The Effect of Microstructure on Local Corrosion Product Formation during Initial SO2-Induced Atmospheric Corrosion of ZnAlMg Coating Studied by FTIR-ATR FPA Chemical Imaging
Round 1
Reviewer 1 Report
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Comments for author File: 
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Author Response
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Reviewer 2 Report
Very interesting investigation. A few comments for your consideration:
1) For Figure 1 I would request that you add labels for the Zn-rich, lamellar binary, and ternary eutectic phases. The labels should clearly indicate on the Figure where each phase is. I would also suggest making reference to this figure earlier on in the introduction when you are talking about the microstructure of this alloy. I would be a useful aid when going through the introduction for the first time.
2) For the exposure conditions, was this stagnant or flushed gas? Was the gas continuously replenished to maintain the specified conditions, or was the gas pumped in and the chamber sealed off for the duration of the experiment? I think this is a crucial detail.
3) For Figure 2 and the supporting text, there are different exposure times mentioned. I see 2.5, 3.5, 10, and 16 hours all mentioned. Please read through and correct all of the exposure times to whatever the correct times were.
4) Around lines 154-156 you make a brief reference to the proposed mechanism that explains your results. I know that this is written as a Results and Discussion section together, but it reads mostly like the Results are presented first and then there is Discussion second. It seems out of place to have this mention of the mechanism without a supporting discussion in the middle of what reads as a results section. I might recommend moving this sentence further down to the discussion-focused section and state the results simply in this earlier results-focused section. For instance, at this point I had several questions about HER, the nature of the electrolyte on the surface, the fate of SO2 in the gaseous vs aqueous phase, etc. But these questions are addressed in part later on in the manuscript in the discussion-focused section. This is why I think this sentence should be moved further down in the text.
5) For Figure 4, I do not see a label for the Sulfate intensity in the Argon condition. Please add that label. I see that there has been some attempt to color-code the labels with the curves in this Figure. I like that and I would like to see it applied to all the labels so that the label color corresponds to the curve color.
6) For Figure 5, I like this figure, it is some nice imaging. I think it would be better if it were bigger on the page. The same is true for most of the figures, they could be bigger. This may be an issue for the editorial staff.
7) For Figure 6, it is difficult to tell exactly where P1 and P2 are on the IR Ara Map. Maybe have a dot or arrow or box indicate the location. Additionally, I would color-code and use arrows to make it clear whick line scan in part b) is for P1 and P2. The same comments apply to Figure 7.
8) Just curious why you say that the corrosion products are soluble in line 262. Isn't the point of the IRRAS and FTIR measurements that there are detectable products on the surface of the material? Indeed, your mechanism in Figure 10 shows the solid products on the surface. Perhaps you are referring to some specific detail of the process. Please elaborate. You mention that pH has some effect on favoring the formation of more soluble corrosion products in the anodic areas. Do you mean that the pH makes it so that products which are normally more soluble are able to crash out as product on the surface or do you mean that the pH makes it so that more soluble corrosion products are formed meaning that they form soluble ions instead of solid compounds.
9) For Figure 10, I believe the HSO32- should be labelled HSO3- in both instances.
10) An added detail to supplement your mechanism. As you say, cathodic reactions ORR and HER increase pH. SO2 is an acidic gas like CO2 and solubility increases greatly with increasing pH. Therefore, the cathodic phase, and especially close to the interface with the anodic phase where there is strong microgalvanic coupling should have the higher pH and also highest local concentration of dissolved SO2. And according to your mechanism highest local concentration of reactive intermediates (assuming that their concentration is proportional to the overall ORR current and not dependent on electrode potential).
11) I believe HER may be more important than you give it credit. Zn is certainly reactive enough (has sufficiently negative corrosion potential) that HER is likely a significant portion of the cathodic current. In fact, it is entirely likely that ORR is in the mass transport limited regime. If so, then the local current density from ORR would be uniform across the sample regardless of microgalvanic coupling. The local HER current density would change however. The local HER current density at the micorgalvanic interface would be higher. To my knowledge, you also produce the same reactive intermediates that you mention for ORR (you would need to find the appropriate references for that) so your mechanistic explanation would still hold for HER as much as it would for ORR. You may want to consider giving HER equal or greater influence in the mechanistic explanation. It is difficult to know for certain without seeing any electrochemcial data, but I suspect that the cathodic kinetics are strongly influenced by HER for the bulk alloy but also for the individual phases, even the Zn-rich phase.
