Corrosion Behavior of Mg-xGd-1Zn-0.4Zr Alloys with Different Gd Additions for Biomedical Application

Round 1

Reviewer 1 Report

Please see the file.

Comments for author File: Comments.pdf

Author Response

Reviewer 1

This paper was well prepared and organized from the point that it showed the corrosion behavior of Mg alloys with different Gd addition for biomedical application. But some part of the manuscript needs to be revised as follows;

1. E-mail address should be changed to your institution’s mail.

Re: We have added the institution’s email of the corresponding author: [email protected].

2. Experimental procedure

1) The analyzed chemical composition of the alloys shall be needed.

Re: We have added Table 1 to show the nominal and analyzed compositions of the alloys. And the analyzed methods were also added “The chemical compositions of the prepared alloys were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP 6300, America)” in the experimental section.

Table 1. Nominal and analyzed compositions of the alloys (wt%).

2) After aging, which cooling method did you use?

Re: After aging, we cooled the samples in air. “After aging, the samples were cooled in air” was added in the revised manuscript.

3) Every instrument used in the manuscript needs the information about Model, Manufacturer, and Country etc.

Re: The information about major instruments was added. Eg. inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP 6300, America), optical microscope (OM, OLYMPUS GX51, Japan), scanning electron microscope (SEM, JSM-6360LV, Japan), energy dispersive X-ray spectroscope (EDS, Oxford Atec X-max 50, UK), electrochemical workstation (PARSTAT2273, Ametek Company, USA).

4) Simulated body fluid’s composition shall be shown in the table.

Re: Table 2 was added to show composition of SBF.

Table 2. Composition of the simulated body fluid

5) Scanning rate shall be described after polarization test, not AC impedance test.

Re: “The scanning speed of the polarization curve is 1 mV/s” was placed in the right position.

6) AC impedance test needs the equivalent circuit model used.

Re: We used the equivalent circuit model as R_s(Q_dl(R_ct(Q_fR_f))).

We have added “In the equivalent circuit model, R_s is the solution resistance, Q_dl is double-layer constant phase angle element, R_ct is charge transfer resistance, Q_f is the constant phase angle element of oxidation film, and R_f the film resistance. R_ct paralleled with Q_dl is corresponded with the solution-substrate interface reaction, and R_f paralleled with Q_f is related to surface oxidation film. Besides, the polarization resistance (R_p) is calculated based on Equation (1), and the higher R_p represents better corrosion resistance.

     (1)”.

7) How did you measure the evolution of hydrogen? Describe the detail method.

Re: We added “A small hole was made on the edge of the sample wafer, and then it was suspended in the funnel. A collecting pipe filled with SBF was placed above the funnel to collect hydrogen” in the experimental section.

3. Figure 1 and Figure 2 and Average grain size: Please explain why Gd increases the grain size first and decreases the grain size or suggest the mechanism of grain refine and growth by Gd addition. Is there any problem in casting process? Why did Gd content reduce the cooling rate in Line 140?

Re: We added “When the Gd content is relatively low, the grain size decreases with the increase in Gd content. The main reason is that the addition of Gd content increases the subcooling at the solid-liquid interface during solidification, which leads to an increase in the nucleation rate and promotes grain refinement [38]. However, when the Gd content continues to increase to a certain amount, the main reason for the grain coarsening is that the increased amount of Gd element reduces the LPSO phase and decreases the formation of grain boundaries, thus increasing the grain size [39]. During solid solution treatment, the LPSO phase is almost completely dissolved into the matrix, and needle-like precipitation phases rich in Zr and Zn elements are formed in grains interiors and grain boundaries. As the Gd content increases, more needle-like phases are precipitated, which are enriched at the grain boundaries and delay the growth of a-Mg grains; nevertheless, when the Gd content exceeds a certain value, more Gd-rich blocks are precipitated by supersaturation, which are distributed in grains interiors and grain boundaries, while the number of needle-like precipitated phases becomes less and some of them are discontinuously distributed at the grain boundaries, which reduces the hindering effect on the grain growth and leads to the grain growth.”

