Effect of Negative Current on the Microstructure of Oxide Coatings Prepared by Hybrid Pulse Anodization
Round 1
Reviewer 1 Report
The paper “Effect of negative current on the microstructure of oxide coatings prepared by hybrid pulse anodization” by Shuo Huang el Al. could be of some interest for the reader. It should be deeply revised before publication on Metals.
In paragraph 2.1, the experimental set-up has to be better shown i.e. schematic of cells used, materials used for electrodes, the volume of electrolytic solution used, the area of electrodes, the distance between them and so on. Was the bath stirred? The supply frequency was 300 Hz so do you used an AC technique? A detailed version of Fig. 6 should be reported here.
Explain in detail the meaning of fig. 1. Which is the difference between the fig. 1a and fig. 1b and between the fig. 1c and fig 1d.
In paragraph 3.2, you should hypnotize a specific model of oxide growth to be able to report the eq. 1 that should be reported in the paper. What does O in square brackets mean?
Which is the usefulness of fig. 4? You should compare it with others obtained using diverse experimental conditions or erase it.
Which is reported in fig. 5 (number of pores?)? In which way have you measured and defined the “diameter” of pores.
Have you any evidence that using the experimental conditions stated (The supply frequency, positive/negative pulse width and positive current density were set as 300 Hz, 300 μs and 5 A/dm2, respectively. The negative current densities varied from 0 A/dm2 to 3 A/dm2 with an increment of 0.5 A/dm2) the ions can follow the electric field variation?
Several references are reported twice.
The papers needs major revisions.
Author Response
Manuscript ID: metals-405742
Title: Effect of negative current on the microstructure of oxide coatings prepared by hybrid pulse anodization
Journal: Metals
Dear reviewer:
Firstly, we would like to thank you for your positive and constructive suggestions for the manuscript. We have made careful major revisions to the manuscript according to your comments and suggestions. Besides, the English-language editing in the revision has been completed by the teacher who has rich experience in technical English writing. All the changes and modifications of the manuscript have been highlighted using the "Track Changes" function in the revision.
We hope you will be satisfied with our responses and the revised manuscript. We look forward to your positive response.
Thank you again for your time and consideration.
Sincerely yours,
Shuo Huang
Response to Reviewer 1 Comments:
Recommendation: Major revisions
Point 1: The paper “Effect of negative current on the microstructure of oxide coatings prepared by hybrid pulse anodization” by Shuo Huang el al. could be of some interest for the reader. It should be deeply revised before publication on Metals.
Response 1: Thanks for your positive comments. Major revision has been made in the revision according to your comments.
Point 2: In paragraph 2.1, the experimental set-up has to be better shown i.e. schematic of cells used, materials used for electrodes, the volume of electrolytic solution used, the area of electrodes, the distance between them and so on. Was the bath stirred? The supply frequency was 300 Hz so do you used an AC technique? A detailed version of Fig. 6 should be reported here.
Response 2: A schematic of experimental set-up has been presented in Figure 1, and more details of the experimental set-up have been added into the revision. Aluminum sample was set as anode, and the stainless steel plate was set as cathode. The distance between them was 5 cm. The area of the aluminum sample for HPA was 20 cm2. The volume of electrolytic solution used was 4 L, and the electrolyte was stirred by an air-blowing device. The bipolar pulse power supply used in the experiment was DC power supply rather than AC supply, so the AC technique was not used. Besides, detailed descriptions of Figure 6 have been added in paragraph 2.1 and Discussion.
Point 3: Explain in detail the meaning of fig. 1. Which is the difference between the fig. 1a and fig. 1b and between the fig. 1c and fig 1d.
Response 3: Figure 1 presented the variation curves of positive and negative voltages with anodization time. This voltage curve could directly show the growth stages of anodic oxidation coating and the influence of different negative current densities on the HPA process. Figure 1a has shown the voltage changes during the whole anodic oxidation process, while Figure 1b only captured the first 60s at the beginning of this process to further study the growth of barrier layer and the appearance of the nanopores, so were Figure 1c and Figure 1d.
