董京京等[4]研究了等轴、魏氏以及双态3种组织结构的Ti-Al-Nb-Zr钛合金在模拟深海环境中的耐蚀性能，结果在表层海水中3种组织的钛合金耐腐蚀性相差不大，但是在深海环境中双态组织的耐腐蚀性优于等轴和魏氏组织。刘俪等[5]提出TC4和TC11钛合金的腐蚀驱动力是不同组织由于成分差异出现电位差，电位差越大，腐蚀越严重，耐蚀性能越差。王可[6]等通过对不同组织的Ti90合金进行电化学测试得出双态组织的耐腐蚀性能大于等轴的耐腐蚀性能，片层组织的耐腐蚀性能最差。Su[7,8]等提出随着退火温度的升高，Ti80合金的β相体积分数增加，α相体积分数减少，Ti80合金的耐蚀性增强，这是由于β相中Nb、Mo和Zr的含量较高，α相中Al和Ti含量较高，β相比α相具有更好的耐腐蚀性能，α相厚度的减小减轻了元素的偏析，进一步抑制了α相和β相之间的微电偶效应。孟康[9]等研究了不同热处理条件下TA31钛合金的腐蚀行为，结果表明腐蚀容易发生在α/β相界面，这是α相与β相因成分不同会形成微区原电池导致的。本文以Ti6321钛合金为原料，研究了该合金在不同热处理条件下显微组织与腐蚀行为的变化，为Ti6321钛合金在海洋领域中生产、应用及使用安全性提供理论指导。
1 实验方法
实验材料为Ti6321合金，其名义成分为Ti-6Al-3Nb-2Zr-1Mo，由6.5%Al、3.0%Nb、1.9%Zr-及0.92% Mo组成 (质量分数)。Ti6321合金的β相转变温度为1000 ℃。在900，980和1020 ℃对Ti6321合金进行不同的参数的热处理，具体工艺为900 ℃×1 h/AC、980 ℃×1 h/AC、1020 ℃×1 h/AC (AC表示空冷)，分别得到等轴、双态以及魏氏3种钛合金组织试样。
实验所用试样通过线切割制成10 mm×10 mm×3 mm的片状试样，用于微观组织表征、电化学测试、硬度测试以及浸泡实验。将电化学测试的试样与铜导线焊接起来置于PVC管内并加入环氧树脂封装，暴露面积为1 cm2。将用于浸泡实验和电化学测试的试样，依次经由240#，400#，800#，1500#，2000#，2500#至5000#水磨砂纸打磨并用抛光机抛光，直到表面光亮无划痕，去离子水冲洗，电吹风机吹干。
使用PGSTAT 302电化学工作站对3种不同组织的Ti6321合金试样进行电化学测试，测试采用三电极体系，Ti6321合金试样为工作电极，铂电极作为辅助电极，参比电极选择饱和甘汞电极 (SCE)，所有电位均相对于饱和甘汞电极。对试样进行电化学曲线测试，动电位极化曲线测量时，电位范围控制在-1.5~3 V，扫描速率1.0 mV/s，电化学阻抗曲线频率测试范围为105~10-2 Hz，交流扰动电压幅值为10 mV。电化学测试所用溶液分别为人工海水溶液 (24.53 g/L NaCl+4.09 g/L Na2SO4+0.201 g/L NaHCO3+0.695 g/L KCl+11.1g/L MgCl2·6H2O+1.16 g/L CaCl2+0.101 g/L KBr) 以及5mol/L盐酸溶液。为保证实验结果的可重复性，每组实验至少重复3次。采用5 mol/L盐酸溶液对三种不同组织的Ti6321试样进行浸泡实验，温度为25 ℃，时间为10 d。使用JSM-6700F扫描电镜 (SEM) 观察样品微观表面形貌，使用Lab.A1金相显微镜和VK-X260K激光共聚焦扫描显微镜 (CLSM) 观察样品表面微观形貌。采用ESCLAB 250Xi X射线分析衍射仪 (XRD) 对不同热处理的Ti6321合金物相构成进行分析，衍射角范围为30°~80°，扫描速率为4°/min。
2 结果与讨论
2.1 热处理对Ti6321合金试样表面形貌以及相组成的影响
图1
图1
3种不同条件热处理后的 Ti6321合金的金相组织
Fig.1
Equiaxed (a), double (b) and widmanstatten (c) structures of Ti6321 alloy after heat treatment under three different conditions
图2
图2
经不同条件热处理后的Ti6321合金的微观组织SEM观察
Fig.2
SEM observations of Ti6321 alloy after heat treatment under three different conditions: (a) equiaxed structure, (b) double structure, (c) widmanstatten structure
