Corrosion Resistance of AlxCoCrFeNiM (M = Ti, V, Si, Mn, Cu) High Entropy Alloys in NaCl and H2SO4 Solutions
, Ahmad Ostovari Moghaddam 2 and Evgeny Trofimov 2Round 1
Reviewer 1 Report
The current study shows something interesting. However, there are some critical issues should be clearly clarified.
- In this study, the authors compare the corrosion resistances for many different HEAs in solutions. Obviously, they exhibit different corrosion resistance. Do you think which is the dominent factor, composition, phases, or else?
- Al0.25CoCrFeNiV exhibited the best corrosion resistance in sulfuric acid. What is the function of the elemental V?
- Whether does the microstructure such as the grain size affect the corrosion resistance?
- We suggest that the authors summarize the effect of the function of different elements in HEAs on the corrosion resistance in different solutions.
- More relevent references should be cited on the corrosion for HEAs, e.g. Metall. Mater. Trans. A 47 (2016) 3312-3321, Journal of Materials Research 33 (2018) 3310-3320, Materials Chemistry and Physics 265 (2021) 124509, Metals 11 (2021)1561.
Author Response
Reviewer 1:
Dear Reviewer!
First of all, we would like to thank you for your attention to our work. Your valuable comments have allowed us to significantly improve the quality of the presentation of the material.
Below we would like to respond to your comments.
1) In this study, the authors compare the corrosion resistances for many different HEAs in solutions. Obviously, they exhibit different corrosion resistance. Do you think which is the dominent factor, composition, phases, or else?
Answer: The main factor can be considered the amount and composition of the phases formed in the alloys. The addition of Ti, V, Si, Mn, Cu led to the formation of a two-phase structure: a phase enriched in Cr and phase enriched in an additional element. A galvanic couple was created, leading to the dissolution of the anode section (enriched in an additional element). At the moment, we are determining the composition of samples for future studies, so that when a two-phase structure appears, both phases contain chromium in sufficient quantities.
2)Al0.25CoCrFeNiV exhibited the best corrosion resistance in sulfuric acid. What is the function of the elemental V?
Answer: As follows from Table 3, although the sample with vanadium showed the best corrosion resistance in a sulfuric acid solution among all the samples we studied, its performance in terms of corrosion current density is much worse than the data presented in the literature for high-entropy alloys without vanadium additives. But for all the samples we studied, the anodic areas with vanadium were dissolved to a lesser extent, possibly due to the formation of a sufficient protective film.
3) Whether does the microstructure such as the grain size affect the corrosion resistance?
Answer: For all samples, corrosion studies were carried out for the as-cast condition. Therefore, the influence of grain sizes was not taken into account in the course of this study. The investigation of the effect of microstructure on corrosion characteristics is part of our future plans.
4) We suggest that the authors summarize the effect of the function of different elements in HEAs on the corrosion resistance in different solutions.
Answer: All the investigated samples in this study showed inferior corrosion behavior (in terms of corrosion potential and current density) than that of AlxCoCrFeNi HEA. Finally, to increase the resistance to electrochemical corrosion, it is necessary to obtain either single-phase alloys, or samples where each of the phases will contain a sufficiently high percentage of corrosion resistant elements.
5) More relevent references should be cited on the corrosion for HEAs, e.g. Metall. Mater. Trans. A 47 (2016) 3312-3321, Journal of Materials Research 33 (2018) 3310-3320, Materials Chemistry and Physics 265 (2021) 124509, Metals 11 (2021)1561.
Answer: Corresponding changes have been made to the text of the manuscript.
All changes made to the text of the manuscript are highlighted in red marker.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This study investigated the corrosion resistance according to the Al content and the addition of Ti, V, Si, Mn, and Cu. Some revisions are suggested before publication.
1. Isn't Figure 2 a backscattered electron (BSE) image? An explanation of BSE is required in the experimental method or text related to Figure 2.
