Investigation on the Effect of a Chromium-Free Sealing Treatment for the Corrosion Resistance of AA2198-T851 after Tartaric Sulphuric Anodizing (TSA)
, Victor Hugo Ayusso 1, Maysa Terada 2,* and Isolda Costa 1
Round 1
Reviewer 1 Report
Comments and suggestions are detailed in the following file.
Comments for author File: 
Comments.pdf
The quality of English is OK.
Author Response
The authors acknowledge the kind considerations. All suggestions were taken into account. Please check the attachment.
Author Response File: Author Response.pdf
Reviewer 2 Report
This is a review of the article “Investigation on the Effect of a Chromium Free Sealing Treatment for the Corrosion Resistance of AA2198-T851 After Tataric Sulphuric Anodising (TSA)”
Summary
The article investigates the effectiveness of sealing and self healing properties of an Al-Li alloy subjected to a hydrothermal process containing cerium. The article presents SEM micrographs for surface morphology investigation and EIS investigations spanned on a duration of 96 hours monitoring the electrochemical behaviour of the samples immersed in 0.5M NaCl aqueous solution.
General comments
The premise of the article is good. There are differences between the unsealed, hydrothermally sealed and sealed with cerium samples. However, some improvements must be made.
Many references are old. If possible, please replace them with newer ones.
For example:
El Arbi Hajjioui, Kenza Bouchaâla, Mustapha Faqir, Elhachmi Essadiqi, A review of manufacturing processes, mechanical properties and precipitations for aluminum lithium alloys used in aeronautic applications, Heliyon, Volume 9, Issue 3, 2023, https://doi.org/10.1016/j.heliyon.2022.e12565.
N.J.H. Holroyd, G.M. Scamans, R.C. Newman, A.K. Vasudevan, Chapter 14 - Corrosion and Stress Corrosion of Aluminum–Lithium Alloys, Editor(s): N. Eswara Prasad, Amol A. Gokhale, R.J.H. Wanhill, Aluminum-lithium Alloys, Butterworth-Heinemann, 2014, Pages 457-500, ISBN 9780124016989, https://doi.org/10.1016/B978-0-12-401698-9.00014-8.
There are small typos throughout the paper (exponents of values).
Specific comments
Experimental
Page 3, line 103. Please provide a better description of the hydrothermal sealing process and specify the solution containing cerium ions.
Results
Figures 1, 2, 3. EDS elemental mapping would be more useful for the identification of the elements from the surfaces of the samples.
Figures 4, 5 and 6 are hard to see. Please provide colored images. The Nyquist plots should have equal values (axes sizes) on the X and Y axes.
Figures 4, 5 and 6. In the figure captions there is writen “Bode IZI diagram” for which there is no actual diagram.
Page 7 lines 184-17. There is a section from the template remaining.
It would be better if the explanation from the second equivalent circuit from page 8 would be moved to page 7, adiacent to Figure 5.
Discussion.
Overall, the discussion section is hard to read.
Perhaps it would be more fitting to make a combined “Results and Discussion” section, or at least move the paragraphs between page 9 line 226 and page 9 line 248 to the “Results” section and begin the “Discussion” section from Figure 7.
Page 9, line 226 states that “white areas on the surface of the samples suggest they are located on the pores of the oxide layer. The cracks all over the surface could be explained by residual tension during the drying process and the precipitation of cerium hydroxide.”
However, the surface shown in Figure 3 is similar to that from Figure 2 which is presumably covered with AlOOH pellets and the white areas (Ce containing particles) are scattered on the surface. EDS elemental mapping would be useful.
Figures 7-11. Please provide colored images for better reading.
Page 9, line 179 “Figures 7 (a-b) and 9 (a-b)” There is no Figure 9 (a-b).
Page 10, lines 271-285. All Rb values have an increasing trend. You state that “each decrease in Rb was followed by a subsequent increase for the aluminum alloy sealed in cerium containing solution”, however the trend of the unsealed sample shows no decrease. Can one presume that the unsealed sample has a better self-healing effect?
Figure 10. The αb parrameter appears only for the unsealed sample. I understand that you used a pure capacitor in the equivalent circuit for the fitting of the data obtained for the sealed samples. You could try to fit the data of the sealed samples using a CPE (CPEb) and if your theory is correct, the αb parameter should remain approximately 1.
Perhaps the overall corrosion behaviour could be also expressed as Tafel plots.
It would be interesting to see SEM images of the samples after the 96 hours of immersion and potentially see the self healing effect.
Author Response
The authors acknoledge the kind considerations and took all of them into account. Please see the attachment. Thank you in advance.
Author Response File: Author Response.pdf
Reviewer 3 Report
Introduction
The comparison of Al-Cu-Mg and Al-Cu-Li alloys appears to be biased in order to present Al-Cu-Li as being more favorable. The comparison of the corrosion resistance and mechanical properties has to presented in a more objective way.
Is stated that “Aluminum alloys containing magnesium (Mg), present low stiffness and high localized corrosion susceptibility”, however reference [2] deals with Al-Cu-Mg not Al-Mg alloys. In line 64 it is further stated that “the susceptibility to localized corrosion has been lately associated to the Al-Cu-Li alloys”. Obviously, both alloying systems are susceptible to localized corrosion. This might be rather due to the Cu, not because of the Mg or Li. Further, the statement in lines 62/63 “The literature also reported the higher corrosion resistance of the AA2198 T851 when compared to the AA2524-T3, suggesting the first one as a potential replacement for the last one.” seems to be wrong. It can be taken from reference [14] that AA2198 shows a less noble corrosion potential compared AA2524. Therefore, Moreto et al. state that the Li-containing alloys are more susceptible to corrosion.
