稀有金属(英文版) 2020,39(12),1449-1456

Corrosion resistance and antibacterial activity of different zones in TA2 weldment by TIG welding

Yan-Ping Zhu Chang-Yi Li Lian-Yun Zhang

Department of Prosthodontics,Tianjin Stomatological Hospital

School of Stomatology,Tianjin Medical University

作者简介：*Lian-Yun Zhang,e-mail:lianyun_zhang@126.com;

收稿日期：18 October 2013

基金：financially supported by the National Natural Science Foundation of China (No.81070871);

Corrosion resistance and antibacterial activity of different zones in TA2 weldment by TIG welding

Yan-Ping Zhu Chang-Yi Li Lian-Yun Zhang

Department of Prosthodontics,Tianjin Stomatological Hospital

School of Stomatology,Tianjin Medical University

Abstract：

The corrosion resistance behavior of TA2 pure titanium processed by tungsten inert gas(TIG) welding was investigated in artificial saliva solution at 37℃.By metallographic examination,electrochemical measurement technology,and electrochemical impedance spectroscopy(EIS),the corrosion resistance of the base metal(BM),heat-affected zone(HAZ),and weld metal(WM) were investigated.Metallographic examination experiments show that welding process would cause the growth of grain size.In addition,phase change happens in the HAZ and WM.The change of grain size and phase would influence the generation of the original passive film.The electrochemical tests show that the BM,HAZ,and WM are all equipped with good corrosion resistance.The welded joint shows a better corrosion resistance than the original TA2.It is shown that the BM with the lowest corrosion potential and the biggest corrosion current has a worse corrosion resistance than WM as well as HAZ.Silver(Ag) nanoparticles can be distributed on the WM zone of Ti uniformly.The WM zone of Ti with Ag coating considerably enhances the antibacterial activity of Ti implants.

Keyword：

Titanium; Tungsten inert gas welding; Corrosion resistance; Artificial saliva solution; Antibacterial activity;

Received： 18 October 2013

1 Introduction

Titanium and its alloy are extensively used in dental application owing to their low density,excellent biocompatibility,corrosion resistance,high strength-to-weight ratio,and relatively low thermal conductivity [ 1, 2, 3] .Grade 2 commercial pure titanium,known as TA2 pure titanium,is one of the most commonly used titanium alloy.TA2 pure titanium is a typicalα-type titanium alloy with a close-packed hexagonal structure [ 4] .Because TiO₂ can easily form on the surface of TA2 pure titanium,it possesses good corrosion resistance.Hence,it c an improve the corrosion resistance in most cases [ 5] .The joining of a metal framework is frequently necessary for dental prostheses to achieve efficient treatment procedures.Laser welding is the conventional method of joining titanium metals in different clinical settings.However,welding quality varies greatly due to the different radiation intensities used.Tungsten inert gas (TIG) welding demanding less expensive equipment is carried out under an argon environment and is extensively used in Ti industrial production;its corrosion resistance is one of the important properties of this welding system.TA2 titanium alloys,in many cases,are fabricated together by TIG welding which is widely used with the thickness of less than 10 mm.However,TIG welding is a complicated physicochemical process in which heat transferring,metal melting and freezing,phase change,welding stress,and deformation are involved [ 6] .The corrosion resistance can be changed by the welding process since the corrosion resistance is sensitive to the change of the microstructure,chemical composition,uniformity,the crystal structure,the grain size,the inner stress,and defects,which are related to the TIG welding process [ 7] .Several studies were reported on the corrosion behavior of welded titanium.Blasco-Tamarit et al. [ 8] studied the galvanic corrosion generated between the titanium-welded titanium pair in heavy brine LiBr solutions at 25,50,and 100℃.The results showed that welded titanium was the anode of the pair so that its corrosion resistance decreased due to the galvanic effect.However,the extremely low galvanic current densities registered by the pair revealed the poor severity of the coupling under the studied conditions.In another study,Atapour et al. [ 9] investigated the corrosion behavior of Ti-6Al-4 V alloy weldment in hydrochloric acid.The results revealed that the corrosion resistance of the base metal (BM) and weld metal(WM) decreased with the temperature and acid concentration increasing,and the WM exhibited higher corrosion rate than the BM in all examined conditions.Orsi et al. [ 10] studied the corrosion behavior of the TA2 CP titanium welded by TIG welding and laser welding,respectively.They draw the following conclusions:the TA2 specimens welded with TIG were more resistant to local corrosion initiation and propagation than those with laser welding,indicating a higher rate of formation and growth of passive film thickness on the surfaces of these alloys compared with specimens welded with laser,which made the occurrence of corrosion more difficult.However,it was indicated that few studies were conducted to determine the corrosion resistance and antibacterial activity of different zones in the TA2-welded joints.

