Effect of Molybdate on Corrosion Performance of Oxide Coating Produced on 7075 Al Alloy Using PEO

Round 1

Reviewer 1 Report

1. In the introduction, the author say” However, the research works investigating the effect of molybdate on PEO coatings at a constant molybdate concentration and, the effect of molybdate concentration in the bath solution and also the distribution of Mo in the coating layers were not found. “ Here first, please rephrase this sentence. Usually, effect of --- on ---- were studied, or effect of – on ---- were not found. Further, the immediate motivation for this study is not clear, as many previous articles have reported on the effects of Mo on corrosion of Al, please show the detail different of this job with previous works.  
2. .In introduction section, “Plasma electrolytic oxidation (PEO) has been considered as one of the most suitable surface treatments for generating a conformal oxide layer on the surface of various valve metals to improve their mechanical and electrochemical properties for industrial applications.” Here I suggest the author to give more introduction of PEO, especially the advantages of this method.
3. Please simplify the conclusion, please give the most important conclusion, not to repeat the results.
4. For the EIS results, is there any Parallel samples, or to obtain the each set of data , how many parallel sample do you measured?
5. I suggest the author to give the error bar for the EIS results.

Author Response

1. In the introduction, the author say” However, the research works investigating the effect of molybdate on PEO coatings at a constant molybdate concentration and, the effect of molybdate concentration in the bath solution and also the distribution of Mo in the coating layers were not found. “Here first, please rephrase this sentence. Usually, effect of --- on ---- were studied, or effect of – on ---- were not found. Further, the immediate motivation for this study is not clear, as many previous articles have reported on the effects of Mo on corrosion of Al, please show the detail different of this job with previous works.  

Thanks for your comments. The sentence is grammatically evaluated and the required corrections are made. Please see the changes on page 2, line 72. Also, the novelty of the work and the detail differences were discussed in page 2, lines 72-76.

2. .In introduction section, “Plasma electrolytic oxidation (PEO) has been considered as one of the most suitable surface treatments for generating a conformal oxide layer on the surface of various valve metals to improve their mechanical and electrochemical properties for industrial applications.” Here I suggest the author to give more introduction of PEO, especially the advantages of this method.

Thanks for your suggestion. Some parts are added to the manuscript in the introduction to satisfy this comment. Please see them on page 1, lines 39-44.

3. Please simplify the conclusion, please give the most important conclusion, not to repeat the results.

The conclusion is simplified to satisfy this comment. Please see the changes on page 14.

4. For the EIS results, is there any Parallel samples, or to obtain the each set of data, how many parallel sample do you measured?

It is usual that the long-term EIS measurements were performed by using duplicate samples. This was mentioned on page 4, line 136.

5. I suggest the author to give the error bar for the EIS results.

Unfortunately, since duplicated samples were used for the EIS measurements, it is not possible to give the standard deviation or error bar for the obtained results.

Reviewer 2 Report

Dear Author's
In the present manuscript, the the PEO process of 7075 Al alloy in a silicate-based electrolyte containing Na2MoO4, to evaluate the change in microstructure, composition, and long-term corrosion performance of the prepared coatings. The incorporation mechanism was considered to explain how the different species inside the electrolytic bath are incorporated in the coating, and what is their role on coating microstructure and corrosion barrier properties. Data are presented in an organized manner and the authors successfully supported their statements. 
Although the study is not novel and many works have been previously done on similar topics, some parts of the manuscript and the way authors have looked at the data are novel and interesting for publication. 
- Is it possible to present an elemental distribution map for the surface presented in Figure 11?
- Whether the PDF 2 database is too poor to analyze this type of material. It is an "old" database. Please, if possible, verify the correctness of the analysis based on the latest ICDD PDF4+ databases 
- In Table 2, values were recorded with different precision (e.g., Al or O with 0.1 precision while, for example, Mg or Si with 0.01 precision). The accuracy of the EDS method is rather around 0.5%, especially is low for light elements. There are no error values, which is particularly important for oxygen measurements, please correct.

Author Response

Dear Author's
In the present manuscript, the PEO process of 7075 Al alloy in a silicate-based electrolyte containing Na2MoO4, to evaluate the change in microstructure, composition, and long-term corrosion performance of the prepared coatings. The incorporation mechanism was considered to explain how the different species inside the electrolytic bath are incorporated in the coating, and what is their role on coating microstructure and corrosion barrier properties. Data are presented in an organized manner and the authors successfully supported their statements. 
Although the study is not novel and many works have been previously done on similar topics, some parts of the manuscript and the way authors have looked at the data are novel and interesting for publication. 

1- Is it possible to present an elemental distribution map for the surface presented in Figure 11?

The elemental distribution map of the coatings after 168 h of immersion in the corrosive solution is added to the revised manuscript in Figure 11. Please also see the highlights on page 12, line 397, and page 13, lines 403-406.

2- Whether the PDF 2 database is too poor to analyze this type of material. It is an "old" database. Please, if possible, verify the correctness of the analysis based on the latest ICDD PDF4+ databases 

Unfortunately, we do not have access to the ICDD PDF4+ database. Fortunately, all the peaks were correctly identified by PDF2.

3- In Table 2, values were recorded with different precision (e.g., Al or O with 0.1 precision while, for example, Mg or Si with 0.01 precision). The accuracy of the EDS method is rather around 0.5%, especially is low for light elements. There are no error values, which is particularly important for oxygen measurements, please correct.

Thanks for this comment. The table is corrected to show all data with 0.01 precision. It is well known that the EDS analysis is a semi-quantitative measurement, but it is usual to present the raw data in papers. As the respected reviewer mentioned, there are no error values for the data and they can be used just for comparisons.