Review Reports - Quaternium-22 as a High-Performance Corrosion Inhibitor for Carbon Steel in Acidic Media: Experimental and Theoretical Insights

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper investigates Quaternium-22 as a High-Performance Corrosion Inhibitor for Carbon Steel in Acidic Media (1M HCL)

The main question addressed by this research is whether this cationic surfactant can serve as an effective and environmentally compatible corrosion inhibitor for carbon steel in acidic media. The topic is highly relevant. Corrosion of carbon steel in acidic media is a persistent and costly problem in many industries and is a major research priority. This study adds a comprehensive, multi-technique case study for a specific surfactant.

My minor concerns and comments are as follows:

The manuscript is generally well-written and scientifically sound. However, the following improvements are recommended:

In the methodology, replication and statistics are insufficient. The authors state that the experiments were performed in triplicate, but the data presentation lacks error bars or standard deviations. Including these is crucial for demonstrating the statistical significance and reproducibility of the results. This is a key requirement for publication.
XPS measurements and evaluations: It would be beneficial to clarify whether any charge correction was performed on the XPS data to ensure accurate peak assignments. This is standard practice and should be mentioned.
There are some typos in the electrochemical equations
In Table 3: It's confusing to have a Blank coloumn. It would be clearer to have the first row as "0" for concentration and then the data for IE% as 0. The Surface Coverage (θ) for the blank should also be 0.
In Table 4, The Rct and Cdl values for the blank are an order of magnitude lower than for inhibited solutions, which is expected. However, the presentation is insufficient. Consider using scientific notation consistently.
In the EIS analyses: The authors mentioned an "inductive dispersion (inductive arc)" at low frequencies and state it was not included in the fitting due to its "weak contribution and instability" (lines 279-285). This is acceptable, but it's worth briefly mentioning what this feature typically represents to acknowledge its presence and justify its exclusion from the quantitative model.
The discussion in sections 3.1-3.4 is acceptable but somewhat segmented. The surface characterization section (3.5) could be integrated into the discussion of the electrochemical results. For instance, when discussing the increase in Rct and decrease in icorr, the authors could immediately reference the SEM/AFM images (Figures 10, 11) and XPS data (Figure 13) as the physical and chemical evidence for the film responsible for those changes.
In Section 3.3, the authors introduce equations for the Freundlich isotherm (Eq. 13) and discuss parameters like n, but then only show data for the Langmuir fit. It would be clearer to state that various isotherms were tested, but the Langmuir model provided the best fit (highest R²), and then focus solely on it. Mentioning other models without showing the data for them can be confusing.

The conclusions are clear, well-structured, and directly answer the main question posed by the research that Q-22 is indeed an effective and potentially eco-compatible inhibitor, and its performance is due to a robust, multi-modal adsorptive film-forming mechanism.

Author Response

Reviewer #1

Comment 1: Replication and Statistics

In the methodology, replication and statistics are insufficient. The authors state that the experiments were performed in triplicate, but the data presentation lacks error bars or standard deviations. Including these is crucial for demonstrating the statistical significance and reproducibility of the results. This is a key requirement for publication.

Response:

We thank the reviewer for this important observation. Standard deviations have been added and highlighted (see Table 3).

Comment 2: XPS Charge Correction

XPS measurements and evaluations: It would be beneficial to clarify whether any charge correction was performed on the XPS data to ensure accurate peak assignments. This is standard practice and should be mentioned.

Response:

Thank you. We agree that this is an essential methodological detail. The following sentence has been added to Section 2.5.1: "All XPS spectra were charge-corrected by referencing the adventitious carbon C 1s peak to 284.8 eV."

Comment 3: Typos in Electrochemical Equations

There are some typos in the electrochemical equations

Response:

Thank you for this careful observation. We have carefully proofread all equations and corrected the typographical errors. Specifically, equation (8) has been reformatted to ensure proper parentheses. All equations have been verified for mathematical accuracy

Comment 4: Table 3 Formatting

In Table 3: It's confusing to have a Blank coloumn. It would be clearer to have the first row as "0" for concentration and then the data for IE% as 0. The Surface Coverage (θ) for the blank should also be 0.

Response:

We appreciate this suggestion for clarity. Table 3 has been completely reformatted. The "Blank" column has been removed, and concentration "0" is now explicitly shown with corresponding IE% and θ values of 0. The revised table now clearly presents the baseline condition.

Comment 5: Table 4 Scientific Notation

In Table 4, the Rct and Cdl values for the blank are an order of magnitude lower than for inhibited solutions, which is expected. However, the presentation is insufficient. Consider using scientific notation consistently.

