Anticorrosive and Microbial Inhibition Performance of a Coating Loaded with Andrographis paniculata on Stainless Steel in Seawater

Round 1

Reviewer 1 Report

I reviewed the paper “Anticorrosive and microbial inhibition performance of a coating loaded with Andrographis paniculata on stainless steel in seawater”. The paper deals on the inihbition effect of an extract of natural plants on the corrosion behavior of stainless steel AISI316L in sea water.

The paper in the present form cannot be published. The authors must clarify some experimental details as following.

 

In introduction: please better explain the phrase in page 2 rows 47-48. Reference 5 is out of theme (is on concrete).

Rows 48-50: please better explain that you are speaking about the degradation of the coating, generally pitting is a localized corrosion of passive metals.

The author should better explain in the introduction why there applied the proposed corrosion inhibitor on AISI316L stainless steel, because generally the passive metals, like stainless steels are not coated.

Page 4 rows 159-169: The author should specify the cell, the test solution, reference electrode and counter electrode.  

Rows 171-172: the polarization potentiodynamic curve is not obtained by application of a current at the samples, but by the application of a potential between the reference and the working electrode and measuring the current that flow between the counter and the working electrode!!!

Row 174; the potential must refer to a standard electrode, i.e. SCE, in alternative the value of potential to start the test should be refer to the open circuit potential (OCP).

Rows 185-186 is a repetition

Page 8 row 289: the authors should better explain the phrase: “Rct acts as the first barrier resistance on 289 the metal surface under the coating layer” because with Rct is generally indicated the charge transfer resistance, in other words the polarization resistance that is proportional to the reciprocal of the corrosion rate.

Figure 7: the authors should explain do they mean with “before immersion”, just dipped? It is not possible to measure EIS without immerging the sample in a conductive solution… In what solution were carried out the electrochemical tests? It is not clear as a passive material - AISI 316L - can show a fully active behavior just dipped. This is unusually, and the authors should comment it with some references. In addition, the values of Cdl - representing in the equivalent circuit the electric double layer - decreases during the test, the authors should comment it. Rows 302-303: the authors should explain how they had carried out the tests! It’s impossible to measure the EIS spectra without immersion of the specimens!

The fitting standard deviation should be showed. How many repetitions were carried out and what was the reproducibility?

The EIS spectra must be reported in ohmxcm2 not ohm. The discussion of the authors on Cdl did not take into account the different volume of solution and exposed area. In bare material all the surface participates to the electrochemical response, in the coated specimens the electrolyte is only in the pores of the coating and the exposed area is the area on the detached coating or in the pores of the coating.

 

The corrosion current must be expressed as current density! During the potentiodynamic curves of the coated specimens was the ohmic drop compensate? The curves reported in figure 8 reports a limiting current both in anodic and cathodic direction. This is not in agreement with the behavior of AISI 316L in sea water. In these conditions the steel is passive and shows localized corrosion, evidented by an increase of the current density from the passive current. These curves are very unusual, and the authors should comment it.

Table 6 does not report the standard deviation of the data obtained. Corrosion rates are expressed in mm with 5 decimal places! this means that the authors can measure the thinning with the accuracy of 10nm. This is absurd. Furthermore, according to what the authors report, the corrosion rate of the C3 sample goes from a practically zero value to over 350 microns / year after 20 days of immersion and then returns to a few microns / year for the rest of the dive.

Author Response

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Reviewer 2 Report

The authors report “Anticorrosive and microbial inhibition performance of a coating loaded with Andrographis paniculata on stainless steel in seawater.” They use a range of measurements to demonstrate the anti-corrosion and anti-microbial properties of coatings modified by Andrographis paniculata in seawater. It’s an interesting work and has a certain novelty. However, it should be major revised before publication in molecules.

1. The English of the paper should be carefully checked and revised. Numerous grammatical problems are seen within the paper.
2. In abstract, the main results of anti-corrosion and anti-microbial should be given.
3. The authors said Andrographis paniculata process anti-oxidation and anti-microbial properties. But, the anti-corrosion performance of Andrographis paniculata in seawater should be evaluated firstly before being filled into the coating.
4. In introduction, the development process of coating and green corrosion inhibitor should be introduced. Here, some articles are beneficial for authors, such as Carbon 159 (2020) 292-302; Carbon 176 (2021) 39-51; Chemical Engineering Journal, 2021, 408, 127367; Journal of Molecular Liquids 321 (2021) 114450.
5. To confirm the influence of various concentrations of Andrographis paniculata for coating, the cross-section of different coating should be given and discussed.
6. In the electrochemical part, EIS plots without origin data are incomplete, and Bode plots are also important when analyzing the anti-corrosion process. Some articles are beneficial for electrochemical analysis, such as Chemical Engineering Journal 406 (2021) 126863; Journal of Materials Science & Technology 52 (2020) 63-71.

