In Vitro Corrosion Performance of As-Extruded Mg–Gd–Dy–Zr Alloys for Potential Orthopedic Applications
Round 1
Reviewer 1 Report
The present work by Liu et al. investigates the corrosion performance of Mg-Gd-Dy-Zr alloys for orthopedic application. The authors prepared several alloys of different chemical composition and investigated them in vitro. The topic is important and the presented results are interesting, however, some points should be addressed.
The authors performed an immersion test. Did they perform an elemental analysis of the solutions by ICP-OES or similar technique in order to determine the selective ion release from the alloys?
Please indicate the reference data used for the identification of crystalline phase by XRD.
Number of typos can be found in the text, e.g.:
line 139 – there is no Figure 1e in the manuscript.
Line 141 - there is no Figure 1f in the manuscript.
Author Response
The present work by Liu et al. investigates the corrosion performance of Mg-Gd-Dy-Zr alloys for orthopedic application. The authors prepared several alloys of different chemical composition and investigated them in vitro. The topic is important and the presented results are interesting, however, some points should be addressed.
Q(1) The authors performed an immersion test. Did they perform an elemental analysis of the solutions by ICP-OES or similar technique in order to determine the selective ion release from the alloys?
Reply: Thank you for your question. We agreed that ion release is very important for the further evaluation on degradation behavior and biosafety of Mg alloys. We will make arrangements in the following study on the biocompatibility and biosafety of Mg-Gd-Dy-Zr alloys.
Q(2) Please indicate the reference data used for the identification of crystalline phase by XRD.
Reply: Thank you for your question. The relevant references have been added in the revised manuscript as follows:
The main purpose of this paper is to improve the corrosion resistance of magnesium alloys by reducing the second phases to control galvanic corrosion. The XRD analysis was performed to preliminarily verify the presence or absence of the second phases in the as-extruded Mg-Gd-Dy-Zr alloys[25], and the results are shown in Fig. 3. No sign of the secondary phases and only the constitution of α-Mg matrix can be detected in all XRD patterns. It indicates that the secondary phase can be efficaciously inhibited by adding alloying elements with high solid solution solubilities in magnesium alloys, in combination with homogenization heat treatment and extrusion deformation [43, 44].
- Li, X.; Liu, C.; Wang, J.; Zhang, C. Tailoring the strength and formability of Mg alloys through rare earth element additions (Gd and Dy) and dynamic recrystallizations. Materials Today Communications 2021, 28, 102627.
- Guang-Li; Bi; Yu-Xiang; Han; Jing; Jiang; Chun-Hong; Yuan-Dong; Li. Microstructural evolution and age-hardening behavior of quasicrystal-reinforced Mg-Dy-Zn alloy. Rare Metals 2019, 38, 29-35.
- Yang, L.; Huang, Y.; Feyerabend, F.; Willumeit, R.; Mendis, C.; Kainer, K.U.; Hort, N. Microstructure, mechanical and corrosion properties of Mg-Dy-Gd-Zr alloys for medical applications. Acta Biomaterialia 2013, 9, 8499-8508.
Q(3) Number of typos can be found in the text, e.g.: line 139 – there is no Figure 1e in the manuscript; Line 141 - there is no Figure 1f in the manuscript.
Reply: Thank you for pointing out this mistake. The typos have been corrected in the revised manuscript as follows:
Fig.2(e) presents the grain size distribution of the as-extruded Mg-Gd-Dy-Zr alloys, which is similar to Gauss distribution for each alloy. The peak area becomes wider when the RE addition is higher, which demonstrates that the grain size changes to non-uniform in the high RE addition samples. Fig.2(f) shows the variation of average grain size with the increase of RE content in the as-extruded Mg-Gd-Dy-Zr alloys. The average grain size is appropriately between 20 μm - 30 μm for all the alloys and gradually increases by adding more RE. Such phenomenon can also be found in other Mg-RE alloys [26, 33, 37, 43]. ……
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This manuscript, entitled „In vitro corrosion performance of as-extruded Mg-Gd-Dy-Zr 2 alloys for potential orthopaedic applications” is relevant to the scope of this journal. It is an interesting study that can provide interesting information to specialists.
