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Cu Composites in 3.5% Sea Water
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ScienceDirect Materials Today: Proceedings 2 (2015) 1166 – 1174
4th International Conference on Materials Processing and Characterization
Microstructure and corrosion behaviour of laser metal deposited Ti6Al4V/Cu composites in 3.5% sea water Mutiu F. Erinoshoa,*, Esther T. Akinlabia, Sisa Pityanab a
Department of Mechanical Engineering Science, University of Johannesburg, Auckland Park Kingsway Campus, Johannesburg, 2006, South Africa. b National Laser Centre, Council for Scientific and Industrial Research (CSIR), Pretoria, 0001, South Africa.
Abstract Titanium alloy applications in the marine industries are due to their excellent corrosion resistance when exposed to surface contaminants. This paper reports the microstructure and corrosion behaviour of laser metal deposited Ti6Al4V/Cu composites in 3.5% artificial sea water.During the deposition process, laser power of 1600 W and scanning speed of 0.3 m/min were kept constant throughout the entire process. The SEM of both Ti6Al4V/Cucomposites shows the formation of Widmanstatten structures and the branches became thinnertoward the fusion zone. This was attributed to the reduction in the distribution and the flow of the heat input. The corrosion resistance behaviour of the Ti6Al4Vcompositesfrom the potentiodynamic polarization curve has been greatly improved with the addition of Cu content and this in turn has enhanced its surface modification. © 2015 2014Elsevier The Authors. Elsevier Ltd. All rights reserved. © Ltd. All rights reserved. Selection peer-review under responsibility of thecommittee conference committee of conference the 4th International conference Selection andand peer-review under responsibility of the conference members of the 4thmembers International on Materials Processing and on Characterization. Materials Processing and Characterization. Keywords:Corrosion; laser metal deposition; microstructures; seawater; Ti6Al4V/Cu composites
* Corresponding author. Tel.: +27747425924. E-mail address: [email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization. doi:10.1016/j.matpr.2015.07.028
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1. Introduction Ti6Al4V is most applicable among the titanium alloys due to its combination of physical, mechanical, and corrosion resistance which have made them necessary for demanding in the industries such as the aerospace, marine, energy and automobile [1-2]. Essentially, seawater is chemically aggressive to materials and plants under it, and these materials are exposed to different corrosion test depending upon the utilizing environment and the nature of the material itself [3]. The corrosion potentials of a laser-alloyed specimens decrease as the titanium (Ti) content increases and the improvement in the corrosion resistance of Ti was attributed to the presence of Ti in the intermetallic and metallic phases for forming the protective oxide [4].Ti6Al4V spontaneously formed a stable, continuous, and adherent oxide film upon exposure to oxygen in water or air which was very vital to its excellent corrosion resistance behaviour. Its corrosion behaviour occurred in aqueous solutions such as seawater and alkalis [5]. The properties of the passive film on Ti alloys are a function of potential and susceptible to various corrosion processes. The stability of the film can be improved, however, if inhibitors could be added to these environments [6]. Gamma titanium aluminide was characterised for corrosion resistance in 3.5% NaCl and seawater using potentiodynamic polarization and electrochemical impedance spectroscopy. The surface of the sample was modified through oxidation in air at 500oC and 800oC. The results indicated that the sample passivated and show good corrosion behaviour in both 3.5% NaCl and seawater solutions [7]. The fabrication of nanocrystalline Al, Al10%Cu, and Al-10%Cu + 5%Ti alloys was conducted for corrosion in natural sea water using the mechanical alloying method. The presence of Cu, Al-10%Cu alloy reduces the corrosion of Aluminium. The addition of 5%Ti to the alloys produced the best passivation to the surface of Al with the lowest corrosion rate [8]. The effect of temperature on the electrochemical corrosion behaviour of Ti and Ti6Al4V was conducted for biomaterial application. The corrosion resistance of Ti was increased with the time of immersion at body temperature (37oC) whilst the corrosion resistance of Ti6Al4V was decreased and occurred as a result of dissolution of the passive film [9].The effectoftitanium (1g to 3g)on AISI 430 grade ferritic stainless has been proved to improve the mechanical properties and corrosion behaviour of the stainless steel. The grain refinement in the weld zone was due to the formation of fine grain microstructure and precipitates [10]. This research work hence is aimed at describing the impactof laser metal deposition (LMD) process on the microstructure and corrosion behaviour of laser metal deposited Ti6Al4V/Cu composites in 3.5% artificial sea water. Three depositions were made which comprises of the Ti6Al4V composite, 3 weight percent of Cu and Ti6Al4V (Ti6Al4V/3Cu) and 5 weight percent of Cu plus Ti6Al4V (Ti6Al4V/5Cu). Nomenclature Ecorr Icorr
corrosion potential current density
2. Experimental techniques The LMD of the Ti6Al4V and Ti6Al4V/Cu composites were conducted at the National Laser Centre of Council of Scientific Industrial Research (NLC-CSIR), Pretoria, South Africa, using the Ytterbium Laser System equipment (YLS-2000-TR). This laser is equipped with a maximum power of 2000 Watts and uses a Kuka robot as the attaching component. 2.1. Materials and methodology A solid plate of 99.6 % titanium alloy (Ti6Al4V) with dimension of 102 X 102 X 7.45 mm3 was used as the substrate. The substrate was sand blasted to remove the contaminants attached to its surface and to prepare the
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surface for firm metallurgical bonding. The sand blasted substrate was cleanedwith acetone and dried up. Ti6Al4V and Cu powders with particle sizes varied between 150 and 200 μmwere used for the deposition. Two separate hoppers (feeders) were setup for the two powders. All the powders flow through the connecting hoses attached to the three way nozzle.Fig.1 shows the schematic view of the LMD process. The laser power of 1600 W and scanning speed of 0.3 m/minwere kept constant for both the Ti6Al4V and Ti6Al4V/Cu composites. 3 and 5 weight percent of Cu powder were deposited with Ti6Al4V. The standoff distance of 12 mm and a beam diameter of 4 mm were used throughout the experimental setup. The flux on the composites was cleaned with a metal brush. According to E3-11 ASTM standard, all the samples were grinded, polished and etched [11].
