Formation and Properties of Oxide Coatings with Immobilized Zeolites Obtained by Plasma Electrolytic Oxidation of Aluminum

Round 1

Reviewer 1 Report

This manuscript describes the formation of composite coating fabricated via plasma electrolytic oxidation (PEO) on pure Al with an addition of either a natural zeolite clinoptilolite or a synthetic zeolite 13X, which are pre-loaded with Ce, into the electrolyte. The authors evaluate the performance of the composite coating in two fronts: catalysis and corrosion properties. Overall, the idea behind this work is sound, but the data presentation and analysis needs to be revised before this manuscript would be publishable in Metals. Below are some detailed issues.

1. The whole Introduction part is repeated twice.
2. There have been few studies composite coatings based on PEO with zeolites. Examples are Prog. In Organic Coatings, 132, 144-147 (2019), Materials and Corrosion, 69, 971-977 (2018), and Surfaces and Interfaces, 26, 101307 (2021) published by the authors of this work.

I recommend referring to some of the works above to point out the singularity of this paper as well as to provide a more in-depth discussion.

3. “Breakdown voltage” must be defined and pointed out in Fig.1. Also, why did the breakdown voltage decrease with particle additions instead of increase as interfacial resistance might be intensified?
4. How did mud-cracks appear in Fig. 3a? In addition, a more substantial explanation in microstructural features should be provided based on Fig. 4.
5. Table 2 shows that thicker coatings were obtained with the presence of particle additive even though the voltage response decreases, contrary to the classical Ohm’s law. Why?
6. The incorporation mechanism of zeolite need to be clarified based on the present data.

Author Response

Reviewer 1

This manuscript describes the formation of composite coating fabricated via plasma electrolytic oxidation (PEO) on pure Al with an addition of either a natural zeolite clinoptilolite or a synthetic zeolite 13X, which are pre-loaded with Ce, into the electrolyte. The authors evaluate the performance of the composite coating in two fronts: catalysis and corrosion properties. Overall, the idea behind this work is sound, but the data presentation and analysis needs to be revised before this manuscript would be publishable in Metals. Below are some detailed issues. 

1. The whole Introduction part is repeated twice.

Thank you for this comment, the repetition is removed.

1. There have been few studies composite coatings based on PEO with zeolites. Examples are Prog. In Organic Coatings, 132, 144-147 (2019), Materials and Corrosion, 69, 971-977 (2018), and Surfaces and Interfaces, 26, 101307 (2021) published by the authors of this work.

I recommend referring to some of the works above to point out the singularity of this paper as well as to provide a more in-depth discussion.

This is a very good comment. Recommended references are added to revised manuscript providing better discussion and outlining its novelty.

1. “Breakdown voltage” must be defined and pointed out in Fig.1. Also, why did the breakdown voltage decrease with particle additions instead of increase as interfacial resistance might be intensified?

We pointed the breakdown voltage in Fig. 1 and added a short discussion explaining the reason for the decrease of breakdown voltage with zeolite addition.

1. How did mud-cracks appear in Fig. 3a? In addition, a more substantial explanation in microstructural features should be provided based on Fig. 4.

We added a discussion after Fig. 4 regarding the mechanism of zeolites incorporation into PEO coatings and its influence on obtained microstructure.

1. Table 2 shows that thicker coatings were obtained with the presence of particle additive even though the voltage response decreases, contrary to the classical Ohm’s law. Why?

We added a discussion after Fig. 4 regarding the mechanism of zeolites incorporation into PEO coatings and its influence on obtained microstructure.

1. The incorporation mechanism of zeolite need to be clarified based on the present data.

We added a discussion after Fig. 4 regarding the mechanism of zeolites incorporation into PEO coatings and its influence on obtained microstructure.

Author Response File: Author Response.pdf

Reviewer 2 Report

The submitted article is devoted to the plasma electrolytic oxidation of aluminum in water solution of sodium tungstate with addition of pure and Ce-loaded zeolites clinoptilolite and 13X and represents a certain interest to the specialists in this field. But the article also has weak points:

Abstract

Abstract includes only common information and does not contain the key research results.

Introduction

1) The lines 22-64 (p.1, 2) and lines 65-107 (p. 2, 3) are absolutely the same. Please check and correct.