However, some points need to be addressed before publication of this manuscript. My comments/suggestions are given:
- In the methods chapter, the authors should specify the potential applied in the electrochemical impedance spectroscopy tests.
- Experimental data obtained using electrochemical impedance spectroscopy must be fitted using equivalent electrical circuits. The obtained values must be presented accompanied by error, so that conclusions can be drawn about the interfaces that occur during the study. From the values obtained, much more concrete observations can be made than simply observing the capacitive semicircles in Nyquist diagrams. Also, from the Nyquist diagram shown in Figure 5, for GD 5.0 and GD 10.0 samples, an inductive loop can be observed, usually associated with adsorption phenomena. This study needs to be deepened and completed to obtain complete information.
- Careful with what Rp represent. It is polarization resistance.
- I don't entirely agree with the above sentence, and I think the reference with corresponds. “Generally, the cathodic polarization curve represents the hydrogen evolution reaction and the anodic polarization curve denotes the magnesium dissolution process [46]. (line 194). Please check and rephrase.
- The expression "higher E0" is not used in electrochemistry. One can say a more electronegative or more electropositive potential. Please correct.
- Analyzing Figure 6, it seems that the corrosion current density value is higher for GD 10.0 than for GD 5.0. In Table 3 the values are exactly the opposite. Please analyze Figure 6 and Table 3 maybe it is a mistake.
- I think some Tafel slope values are high 491, 410.9, maybe you should check how the slopes are drawn.
- The authors should explain more Figure 8. Why is the corrosion rate after 14 days of immersion lower than that after 7 days?
- I believe that the values obtained for corrosion rate by accelerated potentiodynamic polarization tests are completely different from those obtained by immersion tests. There is a difference of 2 orders of magnitude in the case of GD 1.2. Also, from the polarization curves the best behavior is recorded for GD 1.2 sample, while from the immersion tests, it appears that the best sample would be GD 0.6. At this point I think it is important to complete the electrochemical impedance spectroscopy studies in order to draw a consistent conclusion from this study.
- The writing of the references is not uniform and does not meet the requirements of the journal.
- The manuscript must be completed with the contribution of each author to its realization.
Author Response
This manuscript, entitled „In vitro corrosion performance of as-extruded Mg-Gd-Dy-Zr alloys for potential orthopaedic applications” is relevant to the scope of this journal. It is an interesting study that can provide interesting information to specialists.
However, some points need to be addressed before publication of this manuscript. My comments/suggestions are given:
Q(1) In the methods chapter, the authors should specify the potential applied in the electrochemical impedance spectroscopy tests.
Reply: Thank you for pointing out this missing information. We have added this experimental details in the revised manuscript as follows:
…… Impedance measurements were carried out at the OCP as the initial potential using a 10 mV root-mean-square perturbation from 100 kHz to 10 mHz. ……
Q(2) Experimental data obtained using electrochemical impedance spectroscopy must be fitted using equivalent electrical circuits. The obtained values must be presented accompanied by error, so that conclusions can be drawn about the interfaces that occur during the study. From the values obtained, much more concrete observations can be made than simply observing the capacitive semicircles in Nyquist diagrams. Also, from the Nyquist diagram shown in Figure 5, for GD 5.0 and GD 10.0 samples, an inductive loop can be observed, usually associated with adsorption phenomena. This study needs to be deepened and completed to obtain complete information.
Reply: Thank you for giving us the detailed guidance and professional advices on analyzing our experiments. We have added those data in the revised manuscript as follows:
…… It can be seen from Fig. 5 that the shape of impedance spectrum changes with the increase of RE content in the alloys. The sizes of capacitive loop of GD0.6 and GD1.2 samples were relatively similar and obviously larger in comparison with those of GD5.0 and GD10.0 samples in the same solution, which demonstrated that GD0.6 and GD1.2 should possess higher corrosion resistance. GD10.0 has the minimum capacitive loop radius and hence the worst corrosion resistance. Besides, for GD 5.0 and GD 10.0 samples, an inductive loop can be observed, usually associated with adsorption phenomena. ……
Q(3) Careful with what Rp represent. It is polarization resistance.