12) I believe there is another mechanistic explanation at play that you should at least address. Oxygen itself is able to oxidize sulfur up to the 6+ valence state (to sulfate). This happens as a natural process. I wonder if the presence of oxygen in the humid air condition allows for oxidation of the SO2 to higher valence states, even before it is dissolved into the electrolyte layer on the material. In this way, there would be less sulfate on the surface in the Argon case because oxidation of SO2 in the gas phase would not be possible in the absence of oxygen. There would still be more sulfate on the anodic phase and near the microgalvanic interface I think due to the higher concentration of Mg produced to react with. I strongly suspect that HER is operative for the Argon condition (and the air condition) and since the reactive intermediates should be available from the HER reaction, the current mechanistic explanation doesn't fully explain the lack of sulfate in the Argon condition. I think it is fine that you have the current mechanism as the main proposed mechanism but I think that this alternative mechanism needs to be mentioned. If the gas in stagnant during the exposure then I think the alternate mechanism is viable. As time goes on the sulfur in the gas becomes more and more oxidized to the sulfate state. If the gas is constantly replenished then the alternate mechanism is less viable since the sulfur in the gas phase should remain in a more reduced valence state. The oxygen dissolved in the electrolyte can also oxidize the SO2/SO3 dissolved in the electrolyte. If you do suspect that there are oxygen concentration gradients across the surface of the sample, then areas of higher oxygen concentration should have correspondingly higher sulfate concentrations too (like the over the anodic phase near the microgalvanic couple). I still suspect that the oxygen is fairly depleted at the interface due to mass transport limited ORR.
1) line 34, we usually say worldwide instead of wide world
2) The title for section 2.1 and 2.5 have a period in front of the title. Like ". Materials" instead of "Materials"
3) Line 98, there should be a subscript 2 in SO2. Please check throughout the manuscript to ensure that subscripts and superscripts are used where necessary.
4) Line 151, why is this sentenced bolded?
Author Response
We greatly appreciate the comments and constructive suggestions for improvement of the paper by the reviewer. We have provided responses the review report and carefully considered the comments and suggestions.
Very interesting investigation. A few comments for your consideration:
- For Figure 1 I would request that you add labels for the Zn-rich, lamellar binary, and ternary eutectic phases. The labels should clearly indicate on the Figure where each phase is. I would also suggest making reference to this figure earlier on in the introduction when you are talking about the microstructure of this alloy. I would be a useful aid when going through the introduction for the first time.
Ans: We have introduced labels for the different phases in Figure 1. We have added a reference to Figure 1 in the introduction
- For the exposure conditions, was this stagnant or flushed gas? Was the gas continuously replenished to maintain the specified conditions, or was the gas pumped in and the chamber sealed off for the duration of the experiment? I think this is a crucial detail.
Ans: The exposures were made with humid air or humid Ar flowing over the sample. The flow rate of the gas mixture was 1 l /min. This information has been added to section 2.2 in the experimental
- For Figure 2 and the supporting text, there are different exposure times mentioned. I see 2.5, 3.5, 10, and 16 hours all mentioned. Please read through and correct all of the exposure times to whatever the correct times were. ’’
Ans: This is corrected
- Around lines 154-156 you make a brief reference to the proposed mechanism that explains your results. I know that this is written as a Results and Discussion section together, but it reads mostly like the Results are presented first and then there is Discussion second. It seems out of place to have this mention of the mechanism without a supporting discussion in the middle of what reads as a results section. I might recommend moving this sentence further down to the discussion-focused section and state the results simply in this earlier results-focused section. For instance, at this point I had several questions about HER, the nature of the electrolyte on the surface, the fate of SO2 in the gaseous vs aqueous phase, etc. But these questions are addressed in part later on in the manuscript in the discussion-focused section. This is why I think this sentence should be moved further down in the text.
Ans: We agree. We have moved the sentences mentioning the effect of oxidants (line 174-175) as this is discussed later
- For Figure 4, I do not see a label for the Sulfate intensity in the Argon condition. Please add that label. I see that there has been some attempt to color-code the labels with the curves in this Figure. I like that and I would like to see it applied to all the labels so that the label color corresponds to the curve color.