Accordingly,in the present study,the corrosion behavior of the BM,heat-affected zone (HAZ),and WM by TIG welding in an artificial saliva solution at 37℃was analyzed using electrochemical measurement technology,metallographic examination,and electrochemical impedance spectroscopy (EIS).Moreover,Ag nanoparticles were also deposited on Ti surface,and the antibacterial property of Ti coated with Ag nanoparticles was evaluated in vitro.

2 Experimental

2.1 TIG welding

The TA2 pure titanium plate was selected as the base metal.The tungsten inert gas arc welding machine should have the following characteristics:(1) the straight polarity direct current,(2) high frequency arc starting,and (3) power supply attenuation device.Argon was introduced into the TA2 pipe,welding torch and gas shielded equipment.The purity of the argon used in the process must be greater than 99.99%.The choice of the gas flow of the argon was determined by whether the needed color could be got on the surface of welding joint.The grade welding wire was used in the welding experiment.Tables 1 and 2 show the BM and Ti welding wire,respectively,used in the present study.It is very important to make sure that the argon atmosphere is maintained during the welding procedure and the next cooling period.A double layer welding method was used and Table 3 displays the welding parameters.

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Table 1 Chemical composition (wt%) and mechanical properties of BM welding wire

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Table 2 Chemical composition (wt%) and mechanical properties of Ti welding wire

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Table 3 Welding parameters

2.2 Metallographic examination

The specimens prepared for the metallographic examination were sectioned nearby the weld joints.All the grinding,polishing,and etching processes followed the ASMEⅨstandard (qualification standard for welding and brazing procedures,welders,brazers,and welding and brazing operators).After that,a metallurgical microscope(OLYMPUS-GX51) was used to observe the metallographic structure of the samples.

2.3 Electrochemical measurement

The specimens(Φ4 mm×4 mm) were prepared for the electrochemical measurement.Since the surface roughness can obviously affect the corrosion behavior of titanium,all the specimens should be polished using 600-mesh sandpaper [ 11] .

The experiments were carried out in artificial saliva solution at 37℃.The artificial saliva composition is shown in Table 4.A conventional three-electrode electrochemical cell system was used in electrochemical measurement.Inside the glass cell,a saturated calomel reference electrode,a Pt counter electrode and a working electrode were placed.The corrosion measurements for each specimen condition were conducted in the following sequences:(1) open circuit potential (OCP) measurement and (2) potential dynamic polarization measurement.The purpose of OCP measurement was to get a stable open circuit potential before the potential dynamic polarization measurement.Potential dynamic polarization measurement started at-2,000 mV below the OCP at a rate of 1 mV·s^-1and terminated at 2,000 mV above the OCP.Qualitative analyses were made for the potential dynamic polarization measurement diagrams.Polarization resistance,cathodic and anodic Tafel slopes were determined from the experiment using the computer software.The corrosion rate(I_corr)of the alloy was determined according to the SternGeary equation [ 12] :

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Table 4 Artificial saliva composition

where R_p is the polarization resistance,β_aandβ_c are the anodic and cathodic Tafel slopes.