Further, it is shown in reference [14] that the elastic modulus of AA2524-T3 is higher compared to AA2198T851, and just as an example, the Li-free alloy AA7050-T7451 shows higher strength than the Li-alloyed AA7081T73511. Hence the statement in lines 40/41 “The addition of Li … increasing strength and elastic modulus in comparison with conventional aluminum alloys.” seems to be not generally true. Possibly, the general heat-treatment condition and also other precipitates might determine the mechanical properties.
In my opinion, it is not necessary to find reasons why the Al-Cu-Li alloys were used in this study. It is most important to make clear that both Al-Cu-Li and Al-Cu-Mg alloys require protection against localized corrosion.
In line 68 (and further on), only the two possibilities of hydrothermal sealing and sealing in “chromium solution” are presented. Hence, it appears that hydrothermal sealing is the only alternative to “chromium” sealing. First, it is required to describe “chromium” sealing more precisely. I guess that we are talking about “hexavalent chromium” sealing or a “chromate” sealing. Further, there are more ways of sealing, e.g., cold sealing (commercially available) or sealing by particle incorporation or a sol-gel layer (both in research). Still, the motivation could remain the same: All of these sealing strategies do not provide a self-healing effect.
Generally, there is more literature on Ce-based sealing of anodic oxide layers. The authors might consider to report about some of the information in:
DOI 10.1016/S0040-6090(02)01038-6,
DOI 10.1179/1743278211Y.0000000025 and
DOI 10.20964/2022.12.23
One paper by Carangelo et al. even directly investigates the differences between hot-water (hydrothermal) sealing Cr(VI) sealing and a Ce-based sealing and should therefore be considered:
DOI 10.1149/2.0231610jes
The statement in lines 46/47 should be underlined with a reference.
Experimental
The experimental section should be written on a detail level that enables the readers to exactly repeat the experiments. This is not completely the case. The exact parameters of the hydrothermal sealing (hot water or vapor?, temperature) and the Ce-based sealing (chemicals and concentrations, temperature) should be added. How was the sample preparation for SEM imaging done (e.g., carbon coating?).
Normally, when giving the suppliers of materials or devices, the location of the companies headquarter (city and country) should be given, e.g. …the field emission gun microscope Quanta 650 (FEI, Hillsboro, USA)…. Maybe, there is some advice in the template of the journal.
Especially in this section, there are a lot of typos and incorrect notations of physical units:
In g.L-1 and mol.L-1, the “.” is not an appropriate multiplication sign, better would be a central point “∙” or even the “*”. Further, the “°C” and the superscript numbers should be checked and corrected (in all chapters).
Results
The authors might optionally consider sub-headings to differentiate between microstructural and corrosion results.
General comment on SEM images: SEM parameters and the scale bar are small and hardly visible. Recommendation: Omit the sub-line with SEM information, give the SEM parameters in the experimental section, and create bigger and clearly visible scale bars.
General comment on graphs (e.g, Nyquist and Bode plots): The font size should be increased.
General comments on tables (which is also valid elsewhere): Physical units like R have to be written in italics, further additions to the unit have to be written in sub-script (no italics), e.g. Rsol, there should not be a space character in “9.56 x10-7” and take care of using superscript for the exponents.
Figure 1a,b: The depression is too dark.
Figure 1 c: Low quality of the cross-section (scratches in the substrate), although the important information is still recognizable. If possible, reprocess the cross-section and take a new image. Low contrast.
Figure 2: too dark
Figure 3 a,b: contrast is too high
Figure 3 c: Consider naming the unmarked peaks, too
Lines 167-169 are rather discussion.
Lines 185-187 sounds like a reviewer's comment that was not yet implemented. Actually, sub-headings are an option, but interpretation and conclusions should be done in separate chapters according to the structure of the paper.
Discussion
The authors decided to discuss all figures separately and one after another. This could alternatively done in a combined "Results and Discussion" chapter immediately after presenting the findings. This would also save space as the main findings would not be repeated a second time before discussing them.
In my opinion, a separate "Discussion" section only makes sense, if complex results of complementary methods have to be discussed in larger context to show superordinate relationships. In this case, I recommend to use a combined "Results and Discussion" chapter.
Line 227: “pores of the oxide layer” - Does "pores" mean the regular pore channels or larger flaws?
Lines 233 and 238: “Figure 5” and “Figure 6” - This should better be related to Table 3 and Table 4.
Line 239: “EIS values increased” - Which?
Line 249 and the following discussion of Figures 7 and 9 - This is a presentation of results that would fit better in a results or results and discussion section.
Line 281: “precipitation at the base of the pores” - Was this seen at the cross-section? At least, that was not reported when presenting the SEM results.
Figures 7 to 11: In case that several repetition measurements were performed for each condition, please include the standard deviations. If there was only one measurement for each condition, the alternating trends of e.g. Figure 7 might not be reproducible.
Conclusions
Optionally, further conclusions might be listed here. For example, evaluating the suitability of the experimental set-up and the application potential of a Ce-based sealing.
Generally, the English language is fine. However, there are some errors regarding the notation of physical units and numbers.
Author Response
The authors acknowledge the kind considerations and took all of them into account. Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Thank you for your hard work and for answering all my questions/sugestions. In my oppinion, the manuscript was improved significantly and I reccomend the publication.
Reviewer 3 Report
You might optionally give references for the sealing processes listed in lines 73/74 and explain that those methods do not provide a self-healing effect comparable to hexavalent chromium sealing (I guess, this is the reason, why you are investigating Ce sealing.)
There are still some careless mistakes that can easily be corrected by another careful proof-reading. Besides the text, please do also check the presentation of numbers and units, again.