Ans: Figure 4 has been corrected. We have used color-coding the label the curves
- For Figure 5, I like this figure, it is some nice imaging. I think it would be better if it were bigger on the page. The same is true for most of the figures, they could be bigger. This may be an issue for the editorial staff.
Ans: The size of figure 5 has been increased. The size of other figures have been increased slighly and been improved for clarity
- For Figure 6, it is difficult to tell exactly where P1 and P2 are on the IR Ara Map. Maybe have a dot or arrow or box indicate the location. Additionally, I would color-code and use arrows to make it clear whick line scan in part b) is for P1 and P2. The same comments apply to Figure 7.
Ans: We have made boxes (diamond shaped) to mark the locations for local FTIR-analysis ”points”
8) Just curious why you say that the corrosion products are soluble in line 262. Isn't the point of the IRRAS and FTIR measurements that there are detectable products on the surface of the material? Indeed, your mechanism in Figure 10 shows the solid products on the surface. Perhaps you are referring to some specific detail of the process. Please elaborate. You mention that pH has some effect on favoring the formation of more soluble corrosion products in the anodic areas. Do you mean that the pH makes it so that products which are normally more soluble are able to crash out as product on the surface or do you mean that the pH makes it so that more soluble corrosion products are formed meaning that they form soluble ions instead of solid compounds.
Ans: Formation of soluble corrosion products, such as Mg-sulfate, may lead to increase of dissolved ions in the thin surface electrolyte. This should lead to increased water up-take in these areas due lowering of water activity in the surface electrolyte layer and promote the corrosion and corrosion product formation in these areas. We have discussed this further in the text.
9) For Figure 10, I believe the HSO32- should be labelled HSO3- in both instances.
Ans: Figure 10 is corrected
10) An added detail to supplement your mechanism. As you say, cathodic reactions ORR and HER increase pH. SO2 is an acidic gas like CO2 and solubility increases greatly with increasing pH. Therefore, the cathodic phase, and especially close to the interface with the anodic phase where there is strong microgalvanic coupling should have the higher pH and also highest local concentration of dissolved SO2. And according to your mechanism highest local concentration of reactive intermediates (assuming that their concentration is proportional to the overall ORR current and not dependent on electrode potential).
Ans: A very relevant comment. We agree that the higher pH, which is expected at cathodic area ( Zn-rich phase) and at the border to the anodic area should lead to higher SO2 uptake and probably also then increased sulfate formation. However, higher pH can also affect the oxidation of sulphite by hydrogen peroxide in solution, which decrease with increasing pH [M. R. Hoffmann and J. O. Edwards, Journal of Physical Chemistry, Vol. 79, No. 20, 1975]. The local pH is important for several physical and chemical processes on the surface ans we have discussed the effect of pH more in the discussion.
11) I believe HER may be more important than you give it credit. Zn is certainly reactive enough (has sufficiently negative corrosion potential) that HER is likely a significant portion of the cathodic current. In fact, it is entirely likely that ORR is in the mass transport limited regime. If so, then the local current density from ORR would be uniform across the sample regardless of microgalvanic coupling. The local HER current density would change however. The local HER current density at the micorgalvanic interface would be higher. To my knowledge, you also produce the same reactive intermediates that you mention for ORR (you would need to find the appropriate references for that) so your mechanistic explanation would still hold for HER as much as it would for ORR. You may want to consider giving HER equal or greater influence in the mechanistic explanation. It is difficult to know for certain without seeing any electrochemcial data, but I suspect that the cathodic kinetics are strongly influenced by HER for the bulk alloy but also for the individual phases, even the Zn-rich phase.
Ans: For very thin surface electrolyte layers, as under atmospheric studies of oxygen reduction reaction on metals with thin electrolyte layers indicate (that the dissolution of oxygen into the electrolyte layer is often rate controlling during atmospheric corrosion e.g Zhong et al, ChemElectroChem 2021, 8, 712–718, ). We agree that HER could contribute to a small extent to cathodic current during the exposures in humid air but the higher rate of the sulfate/(and sulfite) formation in humid air compared to humid Argon suggests that the dominating cathodic reaction is oxygen reduction. We believe that it is not possible that SO2 is oxidised in gas-phase for a significant amount, prior to entering the water ad-layer, under these conditions (see comments below) and that oxidation of suphite is mediated by the metal surface.
