Microstructural Precipitation Evolution and In Vitro Degradation Behavior of a Novel Chill-Cast Zn-Based Absorbable Alloy for Medical Applications

Round 1

Reviewer 1 Report

To my point of view, this is a well written article that fits within the scope of this Journal. The topic addressed is very interesting and the analysis and discussion are adequate. I only have some comments/suggestions, which are given hereafter:

1. In the introduction, line 57: When refer to a corrosion potential you should also mention the reference electrode and solution.
2. In the introduction, line 86: Could you be more specific on what you consider ‘desirable microstructure’ (e.g. grain refinement, intermetallic size and distribution etc.)?
3. In the Experimental, line 112: Can you quantify the ‘significant grain refinement’?
4. In the Experimental, Mechanical characterization: How many tensile tests did you perform per material? Also, in the ASTM E8 – 08 – 16a standard the strain rates can range from 0.05 and 0.5 mm/mm/min. Can you be more specific on the selected tensile test conditions?
5. In the results and discussion, Figure 9a,b. As-extruded pure Zn has higher hardness, but lower UTS that Zn-Mg. In many cases, the higher the hardness of a material the higher its yield point (HV ≈9 Re) and UTS.
6. In the results and discussion, lines 341-344: You discuss about the influence of strain rate and microstructure on the deformation mechanism. Therefore, it is very important to know in which range you operated.
7. In the fractographies that you present, you should mention the location that you made the picture. In extruded / surface treated alloys a different morphology can exist between the bulk and surface layers.
8. You mention that Mg4Zn7 and Mg0.971Zn0.029 precipitates are known to be highly corrosion resistant when compared with the matrix. Wouldn’t that result in localized galvanic coupling? The craters that you observe, might also due to this phenomenon.
9. From the corrosion morphologies, I believe that there is also pitting, due to the presence of halide ions in your corrosion solution.
10. It would be very interesting if in your future work you considered investigating stress corrosion cracking, as in the actual application these materials will be exposed simultaneously to mechanical loading and corrosion. Investigating these properties separately is important to understand the mechanisms and provides a relative ranking, but does not fully reflect on reality.

Author Response

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Reviewer 2 Report

Dear authors,

I enjoyed reading the manuscript because the paper is well-described and the results are clearly demonstrated. ​ The authors have made a good synthesis of the literature that provides an overview of the research evolution in this area.

This manuscript, entitled „Microstructural precipitation evolution and in vitro degradation behavior of a novel chill-cast Zn–based absorbable alloy for medical applications” is considered to be relevant to the scope of this journal.

However, several points need to be addressed prior to publication of this manuscript. My comments/suggestions are given:

1. Authors should check the writing mode, for example:

• authors' names are not written correctly
• the references are numbered twice ...
• not “Bio – logic VSP”, but “BioLogic VSP”
• With Regards!!! On the line 349

2. Too many details in the section - Abstract. It must be written summary.
3. I don't understand reference 26! It should be checked.
4. “Potentiodynamic tests carried out in Zn–12.5Ag–1Mg, Zn–1Mg and Zn in NaCl indicated an increasingly cathodic behaviour (SCEs = –1.044 V, –1.056 V and –1.092) with alloying additions.” This phrase needs to be reformulated!

Author Response

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Reviewer 3 Report

The subject is quite interesting as it deals with the development of a new alloy for absorbable biomedical applications. The experimentation is well-organized and presented. Some minor English mistakes are noticed throughout the text. The work is worthy to be published, provided that the authors respond to the following comments:

Introduction

1) The authors should explain why did they select pure Zn and Zn-1% Mg for comparison with Zn-12.5Ag-1Mg.

2.1

2) Lines 94-95: “In the present work, a Mg–Zn–Ag ternary alloy was chosen based 94 on crystallographic and microstructural considerations [23].” The authors should be more specific: What are these crystallographic and microstructural considerations?

2.4

3) Lines 136-137: “The shape of the exposed sample should also be reported, namely: Was it a disk (cross-section of the extruded rod?) or was it a cylindrical sample cut from the extruded rod? Dimensions should be given.