4. Figure 3: The red color is not clear. Please change another color.

Re: We have changed the color.

5. SF and LPSO are not usual. Please the full name.

Re: We have given the full name in the introduction “long period stacking ordered (LPSO) or stacking fault (SF)”.

6. Figure 4: Vertical axis – Potential, V(SCE). Table 1 – Ecorr, V(SCE)

Re: The vertical axis has been modified.

Figure 4. Polarization curves of T6-treated alloys in SBF.

7. Figure 5 needs the analysis by the equivalent circuit.

Re: We have fitted the results of electrochemical parameters obtained from EIS. And analyzed the results by the equivalent circuit. We added equivalent circuit in Figure 5 and Table 4. The description has been added “Figure 5 displays EIS plots of the specimens in SBF. EIS Nyquist plots are composed of two capacitive loops, indicating that Mg alloys exhibit similar corrosion behavior but different corrosion resistance. EIS plots are fitted with equivalent circuit model R_s(Q_dl(R_ct(Q_fR_f))), and the fitting electrochemical parameters are listed in Table 4. In the equivalent circuit model, R_s is the solution resistance, Q_dl is double-layer constant phase angle element, R_ct is charge transfer resistance, Q_f is the constant phase angle element of oxidation film, and R_f the film resistance. R_ct paralleled with Q_dl is corresponded with the solution-substrate interface reaction, and R_f paralleled with Q_f is related to surface oxidation film. Besides, the polarization resistance (R_p) is calculated based on Equation (1), and the higher R_p represents better corrosion resistance.

     (1)

Hence it can be concluded that the GZ61K alloy has the best corrosion resistance and the GZ91K alloy presents the worst corrosion resistance, which is consistent with the results obtained by polarization curves.”

Figure 5. Nyquist plots of Mg alloys in SBF solution and their equivalent circuit model

Table 4 Fitting results of electrochemical parameters obtained from EIS

8. Figure 9 and Figure 10 were the results on the surface, is that right? However, as shown in Figure 8, the corrosion products of the alloys were different and thus the EDS analysis on the cross section is needed. Please check it.

Re: Yes, the former Fig. 9 and Fig. 10 are corroded surfaces to show the corrosion sequence at the initial stage. And Fig. 8 presents cross-section corroded morphologies of the alloys after immersion tests which can reveal corrosion mode. As can be seen from Fig. 8, block Mg₅Gd was not corroded and remained in the corrosion product layer. We have studied corrosion products of the Mg-Gd-Zn-Zr and Mg-Nd-Zn-Zr alloys before, therefore, we didn’t analyze corrosion products by EDS or XRD in this work. In order to clarify this comment, we have added “Previous studies have shown that the corrosion products on the surface of Mg-Gd-Zn-Zr magnesium alloy are mainly composed of two parts: the dense film on the substrate surface and the bright particles on the film. Firstly, the dense protective film formed on the substrate surface can prevent the substrate from being immersed in the corrosion solution to a certain extent. In addition, the film on the substrate surface is separated by a large number of cracks, among which the cracks are mainly caused by drying after the immersion experiment. EDS results showed that the bright particles and dense membrane mainly contained Mg, O, P and Ca elements, and the bright particles contained slightly more O, P and Ca elements, but less Mg [42]. In addition, it has been reported that the corrosion products were mainly MgO, Mg (OH)₂, hydroxyapatite (HA), and calcium-magnesium phosphates in SBF [43-45]” in the revised version.

9. Therefore, Figure 11 shall be modified according to Figure 8 and SEM-EDS additional results on the cross section.

Re: We have explained it in comment 8. The schematic diagram was illustrated according to the cross section images.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript reports the corrosion behavior of the Mg-Zn-Zr alloy with Gd additions. The results are interesting, novel and high-quality. However, minor revision should be made by the authors before the publication:

Please, provide the size and shape of the samples and describe surface preparations for all samples in the Experimental procedure section.