HER could be more important in humid Argon where sulfate formation is limited.
12) I believe there is another mechanistic explanation at play that you should at least address. Oxygen itself is able to oxidize sulfur up to the 6+ valence state (to sulfate). This happens as a natural process. I wonder if the presence of oxygen in the humid air condition allows for oxidation of the SO2 to higher valence states, even before it is dissolved into the electrolyte layer on the material. In this way, there would be less sulfate on the surface in the Argon case because oxidation of SO2 in the gas phase would not be possible in the absence of oxygen. There would still be more sulfate on the anodic phase and near the microgalvanic interface I think due to the higher concentration of Mg produced to react with. I strongly suspect that HER is operative for the Argon condition (and the air condition) and since the reactive intermediates should be available from the HER reaction, the current mechanistic explanation doesn't fully explain the lack of sulfate in the Argon condition. I think it is fine that you have the current mechanism as the main proposed mechanism but I think that this alternative mechanism needs to be mentioned. If the gas in stagnant during the exposure then I think the alternate mechanism is viable. As time goes on the sulfur in the gas becomes more and more oxidized to the sulfate state. If the gas is constantly replenished then the alternate mechanism is less viable since the sulfur in the gas phase should remain in a more reduced valence state. The oxygen dissolved in the electrolyte can also oxidize the SO2/SO3 dissolved in the electrolyte. If you do suspect that there are oxygen concentration gradients across the surface of the sample, then areas of higher oxygen concentration should have correspondingly higher sulfate concentrations too (like the over the anodic phase near the microgalvanic couple). I still suspect that the oxygen is fairly depleted at the interface due to mass transport limited ORR.
Ans: We think that it is very unlikely that the SO2 is oxidised in the gas phase before entering the surface electrolyte layer. The oxidation of SO2 has studied extensively due to its importance in atmospheric chemistry and it is known that the oxidation rate is very slow in the absence of oxidants or catalysts (Calvert and Stockwell in SO2, NO and NO2 oxidation mechanisms (1984)). For gas phase oxidation in the atmosphere hydroxyl radical are crucial and these should not be present in the SO2 containg humid air flow. Furthermore, if oxidation to sulfate in gas phase would happen sulfate formation should be observed on other metal substrates exposed under similar conditions. In the previous investigation cited in the article (ref 14, Persson et al 2015) it was seen that sulfate formation was negliable for pure Zn and Mg exposed to pure humid air containing SO2 and only sulfite containing corroson products were formed.
Comments on the Quality of English Language
- line 34, we usually say worldwide instead of wide world
Ans: Corrected
- The title for section 2.1 and 2.5 have a period in front of the title. Like ". Materials" instead of "Materials"
Ans: Seems that some errors are introduced in the manuscript when it was loaded into the system
3) Line 98, there should be a subscript 2 in SO2. Please check throughout the manuscript to ensure that subscripts and superscripts are used where necessary.
4) Line 151, why is this sentenced bolded?
Ans: corrected
Reviewer 3 Report
I read the article “The effect of microstructure on local corrosion product formation during initial SO2 induced atmospheric corrosion of ZnAlMg coating studied by FTIR-ATR FPA chemical imaging”, by Dan Persson, Dominique Thierry , Nathalie Leboze, carefully.
If the article is well written, relying on reference n°14 leads the reader to read it. The question then arises: what does this article bring in relation to this reference? If the techniques are different, the novelty is not highlighted enough, which is materialized by a figure 10 very little different from the diagram of reference 14. On the other hand, the article insists on the fact that it can appear a galvanic coupling. However, the results show a preferential attack of the "binary eutectic" phase. A hypothesis may be a galvanic coupling but the results that are presented do not show it. In this sense I classify the article in major revision.
In addition, I have several comments:
1- The references from 1 to 9 are useless, as they stand in the text. Number 10 is self-citing, as is number 14. However, what about literature? the introduction should be revised to be more descriptive of the literature
2- In the introduction, it is not explained what a sulphite or a sulphate is (think of young readers or inexperienced readers!)