Fig.1. Schematic view of the LMD process [12]
Table 1 shows the matrix used for the experiments. The powder flow rate of the Ti6Al4V was varied between 2.4 and 2.5 rpm; and the gas flow rate was also varied between 2.9 and 3 l/min respectively. Table1. Experiment matrix Sample Designation
Powder flow rate (rpm) Ti6Al4V
Ti6Al4V
2.4
Ti6Al4V/3Cu
2.4 0.1
Ti6Al4V/5Cu
2.5
Nil
0.1
Cu
Gas flow rate (l/min) Ti6Al4V
Cu
2.9
Nil
3
1
3
1
2.2. Microstructure The Kroll’s reagent was prepared with 100 ml of H2O, 2-3 ml HF and 4-6 ml HNO3 for the etchant prior to the SEM observation. This was prepared according to the Struers application note formetallurgical preparation of titanium [13]. The samples were etched for 10-15 seconds, sprayed with acetone, rinsed under clean running water and dried off. The microstructures of all the etched samples were observed under the SEM TESCAN machine equipped with VEGA TC software.
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2.3. Corrosion test The electrochemical measurements for the corrosion rate were conducted using the standard practice according to the G102-89ASTM [14]. A Potentiostat (PSTAT), equipment wasused for the corrosion measurement. The equipment is coupled with a computer and uses GPES Manager Version 4.9 software for its function. A bridge tube beaker was prepared for the experiment and the reference electrode (RE) tube was filled with a 3 Molar Potassium Chloride (3M KCL). The reference electrode and the counter electrode were inserted into the bridge tube beaker compartment. The samples were also inserted via the bridge tube to the compartment and facing the RE in order to sense the applied potential.All the samples were first ran in Open Circuit Potential (OCP) for 7200 s and a scan rate of 2 mV/s before running the complete polarisation. The potentiodynamic polarisation curves were measured from 1.5 V to 2 V at a scan rate of 2 mV/s. The three samples were run with freshly prepared electrolyte (sea water). The constituents used for the preparation of the sea water are shown in Table 2 and masses of the chemical compounds are similar to what Chen and Yan, 2012 prepared for their sea water [15]. A digital pH meter, ORION Model 520A was used to measure the pH of the solution. The pH of the 3.5 % sea water prepared is between 7.95 and 8.10. Table 2. Sea water constituents, appearance and the mass of chemical compounds Chemical Compounds Appearance Mass (g) NaCl
White powder
49.06
Na2SO4
White powder
8.18
CaCl2
White dry granulated
3.16
MgCl2
White granulated
10.4
KCl
White powder
1.39
NaHCO3
White powder
0.40
NaF
White granulated
0.006
KBr
White granulated
0.20
3. Results and discussion The microstructural results, open circuit potential andpotentiodynamic polarisation curves are presented and discussed in this section. 3.1. Microstructural evaluation An analysis was carried out on the Ti6Al4V and Cu powders under the SEM. Figs. 2 (a) and (b) show the morphologies of the Ti6Al4V and Cu powder used for the deposition.The morphologies of the two powders are spherical and equiaxed in structure. The Cu particles were observed to be heavier and denser than that of the Ti6Al4V particles during the flow rate measurements.
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(a)
(b)
Fig. 2. (a) The SEM morphology of Ti6Al4V powder, (b) The SEM morphology of Cu powder
The microstructures of the Ti6Al4V/Cu samples were observed under the SEM. Figs. 3 (a) and (b) show the SEM of Ti6Al4V/3Cudeposited at a powder flow rate of 2.4 rpm and Ti6Al4V/5Cuproduced at a powder flow rate of 2.5 rpm.