2) The novelty of this work is not quite obvious. The analysis of the published work is required. Describe your similar article (https://doi.org/10.1016/j.surfin.2021.101307) to justify your new work and show novelty versus it.

3) The choice of the electrolyte composition (Na₂WO₄) for PEO and substrate (aluminum) is not justified. The disadvantages and advantages of titanium dioxide photocatalysts are discussed in the introduction. It would be logical to use titanium as a substrate.

Materials and Methods

1. In the experimental part, there is no information on the shape and size of the particles introduced into the electrolyte.
2. It is difficult to assess the efficiency of the photocatalytic activity of the samples, since the volume of the MO solution is small. Please, indicate the dimensions of the reactor, the area of the test sample and the volume of the aliquot.

Results and discussion

The conclusions about the photocatalytic activity of the samples are very doubtful.

- Ce was not found on the surface of the coatings, so the correlation between the composition of the coatings and their properties is not obvious. The conclusion about the incorporation of cerium in the coatings only on the basis of luminescence spectra is not entirely justified. 

- The difference between the degrees of conversion of methyl orange in the presence of various samples is negligible. Figure 11 needs the error bars. Additionally, please provide the information about methyl orange degradation in the absence of any samples.

- The discussion about the mechanism of MO degradation is absent.

I am not convinced that potentiodynamic polarization measurements are necessary in this work to assess the stability of PEO coatings. Here, the data on the reproducibility of the results are more relevant. How can you relate the results of potentidynamic polarization measurements to photocorrosion?

Therefore, in my opinion, this work in the present form requires a major revision according to the comments before acceptance.

Author Response

Reviewer 2

The submitted article is devoted to the plasma electrolytic oxidation of aluminum in water solution of sodium tungstate with addition of pure and Ce-loaded zeolites clinoptilolite and 13X and represents a certain interest to the specialists in this field. But the article also has weak points:

Abstract

Abstract includes only common information and does not contain the key research results.

We amended the abstract according to suggestion.

Introduction

1) The lines 22-64 (p.1, 2) and lines 65-107 (p. 2, 3) are absolutely the same. Please check and correct.

Thank you for this comment, the repetition is removed.

2) The novelty of this work is not quite obvious. The analysis of the published work is required. Describe your similar article (https://doi.org/10.1016/j.surfin.2021.101307) to justify your new work and show novelty versus it.

Thank you for this comment. We added a discussion about the novelty of this work in the introduction section.

3) The choice of the electrolyte composition (Na₂WO₄) for PEO and substrate (aluminum) is not justified. The disadvantages and advantages of titanium dioxide photocatalysts are discussed in the introduction. It would be logical to use titanium as a substrate.

We corrected the introduction section according to suggestions.

Materials and Methods

1. In the experimental part, there is no information on the shape and size of the particles introduced into the electrolyte.

We added the information about the average zeolite particle size and shape.

1. It is difficult to assess the efficiency of the photocatalytic activity of the samples, since the volume of the MO solution is small. Please, indicate the dimensions of the reactor, the area of the test sample and the volume of the aliquot.

All required dimensions are added to amended manuscript.

Results and discussion

The conclusions about the photocatalytic activity of the samples are very doubtful.

- Ce was not found on the surface of the coatings, so the correlation between the composition of the coatings and their properties is not obvious. The conclusion about the incorporation of cerium in the coatings only on the basis of luminescence spectra is not entirely justified. 

Unfortunately, here we do not agree with the reviewer. We have proven that Ce is present in obtained PEO coatings not only by photoluminescence, but line EDS scans (Fig. 9) also show the presence of Ce, which is most certainly detectable but cannot be quantified because of its small concentration. Furthermore, line scan for Ce presented in Fig 9b clearly shows that Ce content changes throughout the coating in the same manner as other elements originating from the electrolyte. So, we believe that conclusions should stay as written.

- The difference between the degrees of conversion of methyl orange in the presence of various samples is negligible. Figure 11 needs the error bars. Additionally, please provide the information about methyl orange degradation in the absence of any samples.

Error bars are added to Fig. 11 and we added Fig. 11b showing that Mo degradation in the dark is much lower that under irradiation for all samples.

- The discussion about the mechanism of MO degradation is absent.

We added the discussion about the mechanism of Mo degradation.

I am not convinced that potentiodynamic polarization measurements are necessary in this work to assess the stability of PEO coatings. Here, the data on the reproducibility of the results are more relevant. How can you relate the results of potentidynamic polarization measurements to photocorrosion?