3- On figure 1, put a legend that details the figure and arrows that guide the reading. I remind you that a figure and its legend must be self-supporting. In addition, the image is blurry. A SEM-EDS maps would be welcome and would make it possible to locate the species which would greatly facilitate the reading and to see the evolution with figure 9.
4- The experimental protocol of references 14 and 15 speaks of NaCl and CO2, which is not the case here. A more detailed protocol needs to be explained.
5- L113 what does the 100mGe snail-shaped symbol represent?
6- Pay attention to indices and exponents
7- SEM what magnification?
8- In the results part, 3.5h are mentioned but in the legends of figures 2 and 3 it is 2.5h?
9- Figure 4 there are 4 curves and only 3 legends!
10- L147, do you highlight a difference in surface finish? Has the impact not been studied in the literature?
11- fig 5 is blurred. Why not show until 16h ? Is there a difference in appearance if the air is argon?
12- fig 6-7 and 8 what does the coloured legend represent? For the inexperienced, it is difficult to understand. The axes are illegible. You can't see P1 or P2 very well.
13- L187 it is difficult to see that the location mentioned corresponds to “eutectic phase” In the text, a precision on where P1 and P2 are located and which phase they represent would help the reader. This lack of clarity is related to a poor description of Figure 1.
14- Fig 6 and fig 7 P1 and P2 are reversed put the same P1 for the same phase
15- Figure 6 is better described it is nice to see the phases clearly mentioned. In this sense the lines making the link are useless if you specify what P1 and P2 represent and that it is always the same thing from one figure to another.
16- L198-199 why do you say “The intensities of the sulphate bands are the highest at the binary eutectic areas and also at areas close to the zinc rich phases.” The zinc-rich phase is far from the location of the max, max relative to the scale!
17- Figure 8 why is there no P2.
18- L211-214: why not but justify it by relying on bibliographical references!
19- Same remark for paragraph L230-233
20- Figure 9: the figure is blurred, the captions are not legible and the Zn label is poorly made and cuts off the image
21- Convey l244 conveys without S
22- The SO2 dissolved in water ad-layer can lead to a slight acidification of the aqueous layer, initial dissolution of surface oxides and the sub-sequent formation of metallo-sulphite complex and sulphite containing surface films. Reference ????????
23- oxidation of sulphite to sulphate: equation?
24- l248 “At the border between the zinc rich dendrites and the binary eutectic, oxidation of sulphite to sulphate can be enhanced due presence of oxidants, such as peroxide or superoxide. “I'm sorry but figure 7 doesn't show it. The white phase is far from the top!
25- “The oxygen reduction on zinc in oxygen saturated sulphate solutions in the pH range 4–11 was studied by Boto and Williams [19] and it was found that a mixture of H2O2 and hydroxide was formed. In a study [20] performed on bare and passive zinc it was observed that hydroperoxyl ions are produced on passive zinc while the reaction proceeds primarily by a direct four electron reduction to hydroxyl ions on bare zinc. By in situ and operando attenuated total reflection infrared (ATR-IR) spectroscopy Nayak et al [18] detected surface bound superoxide during ORR on electrodeposited zinc hydroxide-based model corrosion products.” Ok it is noted but what about the anode in all these studies??? and zinc passivation?
26- Figure 10 What does the gray domain represent on the Zn surface? the electrolyte? a passivation?
27- More generally for the discussion, proposing a mechanism is a good idea which must open on the hypotheses which is here a potential galvanic coupling (even if for the moment I am not convinced) and not be based on the hypothesis for make the mechanism. Here we can talk about preferential attack for example.
In conclusion, clarifications on all the figures should be made to gain clarity and ensure that the message of the text is truly illustrated by the figures. We must expand the bibliography which is really poor and try to rely more on what already exists when hypotheses are made (without self-citing!). The discussion should not be based on a hypothesis (potential coupling) but on the actual results. The mechanism is self-supporting but can open on the hypothesis of a coupling.
The article must be reworked before being published.
Author Response
We greatly appreciate the comments and constructive suggestions for improvement of the paper by the reviewer. We have provided responses the review report and carefully considered the comments and suggestions.
I read the article “The effect of microstructure on local corrosion product formation during initial SO2 induced atmospheric corrosion of ZnAlMg coating studied by FTIR-ATR FPA chemical imaging”, by Dan Persson, Dominique Thierry , Nathalie Leboze, carefully.