Reply: Sorry for the misspelling. We have corrected it in revised manuscript as follows:
Fig.6 shows the polarization curves of the as-extruded Mg-Gd-Dy-Zr alloys samples immersed in Hank’s solution. The corrosion potential (E0), the current density (I0), polarization resistance (Rp) and corrosion rate (CR) calculated according to the ASTM-G102-89 standard are listed in table 3.
Q(4) I don't entirely agree with the above sentence, and I think the reference with corresponds. “Generally, the cathodic polarization curve represents the hydrogen evolution reaction and the anodic polarization curve denotes the magnesium dissolution process [46]. (line 194). Please check and rephrase.
Reply: Yes, the previous description was indeed not accurate. The questionable sentence has been deleted in the revised manuscript as follows:
…… The corrosion rate is judged under the influence of both anode and cathode reaction. Generally, the cathodic polarization curve represents the hydrogen evolution reaction and the anodic polarization curve denotes the magnesium dissolution process [46]. ……
Q(5) The expression "higher E0" is not used in electrochemistry. One can say a more electronegative or more electropositive potential. Please correct.
Reply: Thank you for your constructive advice. We have made the revision in the manuscript as follows:
…… In general, excellent corrosion resistance can be obtained with a more electropositive potential, lower I0 and higher Rp at the same time. ……
Q(6) Analyzing Figure 6, it seems that the corrosion current density value is higher for GD 10.0 than for GD 5.0. In Table 3 the values are exactly the opposite. Please analyze Figure 6 and Table 3 maybe it is a mistake.
Reply: Thank you very much for pointing out the controversy. We have carefully checked the experimental data again and found no mistakes for the curves. So the table 3 has been corrected. The original Tafel curves and fitting data of GD5.0 and GD10.0 are shown as the following figures:
Fig.1s Tafel curve (left) and fitting data (right) of GD5.0
Fig.2s Tafel curve (left) and fitting data (right) of GD10.0
Q(7) I think some Tafel slope values are high 491, 410.9, maybe you should check how the slopes are drawn.
Reply: Thank you for your kind reminding. The original Tafel slopes of as-extruded Mg-Gd-Dy-Zr alloys are shown as following figures
GD0.6 GD1.2
GD5.0 GD10.0
Fig.3s The original Tafel slopes of as-extruded Mg-Gd-Dy-Zr alloys
Q(8) The authors should explain more Figure 8. Why is the corrosion rate after 14 days of immersion lower than that after 7 days?
Reply: Thank you for the question. We have added the explanations of the reasons in the revised manuscript as follows:
After cleaning the surface products of the samples with chromic acid, corrosion performance evaluations by weight loss after immersion of 7 days and 14 days were analyzed and illustrated graphically as corrosion rates in Fig. 8. The results show that the corrosion rates of all the as-extruded Mg-Gd-Dy-Zr alloys after 14 days of immersion lower than those after 7 days. This is mainly because the progress of corrosion produces degradation products on the sample surface which could slow down the corrosion rate to a certain extent. These results are consistent with the pH variation shown in Fig.7. The in vitro corrosion rates of as-extruded Mg-Gd-Dy-Zr alloys display an upward trend with increasing amounts of RE contents after both 7 days and 14 days immersion.……
Q(9) I believe that the values obtained for corrosion rate by accelerated potentiodynamic polarization tests are completely different from those obtained by immersion tests. There is a difference of 2 orders of magnitude in the case of GD 1.2. Also, from the polarization curves the best behavior is recorded for GD 1.2 sample, while from the immersion tests, it appears that the best sample would be GD 0.6. At this point I think it is important to complete the electrochemical impedance spectroscopy studies in order to draw a consistent conclusion from this study.