2. It will be easier to understand if Figure 2a is divided into before and after corrosion. It seems to be talking about the degree of corrosion for the components of the D area and the ID area, but it is necessary to match the figure and each table.
3. Have you ever done component mapping through EDS and EPMA to analyze the components before and after corrosion in D and ID areas? In fact, component mapping is needed to directly show that there are many specific elements in a specific area.
4. It seems that the main point of this article is the effect of Ti, V, Si, Mn, Cu on corrosion properties. Is there any other reason for giving different Al fractions?
5. As a result, all HEAs appear to be dual phase FCC+BCC. XRD analysis is essential to confirm the phase.
Author Response
Reviewer 2:
Dear Reviewer!
First of all, we would like to thank you for your attention to our work. Your valuable comments have allowed us to significantly improve the quality of the presentation of the material.
Below we would like to respond to your comments.
- Isn't Figure 2 a backscattered electron (BSE) image? An explanation of BSE is required in the experimental method or text related to Figure 2.
Answer: Photographs of the microstructure of experimental samples on a scanning electron microscope were taken in two modes. The microstructure of the as-cast samples was studied in backscattered electrons, since this mode makes it possible to obtain a contrast in atomic mass, which gives a clear picture of the phase distribution. The microstructure of the samples after corrosion tests was studied in secondary electrons, since this mode makes it possible to obtain a better image of the morphology of corroded surfaces, the picture in backscattered electrons was blurry and unreadable. The necessary clarifications are made in the caption of Figure 2.
- It will be easier to understand if Figure 2a is divided into before and after corrosion.It seems to be talking about the degree of corrosion for the components of the D area and the ID area, but it is necessary to match the figure and each table.
Answer: We have correlated Figure 2 with Tables 4–6. Table captions indicate which EDS analysis belongs to which sample. Also in Fig. 2 for each of the photos at the top in the center of the photo it is indicated whether this is as-cast sample or after corrosion tests (and in which environment). From our point of view, it is this format of presenting the results that will make it easier for the reader to make a comprehensive comparison. Correspondence of figures and results of EDS analysis is given in the captions of the tables:
Table 5. EDS analysis after corrosion test in 1М NaCl solution (at. %) measured from the different regions indicated in Fig. 2 with red squares.
Table 6. EDS analysis after corrosion test in 0.5M H2SO4 solution (at. %) measured from the different regions indicated in Fig. 2 with yellow squares.
- Have you ever done component mapping through EDS and EPMA to analyze the components before and after corrosion in D and ID areas? In fact, component mapping is needed to directly show that there are many specific elements in a specific area.
Answer: You are absolutely right. Mapping certainly perfectly visualizes the distribution of elements. And we did it. For as-cast samples, the maps turned out to be of good quality. For corroded surfaces (in ulcers, irregularities), after testing, the maps of the samples turned out to be of inadequate quality; the areas lying below the surface of the section turned out to have holes (black spots) on the maps, even where there are just small corrosion pits. It is for this reason that we did not present the mapping results. The results of EDS in Tables 4–6 quite well reflect the distribution of elements over phases.
- It seems that the main point of this article is the effect of Ti, V, Si, Mn, Cu on corrosion properties. Is there any other reason for giving different Al fractions?
Answer: We agree that the increased aluminum content in the Al0.5CoCrFeNi1.6Ti0.7 and Al0.45CoCrFeNiSi0.45 alloys could affect their corrosion properties. However, in view of the fact that these materials did not ultimately show high results in corrosion resistance, such an excess is not critical. However, in subsequent works, we plan to improve the smelting technology in order to prevent such fluctuations in the composition.
- As a result, all HEAs appear to be dual phase FCC+BCC. XRD analysis is essential to confirm the phase.
Answer: XRD spectra added to the text of the manuscript.
All changes made to the text of the manuscript are highlighted in yellow marker.
