Corrosion Protection of ZK60 Wrought Magnesium Alloys by Micro-Arc Oxidation
, Sheji Luo 1 and Yi Shen 1
Round 1
Reviewer 1 Report
Comments:
- The novelty should be highlighted more in the introduction, compared to the literature review.
- What was the basis of selecting 250, 300, and 450 C for forging? Why not 350 or 400 C?
- The scale bar should be added to Figure 1(b).
- A reference should be added for the etchant.
- Is there any standard for testing in Figure 3? What is the rate of loading when the sample is under the corrosive medium? What is the time of tensile testing? How many days are the samples under this loading condition, both tensile loading and the corrosive environment?
- The scale bar and the text on images of Figure 4 is not clear. Please change the yellow color. The problem should be addressed in all other images.
- How were the phases validated in Figure 6? The EDX map should be reported.
- The average thickness of MAO coating needs the standard deviation.
- In the research method, the corrosion time was mentioned as 35 days. However, this time is different in the results part for SEM images. It is confusing.
- Presenting results is not in a proper trend! It is better to report all SEM images for all processes and all studied materials. Corrosion results should be reported for all processes and all studied materials. In this form, some data were reported for 250 C; some others are related to 300 C, etc.
- References could be updated based on recent articles in 2015-2022.
- The novelty should be highlighted compared to the following publications.
Micro-arc oxidation (MAO) to improve the corrosion resistance of magnesium (Mg) alloys
Degradation Behavior of Micro-Arc Oxidized ZK60 Magnesium Alloy in a Simulated Body Fluid
Microarc oxidation of magnesium alloys: A review
Stress corrosion cracking behavior of micro-arc oxidized AZ31 alloy
Influence of Plasma Electrolytic Oxidation on Fatigue Behavior of ZK60A-T5 Magnesium Alloy
Author Response
Reviewer 1:
(1) The novelty should be highlighted more in the introduction, compared to the literature review.
The introduction has been revised and included the publications recommended by the reviewer (No.12). The following paragraph has been added (highlighted in blue font in the revised manuscript) to the introduction section to explain the novelty of the work.
Magnesium alloy treated by MAO process have attracted extensive attention as a kind of load bearing material in biomedical field. As biological materials, the stress is relatively light (＜15 MPa). However, as a general structural parts (automobile components), the applied maximum stress is about 30% (80 MPa) of the yield stress of magnesium alloys. Research on corrosion of magnesium alloys and MAO coatings under load condition (corrosion fatigue and stress corrosion cracking) is gaining popularity. Limited research has been done to investigate the static and stress corrosion performances of wrought Mg alloys with MAO coating.
(2) What was the basis of selecting 250, 300, and 450 C for forging? Why not 350 or 400 C?
The forging temperature was selected mainly based on the deformation characteristics of the magnesium alloy.
The forging process and fatigue properties of ZK60 forged magnesium alloys forged at 250, 300, and 450 ℃ were investigated in a previous study done by other research team members (S.M.H. Karparvarfard’s Paper-International Journal of Fatigue, 2019, 118, 282-297). And this work studied the corrosion properties of the alloy forged under the same temperatures, to be consistent.
(3) The scale bar should be added to Figure 1(b).
The scale bar has been added to Figure 1(b) in the revised manuscript.
(4) A reference should be added for the etchant.
The reference [35] for the etchant (ASTM E340-001 Standard test method for macro-etching metals and alloys1) has been added in the revised manuscript.
(5) Is there any standard for testing in Figure 3? What is the rate of loading when the sample is under the corrosive medium? What is the time of tensile testing? How many days are the samples under this loading condition, both tensile loading and the corrosive environment?
Figure 3 shows a customized stress corrosion testing method that was designed to assess the stress corrosion properties of the magnesium alloy quickly. Once the specimen was mounted on the tensile test machine and exposed to the corrosive medium, it was loaded to a stress level of 80 MPa at a relatively fast rate, and then maintained under this constant load for corrosion. The time to fracture for each specimen was recorded.
(6) The scale bar and the text on images of Figure 4 is not clear. Please change the yellow color. The problem should be addressed in all other images.
Figure 4 and other unclear Figures has been revised to show images with clear scale bar and text.
(7) How were the phases validated in Figure 6? The EDX map should be reported.
The phases of ZK60 cast alloy have been validated in previous studies published by myself and other group
members, using XRD and EDS line-scanning analysis. For ZK60, the secondary phases identified were MgZn2 and Zn2Zr intermetallic compounds. The two published papers have been added in the revised manuscript as references [38] and [39].
(8) The average thickness of MAO coating needs the standard deviation.
