Corrosion Resistance, Interfacial Contact Resistance, and Hydrophobicity of 316L Stainless Steel Bipolar Plates Coated with TiN/Amorphous Carbon Double Layer under Different Carbon Target Currents

Round 1

Reviewer 1 Report

The manuscript titled: Corrosion, Interfacial Contact Resistance and Hydrophobicity of 316L Stainless Steel Bipolar Plates Coated with TiN/Amorphous Carbon Double Layer under Different Carbon Target  Currents is devoted to improving the corrosion, interfacial contact resistance, and hydrophobicity of bipolar plates used in proton exchange membrane fuel cells.

In this work, the series of TiN/amorphous carbon double-layer coatings was prepared on 316L stainless steel using magnetron sputtering.

 The publication is well-written, interesting, and useful. I have only two comments:

(1) Check the caption for Figures 7 and 8.

Figure 7. illustrates (?) the interfacial contact resistance of amorphous carbon coatings at different sputtering currents before and after corrosion.

 Fig.7. The interfacial contact resistance of amorphous carbon coatings at different sputtering currents before and after corrosion.

 The same comment for the caption of Fig.8.

 (2) Prepare the references list in accordance with MDPI Instructions for the authors

For example,

 

Journal Articles:
1. Author 1, A.B.; Author 2, C.D. Title of the article. Abbreviated Journal Name Year, Volume, page range.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The paper entitled «Corrosion, Interfacial Contact Resistance and Hydrophobicity of 316L Stainless Steel Bipolar Plates Coated with TiN/Amorphous Carbon Double Layer under Different Carbon Target Currents» includes the microstructural characterization, analysis of chemical bonds, corrosion performance and other related properties (interfacial contact resistance, contact angle) of the double-layer C/TiN coatings. The carbon target current was chosen as the main parameter governing the both properties and morphology of the coatings. I have some comments described below:

1) In the abstract (line 17) the authors declare that the carbon target current is responsible for surface roughness and formation of grain boundaries. However, in the manuscript text there are no experimental results on the average surface roughness (Ra) or evaluation of the length of boundaries. It’s desirable to outline whether the best combination of surface quality and properties could be achieved by alteration of the current.  

2) In the Introduction, it’s quite complicated to understand why the authors propose a multilayer structure of the coating instead of a single layer. Are there any advantages between the amorphous and crystalline structure of an outer C-bearing layer of the coating? Number of the fabricated layers was also chosen for an unknown reason.

3) Composition of the 316L stainless steel should be added as well as the purity of the used targets (lines 117-118).

4) Am I right that the authors deposited a Ti substrate layer in order to improve the adhesion of the coating to steel (lines 123-124)? Please, explain.

5) No details are given for XRD analysis. What geometry (Bragg-Brentano, grazing incidence) did you employ? What was the current (voltage, type of Soller (?) slits)? What method did the authors use to refine peak shapes and peak positions? The same ussie with an XPS description.

6) Amorphous structure of the carbon layer is not proved experimentally in this study. The corresponding XRD pattern (Fig. 4) must contain a diffuse scattering region that belongs to the amorphous carbon. Nevertheless, the authors omit this moment.

7) I have found some issues considering a terminology (lines 162–164, lines 167-168, lines 170-172). The authors describe the morphology using the terms «grain size», «grain structure», «cell-like structure», «cluster structure». It seems that all mentioned features are related with the deposited coatings shown in Fig. 1. I disagree with this point, because under different currents the only cellular-like morphology is observed. Obviously, the voids between the cells are undesirable defects, so «a good coating quality» (lines 180-182) is not achieved even at 5A. Also, columnar crystal growth is associated with crystalline-type coatings, but not with amorphous coatings.

8) In the XPS studies, the amorphous structure of the carbon coatings (line 219) could not be defined. Please, provide an explicit examination of the glassy nature of the carbon coatings.

9) Lines 220-221, the fitting function using a combination of Lorentzian and Gaussian is not common. How can the authors explain the choice of the fitting function?

10) Again, examination of XRD pattern is very poor. No structural information (lattice parameters, domain size or macrostrain) was given for the defined phases. Some broad peaks are not defined (2theta is around 45, 64, 73 deg.) and ignored. They may be associated with amorphous phase. It’s not appropriate to show the XRD examination in the current form.

11) Fig. 5, the «passivation zone» is described (lines 249-251). Please, add the following graphics in the Fig. 5 to outline the region of the passivation.

12) I can not agree with the statement that «uniform and dense coating on the 316L substrate through magnetron sputtering, effectively protecting the substrate» (lines 258-259). All coatings suffer from drawbacks corresponding to the boundaries between the cells (Fig. 1). Also, polarization curves exhibit a clear feature of anodic dissolution (for example, green curve at higher (>1 V) potential) and a very narrow range of repassivation (red curve, for instance). Hence, on the coating’s surface some undesirable oxidation reactions occur, most likely, at C/TiN interface. Please, discuss this moment.

13) Misprint in the line 282: «potentiostatic polarization curve»

14) Formation of a passive layer on the surface could be achieved on the metallic or ceramic surface. In this regard, about what passive layer is discussed (line 286)?

Author Response

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Reviewer 3 Report

This paper was well prepared and organized from the point that it showed the effect of target current on the properties of TiN/amorphous carbon layer coated on 316L. But some part of the manuscript needs to be revised as follows. 

1. Title is not clear and it needs the revision.

2. line 113; 316L used needs the chemical composition and whether the commercial product or not. If it is commercial, the manufacturer's information must be added.

3. All of the instruments used needs the information about 'model', manufacturer', 'country'.

4. line 189; G peak D peak and ID/IG ratio need the detail definition.

5. Figure 2; Check 'Roman shift'?

6. Figure 3; Binding Energy, NOT Bonding energy. (b) must show the ratio of sp2/sp3.

7. Figure 4; Check the location of the symbols.

8. Figure 8; The unit of potential must be V(SSC). Change the x-axis from 1E-3 to 10^-3 etc.

9. Table 1; Ecorr, mV(SSC), NOT V

10. Table 2; Why did you show the current density with different reference electrode like as SCE or SHE?

11. Figure 6; i-t curve at +0.6V(SCE) NOT potentiostatic polarization and etc.

12. Based on the above points, the conclusions need to be deeper and clearer on also the drawbacks of this process, it may be fine to generally promote this process, but the authors should provide also a comprehensive and objective list of conclusions with the good the bad and the neutral conclusions, NOT 1 paragraph.

Author Response

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Round 2

Reviewer 2 Report

I carefully checked the revised version of the manuscript and found out that the description of the experimental results was improved. My comments and suggestions were taken into account, so I may recommend this paper for publication.