If the article is well written, relying on reference n°14 leads the reader to read it. The question then arises: what does this article bring in relation to this reference? If the techniques are different, the novelty is not highlighted enough, which is materialized by a figure 10 very little different from the diagram of reference 14. On the other hand, the article insists on the fact that it can appear a galvanic coupling. However, the results show a preferential attack of the "binary eutectic" phase. A hypothesis may be a galvanic coupling but the results that are presented do not show it. In this sense I classify the article in major revision.
Ans: The article employs spectrochemical imaging by FTIR-ATR FPA to provide information how the chemical composition of the corrosion products is related to the coating microstructure. To our best we have not seen that FTIR-imaging has been applied for studies aiming to relate the chemical composition of corrosion products (here the speciation of S) to the microstructural composition of metal alloys. We have modified figure 10 to better reflect the results obtained by FTIR-FPA imaging
In addition, I have several comments:
- The references from 1 to 9 are useless, as they stand in the text. Number 10 is self-citing, as is number 14. However, what about literature? the introduction should be revised to be more descriptive of the literature
Ans: The introduction has been revised and more articles added to the bibliography. It should be noted that we have not found any publications where this technique (FTIR-FPA imaging) has been employed to relate the composition of corrosion products to the microstructure of a metal / metal coating, which mentioned now in the introduction.
- In the introduction, it is not explained what a sulphite or a sulphate is (think of young readers or inexperienced readers!)
Ans: Done
- On figure 1, put a legend that details the figure and arrows that guide the reading. I remind you that a figure and its legend must be self-supporting. In addition, the image is blurry. A SEM-EDS maps would be welcome and would make it possible to locate the species which would greatly facilitate the reading and to see the evolution with figure 9.
Ans: We have introduced labels for the different phases in Figure 1. SEM studies of this and similar alloys have been reported in several publications before [
4- The experimental protocol of references 14 and 15 speaks of NaCl and CO2, which is not the case here. A more detailed protocol needs to be explained.
Ans: A more detailed experimental protocol is provided
5- L113 what does the 100mGe snail-shaped symbol represent?
Ans: It is 100 mm diameter Ge ATR crystal used. Have been corrected
6- Pay attention to indices and exponents
Ans: Corrected
7- SEM what magnification?
Ans: x1200 has been added to figure text
8- In the results part, 3.5h are mentioned but in the legends of figures 2 and 3 it is 2.5h?
Ans: Corrected
9- Figure 4 there are 4 curves and only 3 legends!
Ans: Corrected
10- L147, do you highlight a difference in surface finish? Has the impact not been studied in the literature?
Ans: In the previous study the
11- fig 5 is blurred. Why not show until 16h ? Is there a difference in appearance if the air is argon?
Ans: In situ time-lapse photography of ZnAlMg was performed in separate experiments, and with a different experimental cell, compared to the in situ IR investigations and photographs for 16 h are not available
12- fig 6-7 and 8 what does the coloured legend represent? For the inexperienced, it is difficult to understand. The axes are illegible. You can't see P1 or P2 very well.
Ans: The figures have been improved for clarity
13- L187 it is difficult to see that the location mentioned corresponds to “eutectic phase” In the text, a precision on where P1 and P2 are located and which phase they represent would help the reader. This lack of clarity is related to a poor description of Figure 1.
Ans: More information is provided in the figures and figure text
14- Fig 6 and fig 7 P1 and P2 are reversed put the same P1 for the same phase
Ans. Corrected
15- Figure 6 is better described it is nice to see the phases clearly mentioned. In this sense the lines making the link are useless if you specify what P1 and P2 represent and that it is always the same thing from one figure to another.
Ans: Figure 6 is updated
16- L198-199 why do you say “The intensities of the sulphate bands are the highest at the binary eutectic areas and also at areas close to the zinc rich phases.” The zinc-rich phase is far from the location of the max, max relative to the scale!
Ans: The locations of the sulphate “hot-spot” is in the binary eutectic is located a couple of micrometers from the Zn phases. This may not be considered as close. We have changed the text and removed statements that is close
17- Figure 8 why is there no P2.