Line 115-116 - As far as I understand, the EDX values are given. How is such accuracy of the values ensured, especially for Zn and Zr? The areas to be examined are quite small, are Zn, Zr and other elements captured from neighboring areas? Please clarify.

Lines 124-142 do not fully explain why the needle structure is formed. In fact, precipitations in supersaturated solid solutions do not necessarily exist in the form of needles. Please clarify or change the logic of the explanation.

Author Response

Reviewer 2

The manuscript reports the corrosion behavior of the Mg-Zn-Zr alloy with Gd additions. The results are interesting, novel and high-quality. However, minor revision should be made by the authors before the publication:

Please, provide the size and shape of the samples and describe surface preparations for all samples in the Experimental procedure section.

Re: We added “Samples with diameter of 12 mm and thickness of 3 mm were cut for microstructure observation and corrosion tests. The surfaces of the samples were successively polished with 600, 1200, and 2000 grid sandpaper, and then polished with diamond paste. After being etched by 4 wt.% HNO₃ alcohol solution and dried by warm air.”

Line 115-116 - As far as I understand, the EDX values are given. How is such accuracy of the values ensured, especially for Zn and Zr? The areas to be examined are quite small, are Zn, Zr and other elements captured from neighboring areas? Please clarify.
Re: Yes, Zn and Zr elements captured from neighboring areas. Therefore, we have explained it as “Furthermore, it is found that the block precipitated phase marked as B is a Gd rich compound (Mg: 43.92wt%, Gd: 55.89wt%, Zn: 0.12wt%, Zr: 0.07wt%), which can be determined as Mg₅Gd according to the atomic ratio of Mg and Gd because trace Zn and Zr captured from the neighboring area can be neglected.”

Lines 124-142 do not fully explain why the needle structure is formed. In fact, precipitations in supersaturated solid solutions do not necessarily exist in the form of needles. Please clarify or change the logic of the explanation.

Re: We added“According to our previous report [18], with the increase of heat treatment temperature, the amount of the β phase with higher Gd and Zn decreases because more and more Gd and Zn atoms dissolve into α-Mg matrix. And during solution treatment, Zr element consumes some Zn, then hinders the formation of LPSO structure with α-Mg matrix. So more Zr precipitates from α-Mg matrix with the increase of the heat temperature, leading to more volume fraction of the precipitates [34]. As can be seen in Fig. 3(b), the Zr-Zn-rich precipitates present needle shape, which can also be found in heat treated Mg-Nd-Zn-Zr alloy [36].”

Author Response File: Author Response.pdf

Reviewer 3 Report

In this paper, the authors studied the effect of Gd additions on the corrosion behavior of Mg alloys. The paper is interesting and novel. It is publishable in Materials subject to revision.

1.It is recommended to list the chemical composition of the alloys in a form of a table. The chemical composition of SBF should also be specified and tabulated.

2.The paper lacks experimental x-ray diffraction (XRD) patterns. The authors speak about “block precipitated phase” or “needle-like precipitated phase”, see, e.g., Fig. 11. However, the phases are not experimentally verified. Diffraction patterns are necessary to confirm the specified phases.

3.Have you measured an open circuit potential of the alloys?

4.The corrosion rate should be calculated from hydrogen evolution volumes and compared with immersion experiments.

5.Corrosion rate should also be calculated from corrosion currents and compared with hydrogen evolution and immersion experiments. Any difference should be discussed.

6.Table 2 should include the corrosion rates obtained in the present work.

7.The post-corroded specimen should also be inspected by XRD. It is difficult to identify the corrosion products by SEM/EDS alone.

Author Response

Reviewer 3

In this paper, the authors studied the effect of Gd additions on the corrosion behavior of Mg alloys. The paper is interesting and novel. It is publishable in Materials subject to revision.

1.It is recommended to list the chemical composition of the alloys in a form of a table. The chemical composition of SBF should also be specified and tabulated.

Re: We added Table 1 to show the nominal and analyzed compositions of the alloys (wt%) and Table 2 to specify and tabulate the chemical composition of SBF.

Table 1. Nominal and analyzed compositions of the alloys (wt%).