Figs. 3. (a) SEM of Ti6Al4V/3Cu deposited at a powder flow rate of 2.4 rpm, (b) Ti6Al4V/5Cu produced at a powder flow rate of 2.5 rpm
Majorly, the Widmanstettan structures were observed in both composites under the SEM. Widmanstettan structures were observed to be robust (thick) at the top of the deposited composites and loses their robustness as the heat input travels down to the substrate. The Widmanstettan structures grows towards the substrate and becomes thinner and faintier as they approach the fusion zone. This occurrence is as a result of the rate at which the heat input sinks and travels into the substrate and the interference between the substrate interface. In other word, as the heat
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input loses its radiating energy. The branches of the widmanstettan structures toward the fusion zone becomes very thin, sharp and closer. This gives an indication of a strong bond that exist between the deposit and the substrate interface. The formation of α-martensite within this boundary predominate and have a significant effect on the strength of the composite most especiallyat the interface. 3.2. Corrosion results The open circuit polarization (OCP) of Ti6Al4V and Ti6Al4V/Cucomposites are presented in Fig.4. It can be observed that the OCP of Ti6Al4V experiences a cathodic fall at the starting point between 2-5 seconds from -0.08 V to -0.62 V; and maintains unstable fluctuation at that potential throughout the 120 minutes. The cathodic fall could be attributed to oxide formed on the surface been weak. The instability of the potentials was in the range between -0.58 V and -0.62 V. These potentials may vary with time due to the variations of oxidation and formation of the passive layer in the nature of the surface of the electrode [16].A similar observation occurred with the OCP of Ti6Al4V/3Cu. A cathodic fall from -0.14 V to -0.52 V was also depicted and maintains unsteadiness of potentials between -0.46 V and -0.58 V.
Fig. 4. Variation in the open circuit potential as a function of time for Ti6Al4V, Ti6Al4V/3Cu and Ti6Al4V/5Cu in 3.5 % sea water
The OCP of Ti6Al4V/5Cu deposited at a laser power of 1600 W and scanning speed of 0.3 m/min demonstrates a slight rise in the corrosion potential directly from -0.28 V at zero point to -0.26 V at 1000 seconds. A considerable cathodic rise of the potential takes place until -0.18 V at 1400 seconds and maintains an inconsistent sinusoidal and continuous cathodic rise till a corrosion potential of -0.13 V with time. This indicates that the corrosion rate with time increases for Ti6Al4V/5Cu composite. This rise gives an indication of film growth on the surface within the solution. The OCP is important and serves as a benchmark for the corrosion behavior [16]. However, the stabilities of Ti6Al4V and Ti6Al4V/3Cu composites are low in the prepared sea water used as observed in the fluctuations of the OCP values in Fig. 4.From the observations, it could be deduced that the corrosion resistances with respect to time of Ti6Al4V/Cu samples were more than the Ti6Al4V sample. In comparing the OCP values, Ti6Al4V/5Cu composite exhibited the highest resistance to corrosion and show good oxidation behaviour.Ti6Al4V/3Cu also shows a corrosion resistance but less than the Ti6Al4V/5Cu composite. In other word, Cu has greatly show an impact in improving the corrosion resistance of Ti6Al4V and also an increase in its content within the surface
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treatment significantly enhanced the resistance to corrosion with respect to time. The metallic Cu improved the corrosion behavior of Ni-Ti alloy and the oxide film interface and increased the susceptibility to pitting corrosion [17]. From the results of the corrosion test conducted, Table 3 shows the corrosion potential Ecorr, the corrosion current density Icorr, and the corrosion rate obtained using both cathodic and anodic branches of the polarisation curves of the Tafel plots. Table 3: corrosion potential Ecorr, the corrosion current density Icorr, Sample designation
Ecorr (V)
Icorr (μAcm-2)
Ti6Al4V
-0.530
8.556
Corrosion Rate (mm/year) 0.0744
Ti6Al4V/3Cu
-0.422
3.227
0.0279
Ti6Al4V/5Cu
-0.394
1.422
0.0123
The potentiodynamic polarisation curves obtained from the test conducted on the Ti6Al4V and Ti6Al4V/Cu composites in prepared artificial sea water are presented. Fig. 5 shows the potentiodynamic polarisation curves of the Ti6Al4V, Ti6Al4V/3Cu and Ti6Al4V/5Cu composites in sea water.