Ans: Corrected
18- L211-214: why not but justify it by relying on bibliographical references!
Ans: Reference added
19- Same remark for paragraph L230-233
Ans: Reference added
20- Figure 9: the figure is blurred, the captions are not legible and the Zn label is poorly made and cuts off the image
Ans: Corrected
21- Convey l244 conveys without S
Ans: Corrected
22- The SO2 dissolved in water ad-layer can lead to a slight acidification of the aqueous layer, initial dissolution of surface oxides and the sub-sequent formation of metallo-sulphite complex and sulphite containing surface films. Reference ????????
Ans: Reference provided and sentence reformulated
23- oxidation of sulphite to sulphate: equation?
Ans: Equation [4] is a simplified “equation” but this is only the exact reaction mechanism since there could be several different equations depending on the involvement of oxidants
24- l248 “At the border between the zinc rich dendrites and the binary eutectic, oxidation of sulphite to sulphate can be enhanced due presence of oxidants, such as peroxide or superoxide. “I'm sorry but figure 7 doesn't show it. The white phase is far from the top!
Ans: We agree to a large extent. We have removed that oxidation is enhanced at the border between Zn rich phases and binary eutectic. The highest intensities are found a in the binary eutectics several micrometes from the Zn dendrite. We have also considered the effect of pH, which for sulphite oxidation is higher at lower pH (ref Hoffman,…). Thus, the formation of sulphate is probably higher at the anodic sites where the pH should be lower, but also linked to the formation of oxidants, such as peroxide, which occur at the cathode. pH variations could also affect absorption of SO2 (higher at higher pH)
25- “The oxygen reduction on zinc in oxygen saturated sulphate solutions in the pH range 4–11 was studied by Boto and Williams [19] and it was found that a mixture of H2O2 and hydroxide was formed. In a study [20] performed on bare and passive zinc it was observed that hydroperoxyl ions are produced on passive zinc while the reaction proceeds primarily by a direct four electron reduction to hydroxyl ions on bare zinc. By in situ and operando attenuated total reflection infrared (ATR-IR) spectroscopy Nayak et al [18] detected surface bound superoxide during ORR on electrodeposited zinc hydroxide-based model corrosion products.” Ok it is noted but what about the anode in all these studies??? and zinc passivation?
Ans: In the cited studies the mechanisms of oxygen reduction were specifically studied with electrochemical techniques, such as ring disc electrodes. We do not understand the relevance of the anode in these cases, for our study. Oxygen reduction was studied on passive Zn, which is important in this case as the zinc rich dendrites probably have a thin oxide layer / surface film.
26- Figure 10 What does the gray domain represent on the Zn surface? the electrolyte? a passivation?
Ans: surface oxide / film on zinc rich phase (“ passive film “). Added to the figure
27- More generally for the discussion, proposing a mechanism is a good idea which must open on the hypotheses which is here a potential galvanic coupling (even if for the moment I am not convinced) and not be based on the hypothesis for make the mechanism. Here we can talk about preferential attack for example.
In conclusion, clarifications on all the figures should be made to gain clarity and ensure that the message of the text is truly illustrated by the figures. We must expand the bibliography which is really poor and try to rely more on what already exists when hypotheses are made (without self-citing!). The discussion should not be based on a hypothesis (potential coupling) but on the actual results. The mechanism is self-supporting but can open on the hypothesis of a coupling.
We have considered the possibility of preferential corrosion of eutectic phases added that to the discussion part. In this case the cathodic process will occur in the eutectics. The cathodic sites could be the for instance be Zn-rich components in the Zn-MgZn2 phase in the binary eutectic. This is a microgalvanic coupling on a finer level.
We have improved and clarified the figures
The introduction have been improved and more references have been added.
The article must be reworked before being published.
Round 2
Reviewer 3 Report
I re-read the edited, corrected version of the article. The article has gained clarity!
I am not convinced about the proximity of the zinc-rich phase and its impact. It would take other experiments to really show it. In this sense I do not understand why it appears in the conclusion? why has the penultimate paragraph of the discussion not been modified???????? we should rather speak of MgSOXnH2O
Figure 8 mentions a P2 which does not exist
If these modifications are taken into account, I authorize the publication!
Author Response
Please see the attachment
Author Response File: 
Author Response.docx