Author Response File: Author Response.pdf
Reviewer 3 Report
The paper is rather interesting but some modifications should be implemented before a final publication to increase its quality and readibility:
1) Introduction should be completed by data about corrosion behavior of other HEA alloys.
2) In the Experimental procedure section, the potentiodynamic polarization technique is poorly detailed. For instance the followed articles should be consulted: https://doi.org/10.1016/j.matdes.2021.110327 and https://doi.org/10.1016/j.electacta.2022.139836.
3) How values of Icorr were estimated? By which method?
4) The Rp values are missing, please complete them.
5) Authors could show changes of Estat in a function of time.
6) What is a motivation to use two different corrosion environments?
7) Please provide an error analysis for polarisation results.
8) What about EDS analysis of tested HEAs before corrosion measurements?
Author Response
Reviewer 3:
Dear Reviewer!
First of all, we would like to thank you for your attention to our work. Your valuable comments have allowed us to significantly improve the quality of the presentation of the material.
Below we would like to respond to your comments.
1) Introduction should be completed by data about corrosion behavior of other HEA alloys.
Answer: Corresponding corrections were made to the text of the manuscript. Tables 2 and 3 also show literature data on the electrochemical behavior of HEAs in the same solutions for comparison of our data and literature data.
2) In the Experimental procedure section, the potentiodynamic polarization technique is poorly detailed. For instance the followed articles should be consulted: https://doi.org/10.1016/j.matdes.2021.110327 and https://doi.org/10.1016/j.electacta.2022.139836.
Answer: Corresponding corrections were made to the text of the manuscript:
“Polarization measurements were carried out in a standard three-electrode YaSE-2 electrochemical cell with a platinum auxiliary electrode using a P-30J potentiostat device. The potentials were measured relative to a saturated silver chloride reference electrode EVL-1M3 at room temperature (25 °C) with a sweep rate of 5 mV/s. Thus, alloys were used as working electrodes, a silver chloride electrode served as a reference electrode, a platinum electrode was an auxiliary electrode. The corrosion tests were carried out in 1M NaCl and 0.5M H2SO4 solutions. Sulfuric acid was used as the acidic electrolyte because it provides high rates for anodic dissolution processes, as well as it inhibits passivation processes on the sample surface and avoids the formation of poorly soluble compounds in the volume of the electrolyte. Before electrochemical tests, the surface of electrodes was mechanically polished with emery paper, degreased with isopropyl alcohol, and then washed with distilled water.
The open circuit potential EOCP was measured for 3600–4400 s in a cell filled with electrolyte solutions without applying current. This time was enough to stabilize each of the working electrodes. The potentials of the samples shifted to a more negative region over time, which indicates the activation of the electrode surface. The obtained EOCP of the samples are presented in Table 1.
When polarization curves were taken with applying current, then the interval for investigation was set on the potentiostat from (EOCP – 1) V to (EOCP + 1) V. Corrosion parameters such as corrosion potential (Ecor), current density (Icor) were determined by the Tafel extrapolation method using both the cathode and anodic branches of the polarization curves. A silver chloride electrode was used as the reference electrode, and then the potentials were recalculated to the scale of a normal hydrogen electrode.
The polarization resistance (Rp) was calculated using the formula:
Please see formula in the attachment file,
where βа and βс are the slope coefficients of the anode and cathode straight lines of the Tafel equation, respectively, obtained by extrapolation.”
3) How values of Icorr were estimated? By which method?
Answer: Corrosion parameters such as corrosion potential (Ecor), current density (Icor) were determined by the Tafel extrapolation method using both the cathode and anodic branches of the polarization curves.
4) The Rp values are missing, please complete them.
Answer: Corresponding corrections were made to the text of the manuscript.
5) Authors could show changes of Estat in a function of time.
Answer: 60–75 minutes was enough to stabilize each of the working electrodes.
Please see figures in the attachment file.
However, we decided not to include these figures directly in the text of the manuscript, since they do not carry additional information and may overload the text.