Fig. 5. Potentiodynamic polarisation curves of Ti6Al4V and Ti6Al4V/Cu composites in artificial sea water under an aerated condition
The polarisation curves are made up of three branches of domain. The cathodic domains found below the corrosion
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potentials; where the corrosion current is determined by reduction process. The constricted region at the locality of the corrosion potential established between the cathodic and anodic transition. The passive regions at the current lines and the partial transpassive regions at the potentials. The primary passive current, breakdown potential and the transpassive current are obtained from the potentiodynamic anodic polarization plots. The anodic branches of the polarization curves exhibit similar trend from the corrosion potential to the transpassivation region except Ti6Al4V that shows a deviation of the passive current. The anodic branch of polarization curve obtained for Ti6Al4V from the test under aerated condition exhibits a low pitting potential. The primary passive current of Ti6Al4V forms a flade potential within the active-passive region at a potential value of approximately 0.8 V. Just before the edge of the flade potential, metastable pits were formed between the potential value of approximately 0.3 V and 0.65 V. Afterwards, the primary passive current triumphs to the transpassivation domain. The Ti6Al4V composite shows the highest current density Icorr of 8.556 μAcm-2 and the lowest potential Ecorr of -5.30 V. The anodic branch of polarization curve for Ti6Al4V/3Cu composite maintains a good primary passive current and prevails to the tranpassivation region at a potential value of approximately 1.45 V without a form of potential breakdown. The Ecorr obtained was -0.422 V which was a bit higher than the Ti6Al4V; and the I corr of 3.227 μAcm-2 was achieved. The anodic branch of polarization curves of Ti6Al4V/5Cu also reveals good primary passive current. Although, a sharp metastable pit occur between the potential values of -0.15 V and 0.1 V. However, the probability for pit balance was minimized within the nucleation site and owing to the continuous passive current trend after the metastable pit. Out of the three composites,Ti6Al4V/5Cu exhibits the highest Ecorr of -0.394 V and the lowest Icorrof 1.422 μAcm-2.
4. Conclusion Ti6Al4V alloy possesses an excellent corrosion resistance due to their high affinity for oxygen when subjected to an aggressive environment. The oxidizing fume formed on their surfaces give rise to a good protection against corrosion attack. The surface modification of the alloy with the addition of 3 and 5 weight percent of Cu was successfully achieved.Amongst the tested composites, Ti6Al4V/5Cu composite exhibits the lowest corrosion rate of 0.0123 mm/year. Ti6Al4V/3Cu unveils a corrosion rate of 0.0279 mm/year while Ti6Al4V shows the highest corrosion rate of 0.0744 mm/year. Invariably, the presence of Cu in the Ti6Al4V lattices significantly enhancedthe rate of corrosion. And in addition, it canalso be deduced and inferred that an increase in the Cu content also leads to the improvement in the corrosion rate of Ti6Al4V. The Ti6Al4V/Cu composites find potential application in the marine industries and other related field.
Acknowledgements This work is supported by the Rental Pool Programme of National Laser Centre, Council of Scientific and Industrial Research (CSIR), Pretoria, South Africa and also, the main author acknowledges the African Laser Centre for the bursary award. References [1] V. N. Moiseyev, Titanium alloys: Russian aircraft and aerospace applications, CRC Press Taylor & Froes Group, (2006) pp 169-180. [2]G. Lutjering and J. C. WILLIAMS, Titanium, Engineering Materials and processes, Springer, Second Edition, (2007) pp 1-449. [3]S. A. Al-Fozan, and A. U. Malik, Effect of seawater level on corrosion behavior of different alloys, Desalination 228 (2008) 61–67. [4]P. K. Wong, C. T. Kwok, H. C Man and F. T. Cheng, Corrosion Behavior of Laser-alloyed Copper with Titanium fabricated by high power diode laser, Corrosion Science, 57, (2012) pp 228-240. [5]Technical data sheet, Carpenter. Titanium alloy. Copyright 2013 Dynamet Holdings Inc., All rights reserved. Accessed 2013. [6]D. W. Shoesmith, Assessing the corrosion performance of high-level nuclear waste containers, Corrosion, 62, (2006) pp 703-722.
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[7]C. Delgado-Alvarado and P.A. Sundaram, A study of the corrosion behavior ofgamma titanium aluminide in 3.5 wt% NaCl solution and seawaterCorrosion Science 49 (2007) 3732–3741. [8]E. M. Sherif, H. R. Ammar, K. A. Khalilb, Effects of copper and titanium on the corrosion behavior of newlyfabricated nanocrystalline aluminum in natural seawater, Applied Surface Science 301 (2014) 142–148. [9]V.A. Alves ,R.Q. Reis , I.C.B. Santos , D.G. Souza , T. de F. Gonçalves , M.A. Pereira-da-Silva, A. Rossi and L.A. da Silva,In situ impedance spectroscopy study of the electrochemical corrosion of Ti and Ti–6Al–4V in simulated body fluid at 25 oC and 37oC,Corrosion Science 51 (2009) 2473–2482. [10] G.Mallaiah, P. Ravinder Reddy, A. Kumar, Influence of titanium addition on mechanical properties, residualstresses and corrosion behaviour of AISI 430 grade ferritic stainlesssteel GTA welds, Procedia Materials Science 6 ( 2014 ) 1740 – 1751. [11]E3-11., 2011. Standard Guide for Preparation of Metallographic Specimens, ASTM International (Accessed from the data base). [12]M. F. Erinosho, E. T. Akinlabi, and S. Pityana, Effect of Powder Flow Rate and Gas Flow Rate On The Evolving Properties Of Deposited Ti6Al4V/Cu Composites, Advanced Materials Research 1016, pp 177-182. [13] Struers Application Note on Titanium. http://www.struers.com/resources/elements/12/104827/Application_Note_Titanium _English.pdf. Assessed 2013. [14]G102 -89 ASTM Internationa standard, Standard Practice forCalculation of Corrosion Rates and Related Informationfrom Electrochemical Measurements1. [15]J. Chen and F. Yan, Tribocorrosion behaviours of Ti6Al4V and Monel K500 alloys sliding against 316 stainless steel in artificial seawater, Transactions of Nonferrous Metals Society of China, Vol. 22, (2012) pp 1356-1365. [16]Y. S. Jiménez, M. T. GIL, M. T. Guerra, L. S. Baltes and J. C. M. Rosca, Interpretation of Open Circuit Potential of Two Titanium Alloys for a long time immersion in Physiological Fluid, Bulletin of the Transilvania, University of Brasov, Vol. 2 (51), (2009) pp 197-204. [17]M. Iijima, K. Endo, H. Ohno and I. Mizoguchi, Effect of Cr and Cu addition on corrosion behavior of Ni-Ti alloys, Journal of Dental Material, 17(1), (1998) pp 31-40.
