High-Temperature Corrosion Behavior of Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂ Coating Prepared by rf Magnetron Sputtering

Round 1

Reviewer 1 Report

High-temperature corrosion of Bi3.75La0.25Ti3O12 y Bi3La1Ti3O12 prepared by rf magnetron sputtering

In the study, Bi3.75La0.25Ti3O12 and Bi3La1Ti3O12 coatings were fabricated and their property were measured. Especially, high-temperature corrosion behavior was measured by electrochemical impedance spectroscopy measurements and potentiodynamic curves, which could be regarded as the main characteristics. This study achieves good originality but is insufficient for acceptance in as-received form. I think that the paper should be improved in these aspects:

1. The title of this study should be changed, for example, “High-temperature corrosion behavior of Bi3.75La0.25Ti3O12 and Bi3La1Ti3O12 coating prepared by rf magnetron sputtering”, or “Comparison of high-temperature corrosion behavior of Bi3.75La0.25Ti3O12 and Bi3La1Ti3O12 coating prepared by rf magnetron sputtering”, or ….

2. In Abstract part, the obtained experimental results are not present and they must be provided.

3. In this study, rf magnetron sputtering was used to fabricate coatings. The advantages and application areas of this surface treatment technology should be provided in Introduction part. In addition, high-temperature corrosion behavior was measured by electrochemical impedance spectroscopy measurements and potentiodynamic curves, which is rarely reported. The authors should introduce the advantages and recent results reported by the authors or other researchers. Therefore, the part could be revised as followings: “Potentiodynamic curves and electrochemical impedance spectroscopy measurements are widely used to evaluate the corrosion resistance or reveal the corrosion mechanism at room temperature (for example, (1)  Corrosion resistance and biocompatibility of calcium-containing coatings developed in near-neutral solutions containing phytic acid and phosphoric acid on AZ31B alloy, Journal of Alloys and Compounds, 2020, 823, 153721; (2) Self-healing epoxy coating based on tung oil-containing microcapsules for corrosion protection, Prog. Org. Coat. 158 (2021) 106236.). However, electrochemical measurements are rarely used at high temperatures (giving corresponding papers). In this study, potentiodynamic curves and electrochemical impedance spectroscopy measurements are applied to measure high-temperature corrosion behavior of ……”.

4. Surface morphology, chemical compositions and cross-sectional morphology of Bi3.75La0.25Ti3O12 and Bi3La1Ti3O12 coating should be provided.

5. According to Figure 2 and Figure 3, the authors should provide the results (for example, corrosion potential, corrosion current density) obtained by potentiodynamic curves in order to compare the high corrosion behavior of Bi3.75La0.25Ti3O12 and Bi3La1Ti3O12 coating.

6. In Conclusions part, at 700°C, 850°C, and 1000°C, which coating, Bi3.75La0.25Ti3O12 or Bi3La1Ti3O12 exhibits better stability and a better life at relatively high temperatures should be provided.

Author Response

Questions Answer

Reviewer 1.

In the study, Bi3.75La0.25Ti3O12 and Bi3La1Ti3O12 coatings were fabricated and their property were measured. Especially, high-temperature corrosion behavior was measured by electrochemical impedance spectroscopy measurements and potentiodynamic curves, which could be regarded as the main characteristics. This study achieves good originality but is insufficient for acceptance in as-received form. I think that the paper should be improved in these aspects:

1. The title of this study should be changed, for example, “High-temperature corrosion behavior of Bi3.75La0.25Ti3O12 and Bi3La1Ti3O12 coating prepared by rf magnetron sputtering”, or “Comparison of high-temperature corrosion behavior of Bi3.75La0.25Ti3O12 and Bi3La1Ti3O12 coating prepared by rf magnetron sputtering”, or ….