图3
图3
经3种不同条件热处理后的Ti6321合金的XRD衍射谱
Fig.3
XRD patterns of Ti6321 alloy after heat treatment under three different conditions
2.2 热处理对Ti6321合金试样开路电位的影响
图4为经3种不同条件热处理后的钛合金在人工海水溶液以及5 mol/L盐酸溶液中的开路电位曲线。开路电位越正，说明材料的腐蚀热力学活性越低，开路电位越负，材料的腐蚀倾向越大。对于Ti6321合金其耐腐蚀性的原因是因为表面容易形成一层氧化膜。氧化膜存在时，钛合金处于钝态，其电位较正。氧化膜消失后，钛处于活化态，其电位较负。
图4
图4
经3种不同条件热处理后的钛合金在人工海水及5 mol/L盐酸溶液中的开路电位
Fig.4
Self-corrosion potentials of Ti6321 alloy samples with three different structures in artificial seawater (a) and 5 mol/L HCl solution (b)
在人工海水溶液中，随着时间的延长，3种钛合金试样开路电位均逐渐正移，表明3种钛合金试样表面均逐渐形成钝化膜，在浸泡刚开始时，钛合金暴露于人工海水溶液中，为裸金属状态，因此开路电位较负，随着浸泡时间的延长，表面开始形成钝化膜，钛合金的耐腐蚀性能增强，因此开路电位明显正移直至趋于稳定即钛合金表面钝化膜的溶解与形成达到动态平衡[13]，说明3种钛合金试样在人工海水溶液中均能自钝化。在人工海水溶液中，双态组织的Ti6321合金试样的开路电位在-0.45 V趋于稳定，等轴组织的试样最后趋近于-0.52 V，魏氏组织的试样开路电位最负，达到-0.57 V，说明在人工海水溶液中，魏氏组织的Ti6321合金试样的腐蚀热力学活性最高，等轴组织的次之，双态组织钝化膜最为致密均匀，腐蚀热力学活性最低。
在5 mol/L盐酸溶液中，随着时间的延长，3种钛合金试样的开路电位逐渐变负，在200 s内，变化速率较大，表明在盐酸溶液中，钛合金表面的钝化膜遭受到破坏而溶解。暴露于盐酸溶液中的钛合金也被腐蚀，说明3种钛合金试样在此环境中处于活化腐蚀状态[9]。在200~800 s之间，在钛合金表面逐渐达到新的平衡状态，降低了反应速率，因此电位变化范围变小，直到800 s后达到趋于稳定的状态。双态组织的开路电位最终趋近于-0.58 V，等轴组织最终趋近于-0.61 V，魏氏组织值最负在接近于-0.65 V稳定，说明在盐酸溶液中，双态组织的Ti6321合金试样的腐蚀趋势依然最小，等轴组织次之，魏氏组织热力学最高。3种组织在盐酸中的稳定状态开路电位均低于在人工海水中的开路电位，说明3种组织在5 mol/L盐酸中腐蚀倾向更大，表面处于活性状态。
2.3 热处理对Ti6321合金试样极化曲线的影响
图5为3种钛合金试样在人工海水及5 mol/L溶液中的极化曲线。在人工海水溶液中，3种钛合金试样均表现出了钝化行为而且均未产生点蚀 (图5a)，这说明3种钛合金试样在人工海水溶液中具有良好的耐蚀性。当扫描电位值达到-0.3 V时，3种钛合金试样都开始进入钝化状态，即在其表面形成稳定致密的氧化膜，当扫描电位到达1.3 V附近时，等轴和魏氏组织的电流密度略为增加，这是由于电子透过Ti6321合金表面氧化膜发生迁移，表面氧化钝化膜被部分溶解[14]。当扫描电位高于1.5 V后，试样又都继续维持在钝化区且维钝电流密度基本保持稳定。对3种钛合金试样的极化曲线进行拟合，数据见表1，魏氏组织的维钝电流密度大于等轴组织的维钝电流密度，双态组织的维钝电流密度最低，也同样说明双态组织的Ti6321合金耐腐蚀性能最高，等轴组织次之，魏氏组织的耐蚀性能最差[15]。
图5
图5
3种钛合金试样在人工海水及5 mol/L盐酸溶液的极化曲线
Fig.5
Polarization curves of Ti6321 alloy samples with three different structures in artificial seawater (a) and in 5 mol/L HCl solution (b)
表1 3种钛合金试样在人工海水中极化曲线的Tafel拟合结果
