Electroless ZnO Deposition on Mg-Al Alloy for Improved Corrosion Resistance to Marine Environments

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

I believe that the study will become more appropriate by taking into account the following comments.

(1) "The AFM and optical images suggested that the corrosion attack on ZnO-AM60 surface was reduced due to the hydrophobicity of the ZnO surface (contact angle of ≈91.6°)."

In this sentence, the word "suggested" is used as a prediction rather than a conclusion. A more precise wording could be used to emphasize the accuracy of this finding.

(2) The results with AFM and optical images require further explanation. It should be explained which corrosion mechanisms the images show and how these mechanisms were evaluated.

(3) The factors causing the decrease of the C1s signal in the XPS analysis should be explained in more detail. Also, the effects of the density variations of other elements in the XPS depth profile on the corrosion mechanism could be more clearly explained.

(4) The conclusion, while summarizing the overall results of the study, seems to have left out some important results. In particular, more emphasis on the EIS and XPS data would be useful. The direct effects of these techniques on corrosion resistance could be further elaborated.

(5) For a better understanding of the results obtained in this study and for comparison with similar studies in the literature, it would be useful to refer to following studies that provide more information on approaches to improve the corrosion resistance of magnesium alloys. It is thought that these studies can make an important contribution to the interpretation and discussion of the results obtained.

https://doi.org/10.1051/metal/2021064

https://doi.org/10.3390/ma14237140

(6) "The Bode impedance |Z|0.01Hz values (Figure 7b) were found to be in good agreement with changes in the capacitive arcs of the Nyquist diagrams..."

"In good agreement" should be replaced by a more specific relationship.      Also, the impact of these results on corrosion resistance should be more clearly explained.

(7) "These facts lead to the decrease in the corrosion progress of the ZnO-AM60, making it difficult for the chloride ions attack towards the Mg-matrix."

This sentence has a grammatical error and could be written more clearly. For example, "These factors led to a decrease in the corrosion rate of the ZnO-AM60, making it more difficult for chloride ions to attack the Mg-matrix."

(8) The abstract states that the hydrophobic properties of the ZnO coating reduce corrosion, but detailed analysis shows that surface roughness is directly related to hydrophobicity. While increasing roughness increases hydrophobicity, the effect of this on corrosion should be more clearly explained. The relationship between the effect of roughness and hydrophobicity on corrosion seems to be expressed in a contradictory way. Please check.

(9) In the summary section, it is stated that the pH change is 13% less on the ZnO-coated surface and the release of Mg2+ ions is reduced by 97%. However, in the detailed analysis, it is said that the release of Mg2+ ions decreased 34 times. There seems to be a numerical inconsistency and contradiction between these statements. Please check.

Author Response

Reviewer 1

Comments and Suggestions for Authors

 I believe that the study will become more appropriate by taking into account the following comments.

(1) "The AFM and optical images suggested that the corrosion attack on ZnO-AM60 surface was reduced due to the hydrophobicity of the ZnO surface (contact angle of ≈91.6°)."

In this sentence, the word "suggested" is used as a prediction rather than a conclusion. A more precise wording could be used to emphasize the accuracy of this finding.

 Comment:

The sentence has been modified (lines 12 -14) as suggested, to precise and emphasize the accuracy of the reported result:

“The AFM and optical images revealed that the corrosion attack on ZnO-AM60 surface was reduced due to the increase in the surface hydrophobicity of the ZnO coating (contact angle of ≈91.6°).”

 (2) The results with AFM and optical images require further explanation. It should be explained which corrosion mechanisms the images show and how these mechanisms were evaluated.

Comments:

For AFM photography (Figure 1) a text was added (lines 154-159):

“The surface of the electroless deposited ZnO on AM60 showed clusters larger than 10 μm in size and agglomerations (dimensions between 2.5 and 5 μm). The average roughness of the AM60 surface was »53 nm, while that of the ZnO coated increased up to »614 nm. The AFM images suggest that the electroless ZnO coating could act as a physical barrier for AM60 surface when exposed to the marine-coastal environment (SME).”

After the Figure 7 (Optical images) a text was added (lines 247-252):

“The corrosion attacks on the uncoated AM60 as Mg-Al alloy surface (Figure 7a) was influenced by the presence of the AlMn intermetallic particles (as local cathodic sites) around which the Mg-matrix (anodic active) suffered degradation (localized corrosion) [45], expressed by the  release. The deposited electroless ZnO coating, as a physical barrier between the medium (SME) and the Mg-matrix, makes more difficult the entry of the SME into the AM60 matrix, and thus, reducing its degradation.

(3) The factors causing the decrease of the C1s signal in the XPS analysis should be explained in more detail. Also, the effects of the density variations of other elements in the XPS depth profile on the corrosion mechanism could be more clearly explained.

Comments:

Lines 183-185: “However, after 30 s of sputtering the C1s signal decreases abruptly, due to the erosion performed by argon ion beam to remove this layer, maintaining this trend until 270 s of sputtering”.