Answer: Suggestion accepted. The manuscript was reviewed and corrected. The new version is:

High-temperature corrosion behavior of Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂ coating prepared by rf magnetron sputtering

2. In Abstract part, the obtained experimental results are not present and they must be provided.

Answer: Suggestion accepted. The manuscript was reviewed and corrected. The new version is:

Using the rf magnetron sputtering technique, Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂. Coatings were formed and obtained as a thin film on Hastelloy substrates. When subjected to high-temperature conditions, the effect of lanthanum on the anti-corrosive properties of the coatings was investigated. The anti-corrosive response was evaluated by electrochemical impedance spectroscopy and potentiodynamic curves, which are rarely reported. Hot corrosion occurs through the electrochemical mechanism, and more information can be obtained through electrochemical corrosion tests, which are very effective and accelerated. The electrochemical behavior at high temperatures was studied by molten salt corrosion test, potentiodynamic polarization curves, and electrochemical impedance spectroscopy. Additionally, the coatings were evaluated by scanning electron microscopy and transmission microscopy to determine their morphology. With X-ray diffraction, the crystallinity of the films was determined. It was determined that the corrosion rate directly correlates with the temperature because the mechanisms induced by the Na₂SO₄ and V₂O₅ salts generated condensation. As the temperature increases, the density of the corrosion current increases in the thin films of Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂. When comparing the two compounds, it is determined that the increase in lanthanum alters the positive acid character, thus reducing the dissolution of the oxides and increasing protection.

3. In this study, rf magnetron sputtering was used to fabricate coatings. The advantages and application areas of this surface treatment technology should be provided in Introduction part. In addition, high-temperature corrosion behavior was measured by electrochemical impedance spectroscopy measurements and potentiodynamic curves, which is rarely reported. The authors should introduce the advantages and recent results reported by the authors or other researchers. Therefore, the part could be revised as followings: “Potentiodynamic curves and electrochemical impedance spectroscopy measurements are widely used to evaluate the corrosion resistance or reveal the corrosion mechanism at room temperature (for example, (1)  Corrosion resistance and biocompatibility of calcium-containing coatings developed in near-neutral solutions containing phytic acid and phosphoric acid on AZ31B alloy, Journal of Alloys and Compounds, 2020, 823, 153721; (2) Self-healing epoxy coating based on tung oil-containing microcapsules for corrosion protection, Prog. Org. Coat. 158 (2021) 106236.). However, electrochemical measurements are rarely used at high temperatures (giving corresponding papers). In this study, potentiodynamic curves and electrochemical impedance spectroscopy measurements are applied to measure high-temperature corrosion behavior of ……”.

Answer: Suggestion accepted. The manuscript was reviewed and corrected. The new version is:

In this study, rf magnetron sputtering was used to fabricate coatings. The advantages and application areas of this surface treatment technology should be provided in introduction part. In addition, high-temperature corrosion behavior was measured by electrochemical impedance spectroscopy measurements and potentiodynamic curves, which are rarely reported. Hot corrosion occurs through the electrochemical mechanism, and more information can be obtained through electrochemical corrosion testing is very effective and accelerated [16]. Electrochemical impedance spectroscopy (EIS) is a technique that has proven effective in investigating the mechanisms and reaction kinetics in hot corrosion induced by molten carbonates, electrochemical testing of the Tafel determination of the corrosion rate using a method known as Tafel extrapolation [17]. As with all electrochemical studies, the environment must be electrically conductive. Therefore, a mixture of Na₂SO₄ and V₂O₅ is required [18]. Therefore, the part could be revised as followings: Potentiodynamic curves and electrochemical impedance spectroscopy measurements are widely used to evaluate the corrosion resistance or reveal the corrosion mechanism at room temperature [19, 20]

4. Surface morphology, chemical compositions and cross-sectional morphology of Bi3.75La0.25Ti3O12 and Bi3La1Ti3O12 coating should be provided.

Answer: Suggestion accepted. The new version is:

3.4. Transmission electron microscopy

Figure 10 (a) and Figure 10 (b)b are images obtained by transmission electron microscopy of the cross-section of the Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂ films, respectively. The results confirm the growth of a columnar structure due to the intense atomic bombardment. In addition, good adhesion of the coatings to the substrate is also observed.

 

(a)	(b)

Figure 10. Micrographs were obtained by transmission electron microscopy. (a) Bi_3.75La_0.25Ti₃O₁₂ film and (b) Bi₃La₁Ti₃O₁₂ film.

3.5. Scanning electron microscopy

Figure 11 shows the surface characterization by scanning electron microscopy of the Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂ coatings as a function of the sintering temperature. In the Bi_3.75La_0.25Ti₃O₁₂ coatings, rough surfaces are evident due to the sputtering process when forming the coatings. The presence of pores of varied sizes is also evident. In the Bi₃La₁Ti₃O₁₂ coatings, the surface is more homogeneous, and the presence of pores is not evident.

