Corrosion and Tensile Behaviors of Ti-4Al-2V-1Mo-1Fe and Ti-6Al-4V Titanium Alloys

Round 1

Reviewer 1 Report

The paper is readable and in the field of journal interest. There are a lot of characterization methods, but the discussion is limited.

The introduction should be improved by a discussion of the Fe and Mo influence on the titanium alloys properties. The motivation of the chose of alloy composition should be discussed in this section, for example, based on the paper [Ouchi, Chiaki, Hideaki Fukai, and Kohei Hasegawa. "Microstructural characteristics and unique properties obtained by solution treating or aging in β-rich α+ β titanium alloy." Materials Science and Engineering: A 263.2 (1999): 132-136; Rahim, Erween Abd, and Safian Sharif. "Investigation on tool life and surface integrity when drilling Ti-6Al-4V and Ti-5Al-4V-Mo/Fe." JSME International Journal Series C Mechanical Systems, Machine Elements and Manufacturing 49.2 (2006): 340-345.]. The microstructure, mechanical, and corrosion properties should be discussed from the Fe and Mo incorporation. There are data on the Fe influence on the corrosion resistance of the titanium alloy compared to Ti-6Al-4V [Lu, Jinwen, et al. "Electrochemical corrosion behavior and elasticity properties of Ti–6Al–xFe alloys for biomedical applications." Materials Science and Engineering: C 62 (2016): 36-44; Kuphasuk, Chotiros, et al. "Electrochemical corrosion of titanium and titanium-based alloys." The Journal of prosthetic dentistry 85.2 (2001): 195-202.]. The only influence of Mo presence on the corrosion properties is not evidenced in the paper. The composition of α-Ti and β-Ti phases should be evaluated by the EDX method, and the difference should be discussed in the details.

Author Response

We appreciate the reviewer’s valuable comments. According to these comments, we have already modified the revised manuscript. The reference provided has been cited in the revised manuscript.

Generally, element alloying is an effective method to improve the corrosion resistance, mechanical properties [17, 18] and bio-compatibility of titanium and its alloys. Since Fe and Al elements are characteristic of low toxicity and cost [19-21], they are added into the Ti and alloys, and the effect of Fe or Al alloying on the mechanical and corrosion properties of Ti alloys has been studied. It is reported that Ti-4.5Al-3V-2Mo-2Fe alloy has superior mechanical properties to Ti-6Al-4V alloy [17] due to its microstructural characteristics and element alloying (i.e., Mo and Fe). Lu [22] investigated the mechanical properties and electrochemical corrosion behaviors of Ti-6Al, Ti-6Al-4V and Ti-6Al-xFe alloys and found that Ti-6Al-4Fe alloy possessed the lowest Young's modulus and exhibits the highest strength/modulus ratios, and Ti-6Al-xFe alloys exhibited a higher corrosion resistance in simulate the human body fluid (SBF) than both the Ti-6Al alloy and the Ti-6Al-4V alloy. However, the corrosion resistance of the Ti-6Al-xFe alloys decreased with the increasing Fe content, suggesting that the content of Fe added into the Ti alloys needs to be controlled at a lower level to achieve a better corrosion performance. It is consistent with the results conducted by N. V. Pimenova et. al [21] and H. C. Hsu et. al [23, 24]. When the concentration of Al was higher than 15 wt.%, Ti-xAl-yFe alloys underwent severe pitting corrosion due to the precipitates of β-Ti phase and uneven distribution of the alloying elements [21]. Thus, more work has to be performed to clarify effect of the Al and Fe alloying on the corrosion and mechanical performances of Ti alloys, especially when Mo is added to improve the resistance of localized corrosion.

The composition of α-Ti and β-Ti phases should be evaluated by the EDX method, and the difference should be discussed in the details.

EDS analysis was conducted to investigate the composition of α-Ti and β-Ti phase of the tested alloys, and the results are shown in Table 2. It is reported that Fe, Mo and V atom has long been recognized as a strong β-stabilizing element [19, 22, 27], thus the atomic ratio of these elements in β-Ti phase is higher than that in α-Ti phase. This is mainly due to the higher element solid solubility and elemental diffusion rate in the β phase [27, 28]. Based on the research [28], the volume fraction of the β-Ti phase in Ti-4Al-2V-1Mo-1Fe alloy may higher than that of Ti-6Al-4V alloy .

Table 2. EDS of of α-Ti and β-Ti phase of i-6Al-4V alloy and Ti-4Al-2V-1Mo-1Fe alloy (wt.%).

