ISSN 20702051, Protection of Metals and Physical Chemistry of Surfaces, 2012, Vol. 48, No. 5, pp. 568–571. © Pleiades Publishing, Ltd., 2012.
PHYSICOCHEMICAL PROBLEMS OF MATERIALS PROTECTION
Effect of Titanium Addition on Corrosion Properties of Al–Si Eutectic Alloys1 S. Zora, M. Zerenb, H. Özkazan¸ca, and E. Karakulakb a
b
Kocaeli University ScienceLiterature Faculty Chemistry Department, 41380 Kocaeli/TURKEY Kocaeli University Engineering Faculty Metallurgical and Materials Engineering Department, 41380 Kocaeli/TURKEY email:
[email protected] Abstract—Aluminium and its alloys are the most used nonferrous metals because of their satisfactory prop erties. Especially the corrosion resistant in different mediums is the most important reason for this. The cor rosion resistance of the aluminium comes from the oxide layer on the surface. Different alloying elements have different effect on corrosion behaviour of the aluminium alloys. In this study effect of different amount of titanium addition on corrosion behaviour of AlSi eutectic alloys was investigated. DOI: 10.1134/S2070205112050176
1
1. INTRODUCTION Aluminium is an important material for many practices such as automotive industry, aviation, house hold appliances, containers and electrical circuits due to its high electrical and thermal conductivity, low density and good resistance against corrosion [1–3]. Aluminium has a quite high resistance against corro sion due to the natural oxide layer formed on the sur face [4–8]. However, pure aluminium has poor mechanical properties. Aluminium should be made alloy for increasing its hardness and strength. The ele ments such as Si, Cu and Mg are frequently used in such alloys. Also, strength of aluminium alloys may be increased by applying thermal processes on them [9, 10]. AlSi alloys have improved mechanical proper ties, good corrosion resistance, low expansion coeffi cient and high abrasion resistance [11, 12]. The effects of the elements included in aluminium alloys on cor rosion behaviour are quite different. It is common practice to add Ti to AlSi foundry alloys because of its potential grain refining effect. The solubility limit of titanium in liquid aluminum is situ ated between 0.12 and 0.15% Ti depending on the dif ferent references. However, an excess of Ti content may cause problems in the liquid metal process and defects in casting [13], due to precipitation of primary TiAlSi coarse intermetallic particles above the liquid temperature. Although there is ample information of TiAl intermetalics in the binary AlTi system most of them are related to the grain refining mechanism, little publication literature exists describing the formation and growth of TiAlSi intermetallics in widely used Al Si foundry alloys [12–14]. Some AlTi alloys are used 1 The article is published in the original.
as low resistance ohmic contacts [15]. Also TiTiAl3 metalintermetallic laminate (MIL) composites have potential application in airplanes [16]. The aim of the present study is to investigate the effect of intermetal lic phases to corrosion properties of different Ti con taining AlSi alloys. 2. MATERIALS AND METODS 2.1. Materials Different Ti containing alloys were obtained by adding 0%, 0.1%, 2% and 5% of titanium into the alu minium alloys containing silicon at the eutectic ratio (12%). An induction melting furnace with graphite crucible was used in manufacturing these alloys. It was found that, intermetallic crystals in microstructure were formed as a result of titanium addition into the AlSi alloys. These intermetallic phases having the combination of TiAl3 have effects on hardness, strength and corrosion properties of the material [12]. Figure 1 shows the microstructures of the alloys con taining 0%, 0.1%, 2% and 5% Ti and the intermetallic crystals existing inside the microstructures. 2.2. The Potentiodynamic Method The system with three electrodes was employed in polarization measurements. In this system, Pt was used as opposite electrode, SCE (saturated calomel electrode) was used as reference electrode and the samples from aluminium alloys were used as working electrode. The surfaces of the working electrode were covered with polytetrafluorineethylene (PTFE) with the exception of that in contact with the solution. The area of the surface of the electrode in contact with the
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EFFECT OF TITANIUM ADDITION ON CORROSION PROPERTIES
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solution is 1.0 cm2. The potential curves taken were obtained within the potential range between –1.0 V and +1.0 V with a scanning speed of 25 mV/s by using EG8G PAR model 263A potentiostat/galvanostat. The measurements were taken at room temperature and in the solutions of 0.1 M HCl and 0.1 M H2SO4. Electrochemical parameters (Icor, Ecor) were specified from the potentiodynamic curves obtained. 10 µm
(а)
10 µm
(b)
TiAl3
10 µm
(c)
10 µm
(d)
Fig. 1. Optical microscope views of the alloys containing (a) Al–Si, (b) 0.1% Ti, (c) 2% Ti, (d) 5% Ti.
1.0 E, (V/SCE)
0.5
Al–Si (0.1 M HCl) Al–Si + %0.1 Ti (0.1 M HCl) Al–Si + %2 Ti (0.1 M HCl) Al–Si + 5 Ti (0.1 M HCl)
0 –0.5 –1.0 –1.5 –2.0
–4.5 –4.0 –3.5 –3.0 –2.5 –2.0 –1.5 –1.0 –0.5 logI (A/cm2) Fig. 2. Potentiodynamic polarization curves of the Al–Si alloys containing titanium at different ratios in 0.1 M HCl solution.