Point 4: In paragraph 3.2, you should hypnotize a specific model of oxide growth to be able to report the eq. 1 that should be reported in the paper. What does O in square brackets mean?
Response 4: Thank you for your advice. Li et al. has deeply studied the mechanism of oxide growth [1]. It is well-known that the eq. 1 happens during anodization process [2], so there is not much explanation about it in the manuscript. Essentially the principle of anodic oxidation is the electrolysis of H2O. In the electrolytic process, oxygen which comes in three forms (molecule O2, atom O and ion O2-) separates from the anode. Afterwards, aluminum is oxidized by the oxygen separated. Therefore, the “[O]” includes the three forms of oxygen.
1. Mohammadi, I.; Afshar, A. Modification of nanostructured anodized aluminum coatings by pulse current mode. Surf. Coat. Technol. 2015, 278, 48-55.
2. Li, J.; Zhang, Z.; Li, Y.; Ma, Y.; Chen, L. Self-organization process of aluminum oxide during hard anodization. Electrochim. Acta 2016, 213, 14-20.
Point 5: Which is the usefulness of fig. 4? You should compare it with others obtained using diverse experimental conditions or erase it.
Response 5: Thank you for your advice. Figure 4 was intended to exhibit the vertically perforated nanopores in the HPA coatings. It has been deleted due to the incompleteness for comparison.
Point 6: Which is reported in fig. 5 (number of pores?)? In which way have you measured and defined the “diameter” of pores.
Response 6: Thanks for your reminding. It has been clearly pointed out in paragraph 3.2 of the revision that the numbers in Figure 5 were based on the statistics of all the nanopores in the whole pictures of Figure 3a, c, e and g. The diameters were measured using the software of Image-Pro Plus 6.0, the SEM image was processed in it, and the picture below was obtained. According to the preset scale, the system automatically calculated the average diameters of every white area.
Point 7: Have you any evidence that using the experimental conditions stated (The supply frequency, positive/negative pulse width and positive current density were set as 300 Hz, 300 μs and 5 A/dm2, respectively. The negative current densities varied from 0 A/dm2 to 3 A/dm2 with an increment of 0.5 A/dm2) the ions can follow the electric field variation?
Response 7: Roshani et al. [3] has demonstrated that the negative current could make H+ move towards anodization coatings and increase the dissolution rate in Formula 2.
3. Roshani, M.; Sabour Rouhaghdam, A.; Aliofkhazraei, M.; Heydari Astaraee, A. Optimization of mechanical properties for pulsed anodizing of aluminum. Surf. Coat. Technol. 2017, 310, 17-24.
Point 8: Several references are reported twice.
Response 8: Thanks for your reminding. Repeated references have been removed in the revised manuscript.
Reviewer 2 Report
This paper is devoted to study the effect of revers polarization onto anodic film formation on Al. The work includes interesting experimental results, which without any doubts, could extend knowledge to this practically important technique. However, discussion can be improved in accordance with notes added in text (see attached file).
Comments for author File: 
Comments.pdf
Author Response
Manuscript ID: metals-405742
Title: Effect of negative current on the microstructure of oxide coatings prepared by hybrid pulse anodization
Journal: Metals
Dear reviewer:
Firstly, we would like to thank you for your positive and constructive suggestions for the manuscript. We have made careful revisions to the manuscript according to your comments and suggestions. All the changes and modifications of the manuscript have been highlighted using the "Track Changes" function in the revision.
We hope you will be satisfied with our responses and the revised manuscript. We look forward to your positive response.
Thank you again for your time and consideration.
Sincerely yours,
Shuo Huang
Response to Reviewer 2 Comments:
Point 1: In line 107. The effect of negative current is main point of this research. However, here you mentioned that it was obvious. If so, why it is important? For me it is not obvious that negative current will affect layer thickness. The mechanism should be provided in discussion.
Response 1: Thank you for your reminder. It has been corrected in the revision, and the mechanism was also provided.