Response:

Following the reviewer's recommendation, we have reformatted Table 4 using consistent scientific notation throughout. All Rct, Y₀, and Cdl values are now presented with the same number of significant figures and uniform exponent formatting.

Comment 6: EIS Inductive Arc Justification

In the EIS analyses, the authors mentioned an "inductive dispersion (inductive arc)" at low frequencies and state it was not included in the fitting due to its "weak contribution and instability" (lines 279-285). This is acceptable, but it's worth briefly mentioning what this feature typically represents to acknowledge its presence and justify its exclusion from the quantitative model.

Response:

We thank the reviewer for this insightful suggestion. The following text has been added to Section 3.2.1: "The small inductive loop observed at very low frequencies is typically attributed to the relaxation of adsorbed intermediates (e.g., FeClₐds or FeOHₐds) involved in the anodic dissolution mechanism [28]. Due to its weak contribution and occasional instability, this component was excluded from the equivalent circuit fitting to focus on the dominant charge-transfer parameters."

[28] Pajkossy, T. (1994). Impedance of rough capacitive electrodes. Journal of Electroanalytical Chemistry, 364(1–2), 111–125. https://doi.org/10.1016/0022-0728(93)03185-5

Comment 7: Integration of Surface Characterization

The discussion in sections 3.1-3.4 is acceptable but somewhat segmented. The surface characterization section (3.5) could be integrated into the discussion of the electrochemical results. For instance, when discussing the increase in Rct and decrease in icorr, the authors could immediately reference the SEM/AFM images (Figures 10, 11) and XPS data (Figure 13) as the physical and chemical evidence for the film responsible for those changes.

Response:

We honestly appreciate the reviewer's helpful recommendation and their favorable evaluation of Sections 3.1–3.4. We completely conclude that the mechanistic knowledge of corrosion inhibition is improved when electrochemical results are correlated with surface characterization.

However, we have updated the discussion to improve the cross-referencing between the electrochemical findings and the surface characterization results in accordance with the reviewer's wise suggestion. We now specifically refer to the SEM and AFM images (Figures 10 and 11) as physical proof of surface protection and the XPS results (Figure 13) as chemical confirmation of the formation and composition of the protective inhibitor film when discussing the increase in charge transfer resistance (Rct) and the decrease in corrosion current density (icorr). While maintaining the established sectional structure, this change enhances the discussion's coherence.

We think that this change resolves the reviewer's issue and strengthens the coherence between the surface analyses and electrochemical behavior described in the text.

Comment 8: Freundlich Isotherm Mention

In Section 3.3, the authors introduce equations for the Freundlich isotherm (Eq. 13) and discuss parameters like n, but then only show data for the Langmuir fit. It would be clearer to state that various isotherms were tested, but the Langmuir model provided the best fit (highest R²), and then focus solely on it. Mentioning other models without showing the data for them can be confusing.

Response:

We agree that this could be confusing. Section 3.3 has been revised to state clearly that multiple isotherms were tested, but only the Langmuir model provided the best fit (highest R²). The discussion of Freundlich parameters and Equation (13) has been removed to maintain focus on the experimentally validated Langmuir model.

Reviewer 2 Report

Comments and Suggestions for Authors

Manuscript ID surfaces-4195586 Title: Quaternium-22 as a High-Performance Corrosion Inhibitor for Carbon Steel in Acidic Media: Experimental and Theoretical Insights Authors: Mohammed Afifi, Nasser Elbasiony, Aziza El-Tabei, Shimaa Abdel Halim, Magdy Ibrahim

General comment The research presents the results of the evaluation of Quaternium-22 as an efficient and environmentally friendly corrosion inhibitor for carbon steel in acidic environments. The performance and inhibition mechanism were evaluated from experimental data obtained from gravimetric and electrochemical measurements, complemented by thermodynamic and adsorption analyses.

Some recommendations:

The Nyquist plots must be represented using orthonormal (orthogonal) scaling, where the axis ratio of real to imaginary components is 1:1 (the length representing one unit on the x-axis is equal to one unit on the y-axis).
The Nyquist and Bode plots should also be described. Insert the corresponding frequency into the Nyquist diagrams on the curves represented for a few points.
Enter in the text the number of the equation you are referring to for easier tracking.
Present for experimental data (gravimetric, electrochemical data) the degree of uncertainty in the experimental data, a quantitative indicator of the reliability of a measurement, (typically denoted by the plus-or-minus symbol followed by the numerical uncertainty.
In table 3, why does the electrode coverage 𝜃 increase in the first 30 minutes of immersion and decrease within 180 min. by approximately 11-12%? Usually, the graph that shows the evolution of 𝜃 with time from immersion, initially shows an increase and reaching a plateau after a certain period.
Reference 39, 41 is not the most appropriate, it presents kinetic equations from adsorption data determined by the difference between initial and final concentrations measured spectrophotometrically rather than electrochemically.
Present the symbol in the same manner (inhibitor concentration for example as Cinh in all equation).
Rewrite the paragraph from R427, the explanation regarding equations 12 and 13 is not very clear. The plot of C/𝜃 vs C gives a straight line with a slope of 1 for a true Langmuir adsorption. Significant deviations from a slope of 1 indicate non-Langmuir behavior due to lateral interactions (Frumkin isotherm), surface heterogeneity (Temkin isotherm) or multi-site adsorption.
Revise your paper to write abbreviations, text, and equations in the same style.
Add the color legend in figures 16 -18.
In Table 4, I think it would be good to pass the electrode coverage fraction with 4 decimals so that the two decimals from IE to be accurate.
The novelty of the work should be emphasis more and specifically positioned with respect to the existing literature.
A schematic representation of the mechanism involved in anti-corrosion protection would be welcome, but is not mandatory.