ScienceDirect Materials Today: Proceedings 2 (2015) 1166 – 1174
4th International Conference on Materials Processing and Characterization
Microstructure and corrosion behaviour of laser metal deposited Ti6Al4V/Cu composites in 3.5% sea water Mutiu F. Erinoshoa,*, Esther T. Akinlabia, Sisa Pityanab a
Department of Mechanical Engineering Science, University of Johannesburg, Auckland Park Kingsway Campus, Johannesburg, 2006, South Africa. b National Laser Centre, Council for Scientific and Industrial Research (CSIR), Pretoria, 0001, South Africa.
Abstract Titanium alloy applications in the marine industries are due to their excellent corrosion resistance when exposed to surface contaminants. This paper reports the microstructure and corrosion behaviour of laser metal deposited Ti6Al4V/Cu composites in 3.5% artificial sea water.During the deposition process, laser power of 1600 W and scanning speed of 0.3 m/min were kept constant throughout the entire process. The SEM of both Ti6Al4V/Cucomposites shows the formation of Widmanstatten structures and the branches became thinnertoward the fusion zone. This was attributed to the reduction in the distribution and the flow of the heat input. The corrosion resistance behaviour of the Ti6Al4Vcompositesfrom the potentiodynamic polarization curve has been greatly improved with the addition of Cu content and this in turn has enhanced its surface modification. © 2015 2014Elsevier The Authors. Elsevier Ltd. All rights reserved. © Ltd. All rights reserved. Selection peer-review under responsibility of thecommittee conference committee of conference the 4th International conference Selection andand peer-review under responsibility of the conference members of the 4thmembers International on Materials Processing and on Characterization. Materials Processing and Characterization. Keywords:Corrosion; laser metal deposition; microstructures; seawater; Ti6Al4V/Cu composites
* Corresponding author. Tel.: +27747425924. E-mail address: [email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization. doi:10.1016/j.matpr.2015.07.028
Mutiu F. Erinosho et al. / Materials Today: Proceedings 2 (2015) 1166 – 1174
1167
1. Introduction Ti6Al4V is most applicable among the titanium alloys due to its combination of physical, mechanical, and corrosion resistance which have made them necessary for demanding in the industries such as the aerospace, marine, energy and automobile [1-2]. Essentially, seawater is chemically aggressive to materials and plants under it, and these materials are exposed to different corrosion test depending upon the utilizing environment and the nature of the material itself [3]. The corrosion potentials of a laser-alloyed specimens decrease as the titanium (Ti) content increases and the improvement in the corrosion resistance of Ti was attributed to the presence of Ti in the intermetallic and metallic phases for forming the protective oxide [4].Ti6Al4V spontaneously formed a stable, continuous, and adherent oxide film upon exposure to oxygen in water or air which was very vital to its excellent corrosion resistance behaviour. Its corrosion behaviour occurred in aqueous solutions such as seawater and alkalis [5]. The properties of the passive film on Ti alloys are a function of potential and susceptible to various corrosion processes. The stability of the film can be improved, however, if inhibitors could be added to these environments [6]. Gamma titanium aluminide was characterised for corrosion resistance in 3.5% NaCl and seawater using potentiodynamic polarization and electrochemical impedance spectroscopy. The surface of the sample was modified through oxidation in air at 500oC and 800oC. The results indicated that the sample passivated and show good corrosion behaviour in both 3.5% NaCl and seawater solutions [7]. The fabrication of nanocrystalline Al, Al10%Cu, and Al-10%Cu + 5%Ti alloys was conducted for corrosion in natural sea water using the mechanical alloying method. The presence of Cu, Al-10%Cu alloy reduces the corrosion of Aluminium. The addition of 5%Ti to the alloys produced the best passivation to the surface of Al with the lowest corrosion rate [8]. The effect of temperature on the electrochemical corrosion behaviour of Ti and Ti6Al4V was conducted for biomaterial application. The corrosion resistance of Ti was increased with the time of immersion at body temperature (37oC) whilst the corrosion resistance of Ti6Al4V was decreased and occurred as a result of dissolution of the passive film [9].The effectoftitanium (1g to 3g)on AISI 430 grade ferritic stainless has been proved to improve the mechanical properties and corrosion behaviour of the stainless steel. The grain refinement in the weld zone was due to the formation of fine grain microstructure and precipitates [10]. This research work hence is aimed at describing the impactof laser metal deposition (LMD) process on the microstructure and corrosion behaviour of laser metal deposited Ti6Al4V/Cu composites in 3.5% artificial sea water. Three depositions were made which comprises of the Ti6Al4V composite, 3 weight percent of Cu and Ti6Al4V (Ti6Al4V/3Cu) and 5 weight percent of Cu plus Ti6Al4V (Ti6Al4V/5Cu). Nomenclature Ecorr Icorr