The standard deviation has been added to the average thickness of MAO coating (in blue font in the revised
manuscript).
(9) In the research method, the corrosion time was mentioned as 35 days. However, this time is different in the results part for SEM images. It is confusing.
The duration of salt spray chamber test is 35 days. However, for each alloy multiple repetitive specimens were put in the chamber. And one or more of the repetitive specimens were extracted at desired time point (e.g., 4hr, 24hr, and 14 days etc.) for SEM imaging and surface analysis. Thus, the SEM results showed images of different time points. Post 35 days of corrosion test, the left over specimens were used for mass loss measurement, generating data for Figure 12. To clarify it, the following explanation was added in the revised manuscript in Section 2.3 (in blue font):
Each day the specimens were examed by naked eyes for signs of corrosion. At certain time point, one of the repetitive specimens was extracted for imaging and surface analysis to document the progression of corrosion.
(10) Presenting results is not in a proper trend! It is better to report all SEM images for all processes and all studied materials. Corrosion results should be reported for all processes and all studied materials. In this form, some data were reported for 250 C; some others are related to 300 C, etc.
For Figure 7, only the EBSD data for the specimen forged at 250 ℃ were presented because only this specimen showed strong twins in microstructure (Figure 6b) and thus it was further analyzed using EBSD. In Figure 8, more SEM micrographs were added to show the surface and cross-section for all the MAO coated ZK60 alloys in the revised manuscript.
Figures 9 and 10 show the corrosion results for all three forging temperatures. The salt chamber corrosion test results have shown that the MAO-coated ZK60EF-300 alloy exhibited the best corrosion resistance, among all MAO-coated ZK60 alloys. Thus, only the uncoated and MAO-coated ZK60EF-300 alloy specimens were selected for further stress-corrosion study. In section 3.3.3, the following sentence was added (in blue font):
Thus, the uncoated and MAO-coated ZK60EF-300 alloy specimens were selected for further stress-corrosion study.
(11) References could be updated based on recent articles in 2015-2022.
The references have been updated based on recently published papers (2015-2022) in the revised manuscript. For example, reference [30], [31], [32] and [33].
Reviewer 2 Report
In the present work, commercial ZK60 extrusion alloy was selected as the starting material for forging. The static and stress corrosion performances of extrusion and forged ZK60 Mg alloys with and without MAO coatings in different environments were characterized.
Nevertheless, I have a few below comments, and please clarify:
1) The optical microscopy images of the microstructure, showing a mixture of small equiaxed grains and large bright elongated grains (Fig.4). Visible strings of second phase particles are seen along the extrusion direction, precipitated around the small grains. Authors mention that similar effects have been observed before, but that there is no explanation for the very mechanism of its formation.
2) It seems that pressing leads to better results, but here also no reference to the presented results (Fig. 5). Why fine grains show smaller disorientation and coarse grains show the larger one. It should be the other way around? I am asking Authors for comment/explanations.
3) At the discussion chapter (378, 379 lines etc.) Authors mentioned that: ...However, when the forging temperature exceeds the melting temperature of the secondary phases (e.g., β-MgZn2 or β-Mg17Al12), the secondary phase particles dissolve and DRXed grains grow coarses, deteriorating the corrosion performance...
On the base of enclosed images, I see that there is no evidence of the β-Mg17Al12 presence. Why exactly? Please comment.
Author Response
Reviewer 2
In the present work, commercial ZK60 extrusion alloy was selected as the starting material for forging. The static and stress corrosion performances of extrusion and forged ZK60 Mg alloys with and without MAO coatings in different environments were characterized.
Nevertheless, I have a few below comments, and please clarify:
(1) The optical microscopy images of the microstructure, showing a mixture of small equiaxed grains and large bright elongated grains (Fig.4). Visible strings of second phase particles are seen along the extrusion direction, precipitated around the small grains. Authors mention that similar effects have been observed before, but that there is no explanation for the very mechanism of its formation.
This microstructure is typical of ZK60 extruded magnesium alloy. In extrusion process, recrystallization determines the grain size and the distribution of the second phase. This ZK60 extrusion alloy was selected as the starting material for forging. The microstructure of ZK60 extruded alloy was observed at different forging processes.
(2) It seems that pressing leads to better results, but here also no reference to the presented results (Fig.5). Why fine grains show smaller disorientation and coarse grains show the larger one. It should be the other way around? I am asking Authors for comment/explanations.
Figure 5 was the EBSD of the initial material of ZK60 extrusion alloy. In section 3.1, the following sentence was revised (in blue font):
The small-angle grain boundaries represent coarse grains that are evenly distributed. The big-angle grain boundaries (40°~90°) mainly refer to fine DRXed grains (i.e., equiaxed fine grains) formed during extrusion [41]. The reference [41] for EBSD has been added in the revised manuscript.