4)The authors should analytically report on how did they determine Icorr? Tafel extrapolation has to conform to some criteria in order to ensure accurate determinations of Icorr. These criteria are reported in literature, such as [A. Lekatou, D. Sioulas, A.E. Karantzalis, D. Grimanelis, Surface & Coatings Technology 276 (2015) 539–556 and G. Bolelli, R. Giovanardi, L. Lusvarghi, T. Manfredini, Corrosion Science 48 (2006) 3375-3397].

3.1

5) Lines 155-157: “Under the solidification conditions employed, a cooling rate of ~80 °C/s was 156 achieved for the Zn–1Mg and ~ 3000°C/s for the Zn–12.5Ag–1Mg alloy as estimated from the expression proposed by Jones [27].” The authors should analytically report on how did they determine these cooling rates and present the respective equations (in section 2.1).

6) Lines 160-163: The authors should report whether Mostaed et al [28] obtained lower solidification rates and provide respective data.

7) Line 166: “A maximum thickness of ~ 2.6 μm was found for this eutectic….” Does the maximum thickness refer to the lamellar plate of eutectic Zn or the n-Zn/Mg2Zn11 microconstituent as a whole?

8) Lines 179-180: “……lead to a reduction in hardness values as shown in Table 3 and Figure 9a”. The reduction is slight for Zn and Zn-1Mg and within experimental error. The reduction trend is more clear in the Zn-Ag-Mg alloy, though still within experimental error. The authors should rephrase.

9) Lines 214-216, Table 2 and Fig. 5d: In Fig. 5d, point E corresponds to ε-AgZn3, however in Table 2, point E corresponds to n-Zn +AgZn solid solution and point F corresponds to ε-AgZn3. Similarly, in Fig. 5d, point F corresponds to Mg-rich AgZn solid solution, however in Table 2, point F corresponds to ε-AgZn3. Corrections are required.

10) AgZn solid solution should be denoted by the format (Ag,Zn) or Ag-Zn; the format AgZn may be mixed up with that of an intermetallic compound.

3.2

11) Fig. 8d and Fig. 5b: Which is the main phase in Zn-1Mg extruded? Zn, as Fig. 5b suggests, or Mg4Zn7, as Fig. 8d suggests? A comment is needed.

12) Fig. 8d: The presence of Mg0.971Zn0.029 (96-152-3361) is highly questionable, as its detection is based on one peak only, which has also been assigned to Zn (96-900-8523).

3.3

13) Lines 282-283: “The reduction in hardness values from 61.6 ± 3.9 HV to 33.3 ± 1.9 HV after extrusion for the Zn–1Mg alloy and from 164.5 ± 8.0 HV to 151.5 ± 4.8 HV for….”  There are slight differences between these values and the respective values in Table 3.

14) Fig. 9b: Can the authors explain why the YS of Zn-1Mg is higher than that of Zn-Ag-Mg?

15) Figs. 9c,d: Comments on the fracture morphologies are missing.

16) Figs. 9c,d: Figs. 9c,d manifest that Zn-1Mg exhibits a more ductile behavior than Zn-Ag-Mg. However, the elongation of Zn-Mg has been measured as almost half than that of Zn-Ag-Mg (Table 4). An explanation is needed.

17) An explanation should be provided on why the ductility of Zn-Ag-Mg is higher than that of Zn-1Mg on the basis of the microstructures (Fig. 5) and the phases present.

18)Table 4: The denotation of (1), (2) exponents should be included in Table 4.

19) Lines 326, 327: “The development of Mg2Zn11 or MgZn2 intermetallics was not detected……” I guess, the authors refer to the Zn-Mg alloy?

20) Line 335: «...at log(ε)=1.3.» It is log(ε)=-1.3.

3.4

21) Lines 359-360: “A comparison of the resultant anodic slopes with the exhibited OCP values (Table 5) shows that pure Zn has the lowest oxidation rates…”It is the Icorr values that show that Zn has the lowest oxidation rates. OCP values show thermodynamic tendency but not corrosion rate.

