Atmospheric Pressure Plasma Treatment of Magnesium Alloy for Enhanced Coating Adhesion and Corrosion Resistance
, Mina Yoon 2, Yi Feng Su 2, John Wade 2, Michael S. Stephens 2 and Jong K. Keum 3Round 1
Reviewer 1 Report
This work evaluates how atmospheric-pressure plasma treatment affects the adhesion of commercial primers to AZ91D magnesium alloy. Although the authors have taken a somewhat novel approach, the apparent best results achieved in this study required the use of CO2 gas, which is not very sustainable, or N2 which is more expensive than other available methods that could produce very similar results. Some sections of the manuscript are unclear, certain information is missing, and the results need to be better organized. Therefore, this manuscript is not suitable for publication in Coatings in its present form.
Major comments:
The authors employed different gas/primer coating combinations for each test in the manuscript. For example, PDP tests employed CO2/ZnP and CO2/epoxy. For the salt spray tests, CO2/MIL23377 was exposed without a cut, whereas Air/MIL23377 was exposed with an X-cut. All results for N2/MIL23377 were left in the supplementary information section, and the AZ91D surfaces appeared to be less attacked than those tested in similar conditions for Air-AP. With this experimental design, it is difficult to compare the results and draw conclusions. If the aim is to emphasize the potential of the Air-AP treatment, all experiments should solely be conducted with this treatment, the primer coatings and compared with the primer-coated 600 grit surface.
The results of the electrochemical tests are unclear. Could you provide information regarding the number of replicates that were conducted for each type of test? Additionally, in the PDP experimental section, it is mentioned that two different exposed areas were tested with different potential windows. Could you please explain the rationale behind this approach? Furthermore, the results of this section do not appear to show the -1.8 V to -1.2 V range. The PDPs for coatings do not provide any additional information other than the fact that the AZ91D surface is physically blocked. Although EIS is considered a suitable technique for this type of assessment, it is not appropriately applied in the manuscript. EIS analysis can be a powerful tool to compare the performance of coatings by extracting certain parameters from the EIS data, such as capacitance of the coating, pore resistance, etc.
The surface of the samples exposed to salt spray tests should have been cleaned prior to observations. To accurately evaluate the morphology of the corrosion attack, the corrosion products/salt deposits must be removed. There is an ASTM standard for such purpose.
Some minor comments:
From lines 74-76: “AP treatments with CO2 or air usually form nano-/micro-scale Mg-O-C particle agglomerated layers, improving the adhesion of primer coatings to the surface” Please add citation(s). Also, the authors state that the nano-/micro-scale Mg-O-C particle agglomerated layers are “usually” formed with these specific AP treatments. Please explain in which type of situation(s) this may not be the case.
Please make sure that the model(s) and supplier(s) for all equipment used in this work is properly mentioned (e.g., in Characterization of materials section, line 128).
Contact angle measurements are not mentioned in the experimental part, neither discussed properly in the results.
From lines 221-223: “Like the ZnP coatings, epoxy primer coatings had excellent electrochemical corrosion resistance for 96 hours in 3.5 wt % NaCl solution (not shown in this study)”. Why do the authors decided to not show those results? Even if they do not consider the results to be relevant for the main manuscript, it would be beneficial to include them in the supplementary information section for the sake of completeness.
Colors in Labels in figure 4a are not visible and the legend in the top right corner is not clear.
Providing the cost per m2 of other types of surface treatments would help readers understand the relative cost-effectiveness of atmospheric-pressure plasma treatment compared to the other available options.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper presents atmospheric-pressure plasma (AP) treatment using CO2, N2, and air, applied to AZ91D Mg alloy to investigate its effects on primer coating adhesion and corrosion resistance. Zinc phosphate primer 16-897 SEYMOUR yellow, epoxy Hard Hat and epoxy MIL23377 primers were used to coat the AZ91D specimens surface of 2.5x2.5 cm. The characterisation of materials was performed using SEM, EDS, XPS, XRD STEM analyses. Salt spray exposure tests were conducted in accordance with ASTM B117 and corrosion assessment was performed by potentiodynamic polarization measurements. The results obtained were presented compared to the AZ91D surface finished with 600 grit.
The article is of interest of Materials journal, but specific aspects mentioned in the following require the revision of the paper (major revision).
Observations:
1. Various water spray conditions have been tested to enhance the adhesion of the coating, but it is not specified on what basis the spray parameters were determined.
