Corrosion behavior of carburized layer of Ti6Al4V titanium alloy in HF

Titanium and titanium alloys have been widely used in biomedical field because of their low density, good biocompatibility and excellent corrosion resistance. Titanium alloy can form osseointegration with bone without adverse reactions. It is a good dental implant material. In recent years, Ti6Al4V titanium alloy has been used in dental implants, orthodontic filaments and crown posts. During use, titanium alloy is dissolved in saliva or gingival crevicular fluid and in direct contact with corrosion products. The corrosion caused by F- in oral cavity is more common, because there are a lot of F- in toothpaste, gel and drinking water, and even F- has been added to cooking salt recently. Fluorinated gels sold in many markets contain up to 10000 × 10-6 (mass fraction,%), fluorine ion with pH value between 7.2 ~ 3.2. Titanium is very sensitive to halides (especially F compounds), h and O, which makes the dense passive film formed on the surface of titanium alloy dissolve or peel off and lose its protective properties. Mabilleug et al. Acted fluoride and oxide on the titanium surface and found that fluoride ion dissolved the oxide layer on the titanium surface and improved the surface roughness. Kumar et al. Analyzed the corrosion behavior of Ti6Al4V, cp Ti and ti15mo titanium alloys under different fluorine ion concentrations. The results show that the current density has a strong correlation with the fluorine ion in the electrolyte. The corrosion current density increases with the increase of the fluorine ion concentration.
Considering the properties of raw materials and service environment, it is necessary to strengthen the surface of titanium alloy to improve its surface strength, hardness and wear resistance. Therefore, it has become a research hotspot to evaluate the service life of modified titanium alloy under different electrolyte, concentration, pH and temperature. Carburizing the surface of 20CrMnTi, 38CrMoAl and other alloys can improve the hardness, wear resistance and corrosion resistance of the metal surface. The effect of carburized layer of Ti6Al4V titanium alloy in fluoric acid under different vacuum induction carburizing conditions was studied? Bubble corrosion and electrochemical corrosion behavior, and the corrosion mechanism was analyzed combined with phase structure and microstructure evolution.

Experimental method

Surface carburization of Ti6Al4V

The experimental material is annealed Ti6Al4V. The diameter of the sample is 15mm and the thickness is 10mm. Ti6Al4V was carburized at different temperatures by vacuum induction pulse heat treatment device. The carburizing process is shown in Figure 1. Put the sample into the vacuum induction pulse heat treatment device. The carburizing temperature is 850 ℃, 880 ℃ and 910 ℃ respectively. The carburizing pressure is – 70kpa and the time is 1H. When the surface temperature of the sample reaches the target temperature, fill the furnace with methane gas (purity greater than 99.99%) – 70kpa and maintain the pressure for 10min for strong infiltration, then extract the gas and keep it for 5min for diffusion. After circulating and holding for 1h, cool it to room temperature with the furnace.

Fig.1 induction carburizing process of titanium alloy Ti6Al4V

Corrosion resistance test

The titanium alloy was immersed in 0.2% HF corrosion solution for accelerated corrosion test, the corrosion solution was changed in 0.5h cycle, and the mass loss was weighed to calculate the corrosion rate; In 1% HNO3 + 1% HF mixed aqueous solution, calomel single salt bridge with saturated potassium chloride solution as electrolyte was used as reference electrode (electrode potential at 25 ℃ was 0.2415v), platinum electrode was used as auxiliary electrode, and biologic electrochemical workstation was used to test polarization curve, impedance and Mott Schottky curve. The scanning speed of polarization curve is 5mv / s and the scanning range EI el is – 1V ~ 0V. Tafel fit the polarization curve, and calculate the corrosion potential and corrosion current of the sample; The scanning frequency of impedance spectrum is 200kHz ~ 100MHz. Zsimdemo3.30d software is used for equivalent circuit fitting. The scanning frequency of Mott Schottky curve is 1kHz ~ 100kHz. Finally, the flat band potential and donor density or acceptor density are obtained by fitting 1kHz.

Characterization of microstructure and corrosion morphology

The phase structure of carburized layer before and after corrosion was analyzed by polycrystalline powder diffractometer (XRD, D8 advance) with Cu (0.154174nm) as target; In order to analyze the cross-section structure of the sample, the cross-section metallographic corrosion was carried out with 5% HNO3 + 5% HF, and the microstructure of the infiltrated layer of the sample after corrosion was analyzed under the metallographic microscope; Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to analyze? Corrosion morphology and corrosion products of carburized layer after bubble.