Point 2: In line 111. Dissolution is not the only possible process. Presence of negative pulses would cause corresponding negative surface charging, which under following positive pulse, has to be discharged. Only after recharging cell potential achieves level of suitable anodic potential. As a result, part of anodic current could be consumed by this recharging rather than electrochemical reaction, thereby reducing current efficiency.
Response 2: Thank you for your comments and explanation about another possible process which will reduce the coating growth rate. In fact, the anodization coating growth rate is mainly controlled by a strong competition between growth rate and dissolution rate [1,2]. This is also the focus of my manuscript. As for aspect you mentioned, further research will be carried out in the future work.
1. Li, J.; Zhang, Z.; Li, Y.; Ma, Y.; Chen, L. Self-organization process of aluminum oxide during hard anodization. Electrochim. Acta 2016, 213, 14-20.
2. Mohammadi, I.; Afshar, A. Modification of nanostructured anodized aluminum coatings by pulse current mode. Surf. Coat. Technol. 2015, 278, 48-55.
Point 3: In line 112. This is also not obvious, since hydrogen ions, once touched the surface of cathode, could be reduced with hydrogen gas evolution. This is possible, since anodic films provide unipolar conductivity, in other words "valve effect".
Response 3: Thank you for your comments. I agree with your opinions, and the discussion has been improved in the revision.
Point 4: In line 137. It is unlikely that O2- are transportable species outside anodic alumina phase. Within liquid phase oxygen is in form of water, hydroxyl or hydroxonium ions. In order to provide growth of anodic alumina, oxygen species have to be in contact with outer surface of the growing oxide, rather than metal substrate. Normaly it is considered that bottom barrier layer is responsible for main electrochemical reactions.
Response 4: Thank you for your correction. I agree with your opinions on these. In fact, the ions that participate in the anodic oxidation process are OH-. However, O2- are usually used instead of OH- in acidic electrolytes [3,4].
3. Roshani, M.; Sabour Rouhaghdam, A.; Aliofkhazraei, M.; Heydari Astaraee, A. Optimization of mechanical properties for pulsed anodizing of aluminum. Surf. Coat. Technol. 2017, 310, 17-24.
4. Zhang, R.; Jiang, K.; Zhu, Y.; Qi, H.; Ding, G. Ultrasound-assisted anodization of aluminum in oxalic acid. Appl. Surf. Sci. 2011, 258, 586-589.
Point 5: It had been much readable if scale X would have the same range: 5-60 for all a-d figures in Figure 5.
Response 5: Thank you for your advice. Figure 5 has been modified in the revised manuscript according to your advice.
Point 6: In line 161. Oxygen and hydrogen migrate to anode and cathode, respectively.
Response 6: Thank you for reminding. This sentence has been corrected in the revision.
Point 7: In line 172. It is unlikely, as cathodic and anodic reactions are quite different. Anodic oxidation is strongly limited by ions migration in oxide, whereas cathodic reaction is much easier on the steel electrode. So we would not expect any H+ accumulation nearly cathode.
Response 7: Thank you for your correction. I quite agree with your comments, and it has been corrected in the revision.
Point 8: In line 182. Could this be caused by hydrogen gas bubbles?
Response 8: Thank you for your kind reminding, and I agree with you. As mentioned in the manuscript, when the negative current density was large, a mass of H+ accumulated on the surface of the oxide coatings. Some of H+ participated in Formula 2, while the others may separate in the form of hydrogen gas during t+ and toff, which would cause damage to the nanopore microstructure. Further study will be carried out in the future work.