Lines 188-192: “The information obtained from the depth profile was specific for the ZnO (of  thickness), since the Mg1s signal didn’t exceed the O1s and Zn2p signals in the region closer to the alloy matrix. The low concentration of Mg1s suggests that the ZnO coating may provide a good physical barrier for the AM60 alloy surface.”

 

(4) The conclusion, while summarizing the overall results of the study, seems to have left out some important results. In particular, more emphasis on the EIS and XPS data would be useful. The direct effects of these techniques on corrosion resistance could be further elaborated.

Comments:

Lines 342-345 (Conclusions): “The XPS depth profile showed that the coating on AM60 was composed by ZnO (of  thickness), which could reduce the contact between the Mg-matrix and the marine environment, since the Mg1s signal didn’t exceed the O1s and Zn2p signals in the region closer to the alloy matrix.”

Lines 358-361 (Conclusions): The EIS results indicated that the polarization resistance ( ) was » 3 times higher for ZnO-AM60 exposed to the aggressive SME solution, than that of the uncoated AM60 alloy. Furthermore,  values associated with charge transfer resistance suggested lower progress in the electrochemical corrosion when the AM60 was coated with the ZnO, a fact confirmed by the optical images”.

(5) For a better understanding of the results obtained in this study and for comparison with similar studies in the literature, it would be useful to refer to following studies that provide more information on approaches to improve the corrosion resistance of magnesium alloys. It is thought that these studies can make an important contribution to the interpretation and discussion of the results obtained.

https://doi.org/10.1051/metal/2021064

https://doi.org/10.3390/ma14237140

Comment:

We appreciate your comment. However, we consider that the mentioned articles are related to biomedical materials. For example, they provide results after testing in the Hank´s solution (the first article) and the second article studies the influence of Ca in Mg-Ca and Mg-Al-Ca alloys (alloys very different in their electrochemical activity).

(6) "The Bode impedance |Z|0.01Hz values (Figure 7b) were found to be in good agreement with changes in the capacitive arcs of the Nyquist diagrams..." "In good agreement" should be replaced by a more specific relationship.    Also, the impact of these results on corrosion resistance should be more clearly explained.

Comment:

Lines 303-309: “The Bode impedance  values (Figure 8b) for ZnO-AM60 exposed to marine environment were higher than those obtained for the uncoated AM60, indicating that the electroless ZnO coating possesses good efficiency as a protector of the Mg-matrix, hindering the electrochemical process of corrosion (Equations 6-9). For this reason, the capacitive arcs observed in the Nyquist diagram were more pronounced for ZnO-AM60 (Figure 8a). This way, the Nyquist and Bode diagrams (Figure 8), were used to measure the efficiency  of ZnO electroless deposited on the AM60 alloys surface [68,69].”

(7) "These facts lead to the decrease in the corrosion progress of the ZnO-AM60, making it difficult for the chloride ions attack towards the Mg-matrix."

This sentence has a grammatical error and could be written more clearly. For example, "These factors led to a decrease in the corrosion rate of the ZnO-AM60, making it more difficult for chloride ions to attack the Mg-matrix."

Comment: See the arrangements above in the sentences.

 (8) The abstract states that the hydrophobic properties of the ZnO coating reduce corrosion, but detailed analysis shows that surface roughness is directly related to hydrophobicity. While increasing roughness increases hydrophobicity, the effect of this on corrosion should be more clearly explained. The relationship between the effect of roughness and hydrophobicity on corrosion seems to be expressed in a contradictory way. Please check.

Comment:

In Lines 229-233 was a text in the submitted article and an explanation was added in its end, to help the reader:

“The changes in the contact angle value before and after electroless deposited ZnO can be explained by the Wenzel [62] and Cassie-Baxter [63] models, considering that at greater surface roughness will correspond higher CA (hydrophobic surface), due to air trapped in the liquid-air and solid-air-liquid interfaces [64]. As a consequence, the contact between the aggressive medium (SME) and ZnO-AM60 surface was reduced”.

(9) In the summary section, it is stated that the pH change is 13% less on the ZnO-coated surface and the release of Mg2+ ions is reduced by 97%. However, in the detailed analysis, it is said that the release of Mg2+ ions decreased 34 times. There seems to be a numerical inconsistency and contradiction between these statements. Please check.

Comment:

The release of magnesium ions was expressed as a percentage and likewise in number of times, presenting the same information. In the abstract the expression below uses both forms of presentation (Lines 14-16):

“The change in time of pH to alkaline values was less pronounced for ZnO-AM60 (in » 13%), while the release of the  ions was reduced 34 times, associated with the……….”

The authors appreciate your comments and suggestions very much, giving us a possibility to improve our manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors used ZnO coating to improve corrosion resistance of AM60 alloy in simulated marine environment and used several characterization methods to investigate the mechanism behind the improvement. Considering the good relevance of this paper’s topic to the journal and the significance of the work, I think it’s appropriate for this paper to be included in this special issue. However, two major points and some minor questions need to be addressed before the paper is ready to be published.

