TiO₂ Microparticles Incorporation in Coatings Produced by Plasma Electrolytic Oxidation (PEO) on Titanium

Round 1

Reviewer 1 Report

This research describes the influences of two types of particles and electrical parameters on coating property including morphology, structure, and corrosion resistance. This study achieves certain originality but is insufficient for acceptance in as-received form. I think that this study should be improved in these aspects:

1. In Abstract part, the given experimental results are too few. Some important results especially the corrosion resistance of PEO coatings fabricated by two types of particles and different electrical parameters should be provided. In addition, soda was not used as the PEO electrolyte and therefore the word “soda” should be replaced.

2. The electrochemical impedance part should be discussed in more depth. Authors should cite some recent works on the area, for example, (1) J.Qin, X.T. Shi, H.Y. Li, et al., Performance and failure process of green recycling solutions for preparing high degradation resistance coating on biomedical magnesium alloys, Green Chem. 24 (2022) 8113-8130; (2) Yongjun Zhang, Xianlong Shen. Facile fabrication of robust superhydrophobic coating for enhanced corrosion protection on AZ91 magnesium alloy by electroless Ni-B/GO plating. Surface and Coatings Technology, 2023, 455(25): 129213.

3. In this study, the influences of electrolyte compositions and electrical parameters on coating corrosion resistance is investigated. Titanium alloys exhibit excellent corrosion resistance and can be widely used without PEO treatment. Therefore, the application areas of titanium alloys should be emphatically given in Introduction part.

4. The properties of PEO coatings fabricated in the solution with rutile and anatase microparticles should be systematically compared and the influencing mechanism should be further discussed.

5. The level of English should be significantly improved. For example, in Abstract part, the sentence of “However, only treatment carried out at 1000 Hz allow to produce a coating which combine a large thickness (up to 50 µm) and an improved anticorrosion behavior” should be changed into “However, only treatment carried out at 1000 Hz allows to produce a coating which combines a large thickness (up to 50 µm) and an improved anticorrosion behavior”.

6. Please check misprints and the sentences about “Error! Reference source not found..” should be carefully revised.

Author Response

dear reviewer, thank you so your comments. hereafter the detailed answers to your rquest

1. Additional information about the electrochemical and corrosion results is added. The word “soda” is replaced by “sodium hydroxide”.
2. The electrochemical part is further commented citing Qin, J.; Shi, X.; Li, H.; Zhao, R.; Li, G.; Zhang, S.; Ding, L.; Cui, X.; Zhao, Y.; Zhang, R. Performance and Failure Process of Green Recycling Solutions for Preparing High Degradation Resistance Coating on Biomedical Magnesium Alloys. Green Chem. 2022 , 24, 8113 8130, doi:10.1039/d2gc02638d
3. In the introduction the possible applicational fields and the conditions under which a surface treatment to improve titanium corrosion resistance is necessary are added.
4. The influence of the two types of microparticles (rutile and anatase) is conferred in the Discussion section. Where a direct comparison is not reported, it is because the particle type does not seem to have influenced the process.
5. English revision was performed.
6. The “Error! Reference source not found” sentences in the PDF file are correct

Reviewer 2 Report

This article describes the effects of adding microparticles of titanium oxide TiO2 to electrolytic solutions used for PEO processing of titanium in order to improve the corrosion resistance of the substrate. Various modes of electric discharge with a frequency of 1000 Hz, 20 Hz and direct current discharges are considered.

It is shown that the most optimal mode is treatment with discharges with a frequency of 1000 Hz. In this case, thick oxides are formed, and, unlike direct current discharges, no significant defects are formed.

The work is interesting and corresponds to the theme of the magazine.

Some remarks should be noted.

1) Throughout the text there are no references to literary sources. It needs to be corrected. Because it makes it difficult to review and read the article.