Author Response

Comment 1: Nyquist plot scaling

The Nyquist plots must be represented using orthonormal (orthogonal) scaling, where the axis ratio of real to imaginary components is 1:1 (the length representing one unit on the x-axis is equal to one unit on the y-axis).

Response:

We thank the reviewer for this important technical observation. All Nyquist plots (Figures) have been regenerated with orthonormal scaling (1:1 axis ratio). A note has been added to each figure caption indicating that the plots are presented with equal scaling to accurately represent the impedance data.

Comment 2: Frequency labeling on Nyquist plots

The Nyquist and Bode plots should also be described. Insert the corresponding frequency into the Nyquist diagrams on the curves represented for a few points.

Response:

Following the reviewer's suggestion, we have added frequency markers at selected points (100 kHz, 1 kHz, 10 Hz, 0.1 Hz) on all Nyquist plots. The direction of decreasing frequency is also indicated with arrows. The figure captions have been updated accordingly.

Comment 3: Equation numbering

Enter in the text the number of the equation you are referring to for easier tracking.

Response:

We have systematically reviewed the manuscript and inserted equation numbers wherever equations are referenced in the text. All cross-references have been verified for accuracy.

Comment 4: Uncertainty quantification

Present for experimental data (gravimetric, electrochemical data) the degree of uncertainty in the experimental data, a quantitative indicator of the reliability of a measurement, (typically denoted by the plus-or-minus symbol followed by the numerical uncertainty.

Response:

We agree that uncertainty quantification is essential. Standard deviations have been added to all tabulated data (see revised Table 3).

Comment 5: Surface coverage decreases over time

In table 3, why does the electrode coverage ? increase in the first 30 minutes of immersion and decrease within 180 min. by approximately 11-12%? Usually, the graph that shows the evolution of ? with time from immersion, initially shows an increase and reaching a plateau after a certain period.

"Why does the electrode coverage θ increase in the first 30 minutes and decrease within 180 min.?"

Response:

This is an excellent observation. We have added a detailed explanation in Section 3.1 discussing possible reasons for this time-dependent behavior, including partial desorption, competitive adsorption, film reorganization, and dynamic equilibrium effects. The revised text (lines X-X) addresses this point comprehensively.

"The surface coverage (θ) values in Table 3 exhibit an initial increase during the first 30 minutes, followed by a gradual decline of approximately 11-12% over 180 minutes. This behavior, while deviating from the ideal plateau typically expected for perfectly stable adsorbed layers, can be rationalized by considering the dynamic nature of the metal–electrolyte interface under continuous acid exposure [X]. Several factors may contribute to this time-dependent decrease in θ:

(i) Partial desorption of inhibitor molecules over extended immersion periods, particularly those bound through weaker electrostatic (physisorptive) interactions. The desorption rate constant and surface residence time are critical parameters governing inhibitor persistency on the metal surface [Y].
(ii) Competitive adsorption between inhibitor molecules and accumulating corrosion products or aggressive chloride ions at the interface;
(iii) Film reorganization under sustained acidic attack, potentially involving local film thinning or defect formation. The inhibitor layer forms at the interphase created between the metal and the aggressive solution, and its stability depends on continuous interaction dynamics [Z].
(iv) Establishment of a dynamic equilibrium where the net adsorption rate decreases as the system approaches steady-state, with the observed θ representing the balance between ongoing adsorption and desorption processes.

Similar time-dependent variations in surface coverage have been reported for other organic inhibitors in acidic media and are often attributed to the complex interplay between inhibitor adsorption, film stability, and ongoing corrosion processes. Importantly, despite this gradual decrease, the inhibition efficiency remains substantially higher than the blank throughout the 180 minutes, confirming the sustained protective action of Q-22."