Author Response File: Author Response.pdf

Reviewer 2 Report

The article focuses on a relevant issue – investigation of the corrosion behavior of newly-developed alloy in the simulated marine environment (3.5 wt.% NaCl solution) and its tensile property. The presented results are of scientific and practical interest.
 However, there are a number of comments on the article, which are given below.
1. It is necessary to expand the introduction. Add more information on the effect of alloying elements on the properties of titanium alloys.
2. A description should be given of how the volume fraction of the beta phase was determined. (Fig. 3b)
3. Please, increase the quality of Figure 4.
4. Please correct the name of the alloy from Ti6-Al4-V to Ti-6Al-4V.

Author Response

1. It is necessary to expand the introduction. Add more information on the effect of alloying elements on the properties of titanium alloys.

    We appreciate the reviewer’s valuable comments. The introduction has been expanded and the effect of alloying elements on the properties of titanium alloys has also been discussed.

     Generally, element alloying is an effective method to improve the corrosion resistance, mechanical properties [17, 18] and bio-compatibility of titanium and its alloys. Since Fe and Al elements are characteristic of low toxicity and cost [19-21], they are added into the Ti and alloys, and the effect of Fe or Al alloying on the mechanical and corrosion properties of Ti alloys has been studied. It is reported that Ti-4.5Al-3V-2Mo-2Fe alloy has superior mechanical properties to Ti-6Al-4V alloy [17] due to its microstructural characteristics and element alloying (i.e., Mo and Fe). Lu [22] investigated the mechanical properties and electrochemical corrosion behaviors of Ti-6Al, Ti-6Al-4V and Ti-6Al-xFe alloys and found that Ti-6Al-4Fe alloy possessed the lowest Young's modulus and exhibits the highest strength/modulus ratios, and Ti-6Al-xFe alloys exhibited a higher corrosion resistance in simulate the human body fluid (SBF) than both the Ti-6Al alloy and the Ti-6Al-4V alloy. However, the corrosion resistance of the Ti-6Al-xFe alloys decreased with the increasing Fe content, suggesting that the content of Fe added into the Ti alloys needs to be controlled at a lower level to achieve a better corrosion performance. It is consistent with the results conducted by N. V. Pimenova et. al [21] and H. C. Hsu et. al [23, 24]. When the concentration of Al was higher than 15 wt.%, Ti-xAl-yFe alloys underwent severe pitting corrosion due to the precipitates of β-Ti phase and uneven distribution of the alloying elements [21]. Thus, more work has to be performed to clarify effect of the Al and Fe alloying on the corrosion and mechanical performances of Ti alloys, especially when Mo is added to improve the resistance of localized corrosion.

2. A description should be given of how the volume fraction of the beta phase was determined. (Fig. 3b)

    The Image-Pro Plus software was used to calculate the volume fraction of the tested alloys.

3. Please, increase the quality of Figure 4.

    Figure 4 with a higher resolution has been provided in the revised manuscript as the reviewer suggested.

4. Please correct the name of the alloy from Ti6-Al4-V to Ti-6Al-4V

    The name of the alloy has been changed from Ti6-Al4-V to Ti-6Al-4V as suggested.

Author Response File: Author Response.pdf

Reviewer 3 Report

In this paper the authors present the results relating to the corrosion behaviour and tensile property of a new Ti-4Al-2V-1Mo-1Fe alloy developed by Institute of Metal Research, Chinese  Academy of Sciences (IMR). This type of alloy has lower contents of aluminum and vanadium but higher contents of molybdenum and iron. The cost of this alloy is relatively low, providing a potential alternative to the Ti6-Al-4V alloy. The difference of alloy property between Ti-4Al-2V-1Mo-1Fe alloy and Ti6-Al-4V alloy was systematically studied. The corrosion behaviour of the alloys was investigated in simulated marine environment (3.5 wt. % NaCl solution) by using polarization curves, electrochemical impedance spectroscopy. The paper is interesting for the journal considering the novelty of the alloy. To consider the paper for the publication on this journal the authors have to consider the following questions/advices:

1) In the manuscript the authors state that the  alloy Ti-4Al-2V-1Mo-1Fe is a new alloy developed by IMR. In literature are reported a 10 years old paper on the same alloy TiAlVMoFe (Prodana, M., Bojin, D., Ioniţǎ, D., Effect of hydroxyapatite on interface properties for alloy/biofluid, UPB Scientific Bulletin, Series B: Chemistry and Materials ScienceOpen AccessVolume 71, Issue 4, 2009, Pages 89-98) characterized in a different environment (biological enviorment) for biomedical purpose. The author have to discuss the differences (if any) between their alloy and the alloy reported on this paper.