2) There are no references to modern research on gas discharges, in particular, for example, works:

Yuanfu Yue et al 2022 Plasma Sources Sci. Technol. 31 125008 DOI 10.1088/1361-6595/acab29

A I Saifutdinov 2022 // Plasma Sources Sci. Technol. 31 094008

DOI 10.1088/1361-6595/ac89a7

3) It is necessary to increase the signatures on the scales in Figures 7 and 8.

4) it is necessary to explain the discharges of alternating current with different frequencies. What voltage signal was used? sinusoidal or pulsed? Usually, sparks in a liquid can also form in the case of alternating current discharges, during a breakdown. Therefore, the formation of cracks in the case of a DC discharge is not convincing. It is necessary to substantiate this question more carefully.

How was the gas/liquid heated in the event of a DC discharge?

After these remarks are eliminated, the work can be published in the journal.

Author Response

Dear reviewer, thank you for your comments. In the following the answers to all tyour requests:

1. It is probably due to the “Error! Reference source not found” present in the PDF version of the manuscript, but not in the W ord one where references are correctly indicated
2. Yue, Y.; Exarhos, S.; Nam, J.; Lee, D.; Linic, S.; Bruggeman, P.J. Quantification of Plasma Produced OH and Electron Fluxes at the Liquid Anode and Their Role in Plasma Driven Solution Electrochemistry. Plasma Sources Sci.Technol. 2022 , 31, doi:10.1088/1361 6595/acab29 citation is added.
3. The scale signature of Figures 7 and 8 is increased. 
4. The type of voltage signal (pulsed) used is further specified in the Materials and Methods.
   The gas/liquid heating during plasma events is commented referring also to
   Yue, Y.; Exarhos, S.; Nam, J.; Lee, D.; Linic, S.; Bruggeman, P.J. Quantification of Plasma Produced OH and Electron Fluxes at the Liquid Anode and Their Role in Plasma Driven Solution Electrochemistry. Plasma Sources Sci.
   Technol. 2022 , 31, doi:10.1088/1361 6595/acab29

Reviewer 3 Report

The research investigates the impact of rutile and anatase microparticles on the morphology, structure, and anticorrosive properties of PEO coatings on titanium in an alkaline solution. The study explores how the electrical regime and working frequency of the anodizing process affect particle incorporation and oxide thickness. Analytical techniques like SEM, EDS, and XRD are used to characterize the coatings. The study finds that under any electrical condition and frequency, particles can be successfully incorporated; however, optimal anticorrosive behavior and coating thickness are achieved at a working frequency of 1000 Hz. Electrochemical behavior is assessed using free corrosion potential monitoring and EIS analysis in sulphuric acid solution.

The paper can be published if the following problems are solved:

1.     Some references display error information: Error! Reference source not found. Please correct them.

2.     The porosity of synthesized material plays an important role in coating. Even though the authors discussed the effect resulting from porosity, it’s better to list a table displaying the porosity of the sample prepared by each processing condition.

3.     What’s the at.% (or wt.%) of the interested elements from EDS analysis?

Some sentences need to be modified. 

Author Response

thank you so much for your clear and helpful revison.

1. The reference error information is correct.
2. A table displaying the porosity of samples is added to the Results
3. Quantitative evaluation of the elements wa s not performed.

Reviewer 4 Report

Submitted manuscript reports on the formation of PEO coatings on Ti grade 2 substrate from the solution containing 2 types of Tio2 particles – rutile and anatase. This topic is suitable for the journal and may be of interest for researchers in the field. On the other hand, some research results are missing, rendering this manuscript not acceptable for publishing in the present form. Authors should be given a chance to resubmit the current manuscript after amendments.

1)       The technical errors through the manuscript like in Lines 82, 111, 134 etc. should be eliminated. The manuscript should be double-checked in the revised version.

2)       The figure 2 is doubled – lines 135 and 143. Please, correct this error.

3)       What is the aim of the authors’ work? In the opinion of the authors, why are the current achieved characteristics of oxide layers on titanium and titanium alloys unsatisfactory? What specific characteristics did the authors want to improve, for which applications? The aim and practical value of the present work should be more clearly stated in the Introduction of the Manuscript.