Experimental results

Immersion corrosion

Place the non carburized and carburized samples with the same surface area in 0.2% HF corrosive solution. After contacting with the corrosive solution, a large number of bubbles are generated, rapid corrosion reaction occurs, and blue compound (titanium trifluoride) is generated in the solution. The corrosion rate (g / (M2 · h)) is

Where M0 is the mass before corrosion (g); M1 is the mass after corrosion (g); A is the surface area of the sample (M2); T is the corrosion time (H). Use equation (1) to calculate the corrosion rate, and the results are listed in Table 1.

Table.1 Tafel fitting parameters of polarization curve

Fig.2 Relationship between quality loss and time

Electrochemical corrosion

Polarization curve

Figure 3 shows the polarization curves in different states. It can be seen that induction carburizing moves the polarization curve to the upper left, indicating that the corrosion tendency is reduced. Obvious passivation transition zone appears after carburizing, indicating that carburizing enhances the surface passivation ability. The non carburized Ti6Al4V titanium alloy is always in the activated solution in 1% HF and 1% HNO3 aqueous solution, but there is no obvious passivation. This shows that the passive film on the carburized surface has good self-healing in the actual service process.

Fig.3 Polarization curves of Ti6Al4V titanium alloy under different states

The corresponding parameters obtained by fitting the polarization curve are listed in Table 1. It can be seen from table 1 that with the increase of temperature, the self corrosion potential (Ecorr) of the non carburized sample increases from -0.94v to -0.68v; The corrosion current (icorr) decreased from 4.10ma · cm-2 to 1.65ma · cm-2; The polarization resistance (RP) increased from 6.36 Ω· cm-2 to 15.8 Ω· cm-2. These results show that induction carburizing improves the HF corrosion resistance of the surface by 2.5 times. Tafel slope of UN carburized samples β a/ β C is less than 1, while the carburized sample β a/ β C is greater than 1. This shows that the reaction process of non carburized samples is mainly controlled by the cathodic polarization process. Rapid carburization changes the situation that the reaction of Ti6Al4V titanium alloy in 1% HF and 1% HNO3 solution is controlled by the cathodic polarization process.

Impedance

Figure 4 shows the AC impedance spectrum. The shape of AC impedance spectrum before and after carburizing is similar, only the radius of capacitive arc is different, indicating that it has the same corrosion mechanism and different corrosion resistance. It can be seen from FIG. 4A that there is inductive arc in the high frequency region, indicating that self-corrosion hydrogen evolution and hydrogen adsorption reaction occur on the surface of titanium alloy during corrosion. The impedance modulus of carburized samples in high frequency region is higher than that in low frequency region to varying degrees, and the impedance modulus increases with the increase of temperature. This shows that the carburized layer increases the resistance, that is, the resistance to corrosion. The high frequency phase angle peak appears in the same frequency range of impedance modulus, indicating that the surface carburized layer can resist corrosion.

Fig.4 Impedance spectrum and equivalent circuit of Ti6Al4V titanium alloy in different states

In the fitted equivalent circuit diagram, each circuit element is not a standard electrical element, but a function of frequency, so the impedance spectrum of the equivalent circuit varying with frequency is obtained. The surface ion telecontrol is not a random Brownian motion, but a periodic or quasi periodic oscillation. The element parameters of the interface are also different with different oscillation periods. According to the equivalent circuit in Fig.4D, the function of electrochemical impedance model equation (2) is used

The impedance values are fitted (Table 2). In formula (2), Z (f) is a complex number; L is the inductive reactance of hydrogen desorption reaction; RS is solution resistance; Q (CPE) is the constant phase angle element of electric double-layer capacitance (Q1 represents the film capacitance of corrosion products adsorbed on the electrode surface, Q2 represents the double-layer capacitance of electrode reaction), and N is the dispersion index (the electrode surface has a certain roughness, and there is a certain dispersion effect in the surface reaction); RF is the film resistance of corrosion products; RT is the charge transfer resistance (Ti Ti + 3); X2 is the relative error value. It can be seen from table 2 that with the increase of carburizing temperature, the charge transfer resistance Rt and corrosion product film resistance RF increase, RT increases from 0.2 Ω· cm2 to 5.7 Ω· cm2 (910 ℃) of non carburized sample, and RF increases from 0.2 Ω· cm2 to 23.6 Ω· cm2. These results show that carburizing increases the electron transfer resistance in the corrosion process and improves the corrosion resistance.