Given the liquid environment in which photocatalytical decomposition of organic pollutants usually takes place, corrosion behavior of formed coatings had to be investigated. Usually, EIS is used for this, but very often potentiodynamic polarization in aggressive electrolyte is also used. By doing so, one can estimate which samples have the lowest corrosion current, and those samples are outlined as corrosion resistant ones.  Therefore, we believe that potentiodynamic polarization gives valuable information about corrosion properties of obtained coatings.

Therefore, in my opinion, this work in the present form requires a major revision according to the comments before acceptance.

Author Response File: Author Response.pdf

Reviewer 3 Report

In this work, a method for deposition of PEO coatings with photo catalytic properties was reported. Authors employed one-step approach using zeolite additives in pure or Ce-doped forms. All coatings demonstrated noticeable rate of the methyl orange decomposition under an artificial sunlight conditions. This is of interest for audience of the Metal journal. However, minor inaccuracies should be corrected as listed below.
1)    Introduction is duplicated
2)    Line 109. Aluminium was 99.0% purity. What is about impurities? 1% is a significant content.
3)    Fig.1. X axis should be in minutes as it is mentioned in the text and other figures.
4)    Fig.2. Peak W – cannot be identified to tungsten. Identification of the phase requires at least three identified peaks.
5)    Reaction (1) – is not a reaction of Al and WO4. Both of them just react with water. In this case it is unclear, what Al3+(solid) is. In the text, discussion is about tungsten incorporation into PEO coating, i.e. into oxide or hydroxide. If so, is oxide transformed into oxide? This reaction does not illustrate incorporation of tungstate into the coating. Where is a coating in the reaction?
6)    Reaction (2) – is opposite reaction to that in (1) in respect to water.
7)    Fig.5 caption “of of”.
8)    Table 4 and 5. Maybe nA can be used. Too many zeros.

Author Response

Reviewer 3

In this work, a method for deposition of PEO coatings with photo catalytic properties was reported. Authors employed one-step approach using zeolite additives in pure or Ce-doped forms. All coatings demonstrated noticeable rate of the methyl orange decomposition under an artificial sunlight conditions. This is of interest for audience of the Metal journal. However, minor inaccuracies should be corrected as listed below.
1)    Introduction is duplicated

We removed the duplicate form the manuscript.

2)    Line 109. Aluminium was 99.0% purity. What is about impurities? 1% is a significant content.

Thank you for this comment! We clearly stated in the revised manuscript that 1050A aluminum samples are used in all experiments.

3)    Fig.1. X axis should be in minutes as it is mentioned in the text and other figures.

Corrected.

4)    Fig.2. Peak W – cannot be identified to tungsten. Identification of the phase requires at least three identified peaks.

We agree with the reviewer that at least three XRD reflections per crystalline phase are required for quantitative phase identification and/or for solving crustal structure. In the manuscript we have presented only qualitative XRD analysis and if one has in mind that we have used high purity Al substrate and p.a. grade chemicals, it is reasonable to detect only phases that can be formed from those precursors. A vast number of articles in the field of PEO is not concerning the number of peaks of each phase appearing in diffractogram, but only their presence.

5)    Reaction (1) – is not a reaction of Al and WO4. Both of them just react with water. In this case it is unclear, what Al3+(solid) is. In the text, discussion is about tungsten incorporation into PEO coating, i.e. into oxide or hydroxide. If so, is oxide transformed into oxide? This reaction does not illustrate incorporation of tungstate into the coating. Where is a coating in the reaction?
6)    Reaction (2) – is opposite reaction to that in (1) in respect to water.

We are sorry that we have omitted the equations for processes occurring at aluminum/oxide and oxide/electrolyte interfaces during the PEO processing. We have added these equations in the form we have used them in previously published papers dealing with PEO on aluminum in tungstate electrolyte.

7)    Fig.5 caption “of of”.

Thank you for pointing out this mistake. It is corrected in the amended manuscript.

8)    Table 4 and 5. Maybe nA can be used. Too many zeros.

I am sorry but we kept microamps, because this is the usual way in which corrosion currents are presented.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The work is publishable in the present form.

Reviewer 2 Report

The authors took into account majority of comments and they proceeded with the necessary revisions.