Table 1
| Structure | Ecorr / VSCE | Ip / A·cm-2 | Epit / VSCE | Icorr / A·cm-2 |
|---|---|---|---|---|
| Double | -0.45 | 4.95×10-6 | --- | 3.39×10-7 |
| Equiaxed | -0.52 | 5.11×10-6 | --- | 4.49×10-6 |
| Widmanstatten | -0.57 | 6.21×10-6 | --- | 5.80×10-6 |
在盐酸溶液中，魏氏组织的Ti6321合金试样的维钝电流密度是1.74×10-4 A/cm2，当电位增加到0.95 V时，电流迅速增大，氧化膜发生溶解；等轴组织的维钝电流密度是1.71×10-5 A/cm2，当电位达到1.38 V时出现活化现象，当电位高于2.15 V后，又继续维持钝化；双态组织的维钝电流密度是1.35×10-5 A/cm2，且在活性-钝性转变区后一直维持在钝性状态。从表2拟合数据可见，魏氏组织在盐酸溶液中的耐腐蚀性能最差，等轴组织次之，双态组织最佳。盐酸溶液中存在大量Cl-，Cl-会通过扩散优先穿过钛合金表面钝化层中较薄弱的区域从而与基体直接接触，魏氏组织相界晶界多，缺陷密度高，氧化膜形成速度不同，因此相对于等轴组织和双态组织在盐酸溶液中的耐蚀性能大大减弱[8]。
表2 3种钛合金试样在5 mol/L盐酸溶液中的极化曲线的Tafel拟合结果
Table 2
| Structure | Ecorr / VSCE | Ip / A·cm-2 | Epit / VSCE | Epp-Epit / VSCE | Icorr / A·cm-2 |
|---|---|---|---|---|---|
| Double | -0.59 | 1.35×10-5 | --- | -0.14-1.44 | 1.35×10-7 |
| Equiaxed | -0.62 | 1.71×10-5 | --- | 0.15-1.37 | 1.29×10-6 |
| Widmanstatten | -0.65 | 1.74×10-4 | 0.95 | 0.18-0.88 | 2.68×10-6 |
2.4 热处理对Ti6321合金试样电化学阻抗谱的影响
在人工海水溶液中，从Nyquist图中可以看出，3种钛合金试样的电化学阻抗谱均表现为单一容抗弧，表明只有一个时间常数，在钛合金表面生成的高度致密的氧化层。当氧化层和人工海水接触时，在界面处发生电荷转移而产生的空间电荷层反映了钛合金与电解液界面上的电荷转移情况。容抗弧半径的大小对应着材料的耐腐蚀性强弱，容抗弧半径越大，说明极化电阻越大，证明材料本身耐腐蚀性能越强；反之，容抗弧半径越小，材料本身更不耐蚀[16,17]。从Bode图看出，3种钛合金试样在人工海水中的Bode曲线只有一个峰，也表明只有一个时间常数。图6a，b也给出了对应的等效电路图，拟合结果如表3所示。其中，Rs代表溶液电阻，Qdl代表双电层电容，Rp代表电荷转移电阻。Rp表明电荷迁跃钝化膜界面引起的阻抗值，反应Ti6321合金表面钝化膜的特性，Rp值越高，钝化膜越稳定，表明合金的耐蚀性越好。双态组织Ti6321合金的Rp值为4.10×105 Ω·cm2，等轴组织和魏氏组织的分别为3.67×105和2.53×105 Ω·cm2，可以看出在人工海水中依然是双态组织的耐腐蚀性能最高，等轴组织的次之，魏氏组织的耐蚀性能最差，这和极化曲线测试结果一致。
图6
图6
3种钛合金试样在人工海水及5 mol/L盐酸溶液中的电化学阻抗谱
Fig.6
Electrochemical impedance spectra of Ti6321 alloy samples with three different structures in artificial seawater (a, c, e) and 5 mol/L HCl solution (b, d, f)
表3 3种钛合金试样在人工海水中阻抗谱拟合结果
Table 3
| Structure | RS / Ω·cm2 | Rp / Ω·cm2 | Qdl / Ω-1·cm-2·Sn | N |
|---|---|---|---|---|
| Double | 21.22 | 4.098×105 | 4.013×10-5 | 0.93 |