 

	700 °C	850 °C	1000 °C
Bi3.75La0.25Ti3
Bi3La1Ti3O12
Substrate

Figure 11. Micrographs by scanning electron microscopy for the Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂ film.

Figure 11 shows the micrographs of the Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂ films deposited on the steel. The micrograph represents the specimen with the highest Lanthanum content, where deterioration is observed in the center and lower region. However, part of the coating was not affected, as seen in the upper part of the micrograph. For Bi_3.75La_0.25Ti₃O₁₂ thin, it can be observed that the percentage of lanthanum is at 0.25. It can be seen that the films are heterogeneous. They present some discontinuities, such as pores (dark regions) [34]. Homogeneity is evidenced in the micrographs of the films with higher lanthanum content and higher intensity values in the X-ray diffraction. Unlike the films with reduced content, the micrographs of the films deposited under the same conditions but varying the amount of lanthanum on steel substrates also showed the same Miller indices. However, they decreased intensity, allowing greater amorphism for the system with lower lanthanum. Figure 11 shows the presence of pores and partial detachment of the coating at 1000 °C, so the coatings are distinguished as compromised. It is observed that the behavior against the attack generated by the electrolyte at the time of the evaluation of the corrosion resistance will be lower for the film with lower lanthanum content. As obtained in the electrochemical tests, these systems are 10 times more susceptible to corrosion damage at high temperatures [35].

The elemental chemical analysis was developed for the two types of coatings. The analyzed areas correspond to the micrographs observed in Figure 11. The results found are recorded in Table 5.

Tabla 5. Chemical analysis for Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂ coatings.

Temperature		C	O	Na	Si	S	La	Ti	Cr	Mn	Fe	Ni	V	Bi
700	Substrate (%)	1	16	4	3	6	0	0	14	3	39	4	10	0
	Bi3.75La0.25Ti3O12 (%)	1	15	1	2	1	6	18	7	1	10	5	8	25
	Bi3La1Ti3O12 (%)	2	13	1	2	1	9	19	8	1	9	5	7	23
850	Substrate (%)	1	22	4	1	2	0	0	4	1	49	5	16	0
	Bi3.75La0.25Ti3O12 (%)	3	19	14	2	4	3	14	2	0	4	5	21	12
	Bi3La1Ti3O12 (%)	4	18	12	1	5	4	16	1	0	7	1	19	12
1000	Substrate (%)	1	18	2	1	2	0	0	2	1	46	2	25	0
	Bi3.75La0.25Ti3O12 (%)	1	22	16	1	2	2	12	0	0	2	1	29	12
	Bi3La1Ti3O12 (%)	1	25	19	2	1	1	11	0	0	2	1	22	15

In the chemical analysis, elements such as Ti, Bi, and O are evident, which correspond to the films. In addition, elements such as the substrate's Cr, Fe, and Ni components are also manifested.

 

5. According to Figure 2 and Figure 3, the authors should provide the results (for example, corrosion potential, corrosion current density) obtained by potentiodynamic curves in order to compare the high corrosion behavior of Bi3.75La0.25Ti3O12 and Bi3La1Ti3O12 coating.

Answer: Suggestion accepted. The manuscript was reviewed and corrected. The new version is:

3.1. High-temperature corrosion

Figure 2 shows the potentiodynamic polarization curves for the substrate as a function of temperatures of 700 °C, 850 °C, and 1000 °C, respectively. Again, the variation of the corrosion potentials is observed. The corrosion potential generally tends to take more positive values at 850 °C. The values of corrosion potential, corrosion current, and corrosion rate for the substrate, according to the heat treatment temperatures, are consolidated in Table 1.

 

Figure 2. Potentiodynamic curves correspond for the substrate evaluated at 700 °C, 850 °C, and 1000 °C.