Author Response

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Reviewer 3 Report

The article investigates Andrographis paniculata or kalmegh leaves extract (KLE) as an eco-friendly component for anti-corrosion coatings. Formulations containing wt.%: 0, 3, 6, 9, and 12 were applied to stainless steel 316L and immersed in seawater for up to 50 days. 6 wt.% KLE was revealed to be the best concentration.

The work is well presented, organized and readable. However, some comments are listed below.

Introduction : the scope of the work in the context described in the introduction is not completely stated.

Section 2.6.1 does not mention the electrolyte of the EIS tests.

Line 217: the text does not mention that Figure 3 refers to KLE (it is mentioned in the figure caption)

Line 283 “resistance uncompensated resistance”, revise.

lines 281-291: Please, add references to support the description of the electric components of the equivalent electric circuits. This is a general journal and the reader may not be familiar with these parameters.

Line 287: defects instead of pores. Rc is affected by pores but also by other defects such as cracks.

Line 312 “C3 gave the best Rc value, which is the lowest among C1, C2, C4, and C5”, revise “Rc” (it seems it should be Cc)

Question: Did you test the same samples that were removed and placed back in the immersion site?

Line 366: “The addition of the optimum value of KLE additive results in lower icorr value, and this represents that the coating is of a mixed-type inhibition, wherein action in both cathodic and anodic sites are inhibited. In addition, C3 that gives the best performances in corrosion resistance has greater tendency to shift Ecorr to the more electropositive value at 10, 40 and 50 days of immersion but shifted to the more electronegative value at 20 and 40 days.” à The reduction of icorr does not mean this is a mixed-type inhibition. The reduction of icorr means there is inhibition, but the type of inhibition (cathodic, anodic or mixed) should be analysed on the basis of Ecorr shifts from bare metal. Moreover, sometimes, Ecorr is not enough to know the type of inhibition. In addition, with only 1 sample is difficult to obtain solid conclusions on this point.

Table 7. How do you explain that Fe is not present? Sample C3 seems to be analysed with NaCl deposits on the surface, these deposits can hide an eventual chromium oxide layer. Did you analyse the bare and the C3 sample before cleaning with water both of them?

General comment: in my view, the term “optimum %” when referring to 6% is not correct. I would recommend to use “the best results obtained here” or something similar. Additional research should be performed in order to find the optimum %, as indicated by the authors in the end of the conclusions.

Author Response

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Round 2

Reviewer 1 Report

Dear Authors, I appreciate your collaboration, but I am not in agreement with some your answer, as follow:

Point 3: The author should better explain in the introduction why there applied the proposed corrosion inhibitor on AISI316L stainless steel, because generally the passive metals, like stainless steels are not coated.

Response 3: As suggested, the sentence to explain the issue on AISI316L is inserted in the intoduction section at the last paragraph. The sentence is: “...on stainless steel grade 316L (SS316L) as to avoid the release of harmful substance like chromate that can harm the aquatic life such as fish.”

This phrase is not correct! The corrosion potential of AISI 316L in seawater is not too high to oxidate the chrome to chromate! At most, the ion release is on Cr3+ not Cr(IV)!!!

Point 4: Page 4 rows 159-169: The author should specify the cell, the test solution, reference electrode and counter electrode.

Response 4: Dear reviewer, the specification has been added in row 163-166: “It employed simple three-electrode cells, which included the reference electrode (RE) for measuring the potential of the working electrode (WE), as the studied sample, and a counter electrode (CE) for allowing the current to pass through.” The test solution is also mentioned for which it is the same seawater collected from the site of the experiment in row 167.

Generally in the experimental is indicated the reference (Standard Calomel Electrode) and Counter electrode (platinum?).

Point 8: Page 8 row 289: the authors should better explain the phrase: “Rct acts as the first barrier resistance on 289 the metal surface under the coating layer” because with Rct is generally indicated the charge transfer resistance, in other words the polarization resistance that is proportional to the reciprocal of the corrosion rate.