corrosion potential current density
2. Experimental techniques The LMD of the Ti6Al4V and Ti6Al4V/Cu composites were conducted at the National Laser Centre of Council of Scientific Industrial Research (NLC-CSIR), Pretoria, South Africa, using the Ytterbium Laser System equipment (YLS-2000-TR). This laser is equipped with a maximum power of 2000 Watts and uses a Kuka robot as the attaching component. 2.1. Materials and methodology A solid plate of 99.6 % titanium alloy (Ti6Al4V) with dimension of 102 X 102 X 7.45 mm3 was used as the substrate. The substrate was sand blasted to remove the contaminants attached to its surface and to prepare the
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surface for firm metallurgical bonding. The sand blasted substrate was cleanedwith acetone and dried up. Ti6Al4V and Cu powders with particle sizes varied between 150 and 200 μmwere used for the deposition. Two separate hoppers (feeders) were setup for the two powders. All the powders flow through the connecting hoses attached to the three way nozzle.Fig.1 shows the schematic view of the LMD process. The laser power of 1600 W and scanning speed of 0.3 m/minwere kept constant for both the Ti6Al4V and Ti6Al4V/Cu composites. 3 and 5 weight percent of Cu powder were deposited with Ti6Al4V. The standoff distance of 12 mm and a beam diameter of 4 mm were used throughout the experimental setup. The flux on the composites was cleaned with a metal brush. According to E3-11 ASTM standard, all the samples were grinded, polished and etched [11].
Fig.1. Schematic view of the LMD process [12]
Table 1 shows the matrix used for the experiments. The powder flow rate of the Ti6Al4V was varied between 2.4 and 2.5 rpm; and the gas flow rate was also varied between 2.9 and 3 l/min respectively. Table1. Experiment matrix Sample Designation
Powder flow rate (rpm) Ti6Al4V
Ti6Al4V
2.4
Ti6Al4V/3Cu
2.4 0.1
Ti6Al4V/5Cu
2.5
Nil
0.1
Cu
Gas flow rate (l/min) Ti6Al4V
Cu
2.9
Nil
3
1
3
1
2.2. Microstructure The Kroll’s reagent was prepared with 100 ml of H2O, 2-3 ml HF and 4-6 ml HNO3 for the etchant prior to the SEM observation. This was prepared according to the Struers application note formetallurgical preparation of titanium [13]. The samples were etched for 10-15 seconds, sprayed with acetone, rinsed under clean running water and dried off. The microstructures of all the etched samples were observed under the SEM TESCAN machine equipped with VEGA TC software.
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2.3. Corrosion test The electrochemical measurements for the corrosion rate were conducted using the standard practice according to the G102-89ASTM [14]. A Potentiostat (PSTAT), equipment wasused for the corrosion measurement. The equipment is coupled with a computer and uses GPES Manager Version 4.9 software for its function. A bridge tube beaker was prepared for the experiment and the reference electrode (RE) tube was filled with a 3 Molar Potassium Chloride (3M KCL). The reference electrode and the counter electrode were inserted into the bridge tube beaker compartment. The samples were also inserted via the bridge tube to the compartment and facing the RE in order to sense the applied potential.All the samples were first ran in Open Circuit Potential (OCP) for 7200 s and a scan rate of 2 mV/s before running the complete polarisation. The potentiodynamic polarisation curves were measured from 1.5 V to 2 V at a scan rate of 2 mV/s. The three samples were run with freshly prepared electrolyte (sea water). The constituents used for the preparation of the sea water are shown in Table 2 and masses of the chemical compounds are similar to what Chen and Yan, 2012 prepared for their sea water [15]. A digital pH meter, ORION Model 520A was used to measure the pH of the solution. The pH of the 3.5 % sea water prepared is between 7.95 and 8.10. Table 2. Sea water constituents, appearance and the mass of chemical compounds Chemical Compounds Appearance Mass (g) NaCl
White powder
49.06
Na2SO4
White powder
8.18
CaCl2
White dry granulated
3.16
MgCl2
White granulated
10.4
KCl
White powder
1.39
NaHCO3
White powder
0.40
NaF
White granulated
0.006
KBr
White granulated
0.20
3. Results and discussion The microstructural results, open circuit potential andpotentiodynamic polarisation curves are presented and discussed in this section. 3.1. Microstructural evaluation An analysis was carried out on the Ti6Al4V and Cu powders under the SEM. Figs. 2 (a) and (b) show the morphologies of the Ti6Al4V and Cu powder used for the deposition.The morphologies of the two powders are spherical and equiaxed in structure. The Cu particles were observed to be heavier and denser than that of the Ti6Al4V particles during the flow rate measurements.