1.0 Al–Si (0.1 M H2SO4) Al–Si + %0.1 Ti (0.1 M H2SO4) Al–Si + %2 Ti (0.1 M H2SO4) Al–Si + 5 Ti (0.1 M H2SO4)
0.5 E, (V/SCE)
3. RESULTS AND DISCUSSION 3.1. Corrosion Behaviour of Alloys Corrosion behaviour of AlSi alloys containing titanium in different amounts (0%, 1%, 2%, 5%) were determined through potentiodynamic polarization method in the solutions of 0.1 M HCl and 0.1 M H2SO4. Potentiodynamic polarization curves of the alloys in the solutions of 0.1 M HCl and 0.1 M H2SO4 are seen on Figs. 2 and 3 respectively. Table 1 shows the values of Icor and Ecor obtained from these curves. According to this, increase in titanium amount in Al– Si alloys did not cause any significant changes in cor rosion potential. Corrosion potential inside HCl ranged between –682 mV and—646 mV and corro sion potential inside H2SO4 ranged between—718 mV and—487 mV. In case of 0.1 M HCl solution, corro sion current density increased as titanium amount increased. In case of 0.1 M H2SO4, corrosion current density increased compared with the AlSi alloy con taining no titanium. However, this value decreased as titanium amount increased. The reason for much more significant increase in corrosion current density in the experiments done inside HCl solution is the active Cl– ions existing in the solution. The effect of Cl– ions decomposing the oxide layer formed on the surface accelerates corrosion and increases corrosion current density. Silicon in Al–Si alloys decelerates local cathode reactions in corrosion. TiAl3 intermetallic phase formed inside the metal as a result of titanium addition into these alloys constructs cathode regions of the localized cell inside the aluminium matrix. The main reason for that TiAl3 phase behaves as cathode is that the oxide film on this phase is thinner than that on the aluminium matrix because it is more inert and it con structs the regions in which electrons' flowing resis tance gets lower. Beginning and spreading of hollow corrosion namely anodic dissolutions are seen in the aluminium matrix surrounding these phases. Figure 4 shows the views of the Al–Si alloys sam ples with different Ti content, taken by scanning elec tron microscope (SEM) after they were kept inside 0.1 M H2SO4 for 120 hours. The TiAl3 intermetallic phase regions behave as cathode and the nearby Al phase
0 –0.5 –1.0 –1.5 –6
–5
–4 –3 logI (A/cm2)
–2
–1
Fig. 3. Potentiodynamic polarization curves of the Al–Si alloys containing titanium at different ratios in 0.1 M H2SO4 solution.
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Electrochemical parameters of aluminium alloys in 0.1 M HCl and 0.1 M H2SO4 Alloys
0.1 M HCl
0.1 M H2SO4
Icor(µA/cm2)
Ecor(mV)
Icor(µA/cm2)
Ecor(mV)
Al–Si
1415
–682
376
–534
Al–Si–0.1Ti
1988
–655
585
–718
Al–Si–2Ti
3229
–646
468
–488
Al–Si–5Ti
3775
–657
385
–546
behaves anodically and dissolves. By increasing the Ti content from 0.1 % to 5 % the average area ratio of the intermetallic phases increases from 0.1% to 7% respectively [12]. So with increasing Ti ratio in the alloy the size of the local cathode areas increases and this leads more dissolution of the Al matrix.
rosion damage in HCl solution is more severe then in the H2SO4 solution. The reason for this is the existence of Cl– ions which decompose the oxide film on the aluminium surface.
Figure 5 shows the SEM images of alloys contain ing 2% and 5% Ti respectively. It can be seen that cor
(a)
18 µm
(b)
18 µm
(c)
20 µm
(d)
10 µm
Fig. 4. SEM views of the alloys containing (a) 0 % Ti, (b) 0.1 % Ti, (c) 2% Ti (d) 5% Ti after the specimens were kept inside 0.1 M H2SO4 for 120 hours.
(а)
50 µm
(b)
50 µm
Fig. 5. SEM views of the alloys containing (a) 2% Ti, (b) 5% Ti after the specimens were kept inside 0.1 M HCl for 120 hours.
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 48 No. 5 2012
EFFECT OF TITANIUM ADDITION ON CORROSION PROPERTIES
4. CONCLUSIONS • Titanium addition to AlSi alloys causes inter metallic phase separation in the composition of TiAl3. • These TiAl3 intermetallic phases construct local cathode regions inside the material and cause dissolu tions at matrix regions close to them. • Therefore, increase in titanium amount in the material leads to the increase in local cathode areas and this caused increase in corrosion current density, in other words, accelerated corrosion in both of the solutions of 0.1 M HCl and 0.1 M H2SO4. • Corrosion damage in 0.1 M HCl solution is more severe than 0.1 M H2SO4 because of Cl– ions. • It is known that, titanium intermetallic phases existing in the Al alloys have different morphologies [14]. More comprehensive studies are required to determine the effects of these different morphologies on corrosion behaviour of alloys. REFERENCES 1. Amin, M.A., Abd El Rehim, S.S., Moussa, S.O., and Ellithy, A.S., Electrochim. Acta., 2008, vol. 53, p. 5644. 2. Capauano, G.A. and Davenport, W.G., J. Electrochem. Soc., 1971, vol. 118, p. 1688.
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