Review Reports - Influence of H<sub>2</sub>S and CO<sub>2</sub> Partial Pressures and Temperature on the Corrosion of Superduplex S32750 Stainless Steel

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Reviewer Comments on CMD Manuscript 3604499

“Influence of H₂S and CO₂ Partial Pressure and Temperature on ” by Iglesias and Diaz

This manuscript describes a limited modelling study of the pH in aqueous solution as a function of CO₂ and H₂S partial pressure and temperature and a comparison with CRA corrosion studies from the literature. While the paper is very well written and seemingly well-structured, I found the contents to be rather uninformative. Therefore, I cannot recommend publication of the manuscript in Corrosion and Materials Degradation.

Main Comments

The only simulations that were done appear to be a prediction of the pH as a function of T and gas pressure, which is a trivial use of the capabilities of the OLI software. Where are the comparisons between predicted and observed corrosion products, or corrosion rates, all of which are capabilities of the OLI software? Much of the discussion in Section 3 can be found in text books on aqueous corrosion and was not very informative.

I was also disappointed in the “validation” against experimental data in the literature. The link between the predicted pH values and the information summarized from the literature was not at all clear to me. The connection between the simulations and the behaviour of superduplex S32750 was unclear.

Author Response

We appreciate your thoughtful comments and the time you have taken to review our manuscript. In response to your observations:

Regarding the use of OLI software:

We acknowledge that predicting pH as a function of temperature and gas pressure is a basic feature of the OLI software. However, our intention was to first establish a clear and validated thermodynamic framework before moving on to more complex interpretations.

Regarding corrosion products and corrosion rates:
We appreciate this aspect. In figure 2 we have incorporated new simulations focused on the prediction of stable corrosion products and have compared them with experimental data available in the literature. In addition, corrosion rates under different conditions have been estimated using the relevant modules of the OLI software, and their implications in relation to the superduplex alloy S32750 are now discussed.

We have added: The results were validated by comparing both the pH values and the predicted corrosion products with published experimental data. Comparing the simulated pH values with those measured by Haghi et al. [24] and Peng et al. [25] for CO₂-H₂O systems, showing good agreement up to 125 °C. At higher temperatures, discrepancies can be attributed to the limited accuracy of thermodynamic models under subcritical conditions.

Regarding corrosion products, the prediction of FeCO₃ and FeS as stable solid phases under specific conditions was consistent with studies by Liu et al. [36], who identified thin FeS layers on duplex steels exposed to H₂S. The partial inhibitory effect of FeCO₃ in CO₂-rich environments, as described by De Waard and Lotz [40], was also reproduced.

These agreements between simulation and experimentation support the validity of the obtained results and allow for a more direct connection between the simulated chemical behavior and the real-world response of superduplex stainless steel S32750 under typical geothermal operating conditions.

Regarding validation with experimental data:

We appreciate this observation, as it helped us identify a weakness in the connection between the simulation results and previously published experimental data. We have restructured the corresponding section to better clarify the link between the predicted pH values, the conditions of the experimental studies, and the electrochemical behavior reported for the S32750 alloy. This strengthens the validation of the proposed model.

We have also added the section 3.4:

3.4. Prediction of Corrosion Products and Rates Using OLI Studi

To complement the pH prediction, additional simulations were conducted in OLI Studio using the chemical equilibrium module with solid phase output, with the goal of identifying potentially stable corrosion products in CO₂-H₂O and H₂S-H₂O systems under representative geothermal operating conditions (25–250 °C, 0–20 MPa).

In the presence of CO₂, the simulations predicted the precipitation of ferrous carbonate (FeCO₃), known for forming passivating layers on carbon steels and stainless steels. At temperatures above 120 °C and pH > 4, FeCO₃ appeared as the predominant solid phase, which is consistent with the findings of Dugstad et al. [41] regarding the stabilization of this compound in "sweet" environments.

In contrast, in atmospheres containing H₂S, the formation of sulfide phases such as FeS and FeS₂ (pyrite) was observed, with the latter predominating at pH values between 3 and 5 and temperatures above 150 °C. These compounds are in agreement with corrosion products experimentally identified in duplex steels exposed to H₂S, according to Zhang et al. [35] and Liu et al. [36].