The two general major points of concern is that:

1. For a coating, one key attribute is its adhesion to the substrate or mechanical durability. A coating that easily delaminate or detach from the substrate has little real-world significance. I strongly recommend the authors to add some adhesion test or mechanical test results to prove the practicality in real-world applications of this coating.

2. The scientific soundness of this work needs improvement in the corrosion test. The corrosion test of AM60 sample is carried out after the silicon carbide sandpaper polishing process, which left a significantly different surface morphology compared to the ZnO coated samples. I would suggest authors uses further methods (polishing by alumina or silica suspension, electropolishing, etc.) to reduce the presence of scratches. Or alternatively, create a similar surface morphology on the coated samples.

A few minor points that need to be revised:

In 3.1. Figure 1, please provide the height legend of the AFM micrographs. This is crucial to the support the text in line 153~154, ‘average roughness … ~53 nm, while the ZnO coated … ~ 614 nm.’

In line 162, ‘The thickness of the film was found to be ~900 nm.’ I’d rather to have the ‘~900 nm’ be expressed in a ‘average +/- error’ (for example, 900nm +/-1000nm) considering the thickness curve is quite wavy. This also provide extra information on the max roughness of the coating.

Figure 5. scale bar needs to be provided here. It’s also better to provide an even more magnified image at the contact point. At current magnification, it is not obvious that the contact angle for the ZnO-AM60 is larger than 90.

Figure 6. It would be better if an image of ZnO-AM60 before exposure to SME could be provided.

Author Response

Reviewer 2

Comments and Suggestions for Authors

The authors used ZnO coating to improve corrosion resistance of AM60 alloy in simulated marine environment and used several characterization methods to investigate the mechanism behind the improvement. Considering the good relevance of this paper’s topic to the journal and the significance of the work, I think it’s appropriate for this paper to be included in this special issue. However, two major points and some minor questions need to be addressed before the paper is ready to be published.

The two general major points of concern is that:

1. For a coating, one key attribute is its adhesion to the substrate or mechanical durability. A coating that easily delaminate or detach from the substrate has little real-world significance. I strongly recommend the authors to add some adhesion test or mechanical test results to prove the practicality in real-world applications of this coating.

Comment:

New Figure 3 has been added, presenting the change in the ZnO electroless coating on AM60, after the adhesion tests and the text in lines 173-178 comments the results:

 “The optical images in Figure 3 show the change in the morphology of ZnO-AM60 (sample pattern, Figure 3a) after the performance of adhesion test at force sweeps of 0.1 mN, 0.05 mN and 0.01 mN (Figures 3b-d, respectively). It can be observed that as the force exerted on the ZnO coating surface decreases, the changes begin to be imperceptible. After the adhesion test, detached agglomerations as a part of the coating were not observed.”

2. The scientific soundness of this work needs improvement in the corrosion test. The corrosion test of AM60 sample is carried out after the silicon carbide sandpaper polishing process, which left a significantly different surface morphology compared to the ZnO coated samples. I would suggest authors uses further methods (polishing by alumina or silica suspension, electropolishing, etc.) to reduce the presence of scratches. Or alternatively, create a similar surface morphology on the coated samples.

Comment:

For the removal of the naturally existing layer (of oxide/hydroxide) on the AM60 surface, the polishing was carried out with different grain size of SiC sandpapers. The posterior deposited ZnO showed better adhesion on surface with higher roughness (scratches). Therefore, the subject was not to create a surface morphology similar to that of ZnO coated AM60.

3. A few minor points that need to be revised:

In 3.1. Figure 1, please provide the height legend of the AFM micrographs. This is crucial to the support the text in line 153~154, ‘average roughness … ~53 nm, while the ZnO coated … ~ 614 nm.’

Comment:

In the Figure 1 was provided the height legend.

In line 162, ‘The thickness of the film was found to be ~900 nm.’ I’d rather to have the ‘~900 nm’ be expressed in a ‘average +/- error’ (for example, 900nm +/-1000nm) considering the thickness curve is quite wavy. This also provide extra information on the max roughness of the coating.

Comment:

The error average was given in line 165: “The thickness of the film was found to be ”

 Figure 5. scale bar needs to be provided here. It’s also better to provide an even more magnified image at the contact point. At current magnification, it is not obvious that the contact angle for the ZnO-AM60 is larger than 90.

Comment:

The scale bar for Figure 6 (ex-figure 5) was added.

Figure 6. It would be better if an image of ZnO-AM60 before exposure to SME could be provided.

 Comment:

We consider that there is not a reason to introduce the non-exposed surface, because  they have a different morphology of those on which the corrosion layers were formed during the to the SME ambience and were consequently removed.

The authors appreciate your comments and suggestions very much, giving us a possibility to improve our manuscript.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for addressing the comments!