| Equiaxed | 21.03 | 3.671×105 | 5.049×10-5 | 0.83 |
| Widmanstatten | 24 | 2.527×105 | 3.451×10-5 | 0.94 |
表4 3种钛合金试样在5 mol/L盐酸溶液中的阻抗拟合结果
Table 4
| Structure | Rs / Ω·cm2 | Qdl / Ω-1·cm-2·Sn | n1 | Rct / Ω·cm2 | Qf / Ω-1·cm-2·Sn | n2 | Rf / Ω·cm2 |
|---|---|---|---|---|---|---|---|
| Double | 4.7 | 0.00028 | 0.90 | 462.5 | 0.03363 | 0.85 | 509.6 |
| Equiaxed | 4.6 | 0.00033 | 0.88 | 317.5 | 0.03968 | 0.83 | 329.7 |
| Widmanstatten | 6.6 | 0.00030 | 0.89 | 290.1 | 0.04318 | 0.87 | 235.6 |
2.5 浸泡实验
在5 mol/L盐酸溶液中浸泡10 d后，3种钛合金试样表面均呈现出均匀腐蚀的特征，而且腐蚀优先发生在α相上以及晶界处。由CLSM形貌 (图7) 和SEM形貌 (图8) 可以看出，相对于等轴组织和双态组织的试样，魏氏组织试样表面剥落的最严重，腐蚀深度更大。这是因为魏氏组织晶粒粗大，钝化膜形核位置更少，而且其相界和晶界的元素偏析以及缺陷密度高，所以优先在高能量位置发生腐蚀。另一方面由于α相和β相间的成分差异，在盐酸溶液中，两相之间存在着电位差，从而产生微电偶腐蚀原电池，加速魏氏组织Ti6321合金试样的腐蚀。所以相对于等轴组织以及双态组织，魏氏组织的Ti6321合金耐腐蚀性能最差[20]。研究表明钛合金中Mo元素在β相中的含量比在α相中高很多，Mo可以极大提高Ti6321合金在盐酸溶液中表面钝化膜的稳定性，所以β相比α相更耐蚀。等轴组织的α相含量最多，因此其耐蚀性能比双态组织的耐蚀性能要差[21-23]。Ti6321合金的耐腐蚀性能取决于组织中合金元素在α和β两相中的分配以及两相界面的数量与分布情况。
图7
图7
在5 mol/L盐酸溶液中浸泡10 d后Ti6321合金试样CLSM表面形貌
Fig.7
CLSM surface morphologies of Ti6321 alloy samples after immersion for 10 d in 5 mol/L HCl solution: (a) equiaxed structure, (b) double structure, (c) widmanstatten structure
图8
图8
在5 mol/L盐酸溶液中浸泡10 d后Ti6321合金试样的SEM形貌
Fig.8
SEM surface morphologies of Ti6321 alloy samples after immersion for 10 d in 5 mol/L HCl solution: (a) equiaxed structure, (b) double structure, (c) widmanstatten structure
3 结论
(1) 经不同条件热处理后，获得具有3种组织结构的Ti6321合金试样。其中，魏氏组织晶粒粗大，α/β相界面多，缺陷密度大；等轴组织中α相所占比例最大，α/β相界面最少；双态组织介于两者之间。
(2) 在人工海水中，3种钛合金试样均处于钝化状态，耐腐蚀性相差不大，电极反应过程由钝化膜控制。在5 mol/L盐酸溶液中，不同组织钛合金均处于活性状态，钛合金腐蚀由电荷转移过程和氢化钛生成过程控制，3种组织的耐腐蚀性能排序：双态组织＞等轴组织＞魏氏组织。
(3) Ti6321合金在5 mol/L HCl溶液中浸泡时，腐蚀优先发生在α相上以及晶界相界处，魏氏组织腐蚀最为严重，等轴组织次之，双态组织的耐腐蚀性能最好。因此，具有不同组织的Ti6321钛合金的耐腐蚀性能主要由α和β两相的含量以及α/β相界面的数量和分布决定。
参考文献
Characterization of hot workability for a near alpha titanium alloy by integrating processing maps and constitutive relationship
[J].