Reviewer 3 Report
It is argued that pore initiation and pore growth take place due to heat generation and accelerated chemical dissolution at the ground of the pores. The argumentation is mainly based on the theories of Keller, Hunter and Mason, which were published in the 1950s. Therefore, these original references should be cited directly. According to the calculation of Hunter and Fowle, boiling sulfuric acid with a concentration of 51 vol% would be required to compensate coating growth at a current density of 1,29 A/dm² at the pore bottom. In other anodizing electrolytes, e.g. oxalic acid, anodic alumina exhibit even lower chemical dissolution rates. Therefore, many authors assess the supposed mechanisms to be unrealistic. Nagayama and Tamura considered Joule heating and heating due to the exothermic oxidation reaction and calculated a maximum temperature increase of 0.06 K at the pore ground. Hoar and Mott already proposed in 1959 that field assisted dissolution might be responsible for the pore growth rather than chemical dissolution. Moreover, Garcia-Vergara et al. analyzed the distribution of slow migrating anions within anodic oxides and performed tracer studies and finally proved that plastic flow of the oxide takes place at the pore bottom under high compressive stresses and that chemical and field assisted dissolution might be negligible. According to Jessensky, self-ordering of pores is determined by the volume expansion during oxidation and the extent of compressive stresses. The theory of field-assisted dissolution and especially the theories of plastic flow at the pore ground and stress-controlled self-ordering should be considered when discussing the results.
It is stated that a negative pulse immediately after the positive pulse helps to eliminate the anodic current flow. This should be proofed by recording the current pulses during anodizing. Furthermore, a negative current flow could be quantified by this method. According to the line of argumentation, an inverted current flow through the barrier layer might also cause Joule heating and therefore impair the self-ordering. Apart from this, different interpretations might be possible, if the authors consider the theory of field-assisted dissolution (no ejection of Al3+-Ions at negative potentials) and the theory of plastic flow at the pore ground.
In general, the results are clearly presented by using adequate figures. However, in figure 4 and especially in figure 3, the scale, the magnification and the parameters of SEM imaging are hardly recognizable. This can be prevented by adding a bigger scale in the figures, giving important information, e.g. acceleration voltage, working distance and detector type, in the experimental section and omitting unimportant information, e.g. the date.
The parameters of anodizing with pulsed current might be clearer to the reader, if figure 6 is already presented within the experimental section. Instead of giving the pulse frequency, the pulse lengths (ton/toff, t+/t-) should be given explicitly.
What do you mean with “negative voltages were recorded manually” (line 75)? Did you use a transient recorder for recording? If so, what type did you use? Did you check, whether positive and negative pulses were generated in good agreement with the schematic illustration in figure 6?
Please add the name of the manufacturer of the thickness gauge and the SEM and the provider of “Image-Pro Plus” (lines 74, 77 and 78). All applied resources should be clearly identified.
Some minor linguistic issues should be considered:
- “duty cycle” instead of “duty circle” in line 53
- “substrate materials” instead of “starting materials” in line 64
- Check the position of the °-Symbol in line 65.Author Response
Manuscript ID: metals-405742
Title: Effect of negative current on the microstructure of oxide coatings prepared by hybrid pulse anodization
Journal: Metals
Dear reviewer:
Firstly, we would like to thank you for your positive and constructive suggestions for the manuscript. We have made careful revisions to the manuscript according to your comments and suggestions. All the changes and modifications of the manuscript have been highlighted using the "Track Changes" function in the revision.
We hope you will be satisfied with our responses and the revised manuscript. We look forward to your positive response.
Thank you again for your time and consideration.
Sincerely yours,
Shuo Huang
Response to Reviewer 3 Comments:
Point 1: It is argued that pore initiation and pore growth take place due to heat generation and accelerated chemical dissolution at the ground of the pores. The argumentation is mainly based on the theories of Keller, Hunter and Mason, which were published in the 1950s. Therefore, these original references should be cited directly. According to the calculation of Hunter and Fowle, boiling sulfuric acid with a concentration of 51 vol% would be required to compensate coating growth at a current density of 1,29 A/dm² at the pore bottom. In other anodizing electrolytes, e.g. oxalic acid, anodic alumina exhibit even lower chemical dissolution rates. Therefore, many authors assess the supposed mechanisms to be unrealistic. Nagayama and Tamura considered Joule heating and heating due to the exothermic oxidation reaction and calculated a maximum temperature increase of 0.06 K at the pore ground. Hoar and Mott already proposed in 1959 that field assisted dissolution might be responsible for the pore growth rather than chemical dissolution. Moreover, Garcia-Vergara et al. analyzed the distribution of slow migrating anions within anodic oxides and performed tracer studies and finally proved that plastic flow of the oxide takes place at the pore bottom under high compressive stresses and that chemical and field assisted dissolution might be negligible. According to Jessensky, self-ordering of pores is determined by the volume expansion during oxidation and the extent of compressive stresses. The theory of field-assisted dissolution and especially the theories of plastic flow at the pore ground and stress-controlled self-ordering should be considered when discussing the results.