Reply: Thank you for the constructive advice. Potentiodynamic polarization is an instantaneous test, and as such represents only a snapshot of the corrosion at the time it is performed. It can only display the corrosion at earlier stages but not necessarily representative of all time points. Different from the potentiodynamic polarization results, the corrosion rates obtained by immersion tests are calculated in terms of mass loss after both 7 days and 14 days immersion in Hank’s solutions. It reflects the corrosion rates of immersion after a period of time. Thus it makes sense that the values obtained for corrosion rate by accelerated potentiodynamic polarization tests and immersion tests are inconsistent.
Q(10) The writing of the references is not uniform and does not meet the requirements of the journal.
Reply: We appreciate for raising the formatting issues. We have carefully checked and revised the writing of the references in the manuscript to meet the requirements of the journal.
Q(11) The manuscript must be completed with the contribution of each author to its realization.
Reply: Thank you for your reminding. The contributions of each author are listed as below:
Chen Liu: Data curation, Formal Analysis; Funding acquisition; Investigation; Writing – original draft; Junsheng Wang: Methodology, Resources, Software, Validation, Writing – review & editing; Yongdong Xu: Funding acquisition, Methodology, Project administration, Supervision, Validation; Yu Fu: Resources, Software, Supervision; Jungang Han: Data curation, Formal Analysis, Visualization, Writing – original draft; Zhaoxun Cao: Data curation, Formal Analysis, Visualization, Writing – original draft; Xiaohu Chen: Validation, Visualization, Writing – original draft; Shuming Zhao: Validation, Visualization, Writing – original draft, Writing – review & editing; Xiurong Zhu: Conceptualization, Project administration, Supervision, Validation, Writing – review & editing.
Author Response File: Author Response.pdf
Reviewer 3 Report
Notes on the article of Chen Liu, Junsheng Wang, Yongdong Xu1, Yu Fu, Jungang Han, Zhaoxun Cao, Xiaohu Chen, Shuming Zhao and Xiurong Zhu “In vitro corrosion performance of as-extruded Mg-Gd-Dy-Zr alloys for potential orthopaedic applications”
The paper reports results of studying the effect of structure on the corrosion resistance of the biodegradable alloys Mg-0.6Gd-0.3Dy-0.2Zr, Mg-1.2Gd-0.6Dy-0.2Zr, Mg-5Gd-2.5Dy-0.2Zr and Mg-10Gd-5Dy-0.2Zr after hot extrusion. The authors showed that all alloys had a single-phase structure after extrusion, according to XRD results. However, the formation of a small amount of the second phase was observed along the grain boundaries, according to the SEM results. It wos also shown that the alloy with the highest content of rare earth elements (Mg-10Gd-5Dy-0.2Zr) had the worst corrosion resistance. The article is interesting and has a high theoretical and practical value. In my opinion this report can be published after revisions that are listed below:
1) How many samples were used to determine the corrosion performance? The experimental error values should be added in Table 3.
2) Why was chromic acid used to clean the surface after immersion tests? Could the metal dissolve together with corrosion products? Weak acid mixtures are usually used (see 1. ASTM G31-21, Standard guide for laboratory immersion corrosion testing of metals)
3) Why was the microstructure assessment performed in the perpendicular direction? Usually, the structure is studied in the direction parallel to the extrusion direction.
4) The most likely reason for the increase in the degradation rate of alloys with a higher content of rare earth elements is an increase in the volume fraction of the precipitated phase. However, due to the fact that in this study it was not possible to determine the volume fraction of the precipitated phase, the issue remains debatable.
5) Typos need to be corrected:
- P.8, L. 285 and L. 297: It should be written: “Mg17Al12” instead of “Mg17Al12”.
6) The article should be formatted in accordance with the requirements of the Journal.
Author Response
The paper reports results of studying the effect of structure on the corrosion resistance of the biodegradable alloys Mg-0.6Gd-0.3Dy-0.2Zr, Mg-1.2Gd-0.6Dy-0.2Zr, Mg-5Gd-2.5Dy-0.2Zr and Mg-10Gd-5Dy-0.2Zr after hot extrusion. The authors showed that all alloys had a single-phase structure after extrusion, according to XRD results. However, the formation of a small amount of the second phase was observed along the grain boundaries, according to the SEM results. It was also shown that the alloy with the highest content of rare earth elements (Mg-10Gd-5Dy-0.2Zr) had the worst corrosion resistance. The article is interesting and has a high theoretical and practical value. In my opinion this report can be published after revisions that are listed below:
Q(1) How many samples were used to determine the corrosion performance? The experimental error values should be added in Table 3.