Response 8: Dear reviewer, the sentence is changed to: Rct acts as the indication of the resistance attributed to the metal surface under the coating layer.

This phrase is not correct! Generally, resistance indicates the ohmic resistance, whereas Rct indicates the charge transfer resistance, that is the resistance to the electron transfer from the metal to the oxidant in the solution. In the case of your experiment, I suppose that the presence of the coating decreases the oxygen apport to the metal surface, and it could trigger a crevice attack. AISI 316L is passive in seawater but, due to its low PRE, is susceptible to localized corrosion, pitting and crevice. For this reason, on my opinion, generally it is not correct to apply a coating to stainless steel. The resistance of AISI 316L to localized attack is higher on very clear surfaces and in flowing conditions with a high oxygen apport. I think that the most helpful of your coating could be to avoid the growth of fouling that depolarizes the metal surface, creating most favorable environment to trigger localized attacks.

Point 9: Figure 7: the authors should explain do they mean with “before immersion”, just dipped? It is not possible to measure EIS without immerging the sample in a conductive solution... In what solution were carried out the electrochemical tests? It is not clear as a passive material - AISI 316L - can show a fully active behavior just dipped. This is unusually, and the authors should comment it with some references. In addition, the values of Cdl - representing in the equivalent circuit the electric double layer - decreases during the test, the authors should comment it. Rows 302-303: the authors should explain how they had carried out the tests! It’s impossible to measure the EIS spectra without immersion of the specimens!

Response 9: Dear reviewer, thank you so much for the clarification of my mistake. What I meant by before immersion is that I have used the same test solution collected from the site of experiment to do the test. I have included this statement in the section 3.2, in row 299-302: It is also important to mention that the result for the ‘before immersion’ period was obtained by testing the specimen directly using test solution collected from the site as soon as the coatings are completely dried. To explain the low value of Cdl, the following sentence is included in row 322-323: All the values are low when compared to the Cdl value of bare metal, which was 37.60 ×10-6 F, indicating the low presence of a delamination area under the coatings.

I don’t understand what the authors means. The valued of the capacitance of double layer in an active system is due to the distribution of the ions in the correspondence of the charged metal’s surface. On passive metals, there is also a contribute of the passive film. At least, on a coated metal, there is also a capacitive contribute of the dielectric (the organic coating that, if dry, does not conduct electrically. The capacitive contribute of the coating decreases and it ohmic resistance increases when the electrolyte permeate inside it.  I suggest to read some text on the interpretation of the electrochemical impedance spectroscopy.

Point 10: The fitting standard deviation should be showed. How many repetitions were carried out and what was the reproducibility?

Response 10: Dear reviewer, I am sorry to inform you that during the experiment, no repetition of the measurement was carried out.

I understand that the tests in natural seawater are complex to realize, but the analysis of the quantitative parameters of EIS has not any scientific meanings, if the authors did not evaluate the reproducibility of the results. I suggest to the authors to make only a qualitative analysis of the results, because their data do not permit a deeper analysis.

Point 11: The EIS spectra must be reported in ohmxcm2 not ohm. The discussion of the authors on Cdl did not take into account the different volume of solution and exposed area. In bare material all the surface participates to the electrochemical

response, in the coated specimens the electrolyte is only in the pores of the coating and the exposed area is the area on the detached coating or in the pores of the coating.

Response 11: Dear reviewer, thank you for pointing out the mistake. The results presented were only calculated by taking into consideration the surface area of the sample for the polarization study, as I explained in section 2.6.2. I have corrected it for the parameter icorr through the whole article as you may have suggested. The volume of of the test solution are the same for all measurement since I am using the same type of holder as showed in the figure below and always ensure that I fill it up when starting the measurement process.

I didn't make myself clear. If the electrochemical test is carried out on the uncoated specimen, all the part exposed surface supplies current. In the case of a specimen coated with a non-conductive organic coating perfectly adherent between the specimen and the counter electrode, no current passes. A current passage begins to register when the solution permeates the coating and reaches the metal surface, in this case the area of ​​the specimen that participates in the exchange of electrons is only the small fraction that comes in contact with the solution, and this depends on the thickness and porosity of the coating. Finally, if the water detaches the coating and a holyday is formed between the coating and the metal, the whole area under the coating holyday participates in the current exchange. The apparent current density value is given by the ratio between the circulating current and the exposed nominal area of ​​the specimen. Obviously, this is maximum for the uncoated specimen, where the whole surface really participates in the reaction and minimum in the case of the perfectly adherent coating. The current increases as the coating deteriorates and allows more and more solution to permeate, increasing the area of ​​metal exposed. If the contact between the solution and the metal triggers a localized corrosion, the current undergoes a strong increase, worsened by the fact that the corrosion products tend to further detach the coating.