2.4 EIS

The variations of the corrosion behavior for the passive film formed on the surface of the samples before and after potential dynamic polarization were studied using EIS technique (GAMRY Interface 1000).The experiments were conducted with voltage perturbation amplitude of10 mV in the frequency range from 100 to 100,000 Hz.

2.5 Antibacterial property of Ti-coated Ag nanoparticles

The TA2 weldment was immersed in 0.03 mol·L^-1 AgNO₃solution (concentrations of NaBH₄ and PVP are 0.026 and0.001 mol·L^-1,respectively) for 3 h to prepare Ag nanoparticles on Ti-surface assembly.The surface morphology of the samples was examined by scanning electron microscopy (SEM,HITACHI S-4800,Japan).The bacterial survival rates on the Ti specimen were assessed by bacterial viability tests on agar plates which contained 106 CFU/plate of inpidual test strains.Escherichia coli (E.coli,ATCC 25922) and Staphylococcus aureus (S.aureus,ATCC 25923) were used as reference strains for antibacterial testing.The samples were wetted with 0.1 ml additional sterile pure water before testing.After air drying for30 min at room temperature,the plates were incubated at37℃for 16 h.Then visible bacterial colonies on the Ti specimen before and after Ag deposition were measured.

3 Results and discussion

3.1 Metallographic examination

Owing to the effects of the welding heat input,martensitic transformation happens.Firstly,the phase transforms toβphase above 883℃during the welding process.Then the igh temperature BCCβphase transforms to the low emperature HCPαmartensite phase during the cooling rocess [ 13] .The metallographic structure of the BM,HAZ,and WM are shown in Fig.1.It can be clearly bserved that the base metal has an equiaxial grains with he smallest grain size.No segregation can be seen in BM.Compared with the BM,the grains of the HAZ show diferent characteristics.The HAZ has a bigger grain size than he BM.In addition,the grain orientation of the HAZ is no onger uniform.Because the area near the weld metal is ubjected to the highest temperature in the welding process,he grains of the weld metal own the biggest grain size.The martensitic structure can be found in the WM and the nhomogeneity of chemical composition appears.

.2 Potential dynamic polarization measurement

igure 2 shows the potential dynamic polarization curves of he BM,HAZ,and WM in artificial saliva solution at 37℃.This procedure was performed in order to analyze the continuity,stability,and intensity of the passive Ti oxide film formation.It is clear that all the samples exhibit a typical active-passive characterization,translating directly into the passive region from the Tafel region.The corrosion potentials (E_corr) can be estimated from these curves as-0.43,-0.34,and-0.17 V for the BM,HAZ,and WM,respectively.The corrosion current densities (I_corr) obtained by Tafel analysis using both anodic and cathodic branches of the polarization curves are listed in Table 5.The results show that HAZ has the smallestI_corr of 1.15×10^-6 A·cm^-2,which is slightly higher than that of the weld(2.95×10^-6 A·cm^-2).The BM exhibits the highest I_corr among the three samples as 3.44×10^-6 A·cm^-2.According to the E_corr and the I_corr,a conclusion can be drawn that the BM with the lowest corrosion potential and the highest corrosion current has a worse corrosion resistance than the WM as well as HAZ.When it comes to evaluating the property of the material with a passive film,however,more attention should be paid to the breakdown potential and breakdown current of the passive film.

Fig.1 OM images of a BM,b HAZ and c WM zones

Titanium is a typical example of a metal endowed with ability to establish a naturally passive film in appropriate environment.Titanium tends to react with air and water to generate TiO₂,and this phenomenon can contribute greatly to improving the corrosion resistance of titanium and titanium alloy.The breakdown potential (E_b) and breakdown current (Ipass) are introduced to evaluate the corrosion resistance of the material with a passive film.We can get E_b and I_pass from the polarization diagrams.Table 6 shows E_b and I_pass of HAZ and weld.The BM shows a wider passivation region than HAZ and WM,as evidenced by the current remaining constant with the increase of potential,which indicates that the passive film forming on the surface of TA2 titanium alloy is very integral and protective to prevent corrosion.No breakdown potential is observed with the potential up to 2.0 V in base metal,which shows a passive film with a different characteristic.Table 6 lists the E_b and I_pass of the HAZ and WM zones by investing the polarization curves.From the data listed in Table 6,it can be judged that HAZ has better passivation properties than WM.