 4)       Why did the authors choose titanium oxide particles (rutile and anatase) to be added to the electrolyte and the mentioned concentration (5 g/l)? Please clarify.

 5)       Regarding particles: it is strongly recommended that the manuscript provide information on the chemical purity and exact size distribution of each type of particle used in the work, their zeta potentials and surface charge densities.

 6)       The authors are also recommended to provide data on oxide layers (the process of their preparation and characteristics) obtained in electrolytes without the addition of rutile or anatase particles for ease of comparison and analysis of the effects of additions rutile or anatase particles to electrolytes.

 7)       It is strongly recommended to present the current-time and voltage-time curves in the manuscript and analyze their changes under the influence of nanoparticles during the PEO process.

 8)       The corrosion performance of the coatings should be measured for long-term EIS test but not only 24 h OCP observations. For example, 3 or 7 or 10 or 14 days (or more) of immersion of obtained samples in the corrosive medium should be used for the estimations of anticorrosive properties by EIS.

 9)       Why was used the 10% solution of H2SO4 as corrosive medium? Please, explain in the paper

 10)     The cross-section porosity and surface porosity as well as the average pore sizes for each sample should be measured, provided and analyzed in the manuscript

 11)     The measurement error and/or error bars should be provided for all measured characteristics in the paper.

 12)     The quantitative phase composition analysis as well as microstructure parameters analysis (size of crystallites, residual strains) should be performed and results should be provided, analyzed and discussed in the p[aper

 13)     According to the opinion of the authors, the broad background peak in the region 28-35 deg of 2Theta on the XRD patterns is reasoned by the amorphous SiO2 part of the layers. However, it is well known, that the amorphous silica (SiO2) gives the maximum of the signal in the XRD pattern in the region 15-30 deg of 2Theta angle. Please, explain the shift of SiO2 broad peak on the observed XRD patterns in the present paper.

 14)     The authors wrote the efficiency of the PEO process as the corrosion mitigation strategy is clearly demonstrated. However, OCP (free corrosion potential or open circuit potential) is not a parameter for evaluation of corrosion resistance. Some materials may have low OCP and small value of corrosion current density - in this case only corrosion current density indicate the corrosion rate of the material (not OCP!). Please, remove the phrase about evaluation of corrosion resistance by OCP and, furthermore, please, provide the Tafel plots of the obtained samples in the paper.

 15)     Continue the previous point, the hydrogen extraction curves during the free exposition of the specimens in the corrosive medium should be given, analyzed and discussed in the paper.

 16)     The equivalent EIS schemes and results of fitting should be provided in the paper. Furthermore, all changes in the R, CPE, C and / or other parameters of EIS fitting data should be analyzed and discussed in the paper in details.  

 17)     The adhesion strength of the obtained coatings is also in very important characteristic which should be measured and provided in the paper. 

-

Author Response

Dear reviewer, thank you so much for all your comments and suggestions, helpful in increasing the quality of the paper. Hereafter the detailed answers to your requests:

1. The technical errors present in the PDF version of the manuscript, but not in the Word one are corrected
2. The technical errors present in the PDF version of the manuscript, but not in the Word one are corrected
3. In the introduction the possible applicational fields and the conditions under which a surface treatment to improve titanium corrosion resistance is necessary are added.
4. The reason why TiO2 microparticles are selected and 5 g/L is used as particles concentration is specified in Materials and Methods
5. The purity of particles has been added.
6. In the Results (Morphology, Structure and Chemical Composition section), a comparison between the samples prepared working with and without microparticles is provided.
7. The comparison of current time curves of samples treated with and without microparticles is provided in the Supplementary Information
8. Long term tests will be the topic of future studies.
9. The reason why the sulphuric acid solution is used is explained in the "Coating Characterization"
10. The porosity data are added in Table 2.
11. Measurement errors concerning the thickness and porosity of coatings are added.
12. Due to the high content of amorphous phase in the coatings it was not possible to perform quantitative evaluation of the crystalline phases.
13. The shift is probably due to the presence of amorphous sodium silicates in addition to silica, references are added
14. Tafel plots are added for samples treated at 1000 Hz.
15. The hydrogen extraction curves will be investigated in future works
16. Only the polarization resistance values graphically extrapolated from Nyquist diagrams are reported. The EIS results are mainly focused on the determination of the characteristic frequency as a key parameter to evaluate the coatings behaviour.
17. The adhesion strength will be investigated in future works.