(3) At the discussion chapter (378, 379 lines etc.) Authors mentioned that: …. However, when the forging temperature exceeds the melting temperature of the secondary phases (e.g., β-MgZn2 or β-Mg17Al12), the secondary phase particles dissolve and DRXed grains grow coarses, deteriorating the corrosion performance….
On the base of enclosed images, I see that there is no evidence of the β-Mg17Al12 presence. Why exactly? Please comment.
The second phases (β-MgZn2 and β-Mg17Al12) mentioned here are the common second phases of magnesium alloys. The second phase of β-Mg17Al12 in AZ series magnesium alloys will be studied in our future work.
Reviewer 3 Report
It is an intersting paper worthy of investigation having original character sustained with relevant enough references . The methodology of the manuscript is properly chosen and the paper is clearly written
My recommendation is minor revision taking into account the results organization and presentation according to the followings :
a ) the manuscript has 15 figures and no tables. A part of the data will be better to be presented in tables
b) it is a need for a statistical data presentation with errors
Author Response
(a) the manuscript has 15 figures and no tables. A part of the data will be better to be presented in tables.
The data of average corrosion rate for uncoated and MAO coated ZK60 alloy after 35 days of salt spray test were listed in Table 1 in the revised manuscript.
(b) it is a need for a statistical data presentation with errors
The standard deviation for average thickness of the MAO coating has been added (in blue font) in section 3.2 in the revised manuscript.
Reviewer 4 Report
- Authors should bring flowcharts of different coating and preparation methods and tests in the Experimental Procedure section.
- More pictures and descriptions of MAO coatings, interface and defects (cracks, holes), and formed phases should be provided.
- The optimal sample in terms of coating conditions and production process and heat treatment must be specified
- Discussion and comparison of the type and method of coating with the amount of corrosion and reaction products should be given in more detail. Doing this, reviewing the following refs could be helpful:
Ceramics International, 40, 2014, 5515-5522
Neural Computing and Applications, 23, 2013, 779-786.
Author Response
(1) Authors should bring flowcharts of different coating and preparation methods and tests in the Experimental Procedure section.
Our previous research mainly focused on the preparation and parameter optimization of the MAO coating on magnesium alloys, and the results have been published in a paper (Corrosion and corrosion fatigue performances of micro-arc oxidation coating on AZ31B cast magnesium alloy, Materials and Corrosion, Vol. 70, Iss. 2, p. 268-280). In the present work, we adopted the optimized parameters for MAO coating process and thus did not give a detail description of the coating process. Our previous study was added as reference [37] in the revised manuscript.
(2) More pictures and descriptions of MAO coatings, interface and defects (cracks, holes), and formed phases should be provided.
We have conducted a series of studies of MAO coatings (i.e., coating preparation, morphology, interface and defects, and phase composition etc.) on Mg alloys, and published the results in a few papers. These papers have been included in the revised manuscript as references [36], [37], and [39].
(3) The optimal sample in terms of coating conditions and production process and heat treatment must be specified.
The MAO coating process has been optimized in our previously published paper (reference [38]). The present work adopted the optimized parameters for MAO coating preparation and focused on investigation the influence of forging temperature on the corrosion resistance of the uncoated and MAO-coated Mg alloy specimens. In section 3.3.3, it was pointed out that “The corrosion resistance of the ZK60 alloys of different processing history is ranked as ZK60EF-300 ＞ ZK60E ＞ ZK60EF-450 ＞ ZK60EF-250 ＞ ZK60C. The MAO-coated ZK60EF-300 alloy also exhibited the best corrosion resistance, among all MAO-Coated ZK60 alloys”.
(4) Discussion and comparison of the type and method of coating with the amount of corrosion and reaction products should be given in more detail. Doing this, reviewing the following refs could be helpful:
Ceramics International, 40, 2014, 5515-5522
Neural Computing and Applications, 23, 2013, 779-786.
The reviewer’s advice is much appreciated. Detailed discussion and comparison of the type and method of coating with the amount of corrosion and reaction products was given in our previous investigations published in references [36], [37], and [39] of the revised manuscript. This paper mainly focused on the influence of forging process on the corrosion resistance of uncoated and MAO-coated ZK60 Mg alloy specimens. Further investigation on the corrosion properties and corrosion performances of the MAO-coated Mg alloys using methods in the reviewer-suggested papers is currently underway.
