Ziziphus joazeiro Stem Bark Extract as a Green Corrosion Inhibitor for Mild Steel in Acid Medium

Round 1

Reviewer 1 Report

Mauro et al reported the utilization of  Ziziphus joazeiro stem bark extract as green corrosion inhibitors for mild steels. The work is interesting and is worth considering for  acceptance after the following revision.

1. The cross-sectional SEM images or AFM 3D topography of the sample surface, with and without inhibitor as well as before and after corrosion tests, are suggested to  be provided. 

2. Following recent reports on the utilization of biomass materials in different fields are suggested to be cited. 

[1] ACS Appl. Mater. Interfaces 2021, 13, 2, 3119–3138. https://0-doi-org.brum.beds.ac.uk/10.1021/acsami.0c18887
[2] Journal of Molecular Liquids 2021, 332, 115852. https://0-doi-org.brum.beds.ac.uk/10.1016/j.molliq.2021.115852
[3] Journal of Colloid and Interface Science 2020, 569,  22-33. https://0-doi-org.brum.beds.ac.uk/10.1016/j.jcis.2020.02.062
[4] Journal of Energy Chemistry 2021, 58, 207-218. https://0-doi-org.brum.beds.ac.uk/10.1016/j.jechem.2020.08.065
[5] Journal of Materials Research and Technology 2020, 9(4), 7104-7122. https://0-doi-org.brum.beds.ac.uk/10.1016/j.jmrt.2020.05.019
[6] Nanomaterials 2020, 10, 346. https://0-doi-org.brum.beds.ac.uk/10.3390/nano10020346

3. Image quality and format standardization should be improved.  The image size of different images in a same figure should be uniform (Figure 6).
Some figures and symbols should be submarked or supermarked. Please check the manuscript carefully.

Author Response

Dear reviewer

First, we would like to thank the Editor and reviewers for evaluation and contribution to our article. We have followed the recommendations to improve the paper.All changes were highlighted in yellow color.

The work is interesting and is worth considering for acceptance after the following revision.

1. The cross-sectional SEM images or AFM 3D topography of the sample surface, with and without inhibitor as well as before and after corrosion tests, are suggested to be provided.

Answer: The analyzes proposed by there viewer are very interesting, however due to the pandemic, carrying out the analyzes is very difficult at our university at this moment. We are doing the work remotely, only Covid-related research is being allowed at UFRJ. Besides, it is important to emphasize that we evaluated the inhibitory action of EJSB and its HMWF towards the mild steel corrosion by different techniques as weight loss measurements, electrochemical impedance and cathodic and anodic polarization curves. All results showed a significative reduction in the corrosion rate, from weight loss measurement data the corrosion rate was around 10 times lower in the presence of 800 mg L⁻¹EJSB and its HMWF, representing na inhibition efficiency of 89% and 91%, respectively. In this context, scanning electron microscopy was used to corroborate these data. This technique was used only to evaluate the morphology of the surface of the corroded plate, showing the pattern of the surface corrosion of the mild steel specimen with or without treatment.

2. 
   Following recent reports on the utilization of biomass materials in different fields are suggested to be cited..

Answer: The recent reports suggested we recited in the new version.

3.Image quality and format standardization should be improved.  The image size of different images in a same figure should be uniform (Figure 6). Some figures and symbols should be submarked or supermarked. Please check the manuscript carefully.

Answer: Image quality and format standardization was revised as requested by the reviewer.

Kind regards,  Mrs. Aparecida Mauro.

Author Response File: Author Response.docx

Reviewer 2 Report

This paper studied the used of the extract of Joazeiro stem barks (EJSB) as a green corrosion inhibitor for mild steel in acid medium. The framework is very interesting, the oil industry is continuously searching for corrosion inhibitors, and low price and environmentally friendly materials are always more than welcome. The EJSB can be a low-cost effective corrosion inhibitor for the mild steel pipeline also at higher temperature. The experimental campaign described in the paper is rich and interesting, and the experimental outcomes are properly discussed. My overall suggestion is to accept the paper after some improvements:

1. The fitting data should be plotted into the plots
2. If you write the formula of Cdl (i.e., eq. 13), the value of f_max should also be reported in the graphs.
3. The Nyquist plots should be redrawn such that X and Y axes have the same scale to reveal the semicircular nature of the Nyquist diagram.
4. The equation of the free energy of adsorption must be reported and the values of ΔGads and Kads must be calculated and discussed. Furthermore if you show the values of each single isotherm, you must also explain their meaning and show the values of the individual components of each isotherm.

Author Response

Dear Reviewer,

First, we would like to thank the Editor and reviewers for evaluation and contribution to our article. We have followed the recommendations to improve the paper. All changes were highlighted in yellow color.