Table 1. The corrosion potential, current density, and corrosion rate were obtained for the steel substrate at 700°C, 850°, and 1000°C

Temperature (°C)	Error (V vs. Pt)	Icorr (mA/cm2)	Corrosion rate (mm/year)
700	-1.47	8.18x10-6	94.28
850	-1.44	16.95x10-6	195.36
1000	-1.49	77.44x10-6	892.55

 

The evaluated coatings at high temperature respond by employing two tests; the first one to determine the rate of deterioration and the corrosion kinetics is the potentiodynamic polarization technique (Figure 3 and Figure 4). The evaluation of the oxidation of the coatings in molten salts simulates severe conditions in atmospheres with the presence of contaminants and materials used in pipes that must withstand high temperatures in combustion processes (580 °C) [22]. Hence, a coated film aims to protect the base material, the same steel previously used for this purpose. Therefore, the degree of protection of the metal-coating interface is determined. In Figure 2 and Figure 3, the corrosion rate was determined to have a direct relationship with the temperature because the mechanisms induced by the Na₂ SO₄ and V₂O₅ salts have generated condensation, and by increasing the temperature, the corrosion current density increases in the thin films of Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂. By comparing the two compounds, it is determined that the increase of La alters the positive acid character, thus decreasing the dissolution of the oxides and increasing the protection [23]. Therefore, having the same Titanium content, the dominant element is La, so increasing the content increases its protection, as observed when comparing the potentiodynamic curves. This curve indicates that the electrochemical action involving combustion gases and corrosive salts depends on the compounds formed at the test temperatures. The corrosion rates in the so-called high-temperature elements are engineered in systems with temperatures above 600 °C; the base steel performs adequately under this temperature. However, at temperatures above 700 °C, layers of molten solids are formed, causing an increase in the corrosion rate and increasing the deterioration of the substrate. The coating protects the substrate from accelerated corrosion processes [24].

Figure 3 and Figure 4, corresponding to the composites generated and applied as a thin film, determined that the damage is minimal for a temperature up to 700 °C and that, at a range increase of 150 °C, the damage is adequate. At 1000 °C, damages are catastrophic due to increased corrosion current density. In all the evaluated cases, it was determined that a general dissolution exists in the anodic region. Therefore there was no evidence of pitting corrosion, typical of steel, which indicates an adequate barrier. When a general dissolution is generated, this is associated with the thin film in contact with the molten salt composed of vanadium, sodium, and sulfur. Therefore, the more ashes of these compounds are generated by the evaluation temperature, the more degradation is determined, so it is related to condensed corrosion causing phase at high temperatures [25].

Depending on the treatment temperatures, the corrosion potential, current, and corrosion rate values for Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂ are consolidated in Tables 2 and 3, respectively.

 

Figure 3. Potentiodynamic curves corresponding to Bi_3.75La_0.25Ti₃O₁₂, which has low lanthanum, were evaluated at three different temperatures.

Table 2. The corrosion potential, current density, and corrosion rate obtained for Bi_3.75La_0.25Ti₃O₁₂ at 700°C, 850°, and 1000°C

Temperature (°C)	Error (V vs. Pt)	Icorr (mA/cm2)	Corrosion rate (mm/year)
700	-0.09	1.01x10-6	1.12
850	-0.10	0.68x10-6	7.82
1000	-0.11	16.10x10-6	185.55

 

Figure 4. Potentiodynamic curves of compound Bi₃La₁Ti₃O₁₂ composite.

Table 3. The corrosion potential, current density, and corrosion rate obtained for Bi₃La₁Ti₃O₁₂ at 700°C, 850°, and 1000°C

Temperature (°C)	Error (V vs. Pt)	Icorr (mA/cm2)	Corrosion rate (mm/year)
700	0.21	0.10x10-6	1.18
850	0.18	0.59x10-6	4.47
1000	0.16	8.14x10-6	93.83

3.2. Electrochemical impedance spectroscopy

Figure 5 and Figure 6 present the Bode diagrams, which are the response of the electrochemical impedance spectroscopy technique using a 5 mV, and a potential signal applied to a Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂ coatings, evaluated in the temperature range 700 °C to 1000 °C, in 150 °C increments, these films are undergoing corrosion processes. The response is measured in current, thus determining the material's response to the degradation processes [26]. Bode diagram analysis allows observation by the logarithm of the absolute value of the impedance versus the frequency. In the two systems, it was determined that at high temperatures, the impedance response is low, and as a result, a series of impedance values is obtained for each frequency studied. This relationship between frequency and impedance is known as the impedance spectrum, where the system with the lowest amount of lanthanum response is about 10 ohm-cm² (Figure 5).

