. Bodunrin, Tribol. Ind. 35 (2013) 25-35. [17] O. B. Fatile, J. I. Akinruli, and A. A. Amori, Int. J. Eng. Technol. Innov. 4 (2014) 251-259. [18] G. K. Sigworth, Int. J. Metalcast. 8 (2014) 7-20. [19] M. M. Boopathi, K. Arulshri, and N. Iyandurai, Am. J. Appl. Sci. 10 (2013) 219. [20] B. A. Obadele, P. A. Olubambi, A. Andrews, S. Pityana, and M. T. Mathew, J. Alloys Compd 646 (2015) 753-759. [21] R. T. Loto and P. Babalola, Cogent Eng. 4 (2017) 1422229. [22] R. Zhang, X. Ai, Y. Wan, Z. Liu, D. Zhang, and S. Feng, Int. J. Corros. 2015 (2015). [23] J. Hu, L. Chen, X. Zhong, S. Yu, Z. Zhang, D. Zeng, et al., "Corrosion Inhibition of Titanium in Hydrochloric Acid containing Na2MoO4," Int. J. Electrochem. Sci. 12 (2017) 8878-8891. [24] Z. Ahmad, A. Farzaneh, and B. A. Aleem, Recent Trends in Processing and Degradation of Aluminium Alloys, ed: InTech, 2011. [25] J. Hou and D. L. Chung, J. Mater. Sci. 32 (1997) 3113-3121. 5 (2003) 493-502. [26] K. A. Lucas, K. A. Lucas, and H. Clarke, Research Studies Pr Ltd, 1993. [27] D. M. Aylor and P. J. Moran, J. Electrochem. Soc. 132 (1985) 1277-1281. [28] I. Gurrappa and V. B. Prasad, Mater. Sci. Technol. 22 (2006) 15-122.
ScienceDirect Materials Today: T Proceedinngs 18 (2019) 382 27–3834
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IICMPC-201 19
An A invesstigation on the Corrosio C on Behav vior of Al A (Mg-T TiFe-SiC C) Matrixx C Composi ite in Accidic and d Chlorid de Mediaa Sam muel Olukaayode Akin nwamidea*, Serge Mu udinga Lem mikaa, Feyik ikayo Adam msc, a b O Ojo Jeremiaah Akinrib bide , Bolannle Tolulop pe Abe , Peter Apataa Olubambiia a
Centre for Trribocorrosion and d Nanoengineerinng, University of Johannesburg, J Jo ohannesburg 20006 South Africa b Tshwa ane University off Technology, Preetoria 0001, South h Africa c American Univversity of Nigeria a, Yola, Nigeria
Absttract This study was connducted to invvestigate the co orrosion of aluuminium matrix x composites reinforced withh ferrotitanium and silicon carbiide. As-cast purre aluminium and a aluminium matrix m composiites were produ uced by stir casting. A fixed voolume of ferrottitanium and varyiing proportionss (0.5, 2, 5, and 7 wt. %) of siliicon carbide weere used for thee fabrication of the composites . Chronopotenttiometry and poten ntiodynamic poolarization meassurements weree used to study tthe corrosion behaviour of the fabricated com mposites. Pits with w irregular shapees and sizes weere observed onn the corroded substrates. s Therre were similarrities between th he polarization curves of the cast c samples in su ulphuric acid (H H2SO4) and soodium chloride (NaCl) solutioons. Compositees with silicon n carbide reinfoorcements exhiibited better corro osion resistancee with lower corrrosion potentiaal values.
© 2019 Elsevier Ltd. A All rights reserveed. Selecction and peer-revview under responnsibility of the 9th h International C Conference of Matterials Processing g and Characterizzation, ICMPC-20 019 Keyw words: Aluminium m matrix composiite; Ferrotitanium m; Silicon carbide ; Potentiodynamiic polarization, Chronopotentiomeetry.