Point 12: The corrosion current must be expressed as current density! During the potentiodynamic curves of the coated specimens was the ohmic drop compensate? The curves reported in figure 8 reports a limiting current both in anodic and cathodic direction. This is not in agreement with the behavior of AISI 316L in sea water. In these conditions the steel is passive and shows localized corrosion, evidenced by an increase of the current density from the passive current. These curves are very unusual, and the authors should comment it.

Response 12: Dear reviewer, as you have suggested, I have corrected the term into current density (A.cm2) throughout the article. Thank you. Following your comment on the unusual curve of the AISI316L, the limitation of current in both anodic and cathodic region is possibly due to the measurement was limited to be in the range of -1.0 to 0.3V as stated in the section 2.6.2. Also, in accordance with your comment that the steel is passive and shows localized

corrosion, evidenced by an increase of the current density from the passive current, the trend was in my humble opinion is observed in the following article: Ahmad, S., & Malik, A. U. (2001). Corrosion behaviour of some stainless steels in chlorinated Gulf seawater. Journal of applied electrochemistry, 31(9), 1009-1016.

The graphs of figure 8 report the current and not current density.

Response 13: Dear reviewer, thank you for the comment. As I have mentioned previously, unfortunately, the measurements were only not repeated during the EIS experiment and hence the standard deviation cannot be calculated. As for the corrosion rate, yes, it is a mistake which I have corrected in the usual 2 decimal places throughout the manuscript. Finally, for the absurd changes of CR, I have included the following explanation: “Another out-of-alignment values were recorded for C3 samples on the 20th day of immersion, possibly due to the sample was damaged during the immersion test. Nevertheless, other samples of C3 throughout the subsequent testing period have managed to retain its integrity by displaying a good performance.”

Without repetition it is possible to make only a qualitative interpretation of the results.

Dear Authors, I appreciate your collaboration, but I am not in agreement with some your answer, as follow:

Author Response

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Reviewer 2 Report

I am very satisfied with the replies of the author.

Author Response

Dear reviewer, thank you for accepting all my response.

Round 3

Reviewer 1 Report

The paper was revised and now it can be published 

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

The authors of the publication use 316L stainless steel as the substrate for their new coating. Such a substrate is resistant to corrosion in itself, and yet the tested coating is to demonstrate its anti-corrosive and anti-microbial corrosion properties. This is evidenced by the salinity value given in Table 2. When conducting research using electrochemical impedance spectroscopy, the authors of the study indicate that they used an amlitude of the sinusoidal perturbation signal at the level of 5 mV. This is a very small value taking into account the testing of stainless steel and the additional application of a polymer-based coating on its surface allows the obtained results to be considered not entirely appropriate. 

The equivalent circuit for the low-frequency part of the spectra obtained should also be verified. The shape of the spectrum indicates the diffusion process rather than the substrate corrosion.

A strange solution seems to be the use of polarization in the Tafel range on the material covered with the coating. Are we able to define on what surface the corrosion process takes place? This is the most important information when calculating the corrosion rate.  The test is probably limited to the damaged areas on the coating, as evidenced by the diffusion nature of the cathode and anode parts of the Tafel curve. What was the rate of potential change during the research .

The results of EDX also require explanation. What was compared with each other ?? The surface of stainless steel without coating and the surface of the coating covering the stainless steel ?? An EDX study on the surface of stainless steel would show primarily the composition of 316L itself. The discussion of the results presented in this table is not convincing.

Table 2 - wrong value of standard deviation - too many numbers after the decimal point.

 

Author Response

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Reviewer 2 Report

General comment

 

The article deals with the preparation of a coating based on the extract from Andrographis paniculate and characterization of its properties including anticorrosion and microbial inhibition. The development of coatings based on natural products addresses the problems associated with harmful effects of corrosion reducing formulations on the environment and human health; as they are not harmful and difficult to undergo biodegradation. It is an interesting addition to the researches underway in this area. The experiments are designed well though some of them needs to be described in more detail for the sake of clarity. Certain interpretations given could also be improved. Some of such specific interpretations are mentioned in the specific comments and suggestions.