Additionally, species concentration data, redox potential, and temperature were used to estimate theoretical corrosion rates using the rate prediction module in OLI. The predicted rates ranged from 0.1 to 1.2 mm/year, depending on temperature, dominant gas, and the stability of the passive layer. These values align with the results reported by Kvarekvål et al. [38], who observed corrosion rate increases of up to sixfold in the presence of small amounts of H₂S.

Regarding the discussion in Section 3:

We agree that some aspects of the discussion may have appeared general or well-known. Therefore, we have revised the text to focus more specifically on the implications of these concepts within the particular context of our study, especially concerning the behavior of superduplex stainless steel in specific environments.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have not done a thorough literature search on relevant data to compare the modelling results thus the conclusion of the study is based on overstatements and are not supported by the results. Also, several errors are found in the text regarding what is stated in the references and what is stated in the manuscript. Additionally, there are numerous errors in statements were the modelling results are used to state the resistance of super duplex stainless steel in geothermal fluid but there is no data to support these claims, not from literature review or from the current study (e.g. corrosion rates, microstructural and chemical composition analysis). Thus, the recommendation from the reviewer is to reject this manuscript. Please see the attached file with further comments from the reviewer to support this recommendation.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Medium quality.

Author Response

Response to Reviewer 2:

Reviewer 2. 2ª round

We sincerely thank the reviewer for the time, dedication, and thoughtful comments provided during the evaluation of our manuscript. We greatly appreciate the careful reading and constructive feedback, which have contributed to improving the quality and clarity of our work.

Line 136. See comment in reference list, the reviewer could not find this paper anywhere, must be incorrect name of author or name of paper??

The reference has beeb changed by: [21] RG Tayactac RG, Basilia B, Corrosion in the Geothermal Systems: A Review of Corrosion Resistance Alloy (CRA) Weld Overlay Cladding Application. IOP Conf. Ser.: Earth Environ. Sci. 2022; 1008 012018.

Line 174: Fix gramar

We have changed by: This may be due to the limitations of existing models for high temperatures and to the fact that low-temperature are much simpler than high-temperature simulations.

Line 225-226: This part of the sentence should be rephrased, i.e. "and can give insight in the connection of pH and partial pressures of H2S and CO2 and the possible formation of corrosion products in H2S/CO2 environments as in geothermal and oil and gas environments".

We have changed the paragraph: "The agreement between simulation and experimental results supports the validity of the findings and provides insight into the relationship between pH and the partial pressures of H₂S and CO₂, as well as the potential formation of corrosion products in H₂S/CO₂ environments, such as those found in geothermal and oil and gas settings."

Line 337: rephrase ...according to the model would be ....

We have changed the phrase: According to the model, when the partial pressure of H₂S predominates over that of CO₂, acidic conditions prevail, and the primary corrosion product formed on superduplex steel is mainly determined by the H₂S content.

Line 340: need for better explanation, i.e. adjusted from

We have changed de figure chaption: Corrosion behavior in the presence of H₂S and CO₂, showing the predominant mechanisms and product formation as a function of the relative partial of both gases [9].

Line 341: As mentioned by Liu et al. [32]

We have changed by Wang et al. [40].

I would like to once again express my sincere appreciation for your patience and the considerable effort you have devoted to the review process. Your careful reading and thoughtful feedback have been truly valuable in helping us improve the m

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors,

the paper sounds very good since you combined the use of models with existing research data.

I propose review these issues in order to improve your paper:

1.- Use correct suscript for compounds, in particular in introduction section ( you write FeCO3 instead FeCO₃) and so on.

2.-The resolution of Figures 2, 3, 4 and 5 should be improved

3.- In conclusion section you say ( line 364) that "the structure of passive layer......". It should be the chemical composition of this passive layer and /or sequence of deposition of different corrosion products and or both issues.

4.- It should be interesting to consider in your analysis/conclusions the potential effect of the particular stainless steel related to the study, since minor variations on its chemical composition could alter the behaviour against corrosion in these particular environment.