Fig.2 Polarization curves of BM,HAZ,and WM zones

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Table 5 Corrosion current density and corrosion potential of BM,HAZ,and WM zones

It can make a conclusion that the HAZ has a more compact passive film than the BM and WM,but the BM shows a more stable passive film by a wider passive region.This consequence is in relation to the change of the microstructure in the welding process.A more compact TiO₂ generates after TIG welding.

3.3 EIS analyses

The Niquist and bode diagrams are placed in Figs.3 and 4,respectively.It is clear from Fig.3 that all the Nyquist plots can be characterized by the incomplete semicircle,a beha^vior of near capacitive response.By observing the incomplete semicircle,it can be found that the highest capacitive semi-arc diameter is displayed for the HAZ,which means the HAZ has the best corrosion resistance.The diameter of the WM is bigger than that of the BM,which indicates a higher corrosion resistance of the WM in comparison with the BM.For bode magnitude plots in Fig.4,two distinct regions for these three simples are observed.In the high frequency (1×10³-1×10⁵ Hz)range,there exists a flat portion (slope≈0) due to the response of electrolyte resistance.In the region with high frequency,the spectra display a linear slope of about-1and it is the characteristic response of a capacitive behavior of passive film [ 14, 15] .

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Table 6 Breakdown potential (E_b) and breakdown current (I_pass) of HAZ and WM zones

Fig.3 Niquist plots in artificial saliva solution at 37℃

Fig.4 Bode plots in artificial saliva solution at 37℃

Fig.5 Equivalent circuit

For the WM,HAZ,and BM,bode phase plots in Fig.4reveal a wide frequency range in which the slope of the phase angle keeps invariable.Such phenomenon can be explained by the formation of single passive oxide film on the surface [ 14] .Since higher impedance and phase angle are conducive to a nobler electrochemical behavior,the curves of the HAZ and WM indicate that they both have a better corrosion resistance compared with the BM.

It is well accepted that titanium is covered with two oxide layers [ 16, 17] .In the present study,the equivalent circuit used to model the EIS data obtained for the polished titanium is presented in Fig.5.The circuit may be regarded as an electrical representation of a two-layer model of the oxide film consisting of a barrier-type compact inner layer and a relatively porous outer layer.In this circuit,R_s is the electrolyte resistance,R₁ and R₂ are the resistances of the outer and inner TiO₂ layers,respectively,and Q₁ and Q₂are parameters related to the capacity of the outer and inner oxide layers,respectively.Here the constant phase element(CPE) was used in the fitting instead of capacitance (C)because the former considers the fact that the barrier film never exhibits the theoretically expected phase shift of 90and a slope of 1 for an ideal dielectric.In fact,the impedance of a CPE is defined as [ 18] :

where C is the capacitance associated to an ideal capacitor,j is the current,w is the angular frequency,and n is a factor accounting for the deviation from the ideal capacitive behavior due to surface inhomogeneity,roughness factors,and adsorption effects.When the value of n is close to 1,the behavior of the surface layer approaches that of an ideal capacitor.The fitting results show that the equivalent circuit used to model the EIS data obtained for each specimen gives a good agreement with the experimental results.

The impedance parameters for the BM,HAZ,and WM in artificial saliva solution at 37℃are presented in Table 7.The low Q₁,Q₂ and high R₁,R₂ values indicate the formation of a highly stable film.In Table 7,the resistance value of the inner barrier layer is significantly larger than that associated to the outer porous layer.This suggests that the corrosion protection of the BM,HAZ,and WM would origin predominantly from the inner barrier layer.