Round 2

Reviewer 1 Report

During revising the manuscript, all points raised by me are suitably addressed. I recommend its acceptance for publication after the following respects have been revised:

1. Soda (Na₂CO₃) was not used as the PEO electrolyte and therefore in Abstract part, “…in alkaline solution based on soda and sodium metasilicates” should be changed into “…in alkaline solution based on NaOH and sodium metasilicates”.

2. In Introduction part, for the sentence of “Nonetheless, as the beneficial effects of using a cathodic current, during PEO treatment on Ti, are well documented here we investigate the use of a bipolar duty cycle imparting an excellent corrosion resistance to Ti oxide coatings. [14,15]”, there is no subject in the sentence and therefore it should be revised.

3. The sentence of “The particles addition aims to reduce the defectiveness of the PEO-coatings improving their corrosion resistance” should be changed into “The particles addition aims to reduce the defectiveness of the PEO-coatings and improve their corrosion resistance”.

4. “The applied signal consists of a sine wave with an amplitude of 10 mVrms and 10 points per decade of frequency are collected in a window between 10⁵÷10⁻² Hz (changing into “10⁵ to 10⁻² Hz”)”; “A characteristic common to all the PEO-samples is, instead, the presence in all the diffactorams of a broad peak around 28 ÷ 35° (changing into “28 to 35°”) that is related to the formation of amorphous material, probably associated to the presence of amorphous silicates and silica. [16–20]”.

5. The fabricated processes of PEO_S_1000 should be listed in Table 1.

During revising the manuscript, all points raised by me are suitably addressed. I recommend its acceptance for publication after the following respects have been revised:

1. Soda (Na₂CO₃) was not used as the PEO electrolyte and therefore in Abstract part, “…in alkaline solution based on soda and sodium metasilicates” should be changed into “…in alkaline solution based on NaOH and sodium metasilicates”.

2. In Introduction part, for the sentence of “Nonetheless, as the beneficial effects of using a cathodic current, during PEO treatment on Ti, are well documented here we investigate the use of a bipolar duty cycle imparting an excellent corrosion resistance to Ti oxide coatings. [14,15]”, there is no subject in the sentence and therefore it should be revised.

3. The sentence of “The particles addition aims to reduce the defectiveness of the PEO-coatings improving their corrosion resistance” should be changed into “The particles addition aims to reduce the defectiveness of the PEO-coatings and improve their corrosion resistance”.

4. “The applied signal consists of a sine wave with an amplitude of 10 mVrms and 10 points per decade of frequency are collected in a window between 10⁵÷10⁻² Hz (changing into “10⁵ to 10⁻² Hz”)”; “A characteristic common to all the PEO-samples is, instead, the presence in all the diffactorams of a broad peak around 28 ÷ 35° (changing into “28 to 35°”) that is related to the formation of amorphous material, probably associated to the presence of amorphous silicates and silica. [16–20]”.

5. The fabricated processes of PEO_S_1000 should be listed in Table 1.

Author Response

in the following our aswer to the reviewr requests

1. Substitution of soda with NaOH was made.
2. The sentence was revised.
3. The sentence was changed as suggested.
4. The suggested changes were made.
5. The PEO_S_1000 was inserted in Table 1.

Reviewer 3 Report

The authors have made all corrections to the issues I raised before. This article can now be published. 

Author Response

thank you so much for your revision

Reviewer 4 Report

The authors responded properly to all reviewers' questions and remarks. And required changes in the manuscript were made. I recommend publication of this paper in MPDI Coatings. 

English is fine , in my opinion

Author Response

thank you