1. In ntroduction Corrrosion which is a form off degradation has created m many problem ms in various fields f where m metallic strucctures are utiliised. Alum minium usagee has become extensive in applications a w where propertties such as lig ght weight, duurability and a high strengtth to weig ght ratio are eessential [1, 2]. Improvemeent in propertiies of aluminiium paved thee way for devvelopment of aluminium a matrix com mposites (AMC C), with goodd chemical, physical p and mechanical properties p [3, 4]. AMC caan be fabricaated using sim mple meth hods (such as casting) and at a low cost in comparisoon with fabrication of otherr metallic matterials (titaniu um, copper) which w are more m complexx and expensivve. Studies haave demonstraated the suscep ptibility of AM MC to corrosiion which can n be a direct reesult from m any of the foollowing. (a) interfacial i phaase formation between matrrix and reinfo orcements (b) galvanic corro osion occurrinng at the interface i of thhe matrix and reinforcementt (c) alteratioon of microstru ucture resultin ng from contaamination duriing the processsing of AMC A [5-7]. Adddition of reinnforcement sho ould howeverr influence corrrosion behaviiour of aluminnium matrix composites.
*Corrresponding authoor. Tel,: +277851990655 E-ma ail address: akinw [email protected] 2214--7853 © 2019 Elssevier Ltd. All rigghts reserved. Selecction and peer-revview under responnsibility of the 9th h International C Conference of Matterials Processing g and Characterizzation, ICMPC-20 019
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S.O.Akinwamide et al./ Materials Today: Proceedings 18 (2019) 3827–3834
Investigations by various researchers assumed the corrosion behaviour of AMC to be unpredictable, as various differences were observed in results obtained for different AMC systems [8-10]. There was an increase or decrease in current densities of aluminium matrix composites reinforced with different types of nanoceramics [11]. Addition of ferrotitanium to reinforcing materials in the fabrication of AMC can enhance its corrosion and wear resistance in several engineering application [12]. Corrosion behaviour of aluminium reinforced with varying proportions of silicon carbide was studied by Zakaria[13], observations showed improvement in the corrosion resistance of aluminium reinforced with silicon carbide in contrast to the pure aluminium matrix. Further observation revealed an increment in corrosion resistance of reinforced composites with increase in volume fraction of silicon carbide. Kenneth et al[14] investigated the corrosion behaviour of thermal cycled aluminium composite with rice husk and silicon carbide as reinforcements. This gave an improved corrosion of reinforced composite in chloride media. Excellent corrosion resistance of aluminium 7075 reinforced with silicon carbide and titanium carbide was also observed by Sambathkumar[15]. Several studies have shown improvement in corrosion resistance of aluminium matrix composites, which confirms the fact that aluminium reinforced with silicon carbide can be a suitable material for engineering applications where excellent corrosion resistant is required [16, 17]. Nevertheless, little or no literature is available on reinforcement of aluminium with silicon carbide and ferrotitanium hence, this work purpose of this present study. Experimental procedure 2.1 Materials An Ingot of aluminium 5083 series (99.9% purity) was used as the matrix, this was obtained from Insibi company in South Africa. Composition of the aluminium matrix is shown in Table 1. Ferrotitanium (Ti-Fe) and silicon carbide (SiC) were used as major reinforcements. 2.2 Methodology An aluminium ingot was placed in graphite crucible and heated until molten in an electric furnace. The temperature of the melt was maintained between 650-670℃. Magnesium weighed at 30 g was added to the melt of aluminium to ensure excellent wettability between the matrix and the reinforcement. Ferrotitanium powder also weighed at 30 g mass was preheated with different masses of silicon carbide in a muffle furnace at temperatures ranging between 850-890℃ prior addition to the melt of aluminium and magnesium. Silicon carbide was added as reinforcement in varying amounts of 0.5%, 2%, 5% and 7 wt.