2. The experiments appear to be conducted empirically, without a planned factorial experiment leading to the determination of an objective function for correlating the main/significant process parameters with surface roughness.
3. It is not clear if the process parameters have been taken from the literature (if so, please indicate where). It is not clear how the swings spaced was establish nor the range of variation of the other parameters. However, it can be narrowed down to find an optimum (e.g. the range for scanning speed from 6 cm/min to 150 cm/min seems too wide).
4. In the case of scanning rate, its expression (the unit of measurement used) is inconsistent (in row 105 it is expressed in cm/min, and in (2), (3) and Table 1 it is expressed in m2/min).
5. Conversion to SI unit of applied pressure (psig), density (lb/L), flow rate etc. must be performed.
6. Some editing error have to be corrected (e.g. line 145 MHz but not mHz; line 167 to be added $)
7. Generally, the figures are not of high quality.
8. Figure 1a is not relevant as it refers to other plate sizes. Why was it introduced, since the actual dimensions of the specimens are different.
9. The scale used in the figures 1b (down), figure 5 etc. seems to be manually placed (but not given by the apparatus for contact angle measurement), so they may be inaccurate.
10. Reference to Table 1 is duplicated. (line 204 and 210 will be referred to Table 2).
11.Conclusions need to be developed. The conclusions are not presented in a structured and synthetic way. Results should be presented according to the AP treatment for the 3 types of gases used (CO2, N2, air). Also, in the conclusions it is not presented what are the specific results obtained with the 3 types of primers: zinc phosphate (ZnP), chromate-containing epoxy, and MIL23377. The results of the adhesion test are also missing from the conclusions. It is not shown (e.g. using percentage) how much the corrosion resistance was improved (in relation to the 600 grit material), and which is the most economically efficient process.
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
Thanks for submitting this paper to the Coatings journal. I found the article worthy of publication after revision.
Comment (1): Please highlight the research's novelty and significance in the introduction section's last paragraph.
Comment (2): In Fig. 2b, label all diffraction peaks with the corresponding phases and add the standard patterns of the identified phases from the JCPDS file to verify the phase matching.
Comment (3): In the PDF file I reviewed, in Fig. 6a, no images could be seen. Please revise it.
Comment (4): In Fig. S1, the authors referred to the EIS test. However, I suggest the authors report the EIS profiles in the supplementary files, including Nyquist plots and the bode plots of phase angle and total impedance magnitude. Please also report the equivalent electric circuit used for simulating the EIS profiles.
Comment (5): Please add the DOI of the cited references.
Comment (6): Please cite some relevant published articles from the Coatings journal MDPI in your paper. I am sure there are many relevant papers.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors have made modifications to improve their manuscript, but it is still difficult to follow and understand at times. However, with a minor revision, the manuscript would be suitable for publication in Coatings. Please address the following comments:
1. The authors still compare different systems, which makes it difficult for readers to interpret the results. To improve consistency, they should compare the same system(s) throughout the manuscript. In this context, a system refers to any surface treatment/gas/coating combination. For example, in Figure 4A, the authors show PDPs of ZnP-coated treated and untreated AZ91D surfaces, as well as a 600 grit AZ91D surface. In Figure 4B, however, the authors show images of ZnP-coated treated and untreated AZ91D surfaces, as well as epoxy-coated treated and untreated AZ91D surfaces. The authors should decide whether to include the PDPs of epoxy-coated treated and untreated AZ91D surfaces or add the images of a 600 grit AZ91D surface. Figures A and B should correlate with each other, as well as with the discussion. The authors should thoroughly check the manuscript again to avoid this type of issue.
2. Figure 1A is not suitable for publication because it is blurred and does not provide valuable information.
3. Figure 6A does not show anything and appears blank. It also does not provide valuable information. If the authors really want to show the surface prior to salt spray tests, they should consider using SEM.
4. In lines 169-170, the authors state that "the frequency window was 200 kHz to 7 MHz". However, these values do not make sense. In EIS, frequency windows typically go from high to low, so it should be 7 mHz instead. Moreover, the authors also wrote in Figure S1 (d) caption that "the lowest frequency is 1.3 Hz for the impedance spectra". This information is contradictory and needs to be clarified.
Author Response
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Author Response File: Author Response.pdf
Reviewer 2 Report
In the paper, starting with the title, there is talk about adhesion, and its growth, but no numerical values of these (obtained from tests) are presented. These should be presented both in the paper and in the conclusions.
Author Response
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Author Response File: Author Response.pdf