Table 2. Composition of the simulated body fluid

2.The paper lacks experimental x-ray diffraction (XRD) patterns. The authors speak about “block precipitated phase” or “needle-like precipitated phase”, see, e.g., Fig. 11. However, the phases are not experimentally verified. Diffraction patterns are necessary to confirm the specified phases.

Re: We have done XRD experiments on T6 samples, and the results show that only the diffraction peak of α-Mg matrix can be observed, and the precipitated phase is very limited, so the diffraction peaks are not observed. We have added “Additionally, neither needle-like Zr-Zn-rich precipitated phase nor block Mg₅Gd phase could be detected by X-ray diffraction because of limited content” in the revised paper.

3.Have you measured an open circuit potential of the alloys?

Re: We have carried out the open circuit potential of the alloys to stabilize the potential, and the curves were not presented in the paper. We have added “At the beginning of the electrochemical testing, an open circuit potential was measured for 15 min to stabilize the potential” in the revised version.

4.The corrosion rate should be calculated from hydrogen evolution volumes and compared with immersion experiments.

Re: This comment is answered in conjunction with Comment 5.

5.Corrosion rate should also be calculated from corrosion currents and compared with hydrogen evolution and immersion experiments. Any difference should be discussed.

Re: We have added the following description in the revision: The corrosion rates of the alloys by mass loss, hydrogen evolution, and polarization tests were calculated and listed in Table 5. It can be seen that the corrosion rates of the three alloys using different methods show the same trend but the values show a little difference. It is clear that the corrosion rate of GZ91K calculated by the mass loss method is higher than that calculated by the hydrogen evolution method. This could be explained that the Gd content in the alloy is high, forming a massive Gd-rich phase, the precipitated phase falls off when the sample is pickling, resulting in the increase of the weight loss. The reason why the corrosion rate of the mass loss method of GZ31K and GZ61K is lower than that of the hydrogen evolution method is that the oxide produced on the surface of the sample is not cleaned, or the sample is slightly oxidized in the process of weighing after pickling, and there is a small amount of oxide on the surface. Among them, the electrochemical corrosion rate of GZ91K is quite different from the data obtained in the immersion experiment. The main reason is that the corrosion rate measured in the immersion experiment is the average rate of the alloy after soaking in SBF for 120 h, while the electrochemical test is the instantaneous corrosion rate of the alloy. In a word, the corrosion rate order of alloys with different Gd content was GZ91K > GZ31K > GZ61K.

Table 5. Corrosion rates (mm/year) of the T6 treated alloys calculated by different methods

6.Table 2 should include the corrosion rates obtained in the present work.

Re: We have added the corrosion rates obtained in the present work into the revised Table 6.

7.The post-corroded specimen should also be inspected by XRD. It is difficult to identify the corrosion products by SEM/EDS alone.

Re: Thanks for this kind comment. Actually, the SEM/EDS in Fig. 8 and Fig. 9 are quasi in-situ corrosion morphologies to demonstrate corrosion sequence of the alloys among various phases but not to inspect the corrosion product. As for corrosion products of Mg-Gd-Zn-Zr alloys in SBF, we have reported in other papers. In the revised manuscript, we have added “Previous studies have shown that the corrosion products on the surface of Mg-Gd-Zn-Zr magnesium alloy are mainly composed of two parts: the dense film on the substrate surface and the bright particles on the film. Firstly, the dense protective film formed on the substrate surface can prevent the substrate from being immersed in the corrosion solution to a certain extent. In addition, the film on the substrate surface is separated by a large number of cracks, among which the cracks are mainly caused by drying after the immersion experiment. EDS results showed that the bright particles and dense membrane mainly contained Mg, O, P and Ca elements, and the bright particles contained slightly more O, P and Ca elements, but less Mg [42]. In addition, it has been reported that the corrosion products were mainly MgO, Mg (OH)₂, hydroxyapatite (HA), and calcium-magnesium phosphates in SBF [43-45]” in the revised version.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The manuscript was well revised.

Reviewer 3 Report

Authors answered most of my comments. The paper is acceptable for publication.