The system with the highest lanthanum contents evaluated at 1000 °C shows a six-fold increase to 67 ohm-cm² (Figure 6) [27,28]. These plots indicate similar results for the systems evaluated at 700 °C and 850 °C due to the material's temperature increase. The resulting signals were obtained due to the frequency variation between 10^-2 Hz and 10¹ Hz, which allowed the diagrams to be adjusted by applying the same input variable to the physical interpretation and the chemical reactions. The diagrams indicate that the generated reactions allow charge transport, i.e., the corrosive salts are transported through the material. As the temperature increases, the rate of charge transport increases, the barrier generated by the formation of a surface coating on the substrates, and the flow of charge through this protective layer, the Bode diagrams indicate that at both 700 °C and 850 °C, the system is suitable because the spectrum does not allow corrosive salts to permeate. It is due to the pores that have not interconnected. However, at 1000 °C, these pores interlock and form cracks and can be therefore highly brittle. The phase that generates the protection is due to the characteristics of the bismuth lanthanum titanate thin film. In this case, an element with a higher amount of lanthanum generates 10 times more protection; however, after surpassing 850 °C, the transfer of charge through the discontinuities and defects of the thin film causes the breakage of the protective layer, resulting in continuous failure [29,30].

 

Figure 5. Bode diagram for the Bi_3.75La_0.25Ti₃O₁₂ composite.

 

Figure 6. Bode diagram for the Bi₃La₁Ti₃O₁₂ composite.

 

The equivalent circuit proposed to model the behavior coatings Bi_3.75La_{0. 25}Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂ have been schematized in Figure 7. The model considers that the corrosion of the films deposited on the substrate is located in the permeable pores where corrosive salts can penetrate them and reach the surface of the substrate. Therefore, the coating can be considered a leaky capacitor (constant phase element). The constituent elements of the electrical circuit in Figure 7 are R.E is the reference electrode, W.E is the working electrode, Rs is solution resistance, Rp1 is porosity charge transfer resistance, CP1 is coating capacitance, CP2 is the capacitance of the exposed metal, and Rp2 is the charge transfer resistance of the substrate-coating interface.

 

 

Figure 7. Equivalent circuit to model the behavior coatings Bi_3.75La_{0. 25}Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂

 

Table 4 shows the values of the electrochemical parameters of the equivalent electrical circuit in Figure 7. Low values of solution resistance (Rs) are observed in the results due to the contribution of the electrolyte. The values for Rp2 (resistance to polarization to charge transfer of the substrate-coating interface) of the substrate are lower. This shows that the films act as an anti-corrosive barrier against attack by the electrolytic solution.

 

Table 4. values of the electrochemical parameters of the equivalent electrical circuit for Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂ coatings

Temperature (°C)	Rs (Ω cm2)	CPE1 (mF cm-2 s-(1-a1)	a1	Rp1 (Ω cm2)	CPE2 mF cm-2 s-(1-a2)	a2	Rp2 103(W cm2)
Bi3.75La0. 25Ti3O12
700	4.10(0.3%)	92.56(4%)	0.75	19.20(3%)	342.83(3%)	0.79	2.43(0.2%)
850	5.24(0.2%)	87.12(4%)	0.69	5.31(2%)	546.23(3%)	0.75	1.02(0.3%)
1000	5.91(0.3%)	92.53(5%)	0.74	2.84(2%)	653.76(4%)	0.67	0.05(0.2%)
Bi3La1Ti3O12
700	1.31(0.4%)	121.05(2%)	0.54	5.3(0.1%)	564.21(3%)	0.56	0.08(0.01%)
850	3.22(0.6%)	243.13(3%)	0.59	2.3(0.2%)	853.28(2%)	0.67	0.04(0.02%)
1000	2.54(0.6%)	322.25(3%)	0.64	1.3(0.3%)	1035.25(2%)	0.64	0.01(0.02%)

 

 

6. In Conclusions part, at 700°C, 850°C, and 1000°C, which coating, Bi3.75La0.25Ti3O12 or Bi3La1Ti3O12 exhibits better stability and a better life at relatively high temperatures should be provided.

Answer: Suggestion accepted. The manuscript was reviewed and corrected. The new version is:

The results allowed to establish that the Bi3La1Ti3O12 coatings evaluated at temperatures of 700 °C, 850 °C and 1000 °C exhibit better stability at relatively high temperatures.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Line sizes are not the same.