6) What is a motivation to use two different corrosion environments?
Answer: Obtaining corrosion-resistant alloys for the operating conditions in sea water is a very important problem in materials science. It is believed that the performance of corrosion resistance in an aqueous solution of NaCl correlates with the resistance of alloys in sea water.
An equally important task is to extend the service life of oil and gas pipelines. To do this, the corrosion behavior of alloys in H2SO4 solution is studied, since the gaseous phase in contact with pipes contains not only sulfur dioxide and H2S, but also water vapor.
We decided to study the behavior of the investigated HEAs in two aggressive environments at once, in order to more fully describe the characteristics of the obtained alloys and not to divide the results of the study into many small reports.
In the future, it is planned to study the corrosion resistance of HEAs in a mixed solution (NaCl/H2SO4). These studies are already being carried out by a number of teams, since at the moment oil production from sea shelves is very relevant, where pipes are exposed to two aggressive environments at the same time.
7) Please provide an error analysis for polarisation results.
Answer: Corresponding corrections were made to the text of the manuscript.
8) What about EDS analysis of tested HEAs before corrosion measurements?
Answer: These measurement data are given in Table 4:
Table 4. EDS chemical composition of as-cast samples prior to corrosion tests (at. %).
All changes made to the text of the manuscript are highlighted with a green marker.
Author Response File: Author Response.pdf
Reviewer 4 Report
The present paper has well written, and having potential contribution. However, the authors have to address the following points before being considered for publications.
- The reasons for improved corrosion resistance in Al0.45CoCrFeNiSi0.45 and Al0.25CoCrFeNiV HEAs are to be incorporated in the abstract.
- The sample manufacturing route and used characterization techniques are also to be addressed in the abstract section
- The chemical composition presented in Table 1 is not clear. I think that the authors have used non-equimolar ratio. However, it is not clear. It has to be checked.
- Based on Figure 1, the authors have to use different contrast colours (Dark one) to differentiate the samples. Present one, some colours are repeated for different composition.
- Why Al0.5CoCrFeNi1.6Ti0.7 alloy produced passive aggressive environment? Need more explanation
Author Response
Reviewer 4:
Dear Reviewer!
First of all, we would like to thank you for your attention to our work. Your valuable comments have allowed us to significantly improve the quality of the presentation of the material.
Below we would like to respond to your comments.
1) The reasons for improved corrosion resistance in Al0.45CoCrFeNiSi0.45 and Al0.25CoCrFeNiV HEAs are to be incorporated in the abstract.
Answer: Corresponding corrections were made to the text of the manuscript.
2) The sample manufacturing route and used characterization techniques are also to be addressed in the abstract section
Answer: Corresponding corrections were made to the text of the manuscript.
According to the questions 1–2:
“Abstract: The electrochemical behavior of as-cast AlxCoCrFeNiM (M = Ti, V, Si, Mn, Cu) high entropy alloys (HEAs) in 1M NaCl and 0.5M H2SO4 solutions is studied. Polarization measurements were carried out in a standard three-electrode electrochemical cell with a platinum auxiliary electrode using a P-30J potentiostat device. The potentials were measured relative to a saturated silver chloride reference electrode EVL-1M3 at room temperature (25 °C) with a sweep rate of 5 mV/s. It is shown that despite a wide passivation region, Al0.5CoCrFeNi1.6Ti0.7 HEA undergoes significant corrosion in both sodium chloride and sulfuric acid solutions and exhibits low corrosion potential and current density. EDS analysis revealed that Ti-containing eutectic areas are the most susceptible regions to corrosion. Intergranular corrosion was found in Al0.25CoCrFeNiMn and Al0.25CoCrFeNiCu HEAs. Moreover, Al0.25CoCrFeNiCu possesses the smallest passivation interval ΔE among all the investigated HEAs. For samples with Ti, Mn, and Cu, a protective film layer is not formed on the surface of the phases enriched in these elements, or it is brittle and crumbles. For samples with Si and V, a passivating film is formed. Thus, Al0.45CoCrFeNiSi0.45 and Al0.25CoCrFeNiV HEAs exhibited the highest resistance in 1M NaCl and 0.5M H2SO4 corrosive environments, respectively”.