Reply: Thank you very much for reminding us. Three duplicates were taken for statistical analysis for each group in order to control the experimental scattering. As the difference between the three duplicates is small, the data of the intermediate curve is selected as the reference data shown in Table 3 in this paper.
Q(2) Why was chromic acid used to clean the surface after immersion tests? Could the metal dissolve together with corrosion products? Weak acid mixtures are usually used (see 1. ASTM G31-21, Standard guide for laboratory immersion corrosion testing of metals)
Reply: Thank you for your kind suggestions. Chromic acid solution is commonly used to clean the corrosion products of biodegradable magnesium alloys [1s. Lin, M. , et al. "A novel biodegradable Mg-Nd-Zn-Zr alloy with uniform corrosion behavior in artificial plasma." Materials Letters 2012, 88:1-4; 2s. Hong D , Saha P , Chou D T , et al. In vitro degradation and cytotoxicity response of Mg-4% Zn-0.5% Zr (ZK40) alloy as a potential biodegradable material[J]. Acta Biomaterialia, 2013, 9(10):8534-8547.]. The chromic acid solution is specifically described as 200 g/L chromium trioxide (CrO3) and 10 g/L silver nitrate (AgNO3). Chromic acid has a passivation effect and does not corrode the magnesium matrix. The more detail has been added in the revised manuscript as follows:
……Weight loss was measured to calculate the corrosion rate of the samples after removal of the corrosion products in chromic acid [200 g/L chromium trioxide (CrO3) and 10 g/L silver nitrate (AgNO3)].……
Q(3) Why was the microstructure assessment performed in the perpendicular direction? Usually, the structure is studied in the direction parallel to the extrusion direction.
Reply: We agreed that the corrosion resistant along the extrusion direction is usually the best and chosen for evaluation for most cases. In this study, the corrosion performance tests of as-extruded Mg-Gd-Dy-Zr alloys are conducted on surfaces perpendicular to the extrusion direction, so the microstructure assessment performed in the perpendicular direction and was consistent with corrosion test surface. In our previous study on the mechanical properties of the as-extruded Mg-Gd-Dy-Zr alloys, the microstructure was studied in the direction parallel to the extrusion direction [Li X, Liu C, Wang J, Zhang C. Tailoring the strength and formability of Mg alloys through rare earth element additions (Gd and Dy) and dynamic recrystallizations. Materials Today Communications 2021;28:102627.].
Q(4) The most likely reason for the increase in the degradation rate of alloys with a higher content of rare earth elements is an increase in the volume fraction of the precipitated phase. However, due to the fact that in this study it was not possible to determine the volume fraction of the precipitated phase, the issue remains debatable.
Reply: Thank you for providing the constructive comments. Due to the fact that in this study, it is speculated that the more Gd and Dy elements dissolved in the α-Mg matrix have a negative effect on the corrosion resistance. This question will be the focus of our following research. Based on previous reports, the meaningful certainty is that the as-extruded Mg-Gd-Dy-Zr alloys with low RE content (GD0.6) allows a satisfactory corrosion resistance in Hank’s solution with a controlled corrosion rate less than 0.5mm/year, which is considered as tolerance limit for corrosion rate of orthopedic implants.
Q(5) Typos need to be corrected:
- P.8, L. 285 and L. 297: It should be written: “Mg17Al12” instead of “Mg17Al12”.
Reply: Thank you for pointing out those mistakes. The typos have been corrected in the revised manuscript and marked in red.
Q(6) The article should be formatted in accordance with the requirements of the Journal.
Reply: Thank you for reminding us the formatting issues. We have re-formatted the article in accordance with the requirements of the Journal
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
I believe that the authors have made all the required corrections and additions, bringing the manuscript into a much improved form. I recommend publication in its present form.
Reviewer 3 Report
The manuscript has been significantly improved and can be accepted in present form.