%. Prior to the addition of silicon carbide, thorough mixing was done for a period of 15 mins to ensure homogeneous dispersion of the reinforcement in the aluminium matrix. Casting of silicon carbide reinforced composite was carried out in a die preheated to a temperature of 700 0C for 2 h to ensure fluidity of the melt. For comparison, reference alloys of pure aluminium and aluminium with magnesium and ferrotitanium were cast using the same method but no silicon carbide was added. Samples for microstructural analysis were cross-sectioned from as-cast composites, then cold mounted in epoxy resin. The grinding of sample surfaces was carried out using 320 – 1200 grits of silicon emery paper and polished to a 1 µm using diamond suspension. Samples were washed with acetone and dried in air. Corrosion testing of the aluminium composites was done with chronopotentiometry and potentiodynamic polarisation techniques in corrosive media of 0.05 M NaCl and 0.3 M H2SO4 solutions with a Versa studio potentiostat equipped with three electrode corrosion cells: working, saturated silver/silver chloride (Ag/AgCl) reference and a graphite counter electrode. Working electrodes were prepared by attaching an insulating wire to the other side of a polished sample with an aluminium tape, before cold mounting in a resin. Prior to mounting, samples used as the working electrode were polished and ground until a mirror-like surface was achieved using an automatic polisher with different sizes of diamond paste suspensions. Open circuit potential (OCP) was measured for 2 h, while potentiodynamic polarisation tests were carried out at a scan rate of 1 mV/s starting from 0.25 - 1.5 V. The electrolyte was replaced after each scan. Results and Discussion 3.1 Microstructural analysis Microstructures of as-cast pure aluminium and composites are shown in Figures 1. Formation of pores and acicular structures are visible in Figure 1a. Dendritic structures which are snowflake like crystals were formed from solid aluminium during liquid to solid transformation[18]. Formed dendrites were observed to disappear upon the addition of silicon carbide and ferrotitanium reinforcements. Even distribution of reinforcement particles in the matrix of aluminium as observed in Figures 1b. and Figure 1c. can improve the mechanical properties of composites[19].
S.O.Akinwamid de et al./ Materialls Today: Proceeedings 18 (2019) 3827–3834 3
(a)
38299
(b) Ti-F Fe Pores
50 µm
50 µm m
(c)
SiC
50 0 µm Fig 1. Opptical micrograph hs of (a) Pure alum minium (b) Al +M Mg + TiFe (c) Al + Mg+ TiFe+ 7 w wt% SiC.
measurementss 2 and 3 show ws the open ciircuit potentiaal (OCP) in 0.05 M NaCl and a 0.3 M H22SO4 media reespectively. From F P values for puure aluminium m were observved to fluctuatte within the first f 4000s, beefore stabilisin ng at a potential of lar trend was observed in the ferrotitannium reinforcced compositee. OCP of coomposites witth silicon carrbide ere observed to stabilize att lower potenttials in compaarison to the stabilizing s pot otential of pure aluminium. The OCP values during d first 40 000s indicatess the formation n and dissoluttion of passivve layers on th he cast samplees in . This could be b as a result of o pores and ccracks which serve as path hs through whhich the solutio on of NaCl ennters yer. This was also a observed in a study byy Obadele et al a [20]. the sample with w highest weight w fractioon of silicon carbide attain ned the higheest potential at -0.507 V. The of silicon carrbide particless can provide resistance to corrosive attack during cheemical dissolu ution of SO42- ions d[21]. The ferrrotitanium reeinforced com mposite stabiliised at a potential of -0.553 V. This caan be ascribeed to ion by the formation f of a dense passsive layer on n the titanium m present in the ferrotitaanium at ambbient 23]. Stability of o OCP in co omposites withh 5 and 7 wt.% of silicon carbide reinfo forcements weere observed after e due to re-paassivation of composites in tthe corrosive media[24]
Fig g. 2. Open circuit potential of samp ples in 0.05 M Na aCl
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Fig. 3. Open circuitt potential of sam mples in 0.3 M H2SO S 4