Investigation on microstructure and texture evolution of Ti-6Al-3Nb-2Zr-1Mo alloy during hot deformation
[J].
Effect of heat treatment on microstructure evolution of Ti-48Al alloy solidified under high pressure
[J].
热处理对高压凝固Ti-48Al合金组织演变的影响
[J].
Electrochemical performance of passive film formed on Ti-Al-Nb-Zr alloy in simulated deep sea environments
[J].
Influence of microstructures of titanium alloy on ultrasound parameters and corrosion-resistance
[D].
钛合金显微组织对其超声参量及耐蚀性能的影响
[D].
Effect of heat treatment on microstructures and properties of Ti90 alloy
[J].
热处理对Ti90钛合金显微组织及性能的影响
[J].
The corrosion behavior of Ti-6Al-3Nb-2Zr-1Mo alloy: effects of HCl concentration and temperature
[J].
Investigation about the corrosion behavior of Ti alloys in different ambient environment is of great significance for their practical application. Herein, we systematically investigate the corrosion behavior of a newfound Ti-6Al-3Nb-2Zr-1Mo (Ti80) alloy in hydrochloric acid (HCl) ranging from 1.37 to 7 M, and temperature ranging from 25 to 55 ℃, by means of electrochemical measurements, static immersion tests and surface analysis. Results manifest that increasing either HCl concentration or temperature can accelerate the corrosion of Ti80 alloy via promoting the breakdown of native protective oxide film and then further facilitating the active dissolution of Ti80 matrix. According to potentiodynamic polarization curves, Ti80 alloy displays a spontaneous passive behavior in 1.37 M HCl at 25 ℃, compared to a typical active-passive behavior under the other conditions. As indicated by cathodic Tafel slope, the rate determining step for cathodic hydrogen evolution reaction is likely the discharge reaction step. The apparent activation energies obtained from corrosion current density and maximum anodic current density for Ti80 alloy in 5 M HCl solution are 62.4 and 55.6 kJ mol-1, respectively, which signifies that the rate determining step in the corrosion process of Ti80 alloy is mainly determined by surface-chemical reaction rather than diffusion. Besides, the electrochemical impedance spectroscopy tests demonstrate that a stable and compact oxide film exists in 1.37 M HCl at 25 ℃, whereas a porous corrosion product film forms under the other conditions. Overall, the critical HCl concentration at which Ti80 alloy can maintain passivation at 25 ℃ can be determined as a value between 1.37 and 3 M. Furthermore, the corroded surface morphology characterization reveals that equiaxed α phase is more susceptible to corrosion compared to intergranular β phase due to a lower content of Nb, Mo, and Zr in the former.
Annealed microstructure dependent corrosion behavior of Ti-6Al-3Nb-2Zr-1Mo alloy
[J].Corrosion resistance of titanium (Ti) alloys is closely connected with their microstructure which can be adjusted and controlled via different annealing schemes. Herein, we systematically investigate the specific effects of annealing on the corrosion performance of Ti-6Al-3Nb-2Zr-1Mo (Ti80) alloy in 3.5 wt.% NaCl and 5 M HCl solutions, respectively, based on open circuit potential (OCP), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), static immersion tests and surface analysis. Results indicate that increasing annealing temperature endows Ti80 alloy with a higher volume fraction of β phase and finer α phase, which in turn improves its corrosion resistance. Surface characterization demonstrates that β phase is more resistant to corrosion than α phase owing to a higher content of Nb, Mo, and Zr in the former; additionally, the decreased thickness of α phase alleviates segregation of elements to further restrain the micro-galvanic couple effects between α and β phases. Meanwhile, the influential mechanisms of environmental conditions on corrosion of Ti80 alloy are discussed in detail. As the formation of a highly compact and stable oxide film on surface, annealed Ti80 alloys exhibit a low corrosion current density (10-6 A/cm2) and high polarization impedance (106 Ω? cm2) in 3.5 wt.% NaCl solution. However, they suffer severe corrosion in 5 M HCl solution, resulting from the breakdown of native oxide films (the conversion of TiO2 to aqueous Ti3+), active dissolution of substrate Ti to aqueous Ti3+ and existence of micro-galvanic couple effects. Those findings could provide new insights to designing Ti alloys with high-corrosion resistance through microstructural optimization.
