Effect of Cationic/Anionic Diffusion Dominated Passive Film Growth on Tribocorrosion
Round 1
Reviewer 1 Report
The study is original and can be published. However, two minor corrections should be made before publication.
These are as follows:
On page 4,at line 124 reference should be given after Mott-Schottky analysis.
On page 18 at line 415 (electric filed) must be corrected as (electric field)
The end of the references doi number should be added( according to spelling rules)
Author Response
The study is original and can be published. However, two minor corrections should be made before publication.
These are as follows:
On page 4, at line 124 reference should be given after Mott-Schottky analysis.
Response：
The reference has been added.
On page 18 at line 415 (electric filed) must be corrected as (electric field)
Response：
This wrong word has been corrected.
The end of the references doi number should be added( according to spelling rules)
Response：
The reference DOI numbers have been added.
Reviewer 2 Report
The manuscript presents the evaluation of the tribocorrosion behavior of Ni and Nb in Na2SO4 aqueous solution. The topic carried out in the work is novel and important for further designing corrosion-resistance alloys. The manuscript is written very clearly, and the introduction provides sufficient background. The results are very interesting and in my opinion, the manuscript should be published in Metals after some minor revision:
- please expand the meaning of abbreviations that you use for the first time (even for chemical formulas), e.g. Ni, Nb, Na2SO4 at the beginning of the abstract etc.
- please explain in the text the difference in the initial contact stress used for Nb and Ni (2.2., line 108).
Author Response
The manuscript presents the evaluation of the tribocorrosion behavior of Ni and Nb in Na2SO4 aqueous solution. The topic carried out in the work is novel and important for further designing corrosion-resistance alloys. The manuscript is written very clearly, and the introduction provides sufficient background. The results are very interesting and in my opinion, the manuscript should be published in Metals after some minor revision:
please expand the meaning of abbreviations that you use for the first time (even for chemical formulas), e.g. Ni, Nb, Na2SO4 at the beginning of the abstract etc.
Response：
The abbreviations had been explained when they first appeared.
please explain in the text the difference in the initial contact stress used for Nb and Ni (2.2., line 108).
Response：
The Nb and Ni have the different elastic modulus and poisson's ratios, thus, the initial contact stress was different. This explanation has been added into the manuscript.
Reviewer 3 Report
- The aim of this work is poorly formulated (lines 66-76). It should be more clearly emphasized (in introduction) what new results the authors expect to receive in comparison with already well-known data.
- There are two absolutely identical paragraphs in the text of manuscript (lines 180-188 and 192-198).
- It is not clear whether and how the authors took into account the different chemical composition of oxide films on Ni (from NiO to Ni2O3) and Nb (NbO, Nb2O3, NbO2 and Nb2O5), their thicknesses, mechanical and chemical properties (e.g., adhesion peculiarity with metal and between different oxide layers, hardness, roughness, Pilling-Bedworth ratios etc.), as well as changes in their compositions under electrochemical conditions.
The changes in chemical state of oxide films on Ni and Nb in PDTT’s (lines 146-148) are of particular interest to explain the results presented in Fig. 2 and to prove conclusions (lines 162-169). Data on the electrochemical generation and growth of Ni and Nb oxides in Ringer’s solutions studied in detail earlier should be quoted.
- The data in the Table 1 are given with unreasonably excessive accuracy (lines 152-155). For instance, the value for Ecorr -0.226±0.32 is absolutely incorrect. The deviation of 0.32 cannot be greater than the average value of 0.226. It is nonsense. The value of -0.292±0.25 (the deviation is almost 90%!!) is also incorrect. Unusually, large deviations mean that the values are poorly defined. But this is not the case with respect to Ecorr. This indicates that the presented results are questionable or even unreliable due to non-reproducibility which is the main requirement for the experimental data to be published.
In my opinion, all the values in Table 1 (preferably without a number) should also be improved. The data should be presented as 14.7±0.2, 7.2±0.2 and so on. It applies to all the numbers in the manuscript as, e.g., in line 85.
If one makes a mistake already in the first significant digit after the decimal point, then the second digit doesn’t make sense at all. It can be any from 0 to 9. So, all the values in Table 1 should be checked and brought into line with common sense.
- The states of the surface of Ni and Nb in Ringer’s solution after electrochemical oxidation are also directly related with changes in the COF’s (lines 170-188). Hydrogen ions appearing during water oxidation react, in turn, with any of oxides Ni and Nb except Nb2O5 and destroy their structures. Oxidation of water, as is known, occurs in a double electric layer and not at the barrier layer-solution interfase (“reaction” 4 in Fig. 15). In this sense, the origin of “reaction” 4 is not clear.
The explanations given by the authors for COF changes are extremely speculative and unreasonable (lines 170-230). The results would be more informative if CVA measurements were performed.
- It is difficult to understand what a tribofilm is, except that it contains some oxygen atoms. Designations in Figs.12, 15 and “reactions” 1-5 are chemically meaning
- 12 isn’t reaction scheme. It would be better to call it a conditional scheme. What is x in MOx/2 to denote NiO and Nb2O5? May be x is electron quantity in 1-3 and 5? What is “reaction” 4? It is absolutely unclear how this “reaction” (decomposition of water) occurs. What k1, k2 and so on are presented for if they are not used?
Author Response
The aim of this work is poorly formulated (lines 66-76). It should be more clearly emphasized (in introduction) what new results the authors expect to receive in comparison with already well-known data.
Response：
This part has been revised according to your comment.
There are two absolutely identical paragraphs in the text of manuscript (lines 180-188 and 192-198).
Response：
The repeated paragraph has been deleted.