Table.2 Fitting results of EIS curve

Sample	L /10-7H·cm2	Rs /Ω·cm2	CPE1		Rf/Ω·cm2	CPE2		Rt /Ω·cm2	X2·10-4
			Y0 /Ω-1·cm-2·S-n	n1		Y0 /Ω-1·cm-2·S-n	n1
Original sample	5.9	5.3	1.3×10-4	1	0.2	2×10-2	1	0.2	1.4
850℃	7.6	6.2	3.1×10-2	1	1.1	1.4×10-2	0.9	0.5	1.7
880℃	8.2	5.5	1.8×10-2	0.8	2.4	9×10-3	1	8.2	2.3
910℃	7.2	4.9	4.2×10-3	1	23.6	1.6×10-2	0.7	5.7	2.0

Mott Schottky curve

The polarization curve shows that the carburized Ti6Al4V titanium alloy surface is easier to passivate than the non carburized sample, and the passivation film has semiconductor characteristics and is closely related to the corrosion resistance. By analyzing the distribution of space charge layer capacitance and bulk potential of surface semiconductor through Mott Schottky curve and donor density or acceptor density results obtained by fitting, the possibility of surface reaction can be judged. When the semiconductor surface is in contact with the electrolyte, a space charge layer appears on the surface, and the sign of the charge is opposite to that in the solution. The relationship between space charge layer, capacitance and potential of n-type semiconductor and p-type semiconductor Mott Schottky is

Where CSC is space charge capacitance (f · cm2); Nd is the donor density (cm-3); Na is the donor density (cm-3); ε Is the dielectric constant, and the value of titanium is about 56f / cm; ε 0 is the vacuum medium constant, and its value is 8.85 × 10-14F/cm； E is the electron charge with a value of 1.602 × 10-19C； E is electrode potential (V); EFB is the flat band potential (V); K is Boltzmann constant; T is the absolute temperature. The slope of the straight-line segment of n-type semiconductor is positive, and the slope of the straight-line segment of p-type semiconductor is negative.
Fig.5 shows the Mott Schottky curve of titanium alloy in HF corrosion solution, and the values of donor density or acceptor density are obtained by fitting the Mott Schottky curve (Table 3). The test voltage is 0V ~ 1V and the test frequency is 1kHz ~ 100kHz. The curve is Mott Schottky curve at 1kHz. As can be seen from Fig.5, the non carburized Ti6Al4V titanium alloy has two different semiconductor characteristics, with n-type semiconductor characteristics in the range of 0V ~ 0.4V and p-type semiconductor characteristics in the range of 0.4V ~ 1V. The surface after rapid carburizing shows n-type semiconductor characteristics, and the donor density decreases with the increase of temperature. The experimental results show that the samples without carburization have two states: surface oxide film and matrix. In HF solution, the surface oxide film is n-type semiconductor and the matrix is p-type semiconductor. The acceptor density of p-type semiconductor increases the adsorption of f-ions on the surface and promotes the corrosion of F-on the surface. The surface of carburized sample is mainly n-type semiconductor. The flat band potential is low in the range of 0V ~ 0.4V, because when the potential is at low voltage, the electrons at the passivation film / solution interface are consumed with the increase of voltage, which increases the donor density, and the flat band potential is lower than the range of 0.4V ~ 1V. The donor density of n-type semiconductor decreases with the increase of temperature, that is, the increase of temperature increases the stability of passive film, reduces the defect concentration on the surface and enhances the corrosion resistance.

Fig.5 Mott-Schottky curve of Ti6Al4V titanium alloy under different conditions

Table.3 Fitting results of donor density or acceptor density of passivation film

Sample	Potential range/V	Flatband potential/V	Donor density/Acceptor density
Original sample	0~0.4	-0.74	2.32×1032 cm-3
	0.4~1	1.80	2.22×1030 cm-3
850℃	0~0.4	-0.08	2.48×1030 cm-3
	0.4~1	-0.35	1.81×1030 cm-3
880℃	0~0.4	-0.18	3.87×1030 cm-3
	0.4~1	0.35	0.51×1030 cm-3
910℃	0~0.4	-0.32	8.26×1030 cm-3
	0.4~1	0.38	0.30×1030 cm-3