22) Lines 370-373: “An even faster corrosion rate is achieved by adding Ag, reaching values of up to 1.251·mm year-1 (see Table 5)…….” The authors should justify the increased corrosion rate based on the cathodic second phases, as they did for Zn-1Mg.

23) Fig. 12: The title of x-axe should be “log of current density”.

24) Table 5: b_a values need to be completed with the respective standard deviation values.

25) The authors should report on why did they determine Icorr on the basis of the anodic Tafel slope. Why did they not utilize the cathodic Tafel slope or both the anodic and cathodic Tafel slopes? This comment, in context with comment 4).

26) Table 5: Potential ranges, current density ranges and linear regression coefficients of the Tafel extrapolation should be included in Table 5 or in a new Table.

27) Comments are needed on whether the corrosion rates of Table 4 are considered acceptable for biomedical applications.

28) Line 393: “…….and corrosion of the base material is not uniform (Figure 13b).” However, the anodic polarization of Fig. 12, supports a uniform corrosion situation, as the authors also claim in page 16, lines 358-359.

29) Line 390: “Moreover, localized corrosion was found in the Zn–1Mg alloy, (Figure 13a),…” According to my opinion, the term “localized corrosion” is not right, as it points to pitting. The “pit” in Fig. 13a is so shallow and wide that it cannot be considered as “localized”. It is rather a case of galvanic corrosion between a precipitate rich zone and a precipitate poor zone, that led to detachment of surface material leaving the shallow crater shown in Fig. 13a.

30) Line 400: “….all grain boundaries act as an anode with respect to the matrix….” The boundaries are rich in Ag0.15Mn1.85. Is this phase anodic or cathodic to the matrix? Similarly, ε-AgZn3/Ag0.15 and MgZn1.85 are anodic or cathodic to the matrix? If they are cathodic to the Zn matrix (possibly, Ag0.15Mn1.85 and ε-AgZn3/Ag0.15 are cathodic to the matrix), then the boundaries have acted as cathodes, also accounting for the high cathodic currents for Zn-Ag-Mg compared to pure Zn and Zn-Mg.

31) Lines 406-407: “These intermetallic compounds possess a relatively high corrosion resistance, thus inducing rapid and local corrosion.” If the intermetallic particles are in a dense and uniform dispersion, corrosion is not local. It may have started as localized around them, but then it has developed to a uniform corrosion leading to material dislodgement, as shown in Fig. 13a,b.

Conclusions

32) Line 450: “…an excellent candidate..” Is the characterization “excellent” in compatibility with the corrosion results? The authors should comment at the end of 3.4.

33) Minor English mistakes in lines: 50, 51, 52, 274, 277, 282, 307

Author Response

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Reviewer 4 Report

The paper is good. The authors need to rewrite the abstract. It is too long, and its style is suitable for a paper introduction.

Author Response

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Round 2

Reviewer 3 Report

Athough the authors have responded appropriately to some of my comments, there are still many comments that have not been responded adequately. Even in the cases of comments that have been responded satisfactorily, the responses have not been added to the manuscript (e.g. comments 1,2,17, 28, 29, 30, 31) as they ought to have been. Thereby, the authors should provide further responses, which should be highlighted in the manuscript, in the following comments:

-Original Comment 3) Lines 136-137: “The shape of the exposed sample should also be reported, namely: Was it a disk (cross-section of the extruded rod?) or was it a cylindrical sample cut from the extruded rod? Dimensions should be given.”

Response 3): Thank you for the comment and the dimensions and shape of samples are given in Fig. 1.

Reviewer’s reply: What I asked for, was to report the shape and dimensions of the specimen that was exposed to the electrolyte (CORROSION TESTING). Fig. 1 has nothing to do with the corrosion tests.  

-Original Comment 4): The authors should analytically report on how did they determine Icorr? Tafel extrapolation has to conform to some criteria in order to ensure accurate determinations of Icorr.

Response 4): Thank you for the comment, however, authors would want to kindly ask if it is possible to reformulate the question because is not completely clear.