Study on corrosion behavior of TA31 alloy in hydrochloric acid solution
[D].
TA31合金在盐酸溶液中的腐蚀行为研究
[D].
Effect of annealing on microstructure and tensile properties of skew hot rolled Ti-6Al-3Nb-2Zr-1Mo alloy tube
[J].
Microstructure and mechanical properties of Ti6321 alloy welded joint by GTAW
[J].
The microstructure and tensile properties of additively manufactured Ti-6Al-2Zr-1Mo-1V with a trimodal microstructure obtained by multiple annealing heat treatment
[J].
A novel biomedical titanium alloy with high antibacterial property and low elastic modulus
[J].β-type titanium alloys have attracted much attention as implant materials because of their low elastic modulus and high strength, which is closer to human bones and can avoid the problem of stress fielding and extend the lifetime of prosthetics. However, other issues, such as the infection or inflammation postimplantation, still trouble the titanium alloy's clinical application. In this paper, we developed a novel near β-titanium alloy (Ti-13Nb-13Zr-13Ag, TNZA) with low elastic modulus and strong antibacterial ability by the addition of Ag element followed by proper microstructure controlling, which could reduce the stress shielding and bacterial infections simultaneously. The microstructure, mechanical properties, corrosion resistance, antibacterial properties and cell toxicity were studied using SEM, electrochemical testing, mechanical test and cell tests. The results have demonstrated that TNZA alloy exhibited an elastic modulus of 75-87 GPa and a strong antibacterial ability (up to 98 % reduction) and good biocompatibility. Moreover, it was also shown that this alloy's corrosion resistance was better than that of Ti-13Nb-13Zr. All the results suggested that Ti-13Nb-13Zr-13Ag might be a competitive biomedical titanium alloy.
Electrochemical activation of passivated pure titanium in artificial seawater
[J].
钝性纯Ti在人工海水中的电化学活化行为研究
[J].
Electrochemical anodizing treatment to enhance localized corrosion resistance of pure titanium
[J].
Corrosion evaluation of Ti-48Al-2Cr-2Nb (at.%) in Ringer's solution
[J].The corrosion behavior of Ti-48Al-2Cr-2Nb (at.%) in Ringer's solution was studied to evaluate its potential as a biocompatible material. Corrosion properties of Ti-6Al-4V were determined under the same conditions for comparison. Two electrochemical techniques, potentiodynamic anodic polarization and electrochemical impedance spectroscopy, were employed to test Ti-48Al-2Cr-2Nb and Ti-6Al-4V. Surface modifications to the samples were made by autoclaving and by oxidation in air at 500 degrees C and 800 degrees C. The results show excellent corrosion resistance for unmodified Ti-48Al-2Cr-2Nb, corroborated by the high values of polarization resistance and corrosion potential and low values of corrosion current and corrosion rate. Ti-48Al-2Cr-2Nb appears to possess corrosion characteristics similar to Ti-6Al-4V. Surface modification rendered the Ti-48Al-2Cr-2Nb material extremely corrosion resistant.
Effect of surface state on corrosion resistance of TC4 Ti-alloy
[J].
表面状态对TC4钛合金的耐蚀性影响
[J].
Effects of pH on the electrochemical behaviour of titanium alloys for implant applications
[J].
Influence of niobium and zirconium alloying additions on the anodic dissolution behavior of activated titanium in HCl solutions
[J].
Corrosion and mechanical properties of titanium alloyed with aluminum, iron, and molybdenum
[J].
The effects of Mo content on microstructure and high temperature tensile behavior of Ti-6.5Al-2Sn-4Zr-xMo-2Nb-1W-0.2Si titanium alloys
[J].
In situ corrosion monitoring of Ti-6Al-4V alloy in H2SO4/HCl mixed solution using electrochemical AFM
[J].