3.4 Potentiodynam P mic polarization measuremeent Corrrosion potentiials and curreent density vaalues obtainedd from Tafel fits f of the pollarisation curvves are given n in Tables 2. and Tablle2. Corrosionn studies in 0..05 M NaCl and a 0.3 M H2S SO4 media sho ows the stabillisation of purre aluminium at a potentiall of 0.83 V and -0.555 V respectivvely, while alu uminium mattrix compositte (AMC) witth the highestt weight perccentage of sillicon carbide was stabiilized at a higgher potentiall of -0.68 V. From Table 2, a decrease was observeed in corrosio on potentials were w obseerved to decreease with incrreasing conten nt of silicon caarbide. This can c be ascribeed to the dom mination of thee cathodic proocess relattive to the annodic processs. Corrosion resistance off aluminium matrix m compo osites is alsoo dependent on o the changge in conccentration of cchloride. A sim milar observaation was madde with pure aluminium haaving the highhest corrosion potential. Sillicon carbide prevents ccorrosion of AMC A by inhibiiting actions oof SO42- and Cl C - ions presen nt in the corrossive media[21 1]. Figu ures 4. and Figure 5. show ws polarizatio on curves forr aluminium and a reinforced d compositess in the as-caast condition. The cathodic range w was observed to t have equal current denssities for all th he samples in n Figure 4. Fuurther observaation showed that com mposites with silicon carbide, ferrotitan nium and maggnesium havee no passive range with aan anodic diissolution in pure alum minium. From m Figure 5, thee polarization scans of all ssamples exhib bited similar anodic a and cat athodic polarissation curves with pseu udo-passive beehaviour in 0.3 0 M H2SO4 solution[20]. Improved co orrosion resistance of the aaluminium maatrix compositte in corro osive media ccan be attributeed to the even n dispersion off silicon carbiide and ferrotitanium reinfoorcements with hin the aluminnium matrrix, as these reeinforcementss inhibit sites where pittingg is likely to be b initiated[25 5] . Migrationn of oxygen an nions in sulphhuric acid solution cann increase the growth rate of aluminium m oxide films, thereby redu ucing the ratee at which hy ydrogen evoluution occu urs in this sollution. The anodic a reactio on of composiites in the solution can theerefore be redduced by the reaction betw ween silicon and oxygeen[21]. The coorrosive action of chloride ions in 0.05 M NaCl coulld be reducedd by presence of silicon carrbide and ferrotitanium m. Presence off reinforcemen nts can result in chemical dissolution d off chloride ionss. It can therrefore be assuumed that the corrosive action of chlooride ions in 0.05 0 M NaCl w was reduced by b the presence of silicon caarbide and ferrrotitanium. Table 1. Tafel polarissation results of pure aluminium annd aluminium maatrix composites in i 0.05 M NaCl
Al Al+ +Mg+TiFe Al+ +Mg+TiFe+0..5%SiC Al+ +Mg+TiFe+2% %SiC Al+ +Mg+TiFe+5% %SiC Al+ +Mg+TiFe+7% %SiC
Ecorr (V) vs (Ag/AgCl) -0.83 -0.74 -0.71 -0.74 -0.69 -0.68
Icorr (µA A) 3.07 0.21 0.39 1.25 0.85 0.59
Table 2. Tafel polarisaation results of pu ure aluminium andd aluminum matrrix composites in 0.3 M H2SO4
All Al+ +Mg+TiFe Al+ +Mg+TiFe+0..5%SiC Al+ +Mg+TiFe+2% %SiC Al+ +Mg+TiFe+5% %SiC Al+ +Mg+TiFe+7% %SiC
Ecorr (V) vs (Ag/AgCl) -0.57 -0.55 -0.53 -0.50 -0.48 -0.47
Icorr (µA A) 81.44 67.62 131.03 162.38 53.35 69.03
S.O.Akinwamid de et al./ Materialls Today: Proceeedings 18 (2019) 3827–3834 3
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Figg. 4. Potentiod dynamic polariization for sam mples in 0.05 M NaCl soluttion
Fig. 5. Poten ntiodynamic ppolarization fo or samples in 0.3 0 M H2SO4 3.5 Optical O microscopy analysiis of substrates Figure 6.. shows typicaal optical micrrographs of thhe substrates after a the corrosion tests. Corrrosion pits arre dominant inn the solid d solution of α-Al and surrroundings off silicon carbiide and ferrottitanium partiicles. Cracks observed in Figure 6a cann be attrib buted to localized corrosionn by pitting in n aggressive chhloride enviro onment[26]. Microstructura M al studies of th he sample surffaces reveealed pits of irrregular shapees and sizes were formed foor both as-castt pure aluminiium (unreinfor orced matrix) and a the reinfoorced com mposites. Corrrosion by pittiing was evideent in fabricatted aluminium m matrix comp posites when they were in the passive state. s Thiss can be as a rresult of disrupptions in the passive p film laayer formed due d to ions preesent in the agggressive enviironments[27, 28]. How wever, the size of pits in the t compositees decreased w with increasin ng addition of o silicon carbbide and a fix xed proportioon of ferro otitanium. Som me reports haave been given n on the decreease of corrossion resistancee in fabricatedd aluminium matrix m composites due to the formaation of Al4C3 phase[29, 30]. The addittion of ferrottitanium particles can decrrease the corrrosion rate off the com mposites througgh the formatiion of a third intermetallic pphase in the composites, c th hereby decreassing the rate at a which the Al A 4C3 phasse is formed.