On the basis of what criteria were the temperatures of 700 °C, 850 °C and 100 °C chosen for measuring electrochemical impedance spectroscopy? Include in the article

fig. 3 Add units - grade C

For the clarity of the article, I recommend supplementing the transverse metallographic analyzes of the coatings and determining their adhesion.

line 155 - Please document the degradation of the coatings.

Fog. 3 - Reword the description below the image

row 186 - At 1000°C, however, these pores become clogged and crack, and therefore can be very brittle. - This claim must be supported by documentation.

Fig.8 - It is not clear which photomicrographs are Bi3.75La0.25Ti3O12 and Bi3La1Ti3O12 films. Places described in the text should be marked with an arrow on the pictures.

Author Response

The answers are in the attached document.

Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript, entitled „High-temperature corrosion of Bi_3.75La_0.25Ti₃O₁₂ y Bi₃La₁Ti₃O₁₂ prepared by rf magnetron sputtering” is relevant to the scope of this journal.

It is an interesting study that can bring valuable information to specialists.

However, some points need to be addressed prior to publication of this manuscript. My comments/suggestions are given:

1.     There was an error in the title! I think instead of “y” it should be “and”.

2.     The last sentence in the abstract needs to be rephrased as it doesn't quite make sense.

3. Line 82 “The details of the deposition process are as follows: Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂ targets [16]”. Reference 16 is not about that! Please check and correct!

4.     Line 97 “Measurements were made in three ranges 700 °C, 850 °C, and 1000 °C. ” If it is about the domain it must be specified. If it is a fixed temperature the measurement error must be given.

5.     Line 107 “100 °C”. I think it's about 1000 °C. Please correct.

6.     Line 114 “range between -0.25 and 0.25 V against the reference electrode, defined for the open-circuit potential”. I think it's the electrode reference potential! I don't understand! Reference electrode potential is the open-circuit potential? Please rephrase to be clearer!

7.     Line 121 “. Using these curves, the oxidation of the alloy used as a coating in the presence of molten salt was determined”. I think it is better to specify that it is the polarization curves. I don't think oxidation but corrosion behaviour was followed. Please rephrase!

8.     I believe that "deterioration rate" is actually corrosion rate and should be renamed as such.

9.     Line 131 “ the corrosion density”. It's actually about "corrosion current density." The authors need to correct everywhere in the manuscript where this term appears!

10.  It was expected that an increase in temperature would increase the corrosion rate. Perhaps a study with activation energy calculation would have been more complete.

11.  It is very important that the kinetic parameters obtained from the polarization curves (Figures 2 and 3) are listed in the text in tabular form. Then some comments on these parameters would be more enlightening.

12.  Line 135 “increasing the content increases its protection, as observed when comparing the potentiodynamic curves”. Without precisely calculated kinetic parameters you cannot say that. For example, at 850 °C, it seems that the corrosion rate is lower for coatings with lower La content.

13.  Polarization curves must be added for the substrate recorded under the same conditions, also kinetic parameters must be calculated for the substrate! Any electrochemical study must be based on the behaviour of the substrate.

14.  Watch out for line 143. “ section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, and the experimental conclusions that can be drawn.”

15.  On the basis of what findings or bibliographical references have the authors established that “damage is minimal for a temperature up to 700 °C, at a range increase of 150 °C, the damage is adequate and at 1000 °C, the damage is considered catastrophic”?

16.  The legend of Figure 3 is wrong! Please correct!

17.  “10 ohm-cm2” The unit of measurement is not correct!

18.  In the Bode diagrams in Figures 4 and 5 the phase angle dependence must be added! The legend of Figure 5 must be rewritten!  In this case also the uncoated alloy tested under the same conditions must be compared!

19.  The EIS data fitting with equivalent electrical circuits is necessary to obtain quantifiable parameters and explanations of the interfaces that occur should complement this electrochemical test!

20.  I don't understand why this way of presenting the XRD spectra in Figure 7 was chosen! I think it was best to show all spectra at the 3 temperatures studied for the two types of coatings.