The reviewer’s reformulation of comment 4: In Table 5, the authors are reporting values of ba and Icorr. In 2.4, they should report on how did they perform these calculations (I assume by Tafel extrapolation). Most important the authors should report on whether did they apply the appropriate criteria to avoid erroneous Tafel calculations.  To be more clear, I am citing respective extracts on the criteria applied to ensure a confident Tafel extrapolation from papers and book:

M.F. Fontana, Corrosion Engineering, Mc Graw-Hill

“In actual practice, an applied polarization curve becomes linear on a semilogarithmic plot at approximately 50 mV more active than the corrosion potential……To ensure reasonable accuracy, the Tafel region must extend over a current range of at least one order of magnitude…….Further, the method can be applied only to systems containing one reduction process……”

Angeliki Lekatou, Athanasios K. Sfikas, Christina Petsa and Alexandros E. Karantzalis, Al-Co Alloys Prepared by Vacuum Arc Melting: Correlating Microstructure Evolution and Aqueous Corrosion Behavior with Co Content, Metals 2016, 6, 46; doi:10.3390/met6030046

“ The corrosion current densities (average of 4–5 values) were calculated by Tafel extrapolation [38]. The extrapolation was performed by linear regression analysis (least squares method) applied to the E versus log (i) data starting from potentials differing from the rest potential by at least 50 mV and extending over a current density range of at least one order of magnitude. Reasonable accuracy was ensured by conforming to several criteria, analytically reported in a previous effort [39]. Epigrammatically, the following criteria were applied:

(a) A sufficiently low scan rate (10 mV/min) was employed;

(b) the Tafel region was extended over a current density range of at least one order of magnitude;

(c) the linear fit was only accepted if the regression coefficient was found greater than 0.98;

(d) the linear regression analysis was started at sufficiently large overpotentials (differing from the rest potential by at least 50 mV);

(e) only one reduction process was considered in the range of linear fit (in neutral solutions, such as naturally aerated 3.5 wt.% NaCl, the cathodic current is almost entirely consumed by the reduction of dissolved oxygen [40]);

(f) if only one of the two polarization curves presented a linear region extending over a current density range of at least one order of magnitude, then Tafel extrapolation was only applied to this curve.”

Giovanni Bolelli, Roberto Giovanardi, Luca Lusvarghi, Tiziano Manfredini, Corrosion resistance of HVOF-sprayed coatings for hard chrome replacement, Corrosion Science 48 (2006) 3375-3397:

“Tafel’s law, as an approximation of the general Buttler–Volmer equation, is valid when considering sufficiently high anodic and cathodic overpotentials (≥50 mV). Even though polarization curves often do not follow Tafel’s law over the entire experimental range, linear sections on the E–log(I) plot can generally be found. Thus, on every anodic and cathodic polarization curve, the potential range where a linear dependence between E and log(I) exists is located, and the regression line which best fits this data range (according to the least squares method) is found, in order to determine the coefficients of the above equation. In every case, the regression coefficient must be ≥0.98 for the linear fit to be considered acceptable. The fit is always performed over data ranges where the applied potential differs from the initial rest potential by more than 50 mV (to enter the validity range of Tafel’s linear approximation). ………………………………………………………………….

The potential ranges for the linear fit on the anodic and cathodic polarization curves is indicated in the tables where polarization test results are illustrated. In a few cases, the linear fit was not possible on the cathodic curve: in this case, to obtain the corrosion current density, the corrosion potential was assumed to be equal to the rest potential at the beginning of the test, and its value was substituted in the linear equation of the anodic curve.”

-Original comment 5): Lines 155-157: “Under the solidification conditions employed, a cooling rate of ~80 °C/s was achieved for the Zn–1Mg and ~ 3000°C/s for the Zn–12.5Ag–1Mg alloy as estimated from the expression proposed by Jones [27].” The authors should analytically report on how did they determine these cooling rates and present the respective equations (in section 2.1).

Response 5): Thank you for the question and in fact, the reference [27] explain in detail the correlation between cooling rate and the secondary arm spacing value. In this way, it is possible to estimate the desired parameter having one of them from the experimental.

Reviewer’s reply: The authors should still respond, since the cooling rate is a paramount factor for the obtained microstructures. I am repeating comment 5):  The authors should analytically report on how did they determine these cooling rates and present the respective equations (in section 2.1).

-Original Comment 11): Fig. 8d and Fig. 5b: Which is the main phase in Zn-1Mg extruded? Zn, as Fig. 5b suggests, or Mg4Zn7, as Fig. 8d suggests? A comment is needed.

Response 11): Thank you for this observation, for Zn-1Mg alloy the main phase is η – Zn solid solution (matrix) and, the presence of Mg4Zn7 was distributed along all the microstructure (Fig. 5b) and detected by XRD too (Fig. 8d, ref 96–152–3103).

Reviewer’s reply: In order to avoid any misunderstanding, an appropriate note should be made within the manuscript.

-Original Comment 12): Fig. 8d: The presence of Mg0.971Zn0.029 (96-152-3361) is highly questionable, as its detection is based on one peak only, which has also been assigned to Zn (96-900-8523).

Response 12): Precipitates in Zn – based alloys have an important impact on their mechanical properties. For this reason, authors would like to live the proposed intermetallic compound and to confirm its presence in future work.

Reviewer’s reply: Still, it is wrong to assume the presence of a phase on one peak only, which has also been assigned to another phase.   At least, an appropriate note should be made within the manuscript.

- Original Comment 14) Fig. 9b: Can the authors explain why the YS of Zn-1Mg is higher than that of Zn-Ag-Mg?

Response 14): As it was aforementioned, the precipitates play an important role on the mechanical properties of Zn – based alloys. In this sense, precipitates in Zn–Mg system induce higher strengthening mechanism than the precipitates present in the ternary alloy.

However, this research is ongoing and a detail work focus on precipitation evolution and their impact on the mechanical properties should be done.

Reviewer’s reply: The author’s should at least make a note in their manuscript for the peculiar observation, that although the tensile strength of Zn-Ag-Mg is higher than the tensile strength of Zn-Mg, the opposite results for the yield strength.  In any case, both alloys experience dispersion strengthening. Maybe, the authors should try an explanation compatible with conclusion 4 and place it  in the manuscript.

-Original Comment 17):  An explanation should be provided on why the ductility of Zn-Ag-Mg is higher than that of Zn-1Mg on the basis of the microstructures (Fig. 5) and the phases present.

Response 17): From Fig. 5 it is evident that Zn–Ag–Mg alloy is conformed by fine – grains which provided an important increment on its ductility. Additionally, the presence of a cubic phase embedded into the Zn–Ag–Mg matrix alloy also favors the ductility of the material.

Reviewer’s reply: Thanks for the response. This should be added to the manuscript.

-Original Comment 18): Table 4: The denotation of (1), (2) exponents should be included in Table 4.

Response 18): The denotation (1) and (2) it is present in Table 4 and they were highlighted in red for practical purposes.

Reviewer’s reply: Denotations still have not been added. To rephrase the comment, the authors should add as a footnote in Table 4, what do exponents (1) and (2) stand for? (please, refer to Fig. 10a)

- Original Comment 21): Lines 359-360: “A comparison of the resultant anodic slopes with the exhibited OCP values (Table 5) shows that pure Zn has the lowest oxidation rates…”It is the Icorr values that show that Zn has the lowest oxidation rates. OCP values show thermodynamic tendency but not corrosion rate.

Response 21): Thank you for the comment and, a comparison of the resultant anodic slopes with the exhibited thermodynamics changes in OCP values.

Reviewer’s reply: The authors have still not responded to my comments. The sentence “ A comparison of the resultant anodic slopes with the exhibited OCP values (Table 5) shows that pure Zn has the lowest oxidation rates “, does not have any meaning. As it is well known, potentials show thermodynamic tendencies (nobility) and not kinetic trends. The kinetic trends (e.g. oxidation RATE) can be shown only by the current density values. The sentence should be replaced by: ““A comparison of the I corr values (Table 5) shows that pure Zn has the lowest oxidation rates”.

-Original Comment 24) Table 5: ba values need to be completed with the respective standard deviation values.

Response 24): Thank you for the observation, chi2 values were included for adjusting Tafel

slops.

Reviewer’s reply: What is Chi²? The authors should write what does X² stand for below Table 5. What I meant by comment 24) is that the authors should present the ba values by the format: e.g. for Zn: 0.167±…… as they did with OCP, Icorr and Corrosion rate.

-Original Comment 25): The authors should report on why did they determine Icorr on the basis of the anodic Tafel slope. Why did they not utilize the cathodic Tafel slope or both the anodic and cathodic Tafel slopes? This comment, in context with comment 4).

Response 25): Thank you for the question but, authors appreciated if the question could be reformulated and clear.

Reviewer’s reply:

According to Tafel law, the corrosion current density is determined by solving the system:

E=a_c+b_c log(Icorr)

E=a_a+b_a log(Icorr)

where the a, c subscripts stand for ‘‘anodic’’ and ‘‘cathodic’’ respectively [please, see the aforementioned paper by Bolelli et al., Corrosion Science 48 (2006) 3375]. Often, when only one of the two polarization curves presents a linear region extending over a current density range of at least one order of magnitude, then Tafel extrapolation is only applied to this curve [Please, see the aforementioned paper by Lekatou et al., Metals 2016, 6, 46; doi:10.3390/met6030046]. You used only the anodic slopes to calculate your Icorr. Is this because you could not perform a linear fit in the cathodic portions? Judging from Fig. 12, I think this is the reason, but I need a proper answer that has to be included in the manuscript..

Original comment 26) Table 5: Potential ranges, current density ranges and linear regression coefficients of the Tafel extrapolation should be included in Table 5 or in a new Table.

Response 26): Resulting in increased activity, in the same trend changes in the slope is observed, indicating that the active behavior and therefore increase the corrosion rate. This reaction is due to decreasing the concentration polarization associated with electrochemical reactions that are manipulated by the diffusion of ions in the electrolyte, typically present in metals with very negative redox potentials such as Zn and Mg. Electrode potential tends to less negative values and metal corrosion rate increases as cathodic control decreases, compared to other ones that is limited to net current due to low alloy speed cathodic reactio by the effect of mass transport of hydrogen ion diffusion through bubbles that are formed by water decomposition which significantly increases the activity of the Zn-Mg and Zn-Mg-Ag alloys.

Reviewer’s reply:  The answer is irrelevant to my comment. Table 5 needs to be completed by some numerical values, namely: Range of potentials in the anodic portion where the linear fit was applied, range of currents in the anodic portion where the linear fit was performed and the regression coefficients which should be higher than 0.98.

-Original Comment 27): Comments are needed on whether the corrosion rates of Table 4 are considered acceptable for biomedical applications.

Response 27): Thank you for the comment. The main evidence of why this alloy is considered acceptable for biomedical applications is due it preserves a good balance in between mechanical behavior and homogeneous and medium degradation.

Reviewer’s reply:  The response should refer specifically to the corrosion rates. How is the corrosion rate of Zn-Ag-Mg compared with corrosion rates of other absorbable alloys? Although the authors have presented enough data to support the good mechanical behavior of this alloy compared to other works (Figure 11) they have not presented enough comparative data on the corrosion.  They should do so, in the manuscript.

-Original Comment 28) Line 393: “…….and corrosion of the base material is not uniform (Figure 13b).” However, the anodic polarization of Fig. 12, supports a uniform corrosion situation, as the authors also claim in page 16, lines 358-359.

Response 28): The polarization technique indicates a controlling reaction mechanism. For the Tafel slop behavior and the earrings that are presented and assumed to comply with the Butler–Volmer equation for overpotential higher that 100 mV.

Reviewer’s reply: I totally agree. However, the authors state (lines 356-358): “In turn, from these results the dominant corrosion mechanism is apparently controlled by charge transfer which facilitates uniform corrosion of the metallic interface”. Elsewhere (lines 392), they state the opposite: “…and corrosion of the base material is not uniform …”. So, in in order for any misunderstanding to be avoided, the authors should add a respective note in the manuscript, as they did in their response (they should also correct their English mistakes).

-Original Comment 29) Line 390: “Moreover, localized corrosion was found in the Zn–1Mg alloy, (Figure 13a),…” According to my opinion, the term “localized corrosion” is not right, as it points to pitting. The “pit” in Fig. 13a is so shallow and wide that it cannot be considered as precipitate poor zone, that led to detachment of surface material leaving the shallow crater shown in Fig. 13a.

Response 29): We agree. Yes, the localized phenomenon occurs by the presence of this amorphous phase, but we do not believe that it is presented by a galvanic couple among the elements of it. The phenomena is presented by difference of cathodic and anodic areas which prevents the passive layer from forming homogeneously.

Reviewer’s reply: Since the authors agree, they should proceed to remove or rephrase the term “localized corrosion”.  As they point out, “The phenomena is presented by difference of cathodic and anodic areas which prevents the passive layer from forming homogeneously.”. This phenomenon, seems quite extensive (Figs. 13a,b) and it cannot be characterized as “localized”.

-Original comment 30) Line 400: “….all grain boundaries act as an anode with respect to the matrix….” The boundaries are rich in Ag0.15Mn1.85. Is this phase anodic or cathodic to the matrix? Similarly, ε-AgZn3/Ag0.15 and MgZn1.85 are anodic or cathodic to the matrix? If they are cathodic to the Zn matrix (possibly, Ag0.15Mn1.85 and ε-AgZn3/Ag0.15 are cathodic to the matrix), then the boundaries have acted as cathodes, also accounting for the high cathodic currents for Zn-Ag-Mg compared to pure Zn and Zn-Mg.

Response 30): Thank you for your comment and authors agreed that the phase possessed a more anodic behavior into the grain boundaries.

Reviewer’s reply: As the authors agreed, why did not they change their expression “…all grain boundaries act as an anode with respect to the matrix….” that the phases at the grain boundaries  act as cathodes with respect to the matrix….” ? They should do so.

-Original Comment 31): Lines 406-407: “These intermetallic compounds possess a relatively high corrosion resistance, thus inducing rapid and local corrosion.” If the intermetallic particles are in a dense and uniform dispersion, corrosion is not local. It may have started as localized around them, but then it has developed to a uniform corrosion leading to material dislodgement, as shown in Fig. 13a,b.

Response 31): The mechanism that controls the corrosion rate presented was a homogeneous type but the corrosion morphology in discrete sites is presented locally when material is detached. Later studies can perform techniques much more sensitive to these localized phenomena such as local electrochemical impedance or electrochemical microscope to better describe the a anodic or cathode behavior of the phases, as well as the mechanism presented in each of them.

Reviewer’s reply: Once again, the authors agree to an extent with my comment. Then, why did not they add a short note to avoid the confusion between uniform corrosion that is the governing form and localized corrosion that exists but it is a secondary form of corrosion? The authors should add their response “The mechanism that controls the corrosion rate presented was a homogeneous type but the corrosion morphology in discrete sites is presented locally when material is detached.” having corrected their English mistakes, to the manuscript.

-Original Comment 32) Line 450: “…an excellent candidate..” Is the characterization “excellent” in compatibility with the corrosion results? The authors should comment at the end of 3.4.

Response 32): Authors agreed that the ternary system is an excellent candidate to be considered as absorbable material because exhibit a balanced between its mechanical properties and corrosion behavior.

Reviewer’s reply: First of all, the authors have not commented at the end of 3.4, as I suggested. Second, the characterization “excellent” is excessive as it is not accompanied by comparative results from previous works on other absorbable alloys regarding the corrosion performance.  As the authors, previously noted (response 27): “The main evidence of why this alloy is considered acceptable for biomedical applications is due it preserves a good balance in between mechanical behavior and homogeneous and medium degradation.” This constitutes a proper statement, drawn and justified by the experimental results, and should replace the excessive statement “From the experimental outcome, the proposed Zn–12.5Ag–1Mg ternary absorbable alloy is found to be an excellent candidate to be considered for cardiovascular applications.”

Author Response

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