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(a)
(b)
Fig. 2. (a) The SEM morphology of Ti6Al4V powder, (b) The SEM morphology of Cu powder
The microstructures of the Ti6Al4V/Cu samples were observed under the SEM. Figs. 3 (a) and (b) show the SEM of Ti6Al4V/3Cudeposited at a powder flow rate of 2.4 rpm and Ti6Al4V/5Cuproduced at a powder flow rate of 2.5 rpm.
Figs. 3. (a) SEM of Ti6Al4V/3Cu deposited at a powder flow rate of 2.4 rpm, (b) Ti6Al4V/5Cu produced at a powder flow rate of 2.5 rpm
Majorly, the Widmanstettan structures were observed in both composites under the SEM. Widmanstettan structures were observed to be robust (thick) at the top of the deposited composites and loses their robustness as the heat input travels down to the substrate. The Widmanstettan structures grows towards the substrate and becomes thinner and faintier as they approach the fusion zone. This occurrence is as a result of the rate at which the heat input sinks and travels into the substrate and the interference between the substrate interface. In other word, as the heat
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input loses its radiating energy. The branches of the widmanstettan structures toward the fusion zone becomes very thin, sharp and closer. This gives an indication of a strong bond that exist between the deposit and the substrate interface. The formation of α-martensite within this boundary predominate and have a significant effect on the strength of the composite most especiallyat the interface. 3.2. Corrosion results The open circuit polarization (OCP) of Ti6Al4V and Ti6Al4V/Cucomposites are presented in Fig.4. It can be observed that the OCP of Ti6Al4V experiences a cathodic fall at the starting point between 2-5 seconds from -0.08 V to -0.62 V; and maintains unstable fluctuation at that potential throughout the 120 minutes. The cathodic fall could be attributed to oxide formed on the surface been weak. The instability of the potentials was in the range between -0.58 V and -0.62 V. These potentials may vary with time due to the variations of oxidation and formation of the passive layer in the nature of the surface of the electrode [16].A similar observation occurred with the OCP of Ti6Al4V/3Cu. A cathodic fall from -0.14 V to -0.52 V was also depicted and maintains unsteadiness of potentials between -0.46 V and -0.58 V.
Fig. 4. Variation in the open circuit potential as a function of time for Ti6Al4V, Ti6Al4V/3Cu and Ti6Al4V/5Cu in 3.5 % sea water
The OCP of Ti6Al4V/5Cu deposited at a laser power of 1600 W and scanning speed of 0.3 m/min demonstrates a slight rise in the corrosion potential directly from -0.28 V at zero point to -0.26 V at 1000 seconds. A considerable cathodic rise of the potential takes place until -0.18 V at 1400 seconds and maintains an inconsistent sinusoidal and continuous cathodic rise till a corrosion potential of -0.13 V with time. This indicates that the corrosion rate with time increases for Ti6Al4V/5Cu composite. This rise gives an indication of film growth on the surface within the solution. The OCP is important and serves as a benchmark for the corrosion behavior [16]. However, the stabilities of Ti6Al4V and Ti6Al4V/3Cu composites are low in the prepared sea water used as observed in the fluctuations of the OCP values in Fig. 4.From the observations, it could be deduced that the corrosion resistances with respect to time of Ti6Al4V/Cu samples were more than the Ti6Al4V sample. In comparing the OCP values, Ti6Al4V/5Cu composite exhibited the highest resistance to corrosion and show good oxidation behaviour.Ti6Al4V/3Cu also shows a corrosion resistance but less than the Ti6Al4V/5Cu composite. In other word, Cu has greatly show an impact in improving the corrosion resistance of Ti6Al4V and also an increase in its content within the surface
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treatment significantly enhanced the resistance to corrosion with respect to time. The metallic Cu improved the corrosion behavior of Ni-Ti alloy and the oxide film interface and increased the susceptibility to pitting corrosion [17]. From the results of the corrosion test conducted, Table 3 shows the corrosion potential Ecorr, the corrosion current density Icorr, and the corrosion rate obtained using both cathodic and anodic branches of the polarisation curves of the Tafel plots. Table 3: corrosion potential Ecorr, the corrosion current density Icorr, Sample designation
Ecorr (V)
Icorr (μAcm-2)
Ti6Al4V
-0.530
8.556
Corrosion Rate (mm/year) 0.0744
Ti6Al4V/3Cu
-0.422
3.227
0.0279
Ti6Al4V/5Cu
-0.394
1.422
0.0123
The potentiodynamic polarisation curves obtained from the test conducted on the Ti6Al4V and Ti6Al4V/Cu composites in prepared artificial sea water are presented. Fig. 5 shows the potentiodynamic polarisation curves of the Ti6Al4V, Ti6Al4V/3Cu and Ti6Al4V/5Cu composites in sea water.