2) Concerning the Electrochemical Impedance Spectroscopy, the authors model EIS spectra by using the equivalent circuit of figure 8. Authors named the CPE element as Q_dl (double layer). The CPE elements takes into account the double layer capacitance and the semiconductor capacitance relating to the passive films. In ref. F. Di Quarto, F. Di Franco, S. Miraghaei, M. Santamaria, F. La Mantia, The Amorphous Semiconductor Schottky Barrier Approach to Study the Electronic Properties of Anodic Films on Ti, Journal of The Electrochemical Society, 164 (9), (2017), C516-C525 the authors model the interface by using an equivalent circuit separating the contributes arising from double layer capacitance (assuming a value of 30 microF cm^-2) and semiconductor capacitance. I suggest the authors to correct this by considering the suggested paper.

Author Response

    1) In the manuscript the authors state that the alloy Ti-4Al-2V-1Mo-1Fe is a new alloy developed by IMR. In literature are reported a 10 years old paper on the same alloy TiAlVMoFe (Prodana, M., Bojin, D., Ioniţǎ, D., Effect of hydroxyapatite on interface properties for alloy/biofluid, UPB Scientific Bulletin, Series B: Chemistry and Materials Science Open Access Volume 71, Issue 4, 2009, Pages 89-98) characterized in a different environment (biological environment) for biomedical purpose. The author have to discuss the differences (if any) between their alloy and the alloy reported on this paper.

Response:

We appreciate the reviewer’s valuable comments. The reference provided has been cited in the revised manuscript. The introduction has been expanded and the effect of alloying elements on the properties of titanium alloys has also been discussed. The results reported in our work are slightly different from the literature provided. Several reasons are given below:

    a). The composition and microstructure of the TiAlVMoFe alloy (namely Ti-7Al-3V-3Mo-2Fe alloy in Prodana’s work) are different from these of the Ti-4Al-2V-1Mo-1Fe alloy developed by IMR;

    b).The potential usages of these two alloys are different (the TiAlVMoFe alloy is for biomedical purpose as the reviewer pointed out, while the Ti-4Al-2V-1Mo-1Fe alloy we studied is for marine use);

    c). The research goals are different: they tried to develop some surface coatings to improve the bio-compatibility of several Ti alloys (i.e., pure Ti, TiAlVMoFe alloy and the TiAlZr); while our goal was to develop a new Ti alloy with similar corrosion performance to commercial Ti-6Al-4V alloy but a lower cost.

2) Concerning the Electrochemical Impedance Spectroscopy, the authors model EIS spectra by using the equivalent circuit of figure 8. Authors named the CPE element as Q_dl (double layer). The CPE elements takes into account the double layer capacitance and the semiconductor capacitance relating to the passive films. In ref. F. Di Quarto, F. Di Franco, S. Miraghaei, M. Santamaria, F. La Mantia, The Amorphous Semiconductor Schottky Barrier Approach to Study the Electronic Properties of Anodic Films on Ti, Journal of The Electrochemical Society, 164 (9), (2017), C516-C525 the authors model the interface by using an equivalent circuit separating the contributes arising from double layer capacitance (assuming a value of 30 microF cm-2) and semiconductor capacitance. I suggest the authors to correct this by considering the suggested paper.

Response:

   We appreciate the reviewer’s valuable comments. The results reported in our work are slightly different from the literature provided. Several reasons are given below:

    a).The materials are different: they used pure Ti, while we investigated either the commercial Ti-6Al-4V alloy or the new-developed Ti-4Al-2V-1Mo-1Fe alloy;      b).The corrosion processes are different and thus the corrosion films formed may be different: F. Di Quarto et. al studied the corrosion process of pure Ti in acid under electrochemically-polarized conditions, while we compared the corrosion performance of the Ti-4Al-2V-1Mo-1Fe alloy to that of the commercial Ti-6Al-4V alloy in neutral chloride solutions under natural corrosion conditions;      c).The features of EIS spectra are different: there were two time constants in the EIS spectra presented by F. Di Quarto et. al; while there was only one time constant in our studies as clearly shown in Figure 6 and Figure 7.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

It is my pleasure to recommend this revised paper to acceptances.

Author Response

Thank you wery much for your kind help!

Reviewer 3 Report

I want to thank the authors for their response.