3) The chemical composition presented in Table 1 is not clear. I think that the authors have used non-equimolar ratio. However, it is not clear. It has to be checked.
Answer: The average composition (in at. %) is added to the Table 4.
To date, research in the field of high-entropy alloys has gone far beyond the limits of equimolar compositions. In the literature, you can see studies of alloys Co1.5CrFeNi1.5Ti0.5Mo0.1, Al0.9CoCrFeNiTi0.5 etc.
4) Based on Figure 1, the authors have to use different contrast colours (Dark one) to differentiate the samples. Present one, some colours are repeated for different composition.
Answer: Figure 1 corrected.
5) Why Al0.5CoCrFeNi1.6Ti0.7 alloy produced passive aggressive environment? Need more explanation.
Answer: According to the Pourbaix diagram, titanium is in a passive state in a wide range of potentials, this is due to the formation of amorphous oxide TiO2. In addition to TiO2, the oxide layer also contains other oxides, Ti2O3 and TiO. In an acidic environment, titanium can be oxidized to the Ti3+ ion, however, in the presence of oxygen, further oxidation to the Ti4+ ion is possible, which in turn is hydrolyzed to TiO2. This contributes to the spontaneous cessation of local corrosion of titanium.
All changes made to the text of the manuscript are highlighted in blue marker.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
It has been revised accordingly. The pictures can be further polished.
Author Response
Dear Reviewer!
First of all, we would like to thank you for your attention to our work. Your valuable comments have allowed us to significantly improve the quality of the presentation of the material.
Below we would like to respond to your comments.
Reviewer 1
It has been revised accordingly. The pictures can be further polished.
Answer:
The quality of the pictures has been improved.
Reviewer 3 Report
The paper is quite good revised, but there is one concern: why log I (A/cm2) has negative values? See Fig. 1 - polarisation curves.
Author Response
Dear Reviewer!
First of all, we would like to thank you for your attention to our work. Your valuable comments have allowed us to significantly improve the quality of the presentation of the material.
Below we would like to respond to your comments.
Reviewer 3:
The paper is quite good revised, but there is one concern: why log I (A/cm2) has negative values? See Fig. 1 - polarisation curves.
Answer:
For most steels and alloys, the current density values are quite small, on the order of 0.0001-0.00000001 A/cm2. Therefore, it is customary to translate them into a logarithmic scale, respectively, and graphs are built from –8 to zero (as variant). For example, see articles:
1) Sun, Y.; Lan, A.; Zhang, M.; Yang, H.; Qiao, J. Influence of lanthanum on passivity behavior of CrMnFeNi high entropy alloys. Mater. Chem. Phys. 2021, 265, 124509. DOI: 10.1016/j.matchemphys.2021.124509
2) Hu, W.; Zhu, H.; Hu, J.; Li, B.; Qiu, C. Influence of vanadium microalloying on microstructure and property of laser-cladded martensitic stainless steel coating. Materials 2020, 13, 826. DOI: 10.3390/ma13040826
3) Kao, Y.F.; Lee, T.D.; Chen, S.K.; Chang, Y.S. Electrochemical passive properties of AlxCoCrFeNi (x = 0, 0.25, 0.50, 1.00) alloys in sulfuric acids. Corr. Sci. 2010, 52, 1026–1034. DOI: 10.1016/j.corsci.2009.11.028
Reviewer 4 Report
The authors have worked based on my previous comments. Hence, I am recommending to accept the revised article.
Author Response
Reviewer 4:
The authors have worked based on my previous comments. Hence, I am recommending to accept the revised article.
Answer:
Thank you for your attention to our work!