Fig. 5. Potentiodynamic polarisation curves of Ti6Al4V and Ti6Al4V/Cu composites in artificial sea water under an aerated condition
The polarisation curves are made up of three branches of domain. The cathodic domains found below the corrosion
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potentials; where the corrosion current is determined by reduction process. The constricted region at the locality of the corrosion potential established between the cathodic and anodic transition. The passive regions at the current lines and the partial transpassive regions at the potentials. The primary passive current, breakdown potential and the transpassive current are obtained from the potentiodynamic anodic polarization plots. The anodic branches of the polarization curves exhibit similar trend from the corrosion potential to the transpassivation region except Ti6Al4V that shows a deviation of the passive current. The anodic branch of polarization curve obtained for Ti6Al4V from the test under aerated condition exhibits a low pitting potential. The primary passive current of Ti6Al4V forms a flade potential within the active-passive region at a potential value of approximately 0.8 V. Just before the edge of the flade potential, metastable pits were formed between the potential value of approximately 0.3 V and 0.65 V. Afterwards, the primary passive current triumphs to the transpassivation domain. The Ti6Al4V composite shows the highest current density Icorr of 8.556 μAcm-2 and the lowest potential Ecorr of -5.30 V. The anodic branch of polarization curve for Ti6Al4V/3Cu composite maintains a good primary passive current and prevails to the tranpassivation region at a potential value of approximately 1.45 V without a form of potential breakdown. The Ecorr obtained was -0.422 V which was a bit higher than the Ti6Al4V; and the I corr of 3.227 μAcm-2 was achieved. The anodic branch of polarization curves of Ti6Al4V/5Cu also reveals good primary passive current. Although, a sharp metastable pit occur between the potential values of -0.15 V and 0.1 V. However, the probability for pit balance was minimized within the nucleation site and owing to the continuous passive current trend after the metastable pit. Out of the three composites,Ti6Al4V/5Cu exhibits the highest Ecorr of -0.394 V and the lowest Icorrof 1.422 μAcm-2.
4. Conclusion Ti6Al4V alloy possesses an excellent corrosion resistance due to their high affinity for oxygen when subjected to an aggressive environment. The oxidizing fume formed on their surfaces give rise to a good protection against corrosion attack. The surface modification of the alloy with the addition of 3 and 5 weight percent of Cu was successfully achieved.Amongst the tested composites, Ti6Al4V/5Cu composite exhibits the lowest corrosion rate of 0.0123 mm/year. Ti6Al4V/3Cu unveils a corrosion rate of 0.0279 mm/year while Ti6Al4V shows the highest corrosion rate of 0.0744 mm/year. Invariably, the presence of Cu in the Ti6Al4V lattices significantly enhancedthe rate of corrosion. And in addition, it canalso be deduced and inferred that an increase in the Cu content also leads to the improvement in the corrosion rate of Ti6Al4V. The Ti6Al4V/Cu composites find potential application in the marine industries and other related field.
Acknowledgements This work is supported by the Rental Pool Programme of National Laser Centre, Council of Scientific and Industrial Research (CSIR), Pretoria, South Africa and also, the main author acknowledges the African Laser Centre for the bursary award. References [1] V. N. Moiseyev, Titanium alloys: Russian aircraft and aerospace applications, CRC Press Taylor & Froes Group, (2006) pp 169-180. [2]G. Lutjering and J. C. WILLIAMS, Titanium, Engineering Materials and processes, Springer, Second Edition, (2007) pp 1-449. [3]S. A. Al-Fozan, and A. U. Malik, Effect of seawater level on corrosion behavior of different alloys, Desalination 228 (2008) 61–67. [4]P. K. Wong, C. T. Kwok, H. C Man and F. T. Cheng, Corrosion Behavior of Laser-alloyed Copper with Titanium fabricated by high power diode laser, Corrosion Science, 57, (2012) pp 228-240. [5]Technical data sheet, Carpenter. Titanium alloy. Copyright 2013 Dynamet Holdings Inc., All rights reserved. Accessed 2013. [6]D. W. Shoesmith, Assessing the corrosion performance of high-level nuclear waste containers, Corrosion, 62, (2006) pp 703-722.
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