By comparing the impedance parameters of the three specimens,it can be concluded that the HAZ with the highest R₁ and R₂ possesses the smallest corrosion rate.In addition,the corrosion resistance of the WM is more excellent than that of the BM.All the results obtained from the EIS tests are in good agreement with the results of the potential dynamic polarization measurement and indicate that the passive film of the BM is not as compact as the passive film of the HAZ and WM.

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Table 7 Impedance parameters for BM,HAZ,and WM zones in artificial saliva composition

Fig.6 SEM images of a original titanium surface of WM zone and b WM zone modified by Ag nanoparticles and c EDS spectrum of selected area in b

Fig.7 SEM images of S.aureus before a and after b Ag modifying WM zone of Ti sample plates;SEM images of E.coli before c and after d Ag modifying WM zone of Ti sample plates

3.4 Antibacterial performance

It is known that the pure Ti coated by Ag nanoparticles exhibits good antibacterial performance [ 19] .However,there are few literatures focused on the antibacterial performance of WM zone.Figure 6a and b shows SEM images of Ti before and after Ag nanoparticles deposition.From Fig.6a and b,it can be seen that the WM zone of original Ti has a rough surface (Fig.6a),while the surface is covered by spherical shape particles with an average diameter of less than 40 nm (Fig.6b) after Ag nanoparticles'deposition,and the nanoparticles are evenly distributed.Figure 6c shows the EDS spectrum of the selected zone in Fig.6b.The surface layer mainly contains Ti and Ag,indicating that Ag nanoparticles can be deposited on the WM zone of pure Ti surface.S.aureus and E.coli are considered to be common causes of implant-associated infections and peri-implantitis [ 20] .The results of bacterial viability tests shown in Fig.7 indicate that the number of viable S.aureus and E.coli bacteria is lower for the WM zone modified by Ag nanoparticles than that for the one uncoated.This indicates that the WM zone of Ti sample with Ag coatings shows improved antibacterial properties.The inhibitory effect of Ag ions that are released from Ag under complex physiological conditions is believed to inactivate the constituents of the bacterial membrane and lead to structural change of the membranes and intracellular metabolic activity,which might prevent bacterial replication [ 21] .For comparison,bacterial viability tests of BM zone of Ti were also conducted.Figure 8 indicates that Ti surface modified by Ag nanoparticles can also exhibit antibacterial properties to suppress microbial proliferation and thereby reduce bacterial counts.However,the antibacterial effect of BM zone of Ti with Ag coating is less obvious than that of WM zone of Ti with Ag coating.It can be seen from Figs.7 and 8 that the BM zone of Ti possesses a rougher surface than WM zone of Ti.A rough surface generally has a larger surface area,thus increasing the possibility of bacterial contact and promoting the colonization by bacteria [ 22] .

Fig.8 SEM images of S.aureus before a and after b Ag modifying BM zone of Ti sample plates;SEM images of E.coli before c and after d Ag modifying BM zone of Ti sample plates

4 Conclusion

In this study,the corrosion behavior of the TA2 weldment in artificial saliva solution by manual argon tungsten arc welding was investigated by metallographic examination,electrochemical corrosion measurement,and EIS test.Metallographic examination experiments show that welding process can cause the growth of grain size.In addition,phase change happens in the HAZ and WM zones.The change of grain size and phase would influence the generation of the original passive film.The results of potential dynamic polarization and EIS measurements indicate that the BM exhibits higher corrosion rate than other zones.The BM has the widest breakdown potential which indicates that the passive film on the BM is stable but not so compact.The generation rate of the passive film on the BM is smaller than that on the HAZ and WM.In addition,the HAZ exhibits the best corrosion resistance.The antibacterial properties of surface coatings containing Ag can suppress microbial proliferation and thereby reduce bacterial counts.The antibacterial effect of WM zone of Ti with Ag coating is better than that of BM zone of Ti with Ag coating.

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