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(b)
(a)
Pit
50 µm
50 µm (d)
(c)
Al4C3
50 µm (e)
50 µm (f)
50 µm
50 µm
Figure 6. Optical micrograph of corroded as-cast aluminium and reinforced aluminium matrix in 0.3 M Sulpuric acid for (a) Pure aluminium (b) Al+Mg+TiFe (c) Al+Mg+TiFe+0.5%SiC (d) Al+Mg+TiFe+2%SiC (e) Al+Mg+TiFe+5%SiC (f) Al+Mg+TiFe+7%SiC
3.5 Scanning Electron Microscopy analysis of corrosion substrates Figure 7 (a-c)shows the scanning electron microscopy (SEM) micrographs of the as-cast pure aluminium and composites after immersion in 0.05 M sodium chloride. Evidence of pit formation was visible across the surfaces of pure aluminium and composites in the corrosive medium. Formation of irregular shaped pits was observed to be more pronounced as they spread across the surface of pure aluminium. Pits formation in composites were observed around silicon carbide particles, which indicates an interruption of pits growth on the surface of the composites. SEM images also shows that pits formed around silicon carbide particles is an indicates of galvanic coupling occurring at the aluminium- silicon carbide and titanium (from ferrotitanium) interface [31]. These interfaces serve as preferential sites for initiation of pitting corrosion as a result of easy breakdown of the formed passive films at these sites. The presence of more aluminium-silicon carbide-titanium interface across the surface of the composites makes distribution of pits more uniform, thereby decreasing the depth of pits formed. Result from energy-dispersive-Xray (EDX) also confirms the presence of various elements present in alloyed aluminium and composites after corrosion.
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(a)
(b)
(c)
Fig. 7. SEM and EDX micrographs of (a) Pure aluminium (b) Al+Mg+TiFe+0.5%SiC (c) Al+Mg+TiFe+7%SiC Conclusion This research investigated the corrosion properties of aluminium and aluminium matrix composites reinforced with ferrotitanium and silicon carbide in 0.05 M NaCl and 0.3 M H2SO4. Corrosion resistance was greatly improved in aluminium composite with a combination of ferrotitanium and silicon carbide as reinforcement in sodium chloride and sulphuric acid media. The shift in potential towards the cathodic region observed from the polarization curves is due to the inhibition of corrosion provided by the reinforcement additions. The SEM micrographs of samples immersed in 0.3 M H2SO4 confirmed the formation of pits across the surface of pure aluminium. Inhibition to formed and propagated pits were also observed on the surface of composites. The lowest corrosion rate was observed in the test conducted in 0.3 M H2SO4 solution for sample with 7 wt.% silicon carbide reinforcement. Acknowledgements The authors wish to thank the National Research Foundation for funding this research and the Global excellence stature (GES) of University of Johannesburg South Africa for funding the doctoral studies of Samuel Olukayode Akinwamide. References [1] B. Obadele, Z. H. Masuku, and P. Olubambi, Powder Technol. 230 (2012) 169-182. [2] J. Hashim, L. Looney, and M. Hashmi, J. Mater. Process. Technol. 92 (1999) 1-7. [3] T. Christy, N. Murugan, and S. Kumar, J. Mineral. Mater. Charact. Eng. 9 (2010) 57. [4] K. Alaneme, Int. J. Mech. Mater. Eng. 7 (2012) 96-100. [5] D. Berkeley, H. Sallam, and H. Nayeb-Hashemi, Corros. sci 40 (1998) 141-153.
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