Response 1: Thank you for your comments and kind reminding. The original references about the theories have been added into the revised manuscript.
Point 2: It is stated that a negative pulse immediately after the positive pulse helps to eliminate the anodic current flow. This should be proofed by recording the current pulses during anodizing. Furthermore, a negative current flow could be quantified by this method. According to the line of argumentation, an inverted current flow through the barrier layer might also cause Joule heating and therefore impair the self-ordering. Apart from this, different interpretations might be possible, if the authors consider the theory of field-assisted dissolution (no ejection of Al3+-Ions at negative potentials) and the theory of plastic flow at the pore ground.
Response 2: Thank you for your advice. I agree with your opinions, and further study on these will be carried out in the future work.
Point 3: In general, the results are clearly presented by using adequate figures. However, in figure 4 and especially in figure 3, the scale, the magnification and the parameters of SEM imaging are hardly recognizable. This can be prevented by adding a bigger scale in the figures, giving important information, e.g. acceleration voltage, working distance and detector type, in the experimental section and omitting unimportant information, e.g. the date.
Response 3: Thank you for your advice. It has been improved in the revision according to your comments.
Point 4: The parameters of anodizing with pulsed current might be clearer to the reader, if figure 6 is already presented within the experimental section. Instead of giving the pulse frequency, the pulse lengths (ton/toff, t+/t-) should be given explicitly.
Response 4: Thank you for your kind reminding. Some modifications have been made in the experimental section according to your advice.
Point 5: What do you mean with “negative voltages were recorded manually” (line 75)? Did you use a transient recorder for recording? If so, what type did you use? Did you check, whether positive and negative pulses were generated in good agreement with the schematic illustration in figure 6?
Response 5: In the process of HPA, the power supply I used was composed of two pulse power sources, which worked independently to generate positive and negative currents respectively and were controlled by a controller. The real-time positive and negative voltages could be directly read and recorded from the display. Therefore, transient recorder was not used.
Point 6: Please add the name of the manufacturer of the thickness gauge and the SEM and the provider of “Image-Pro Plus” (lines 74, 77 and 78). All applied resources should be clearly identified.
Response 6: Thank you for your reminder. The information needed has been added into the revision.
Point 7: Some minor linguistic issues should be considered:
- “duty cycle” instead of “duty circle” in line 53
- “substrate materials” instead of “starting materials” in line 64
- Check the position of the °-Symbol in line 65.
Response 7: Thank you for your corrections. These issues have been modified in the revision.
Round 2
Reviewer 1 Report
the paper is ok
Author Response
Manuscript ID: metals-405742
Title: Effect of negative current on the microstructure of oxide coatings prepared by hybrid pulse anodization
Journal: Metals
Dear reviewer:
We appreciate your valuable suggestions and positive comments on our manuscript. It has been greatly improved under your guidance, and we are very happy to finally get your recognition.
Thank you again for your time and consideration.
Sincerely yours,
Shuo Huang
Reviewer 2 Report
The corrections significantly improved readability of this paper.
Author Response
Manuscript ID: metals-405742
Title: Effect of negative current on the microstructure of oxide coatings prepared by hybrid pulse anodization
Journal: Metals
Dear reviewer:
We appreciate your valuable suggestions and positive comments on our manuscript. It has been greatly improved under your guidance, and we are very happy to finally get your recognition.
Thank you again for your time and consideration.
Sincerely yours,
Shuo Huang