[X] McCafferty, E. (2000). An eigenvalue analysis of coupled differential equations in regard to corrosion inhibition. Journal of the Electrochemical Society, *147*(1), 125–131. https://doi.org/10.1149/1.1393166

[Y] Woollam & Betancourt (2021). Utilization of Adsorption Kinetics Rate Constants to Better Define Corrosion Inhibitor Availability. SPE/OnePetro. https://search.spe.org/i2kweb/SPE/doc/onepetro:CEADACE9

Utilization of Adsorption Kinetics Rate Constants to Better Define Corrosion Inhibitor Availability

Richard C. Woollam; Daniel Betancourt

Paper presented at the CORROSION 2021, Virtual, April 2021.

Paper Number: NACE-2021-16861- Published: April 19 2021

[Z] Ogukwe, C. E., Akalezi, C. O., Chidiebere, M. A., Oguzie, K. L., Iheabunike, Z. O., & Oguzie, E. E. (2012). Corrosion inhibition and adsorption behaviour of Anthocleista djalonesis leaf extract on mild steel in acidic media. Portugaliae Electrochimica Acta, *30*(3), 189–202.

Comment 6: Inappropriate references 39, 41

Reference 39, 41 is not the most appropriate, it presents kinetic equations from adsorption data determined by the difference between initial and final concentrations measured spectrophotometrically rather than electrochemically.

"Reference 39, 41 is not the most appropriate..."

Response:

The references have been checked carefully, and consequently, references [39] and [41] have been replaced with more suitable corrosion-focused adsorption studies.

Comment 7: Consistent symbol formatting

Present the symbol in the same manner (inhibitor concentration for example as Cinh in all equation).

Response:

We have standardized all symbol formatting throughout the manuscript. Inhibitor concentration is now consistently presented as C_inh in all equations and text. Other symbols (R_ct>, i_corr, θ, IE%) have been similarly harmonized.

Comment 8: Unclear explanation of Equations 12-13

Rewrite the paragraph from R427; the explanation regarding equations 12 and 13 is not very clear. The plot of C/? vs C gives a straight line with a slope of 1 for a true Langmuir adsorption. Significant deviations from a slope of 1 indicate non-Langmuir behavior due to lateral interactions (Frumkin isotherm), surface heterogeneity (Temkin isotherm) or multi-site adsorption.

Response:

We have completely rewritten this paragraph to provide a clearer explanation of Langmuir isotherm interpretation, including a discussion of slope significance and deviations indicating non-Langmuir behavior. The revised Section 3.3 has been given and highlighted.

Comment 9: Inconsistent style

Revise your paper to write abbreviations, text, and equations in the same style.

Response:

We have thoroughly reviewed the manuscript and standardized all stylistic elements, including:

Unit formatting (µmol L⁻¹, 1.0 M HCl, 20 °C)
Equation numbering style
Abbreviation definitions at first use
Figure and table referencing format

Comment 10: Color legend in Figures 16-18

Add the color legend in figures 16 -18.

Response:

Clear color legends have been explained as given in page 29, lines 701 - 713, and added to Figures 16-18 (DFT studies).

Comment 11: Decimal places for θ in Table 4

In Table 4, I think it would be good to pass the electrode coverage fraction with 4 decimals so that the two decimals from IE are accurate.

Response:

We have increased the decimal places for θ from 3 to 4 in Table 4 to ensure accurate calculation of IE% to two decimal places. The revised table now presents θ with four decimals throughout.

Increase decimal places for θ from 3 to 4 decimals to maintain precision when calculating IE% (IE% = θ × 100).

Comment 12: Strengthen novelty emphasis

The novelty of the work should be emphasis more and specifically positioned with respect to the existing literature.

Response:

We have strengthened the novelty statement in both the Introduction and Conclusion sections. The revised introduction now explicitly positions Q-22's unique molecular architecture relative to existing literature, and the Conclusion highlights the structure–property relationships established through systematic comparison with Q-15 (Surfactant/Inhibitor). Both are clearly highlighted.

Comment 13: Schematic mechanism (optional)

A schematic representation of the mechanism involved in anti-corrosion protection would be welcome, but is not mandatory.

Response:

We are truly grateful to the reviewer for this insightful recommendation. We agree that a schematic representation of the corrosion inhibition mechanism could provide additional visual clarity. But this point is optional, as the reviewer pointed out. The revised version does not include a schematic picture due to time constraints and to preserve the manuscript's current structure and purpose.

However, electrochemical, surface, and computational results have been used to support the inhibitory mechanism, which has been thoroughly addressed. Together, these results offer a clear comprehension of the protective function of the examined inhibitor.

We value the reviewer's recommendation and will think about incorporating a schematic like that in subsequent development.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Accept in present form.