 

Specific comments and suggestions

 

Line 107: Reflection spectra are not gathered since the measurement is carried out in the transmission mode. The term reflectance can be omitted.

Line 109-110:  …. functional groups of the constituents in the extract and coatings prepared.

Line 40: Inflammation of metal parts? What is that to describe for in this context? Not clear for me.

Line 112-113: XRD measurements on the paint and coating samples were conducted using a Rigaku…..

How were the samples prepared? Some descriptions of the measurement conditions?

Line 113: There are different models of HPLC from Shimadzu. Which model is used in this experiment? What is the detector used? How about other additional parameters that need to be described like the solvent composition and elution condition?

Line 155: JSM-6390LA can be omitted here

Line 157: … changes in the surface morphologies …

Line 158: Using a coater machine (its name can be mentioned if possible)

Line 159: The observation in the SEM analysis: In SE or BSE mode? Or both?  What about EDX? Was it used? If so, brief description about it and the measurement conditions are needed.

Line 190:  … the sharpness of O-H …. Does it to mean intensity in the O-H band? In figure 4, this band is visible only in then case of C3. How is it compared? Do the close-up views in that region show the observation descried as ‘increased until C3’? It would be great if the wavenumbers are given for the prominent IR bands mentioned in the discussion.

Line 194:  … sharpness? There are different sources of the carbonyl functional group from the diverse ingredients of the coatings? What is the C=O attributed to since the intensity has shown a decrease as we go from C1 to the other coatings prepared? Again, it would be great if the wavenumbers to such bands are given. The spectra from the extract (KLE) and some of the coatings prepared can also be compared to facilitate the interpretation and band assignments.

Line 208: The abscissa is not in wavelength, but in wavenumber (cm^-1)

Line 212: … with the angle of 2θ at ….

Line 214: …. Its presence should increase ….

Line 217: Which polymer in which ingredient of the coatings? Does it refer to WW Rosin?

Line 217-218:  The diffraction pattern of ZnO appeared in the coating with ……

Line 218: 6 wt.% of KLE but diminished as the additive concentrations increased to  9 wt.% and 12 wt.%.

Line 219: The trend is proportional to

Line 223: What is the role of calcium carbonate?

Line 228: Why is calcium carbonate not identified by its diffraction pattern in the XRD analysis? Its concentration in the coatings too low to be detected?

Line 232: Andrographolide was observed …..at a retention time of 4.65 min. That means the extract of 85% ethanol is entirely composed of this compound? No other peaks noted in the chromatographic analysis?

Line 271: …. was observed in the case of C5…..

Line 305: What could be the possible reason for this shift in the case pf C4 and C5?

Line 318: to the more electropositive value ….

Line 319: … value at 20 and 30 days? Figure 8 needs labeling the plots. Only a and b are  given. It could be confusing.

Line 323: See comment above

Line 357: What is the origin of chromium detected? From the metal or surface deposition after immersion? It would be great to compare the composition of the chromium in the polished metal before immersion.

Line 362: SE or BSE image? Elemental mapping could be more informative when it comes to the distribution of the different elements in the coatings prepared and the changes after immersion in different durations. C5 should be f, not e.

Line 369: How is this percentage calculated? Are the other elements detected ignored for the sake of comparison of only these four elements? The C3 has then accumulated Na and Cl. Wouldn’t this exacerbate corrosion of the metal?

Line 420-421:  Described as major components here because of their potential roles as sites of adsorption on the metal surface or just because of the relative band intensities of these functional groups in the IR spectrum? Identification of andrographolide from the IR spectrum needs also to consider the finger print region.

Line 422-423: Is the retention time mentioned a unique identifier to the compound? Identification of a single component from the extract can be implied. Lines 420-223 need rewriting.

Author Response

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Round 2

Reviewer 1 Report

The authors of the publication supplemented the substitute scheme with an element describing diffusion, but they did not recalculate the results. They are identical in the tables in both versions of the work. This should be verified.

I still don't understand how the Tafel measurements were taken on the coated samples. If the coating is sealed, there is no possibility of any flow of current. If the coating is not tight then it is difficult to define the corrosion rate per unit area, because it is not known what it really is.

The EDX result for pure metal, if other components are omitted, shows the composition of AISI 316L steel used in the tests. And chromium oxide on this type of steel is always present as a layer responsible for its resistance (stainless steel) !!!????

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