It is not clear whether and how the authors took into account the different chemical composition of oxide films on Ni (from NiO to Ni2O3) and Nb (NbO, Nb2O3, NbO2 and Nb2O5), their thicknesses, mechanical and chemical properties (e.g., adhesion peculiarity with metal and between different oxide layers, hardness, roughness, Pilling-Bedworth ratios etc.), as well as changes in their compositions under electrochemical conditions.
Response：
First, we must admit that the properties and performances of oxide films formed on the two metals are different and this may influence the tribocorrosion behavior. Nevertheless, these two metals showed similar tribocorrosion behavior in most cases (at low potential). For example, the two oxide films could be formed during tribocorrosion to maintain the passivation of the metals. However, when the potential was increased to some value, the early breakdown of passivation occurred on Nb. At this time, the mentioned properties and performances could not be obviously changed. Thus, we deduced the passive film growth mechanism was the key factor. Secondly, we want to investigate the influence of passive film growth mechanism on tribocorrosion behavior. Different metals must be used and the difference in property and performance of oxide films is inevitable.
The changes in chemical state of oxide films on Ni and Nb in PDTT’s (lines 146-148) are of particular interest to explain the results presented in Fig. 2 and to prove conclusions (lines 162-169). Data on the electrochemical generation and growth of Ni and Nb oxides in Ringer’s solutions studied in detail earlier should be quoted.
Response：
Thank you for your suggestion. The relation between the chemical state of oxide films and PDTT results has been mentioned in this paragraph. But the detail discussion needs subsequent experimental results (such as the Mott–Schottky analysis) to improve, thus, it was put into section 3.4.
The data in the Table 1 are given with unreasonably excessive accuracy (lines 152-155). For instance, the value for Ecorr -0.226±0.32 is absolutely incorrect. The deviation of 0.32 cannot be greater than the average value of 0.226. It is nonsense. The value of -0.292±0.25 (the deviation is almost 90%!!) is also incorrect. Unusually, large deviations mean that the values are poorly defined. But this is not the case with respect to Ecorr. This indicates that the presented results are questionable or even unreliable due to non-reproducibility which is the main requirement for the experimental data to be published.
In my opinion, all the values in Table 1 (preferably without a number) should also be improved. The data should be presented as 14.7±0.2, 7.2±0.2 and so on. It applies to all the numbers in the manuscript as, e.g., in line 85.
Response：
Sorry for the mistakes in the deviation of Ecorr. The correct values of Ecorr should be -0.226±0.032, -0.292±0.025, -0.367±0.025 and -0.666±0.022, respectively. The large deviation in Eb on Nb under condition with wear was due to the randomness of early breakdown of passivation.
If one makes a mistake already in the first significant digit after the decimal point, then the second digit doesn’t make sense at all. It can be any from 0 to 9. So, all the values in Table 1 should be checked and brought into line with common sense.
Response：
Thank you for your suggestion, the Table 1 has been carefully checked.
The states of the surface of Ni and Nb in Ringer’s solution after electrochemical oxidation are also directly related with changes in the COF’s (lines 170-188). Hydrogen ions appearing during water oxidation react, in turn, with any of oxides Ni and Nb except Nb2O5 and destroy their structures. Oxidation of water, as is known, occurs in a double electric layer and not at the barrier layer-solution interfase (“reaction” 4 in Fig. 15). In this sense, the origin of “reaction” 4 is not clear.
Response：
When the barrier layer (oxide film) has well adhesion with the substrate, the double electric layer will appear on the film/solution interface. In this study, the oxide film was grown from the metal surface, thus the reaction would occur on the barrier layer-solution interface.
The explanations given by the authors for COF changes are extremely speculative and unreasonable (lines 170-230). The results would be more informative if CVA measurements were performed.
Response：
In the experimental stage, we have tried to use CVA to investigate the influence of potential on COF. We found that when the potential was over the breakdown potential, the surface would be seriously worn. The worn surface was very rough that influenced the subsequent reverse scanning. The lubricating effect of oxide passive film has been reported in many researches, such as Wear 2015, 338-339:1-10 and Wear 2007, 263:192-500. In the PDTT, the COF in the passive region was lower than that in the cathodic region, this indicated the formation of passive film also has lubricating effect in this study.
It is difficult to understand what a tribofilm is, except that it contains some oxygen atoms.
Response：
Tribo-film is formed under the combination of corrosion and mechanical force. It has been found in many fields, such as artificial joints and engine. Normally, it is mechanically mixed with corrosion products (oxide, hydroxide) and wear debris (oxide, hydroxide, metallic particle), thus it contains some oxygen atoms.
Designations in Figs.12, 15 and “reactions” 1-5 are chemically meaning
12 isn’t reaction scheme. It would be better to call it a conditional scheme. What is x in MOx/2 to denote NiO and Nb2O5? May be x is electron quantity in 1-3 and 5? What is “reaction” 4? It is absolutely unclear how this “reaction” (decomposition of water) occurs. What k1, k2 and so on are presented for if they are not used?
Response：
“Reactions” may be not proper in Figs. 12 and 15. They are physicochemical processes. Related contents have been revised.
The x in MOx/2 denote the actual coordination number. In the oxide film, the metal atoms are not completely oxidized during the growing process. Thus, MOx/2 does not donate NiO or Nb2O5, but the intermediate oxide products.
Only the occurrence of reaction 4 can annihilate oxygen vacancy (Schottky defects). (J. Electrochem. Soc. 1992, 139, 3434-3449).
k1 to k5 are the rate constants for each reaction and not used in this this study. After consideration, they were removed in the revised manuscript.