5.- Review the reference "Fuentes et al 2007"( line 165). It appears without related number neither at references section

Author Response

Response to Reviewer 3:

We sincerely thank the reviewer for their constructive and insightful comments, which have helped us to improve the clarity and scientific quality of the manuscript.

1.- Use correct suscript for compounds, in particular in introduction section ( you write FeCO3 instead FeCO₃) and so on.

We appreciate this valuable observation. The formula of the compounds has been written correctly

2.-The resolution of Figures 2, 3, 4 and 5 should be improved

Thank you for pointing this out. The resolution of figures 2, 3, 4 and 5 has been improved

Thank you for your valuable observation. We agree that referring to the "structure" of the passive layer may be ambiguous or insufficient in the context of corrosion science. In response, we have revised the text in line 364 to specify that our discussion refers to the chemical composition of the passive layer and/or the sequence of deposition of corrosion products, as these are more relevant parameters when analysing the protective properties and stability of passive films. This clarification strengthens the interpretation of our findings.

Original:
"The structure of the passive layer was found to..."

Revised:
"The chemical composition of the passive layer, and the sequential deposition of corrosion products, were found to..."

Thank you for your insightful comment. We fully agree that minor variations in the chemical composition of stainless steels can influence their corrosion resistance, particularly under the specific environmental conditions addressed in this study. In response, we have included a paragraph in the Conclusions section acknowledging this aspect and highlighting the need for further investigations to assess the role of compositional differences in corrosion behaviour. This is consistent with previous findings indicating that even small changes in alloying elements (such as Mo, Cr, or N) can significantly affect localized corrosion resistance

5.- Review the reference "Fuentes et al 2007"( line 165). It appears without related number neither at references section

Thank you for this helpful comment.

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

The authors calculated the pH values with different partial pressure and temperature of H2S and CO2 in geothermal fluids used OLI software. The pH of the fluids influence the corrosion of super duplex S32750 steel. Super duplex S32750 is susceptible to localized corrosion. Have you simulated the repassivation potentials with different partial pressure and temperature of the gases in the thermal fluids?

Questions:

Line 46: is seawater involved in the internal pipe corrosion?

Delete the solid lines between lines 270 and 271.

Line 289: Table caption should be above the table.

Check the exponents of the H2S vs CO2 in the Table 1. It should be positive, right?

Author Response

Response to Reviewer 4:

We sincerely thank the reviewer for their constructive and insightful comments, which have helped us to improve the clarity and scientific quality of the manuscript.

Line 46: is seawater involved in the internal pipe corrosion?

We appreciate this observation. We have added: However, when this passive layer is altered or destroyed by factors such as salinity, the presence of microorganisms, and the electrochemical conditions of seawater, both internal and external corrosion of the pipes can accelerate, compromising the integrity of the metallic material over time.

Delete the solid lines between lines 270 and 271.

Thank you for your valuable observation, continuous lines have been deleted

Line 289: Table caption should be above the table.

We appreciate this observation, the table caption has been changed

Check the exponents of the H2S vs CO2 in the Table 1. It should be positive, right?

Than you for your observation, the exponents are negative. CO₂ is poorly soluble and its conversion to H₂CO₃ is limited and H₂S is a moderately soluble gas.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed my original comments.

Author Response

Thank you. I would like to once again express my sincere appreciation for your patience and the considerable effort you have devoted to the review process.

Reviewer 2 Report

Comments and Suggestions for Authors

Please see the attached file for the reviewer comments.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Could be improved.

Author Response

Reviewer 2. 2ª round

Line 136. See comment in reference list, the reviewer could not find this paper anywhere, must be incorrect name of author or name of paper??

Line 174: Fix gramar

We have changed by: This may be due to the limitations of existing models for high temperatures and to the fact that low-temperature are much simpler than high-temperature simulations.

Line 337: rephrase ...according to the model would be ....

Line 340: need for better explanation, i.e. adjusted from

Line 341: As mentioned by Liu et al. [32]

We have changed by Wang et al. [40].

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed the comments from the reviewer thus the recommendation is to accept in present form.