Analysis and discussion

Phase structure

Fig.6 shows the XRD patterns of carburized samples under different conditions. Fig.6A shows the XRD patterns of Ti6Al4V titanium alloy and non corroded samples after carburization. It can be seen from Fig.6A that a layer of TiC ceramic phase and cti0.42v1.58 composite compound phase can be prepared on the surface of Ti6Al4V titanium alloy by carburizing. In the process of induction carburizing, methane decomposes activated carbon atoms, titanium atoms and alloy elements to react to form mixed compound phase. Because the radius of carbon atom (less than 0.077 nm) is smaller than that of titanium (0.147 nm), carbon is easy to diffuse to the surface of titanium and form compounds with titanium. At the same time, with the progress of carburizing process, part of activated carbon diffuses from the surface and inside to form solid solution. Compared with non carburized samples, the characteristic peaks of titanium in carburized samples at 910 ℃ decreased significantly, while the XRD peaks of carburized samples at 850 ℃ and 880 ℃ were similar to those of non carburized samples. The reason is that when carburizing before 885 ℃, titanium is mainly dense HCP structure, which has little solubility and slow diffusion to carbon atoms, and a small amount of carbon atoms have little effect on the lattice of titanium; At above 885 ℃, the carburized titanium transformed into a relatively loose bcc structure, the diffusion rate increased, and the composite phase of tic and cti0.42v1.58 increased.

Fig.6 XRD pattern of Ti6Al4V titanium alloy under different conditions

Fig.6B shows the XRD pattern of the corresponding sample in Fig.6A after immersion corrosion. For HF solution? The XRD peak basis analysis results of the samples after bubble corrosion show that the residual phases after corrosion are mainly Ti and CTI 0.42v1.58, but there is no TC. Where? In the process of bubble corrosion, HF has and strong corrosivity to titanium. With the progress of the corrosion process, the surface carbides are corroded and peeled off.

Microstructure

Fig.7a, B and C show the microstructure after induction carburizing at 850 ℃, 880 ℃ and 910 ℃ respectively. Surface carburization causes the surface to form an equiaxed shape α Phase and flake α Phase, equiaxed with the increase of temperature α With the increase of phase thickness, the microstructure becomes thicker. Because carbon is α One of the phase stable elements, with the increase of carburizing temperature, the surface carbon content increases and promotes the surface β Opposite α Phase transition. The excessive between the infiltrated layer and the matrix is relatively mild, and there is no sudden change in the distribution of interlayer structure, indicating that the adhesion between the infiltrated layer and the matrix is strong. The dense structure and infiltration layer with a certain thickness can prevent the invasion of anions in the corrosive medium.

Fig.7 Cross-section structure of Ti6Al4V titanium alloy under different conditions (a) raw, (b) 850℃, (c) 880℃, (d) 910℃

Corrosion morphology

Fig.8a and b show the surface corrosion morphology of non carburized samples at different multiples, FIG. 8C and d show the surface corrosion morphology of carburized samples at 880 ℃ at different multiples, FIG. 8e shows the energy spectrum within region (E) in Fig.8C, and Fig.8F shows the line scanning energy spectrum within region (f) in Fig.7d. It can be seen from Fig.8 that the titanium alloy is locally dissolved in HF corrosion solution, and a large number of cracks appear on the corrosion surface α、β- The depression area of Ti and the mixed projection area of white loose (mainly containing F, C, O and Ti elements). Titanium alloy forms a dense passive film in most organic acid (alkali) solutions and oxidizing media and is not easy to corrosion. However, in reducing acid solution (HF), the surface strengthening layer of titanium alloy is easy to be damaged (or dissolved) to form a multi empty non protective film, which improves the corrosion rate of titanium. A small amount of carbides and oxides can be observed from the element composition in the local area, indicating that the main corrosion mechanism of Ti6Al4V titanium alloy in HF corrosion solution is hydrogen evolution corrosion.

Fig.8 Surface corrosion morphology and EDS composition analysis

Conclusion

• (1) Ti6Al4V titanium alloy has rapid overall corrosion in 0.2% HF solution before and after carburizing. With the increase of carburizing temperature, the corrosion resistance increases.
• (2) The non carburized Ti6Al4V titanium alloy shows p-type semiconductor characteristics, and the carburized layer shows n-type semiconductor characteristics in 1% HF and% HNO3 aqueous solution. The corrosion current density decreases and the electron transfer resistance increases, which improves the corrosion resistance.
• (3) The carburized layer on the surface of Ti6Al4V titanium alloy is composed of tic and cti0.42v1.58 compound phase, and the surface structure is a large number of α- Ti phase and a small amount β- Ti phase.

A large amount of carburized layer is formed after corrosion in HF solution α- Ti and β- The corrosion mechanism of Ti concave area and white loose convex area (mainly containing F, C, O and Ti) is mainly hydrogen evolution corrosion.

Authors: Li kunmao, Liu Jing, Zhang Xiaoyan, Li Hong, Dai Yan DOI:10.11901/1005.3093.2018.650

Source: China Titanium Flange Manufacturer: www.titaniuminfogroup.com