- Concerning the equivalent circuit employed to model the EIS spectra, I want to stress that the authors have to consider that CPE element take into account for two contributes, one arising from double layer capacitance and one arising from semiconductor capacitance relating to the passive film. In ref. F. Di Quarto et al., The Amorphous Semiconductor Schottky Barrier Approach to Study the Electronic Properties of Anodic Films on Ti, Journal of The Electrochemical Society, 164 (9), (2017), C516-C525, the authors model the interface metal/passive film/electrolyte by using an equivalent circuit that separates the contributes arising from double layer capacitance (assuming a value of 30 microF cm^-2 as reported in aqueous solutions) and semiconductor capacitance from passive film. In their reply the authors state that “there were two time constants in the EIS spectra presented by F. Di Quarto et. al; while there was only one time constant in our studies as clearly shown in Figure 6 and Figure 7”. However the authors have to explain why they don’t have the presence of double layer capacitance or passive films in their eis spectra. Which of these contributes don’t authors consider? Which is the physical reason?

- The authors didn’t discuss about pitting corrosion. They have to discuss why pitting corrosion doesn’t occur in their experiment in sea water.

Author Response

Concerning the equivalent circuit employed to model the EIS spectra, I want to stress that the authors have to consider that CPE element take into account for two contributes, one arising from double layer capacitance and one arising from semiconductor capacitance relating to the passive film. In ref. F. Di Quarto et al., The Amorphous Semiconductor Schottky Barrier Approach to Study the Electronic Properties of Anodic Films on Ti, Journal of The Electrochemical Society, 164 (9), (2017), C516-C525, the authors model the interface metal/passive film/electrolyte by using an equivalent circuit that separates the contributes arising from double layer capacitance (assuming a value of 30 microF·cm^-2as reported in aqueous solutions) and semiconductor capacitance from passive film. In their reply the authors state that “there were two time constants in the EIS spectra presented by F. Di Quarto et. al; while there was only one time constant in our studies as clearly shown in Figure 6 and Figure 7”. However the authors have to explain why they don’t have the presence of double layer capacitance or passive films in their EIS spectra. Which of these contributes don’t authors consider? Which is the physical reason?

We agree with the comments raised by the reviewer.

Since one time constant is observed in the spectrum, the plot is fitted with a so-called Randles-type RC-R electric circuit [K. Yanagisawa, T. Nakanishi, Y. Hasegawa, K. Fushimi, Passivity of Dual-Phase Carbon Steel with Ferrite and Martensite Phases in pH 8.4 Boric Acid-Borate Buffer Solution, Journal of The Electrochemical Society 162(7) (2015) C322-C326.]. The CPE element in equivalent circuit employed to fit the EIS spectra should take into account for two contributes: one arising from double layer capacitance and one arising from semiconductor capacitance caused by the formation of passive film. The reason that we chose to use the equivalent circuit in our work is below. Since the CPE element is taken into account for two contributes, double layer capacitance (C_H) and the passive film capacitance (Csc), the total capacitance can be obtained using Eq. 1 and Eq.2.

In our work the values for Ti-6Al-4V and Ti-4Al-2V-1Mo-1Fe alloys are 0.93 and 0.95 respectively, and thus C_sc is approximately equal to Q_SC . If C_H is 30μF·cm⁻², Q_SC is ~10⁻⁶ S·s^α ·cm⁻², and the capacitance of double layer seems to be neglected [B. Hirschorn, M.E. Orazem, B. Tribollet, V. Vivier, I. Frateur, M. Musiani, Determination of effective capacitance and film thickness from constant-phase-element parameters, Electrochimica Acta 55(21) (2010) 6218-6227.]. So the capacitance behavior of Ti-6Al-4V and Ti-4Al-2V-1Mo-1Fe alloys is dominated by the passive film [Z.B. Wang, H.X. Hu, C.B. Liu, Y.G. Zheng, The effect of fluoride ions on the corrosion behavior of pure titanium in 0.05M sulfuric acid, Electrochimica Acta 135 (2014) 526-535].

The authors didn’t discuss about pitting corrosion. They have to discuss why pitting corrosion doesn’t occur in their experiment in sea water.

The corrosion resistance of Ti and its alloys in aggressive environments is ensured by the formation of a compact and chemically-stable oxide film, mainly composed of titanium oxide, TiO₂, which spontaneously covers the metal surface to protect the metal substrate. Basame et al. clearly presented the dependence of the pitting potential on anion concentration in solutions containing Cl^- [S.B. Basame, H.S. White, Pitting corrosion of titanium - The relationship between fitting potential and competitive anion adsorption at the oxide film/electrolyte interface, Journal of The Electrochemical Society 147(4) (2000) 1376.], showing that pitting potential is very positive. It is far beyond the corrosion potential measured in Figure 5, and thus pitting corrosion was not observed in our work.

Fig. 1 The dependence of the pitting potential on anion concentration in solutions containing KCl

Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

I want to thank the authors for their response and their changes in the manuscript according to my comments. I think that the paper now is acceptable in the present form for the publication.