The framework is very interesting, the oil industry is continuouslysearching for corrosion inhibitors, and low price and environmentally friendly materials are always more than welcome. The EJSB canbe a low-cost effective corrosion inhibitor for the mild steel pipeline also at higher temperature. The experimental campaign described in the paper is richand interesting, and the experimental out comes are properly discussed. My overall suggestion is to accept the paper after some improvements:

1. 
   The fitting data should be plotted into th eplots

Answer: The fitting data were plotted on the graphs as suggested.

2. If you write the formula of Cdl (i.e., eq. 13), the value off Max should also be reported in the graphs.

Answer: The value off Max were reported in the graphs as suggested.

3. The Nyquist plots should be redrawn such that X and Y axés have the same scaletoreveal the semicircular nature of the Nyquist diagram.

Answer: The Nyquist plots were redrawn as suggested by there viewer.

4. The equation of the free energy of adsorption must be reported and the values of ΔGads and Kads must be calculated and discussed. Furthermore if you show the values of each single isotherm, you must also explain their meaning and show the values of the individual components of each isotherm.

Answer: We discussed and showed the individual components of each isotherm, as required by the reviewer and the Kads was calculated. However, the DG°ads calculation can not be done since the molecular mass of the component responsible adsorption processis unknown. Please, see the following reference Chem. Eng. Comm., 201:1459–1467, 2014.

In the corrosion inhibition process, the adsorption of adsorbate onto the adsorbent-solution interface is denoted as a substitutional adsorption process between the solute molecules in the aqueous solution (Solute(sol)) and the water molecules on the adsorbent surface (H₂O(ads)). The standard equilibrium constant K° (dimensionless) for calculating the standard free energy changes (DG°) is used according to the following equation:

Where the standard equilibrium constant (dimension less, K°) for calculating the standard free energy changes (DG°) of the adsorption processes is equal to 55.5 K (where K is given in L mol^-1). If the K unityis L mg^-1, K should be:

K° = K x (Madsorbate x 1000) x 55.5

Where Madsorbate is the molar mass of the inhibitor. So the thermodynamical calculation involving natural products extracts is wrong. Please read the manuscript “The Unit Problem in the Thermodynamic Calculation of Adsorption Using the Langmuir Equation” in Chem. Eng. Comm., 201:1459–1467, 2014.

Kind regards,  Mrs. Aparecida Mauro.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

I have read this paper (i.e., The Unit Problem in the Thermodynamic Calculation of Adsorption Using the Langmuir Equation).

Determining Gibbs energies of adsorption from corrosion inhibition efficiencies: Is it a reliable approach? Corrosion Science Volume 155, 15 July 2019, Pages 182-185

Is the Free Energy Change of Adsorption Correctly Calculated? J. Chem. Eng. Data 2009, 54, 1981–1985

However, in this paper, they reported that if the unit of K is dm³/mol you use K^o=K55.5.

55.5 are the mol water per liter (i.e.,1000 g divided by the molar weight of pure water).

If the unit of K is g/L they reported that you have to multiply by Mads. The article does not explain why Mads.  Why not just converting 55.5 mol/l in grams? 1000g, the mass of water, per 1 liter? You will have the unit in g/L.

But that is not even the problem, you can omit the DG°ads. The temperature is the best way to assess an inhibitor, and you did it. I asked you to calculate the other parameter become I thought that the Langmuir’s isotherm wasn’t ideal for your inhibitor. The structure of your inhibitor is too big.

For a given adsorption model, the goodness of the linear correlation coefficient may not offer any insight into possible adsorption mechanisms. For instance, Langmuir isotherm was developed according to some specific assumptions such as the following: (i) there is no interaction between the adsorbed molecules; (ii) there is a fixed number of adsorption sites on the metal surface, and each of them holds one adsorbed species (copied from “Guar Gum as an Eco-Friendly Corrosion Inhibitor for Pure Aluminium in 1-M HCl Solution”). But you can find also it in other papers.

You reported that:

Flory-Huggins and El-Awady isotherms provide supplementary information. If the parameter x is greater than one unit, there is an indication that more than one molecule of water is displaced by a single molecule of the inhibitor. Similarly, if the parameter y is less than one unit, it shows that a single molecule of the inhibitor may be adsorbed on more than one active site of the metal [1; 39-40; 42]. Our results (x = 1.932 and y = 0.5540 for EJSB and x = 2.983 and y = 0.108 for its HMWF) suggest that the inhibitory molecules are likely bulky. Besides the parameter a from Temkin isotherm is negative, indicating the repulsive nature of the interaction between the adsorbed molecules.

Your results contradict Langmuir’s isotherm. For bulky molecules or macromolecules, Langmuir’s isotherm is meaningless. I am not saying to remove it, but just point out the inconsistencies

Author Response

Dear Reviewers,

First, we would like to thank the Editor and the reviewer for your evaluations
and contributions to our article. We have followed your recommendations to
improve the paper. All the changes were marked with yellow color.

Kind regards

Author Response File: Author Response.docx