Round 2
Reviewer 1 Report
Several comments were properly addressed in the revised article. However, some comments still need to be addressed in the text. It is not enough to answer the comments and the answers should be added to the text. These comments are as follows,
(2) What was the basis of selecting 250, 300, and 450 C for forging? Why not 350 or 400 C?
(5) Is there any standard for testing in Figure 3? What is the rate of loading when the sample is under the corrosive medium? What is the time of tensile testing? How many days are the samples under this loading condition, both tensile loading and the corrosive environment?
(9) In the research method, the corrosion time was mentioned as 35 days. However, this time is different in the results part for SEM images. It is confusing.
Author Response
Reviewer 1
Several comments were properly addressed in the revised article. However, some comments still need to be addressed in the text. It is not enough to answer the comments and the answers should be added to the text. These comments are as follows,
(2) What was the basis of selecting 250, 300, and 450 C for forging? Why not 350 or 400 C?
(5) Is there any standard for testing in Figure 3? What is the rate of loading when the sample is under the corrosive medium? What is the time of tensile testing? How many days are the samples under this loading condition, both tensile loading and the corrosive environment?
(9) In the research method, the corrosion time was mentioned as 35 days. However, this time is different in the results part for SEM images. It is confusing.
The answers of these three comments have been added to the corresponding section ((2) is shown in Section 2.1; (5) is shown in Section 2.4; (9) is shown in Section 2.3).
Reviewer 4 Report
In response to the reviewers' comments, the authors referred to their previous articles and avoided giving a full response to the reviewers' requests. The text of the article should be rewritten and corrected based on the opinions of the reviewers. The authors should follow the provided comments by all the reviewers.
The authors refer to more than 20 articles from their previous articles and this number should be reduced.
State of the art to be improved further. The authors use mostly self-citation, it should be reduced.
Author Response
Dear Reviewer,
This work is a continuation of our previous work. In previous work, we have been focusing on the influence of MAO process parameters on the microstructure (including interface and defects (cracks, holes)) and corrosion properties of MAO coating, and optimizing the process parameters of MAO coating.
Our present work is to study the influence of substrate properties on corrosion resistance of MAO coating based on MAO coating optimization.
(1) Authors should bring flowcharts of different coating and preparation methods and tests in the Experimental Procedure section.
This paper mainly adopts the parameters optimized by our previous research. For this work, the preparation parameters and process of all MAO coating are the same, and the effect of the substrate alloy on the corrosion resistance of MAO coating is mainly studied.
The schematic diagram of MAO process has been added as Figure 3 in the revised manuscript (in blue font in Section 2.2).
(2) More pictures and descriptions of MAO coatings, interface and defects (cracks, holes), and formed phases should be provided.
We have conducted a series of studies of MAO coatings (i.e., coating preparation, morphology, interface and defects, and phase composition etc.) on Mg alloys, and published the results in a few papers.
The preparation parameters of MAO process in this present work are same for all studied specimens. We study the influence of the substrate alloy on the corrosion resistance of the MAO coating under the condition of guaranteeing the same coating state.
(3) The optimal sample in terms of coating conditions and production process and heat treatment must be specified.
The MAO coating process has been optimized in our previously published paper. The present work adopted the optimized parameters for MAO coating preparation and focused on investigation the influence of forging temperature on the corrosion resistance of the uncoated and MAO-coated Mg alloy specimens. In section 3.3.3, it was pointed out that “The corrosion resistance of the ZK60 alloys of different processing history is ranked as ZK60EF-300 ＞ ZK60E ＞ ZK60EF-450 ＞ ZK60EF-250 ＞ ZK60C. The MAO-coated ZK60EF-300 alloy also exhibited the best corrosion resistance, among all MAO-coated ZK60 alloys”.
(4) Discussion and comparison of the type and method of coating with the amount of corrosion and reaction products should be given in more detail. Doing this, reviewing the following refs could be helpful:
Ceramics International, 40, 2014, 5515-5522
Neural Computing and Applications, 23, 2013, 779-786.
The reviewer’s advice is much appreciated. These papers have been as the references in the revised manuscript. Detailed discussion and comparison of the type and method of coating with the amount of corrosion and reaction products was given in our previous investigations. This paper mainly focused on the influence of forging process on the corrosion resistance of uncoated and MAO-coated ZK60 Mg alloy specimens.
Further investigation on the corrosion properties and corrosion performances of the MAO-coated Mg alloys using methods in the reviewer-suggested papers is currently underway.
(5) The authors refer to more than 20 articles from their previous articles and this number should be reduced.
State of the art to be improved further. The authors use mostly self-citation, it should be reduced.
The references have been revised and optimized in the revised manuscript.
Round 3
Reviewer 4 Report
The second revision is recommended for publication.