21.  The legend of Figure 8 is unclear! It is about what coating and at which temperature studied for each case?

22.  The images in Figure 8 need to be replaced with clearer ones and perhaps at higher magnifications!

23.  Some EDS analysis might elucidate which type of corrosion products were formed!

24.  The conclusions state that “as the temperature increases, the corrosion rate increases, and this change is mainly observed if the temperature exceeds 850 °C.”  What does that mean? Explain!

25.  “Amorphous processes are generated due to the increase in corrosion density.” What do you mean amorphous processes? Please explain!

Author Response

The answers are in the attached document.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

After revision, the manuscript has been improved significantly. However, I think that the paper should be improved in these aspects:

1. In Abstract part, the sentence of “Using the rf magnetron sputtering technique, Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂. Coatings were formed and obtained as a thin film on Hastelloy substrates” should be changed into “Using the rf magnetron sputtering technique, Bi_3.75La_0.25Ti₃O₁₂ and Bi₃La₁Ti₃O₁₂ coatings were formed and obtained as a thin film on Hastelloy substrates”.

2. In Abstract part, the sentence of “It was determined that the corrosion rate directly correlates with the temperature because the mechanisms induced by the Na₂SO₄ and V₂O₅ salts generated condensation” should be changed into “It was determined that the corrosion rate directly correlates with the temperature attributed to the mechanisms induced by the Na₂SO₄ and V₂O₅ salts generated condensation”.

3. In Introduction part, the sentence of “Therefore, trivalent elements such as lanthanum can be used to replace bismuth, increases in remanent polarisation have been reported, and even fatigue values close to zero have been achieved, a parameter sought to be reduced” has been changed into “Therefore, trivalent elements such as lanthanum can be used to replace bismuth, increasing remanent polarisation has been reported, and even fatigue values close to zero have been achieved, a parameter sought to be reduced”.

4. In Introduction part, the sentence of “In this study, rf magnetron sputtering was used to fabricate coatings. The advantages and application areas of this surface treatment technology should be provided in introduction part. In addition, high-temperature corrosion behavior was measured by electrochemical impedance spectroscopy measurements and potentiodynamic curves, which are rarely reported.” should be changed into “In this study, rf magnetron sputtering was used to fabricate coatings”.

5. In Introduction part, the sentence of “Therefore, the part could be revised as followings: Po-tentiodynamic curves and electrochemical impedance spectroscopy measurements are widely used to evaluate the corrosion resistance or reveal the corrosion mechanism at room temperature [19, 20]” should be changed into “Although potentiodynamic curves and electrochemical impedance spectroscopy measurements are widely used to evaluate the corrosion resistance or reveal the corrosion mechanism at room temperature [19, 20], electrochemical measurements are rarely used at high temperatures. In this study, high-temperature corrosion behavior was measured by electrochemical impedance spectroscopy measurements and potentiodynamic curves.”

6. In “2. Materials and Methods” part, the sentence of “…using a cell with a working electrode and an exposed area of 1 cm², a platinum wire counter electrode (CE), and a platinum wire reference electrode (RE)” should be changed into “…using a cell containing a working electrode with an exposed area of 1 cm², a platinum wire as counter electrode (CE), and a platinum wire as reference electrode (RE)”.

 

7. In “3. Results and Discussion” part, the sentence of “The constituent elements of the electrical circuit in Figure 7 are R.E is the reference electrode, W.E is the working electrode, Rs is solution resistance, Rp1 is porosity charge transfer resistance, CP1 is coating capacitance, CP2 is the capacitance of the exposed metal, and Rp2 is the charge transfer resistance of the substrate-coating interface” should be changed into “The constituent elements of the electrical circuit in Figure 7 are as followings: R.E the reference electrode, W.E the working electrode, Rs solution resistance, Rp1 porosity charge transfer resistance, CP1 coating capacitance, CP2 the capacitance of the exposed metal, and Rp2 the charge transfer resistance of the substrate-coating interface”.  

8. Table 4 and Table 5 are irregular. A format of three-line table should be used.

9. In Figure 8 and Figure 9, the phase patterns obtained under different temperatures cannot be clearly distinguished and therefore these figures should be revised.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors have made all the corrections and additions requested.

One more small recommendation would be before the manuscript can be published:

In Bode diagrams, the phase angle notation on the axes should be rotated by 180 degrees. This is the standard representation.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf