Alternating Current Interference as a Plausible Dominant Factor Affecting Corrosion Risk in a Mixed Steel/Polyethylene Urban Gas Distribution Pipeline: A Field Case Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript presents an interesting and relevant field case study on AC interference in mixed steel/polyethylene gas pipelines. The topic is important from a practical engineering perspective and fits well within the scope of the journal. However, the paper requires several improvements before it can be considered for publication.

First, the title should be revised. It currently starts with an acronym (“AC”), which is not recommended. The full term “Alternating Current” should be used instead. Also, the title is a bit long and could be simplified.

Figure 1 is of low quality and needs to be redrawn. The schematic is not very clear, labels are difficult to read, and overall it looks more like a draft figure than a publication-ready one. This should be significantly improved.

The methodology is generally acceptable, but some aspects could be described in more detail. In particular, more information about soil conditions and environmental parameters would strengthen the analysis. Also, the interpretation of corrosion risk is based mainly on indirect indicators (potential, AC voltage), without direct validation (e.g., metal loss, inspection data), which should be clearly acknowledged and slightly toned down.

The results are interesting, but the discussion is sometimes a bit repetitive and could be more concise. In some places, the conclusions seem slightly stronger than what is directly supported by the data (e.g., “dominant corrosion driver”).

The proposed mitigation strategy is relevant, but it is not validated with post-installation data. At this stage, it should be presented more as a proposed or conceptual approach rather than a confirmed solution.

The English language is generally good, but the text could be improved for clarity. Some sentences are quite long and could be simplified.

Overall, the manuscript has good potential, but requires revision, especially in terms of figure quality, clarity of presentation, and careful interpretation of results..

Author Response

The manuscript presents an interesting and relevant field case study on AC interference in mixed steel/polyethylene gas pipelines. The topic is important from a practical engineering perspective and fits well within the scope of the journal. However, the paper requires several improvements before it can be considered for publication.

Response:
We sincerely thank the Reviewer for the careful reading of the manuscript and for the constructive and encouraging comments. We appreciate the positive assessment of the practical relevance of the study and its fit within the scope of the journal. We have revised the manuscript accordingly and addressed the Reviewer’s comments point by point, as detailed below.

Comment 1

First, the title should be revised. It currently starts with an acronym (“AC”), which is not recommended. The full term “Alternating Current” should be used instead. Also, the title is a bit long and could be simplified.

Response:
Thank you for this observation. The title was revised so that the acronym "AC" was replaced by the full term "Alternating Current", and the wording was adjusted to better reflect the risk-oriented interpretation. We also reconsidered the wording of the title in order to improve readability while preserving the central focus of the paper.

Comment 2

Figure 1 is of low quality and needs to be redrawn. The schematic is not very clear, labels are difficult to read, and overall it looks more like a draft figure than a publication-ready one. This should be significantly improved.

Response:
We agree with the Reviewer. Figure 1 has been redrawn and improved in order to increase clarity, readability of labels, and overall publication quality. The revised version is cleaner, more compact, and better aligned with the graphic standard expected for a journal manuscript.

Comment 3

The methodology is generally acceptable, but some aspects could be described in more detail. In particular, more information about soil conditions and environmental parameters would strengthen the analysis. Also, the interpretation of corrosion risk is based mainly on indirect indicators (potential, AC voltage), without direct validation (e.g., metal loss, inspection data), which should be clearly acknowledged and slightly toned down.

Response:
We thank the Reviewer for this important remark. The manuscript was revised in two ways.

First, additional information was introduced regarding the local soil conditions in the buried zone. The revised text now specifies that the investigated buried section was located in a roadside urban corridor with paved road infrastructure and adjacent green areas, and that the local soil was a yellow clayey material. We also added compositional information from a representative soil sample, showing a clay-rich matrix, and clarified its possible relevance to moisture retention and electrochemically unfavorable local conditions. This addition was intended to strengthen the environmental description without overstating the available dataset.

Second, we agree that the corrosion-risk interpretation is based on indirect electrical and electrochemical indicators, rather than on direct wall-loss evidence. For this reason, the relevant passages in the manuscript were revised and toned down. In addition, the Study Limitations subsection was strengthened to state explicitly that:
(i) the study is based on a single field case;
(ii) monitoring was limited to one 24 h period;
(iii) no direct metal-loss, excavation-based, or wall-thickness validation data were available; and
(iv) the present interpretation should therefore be regarded as risk-oriented rather than deterministic.

Comment 4

The results are interesting, but the discussion is sometimes a bit repetitive and could be more concise. In some places, the conclusions seem slightly stronger than what is directly supported by the data (e.g., “dominant corrosion driver”).

Response:
We appreciate this comment and have revised the manuscript accordingly. Several passages in the Results, Discussion, Prognosis, and Conclusions sections were made more concise and more cautious in tone. In particular, formulations implying direct proof of corrosion severity were replaced by wording such as “consistent with elevated corrosion risk”, “most plausible principal source of the recorded electrical perturbation”, and similar expressions that more accurately reflect the evidential basis of the study. The revised manuscript therefore presents the findings in a more balanced and defensible manner.

Comment 5

The proposed mitigation strategy is relevant, but it is not validated with post-installation data. At this stage, it should be presented more as a proposed or conceptual approach rather than a confirmed solution.

Response:
We fully agree. The manuscript was revised to clarify that the mitigation system is a proposed engineering concept, not a validated field-confirmed solution. The revised text consistently presents the mitigation strategy as a proposed mitigation and validation approach, and the limitations section now explicitly states that post-installation validation data are not yet available. In addition, the workflow shown in Section 5 was introduced to clarify that the proposed solution is intended to be verified through before-and-after monitoring rather than assumed to be confirmed in advance.

Comment 6

The English language is generally good, but the text could be improved for clarity. Some sentences are quite long and could be simplified.

Response:
Thank you. The manuscript was carefully revised for clarity, readability, and language consistency. Several long or overly dense sentences were simplified, and the text was edited to improve overall fluency. Particular attention was given to the Abstract, Introduction, Results interpretation, and Conclusions in order to make the argumentation more direct and easier to follow.

Final response

We once again thank the Reviewer for the constructive and helpful comments. We believe that the revised manuscript is clearer, more balanced in interpretation, and stronger both in presentation and in methodological transparency as a result of these revisions.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript reports a field case study of a mixed steel/polyethylene gas distribution pipeline near an electrified railway, using 24 h monitoring of pipe-to-soil potential, AC pipe-to-soil voltage, waveform visualization, and coating insulation resistance to argue that AC interference is the main corrosion driver. The major concerns are listed below.

1. The study is fundamentally based on a single field case without any comparative analysis, parametric variation, or benchmarking against similar systems.
2. The manuscript infers that AC interference is the dominant corrosion driver, but it does not present direct corrosion evidence. The dataset only shows electrical disturbance and a non-protective potential range, which is not sufficient to prove active corrosion dominance.
3. The monitoring duration is restricted to a single 24 h period, which is inadequate to capture temporal variability related to train operation, seasonal soil conditions, or long-term electrochemical evolution.
4. The nearby electrified railway is assumed to be the principal interference source mainly because the pipeline runs parallel to it. However, the manuscript admits other hazards may exist, yet still presents the railway as dominant without adequate proof.
5. The manuscript lacks the physical parameters needed to support the proposed mitigation logic. The proposed polarized decoupling and dedicated grounding scheme is not grounded in enough site-specific engineering data.
6. The presentation of the Figures can be improved. Figure 1 takes up an entire page and needs to be adjusted. It is suggested to keep only one of Figures 3 and 4.
7. There is an issue with the chapter numbering; after Section 3.3, it changes to 4.3.
8. The text for the result analysis is too much. A flowchart is recommended in Section 5.
9. There are so many references in the Abstract.

Author Response

The manuscript reports a field case study of a mixed steel/polyethylene gas distribution pipeline near an electrified railway, using 24 h monitoring of pipe-to-soil potential, AC pipe-to-soil voltage, waveform visualization, and coating insulation resistance to argue that AC interference is the main corrosion driver. The major concerns are listed below.

Response:
We sincerely thank the Reviewer for the careful reading of the manuscript and for the detailed and constructive comments. We appreciate the concerns raised regarding the scope, level of proof, and presentation of the study. In response, the manuscript was revised substantially in order to improve clarity, moderate the interpretation, strengthen the discussion of limitations, and better define the paper as a field-based risk-oriented case study rather than a deterministic corrosion proof. Our detailed responses are provided below.

Comment 1

The study is fundamentally based on a single field case without any comparative analysis, parametric variation, or benchmarking against similar systems.

Response:
We agree with the Reviewer. The study is indeed based on a single field case, and we do not claim statistical generality or universal threshold derivation. To address this point explicitly, the Study Limitations section was strengthened to state that the manuscript is based on one site and should therefore be interpreted as a field-based case study and as a contribution to diagnosis and mitigation planning under realistic service conditions, rather than as a comparative or universally predictive study. The text was revised accordingly.

Comment 2

The manuscript infers that AC interference is the dominant corrosion driver, but it does not present direct corrosion evidence. The dataset only shows electrical disturbance and a non-protective potential range, which is not sufficient to prove active corrosion dominance.

Response:
We appreciate this important observation and agree that the original wording was too strong in some places. The manuscript has therefore been revised to moderate the interpretation and to avoid implying direct proof of active corrosion dominance. In the revised version, the findings are expressed in terms such as “electrochemical conditions associated with elevated corrosion risk”, “most plausible principal source of the recorded electrical perturbation”, and “conditions favorable to localized corrosion development”, rather than as direct evidence of corrosion rate or metal loss. We also strengthened the limitations section to clarify that no direct wall-loss, excavation-based, or metal-loss validation data were available. The revised manuscript now consistently presents the results as risk-oriented and indirect, not deterministic.

Comment 3

The monitoring duration is restricted to a single 24 h period, which is inadequate to capture temporal variability related to train operation, seasonal soil conditions, or long-term electrochemical evolution.

Response:
We agree. The revised manuscript now explicitly states that the 24 h monitoring interval is sufficient for baseline diagnosis but not for full characterization of seasonal variability, longer-term railway operating fluctuations, or long-term electrochemical evolution. This limitation is now stated clearly in Section 6.3. In addition, Section 5 and the newly added workflow figure clarify that the proposed engineering approach is intended to be followed by post-installation and repeated follow-up monitoring, rather than interpreted as a one-step final diagnosis.

Comment 4

The nearby electrified railway is assumed to be the principal interference source mainly because the pipeline runs parallel to it. However, the manuscript admits other hazards may exist, yet still presents the railway as dominant without adequate proof.

Response:
Thank you for this remark. We revised the manuscript in order to make this interpretation more cautious and more explicitly justified. The revised text now states that no other nearby electrical infrastructures with interference potential comparable to that of the adjacent electrified railway were identified within the analyzed site perimeter. On this basis, the railway is now described as the most plausible principal source of the recorded alternating-current perturbation, rather than as an absolutely proven exclusive source. This wording was adopted specifically to avoid overstatement while preserving the engineering interpretation supported by the site configuration and field measurements.

Comment 5

The manuscript lacks the physical parameters needed to support the proposed mitigation logic. The proposed polarized decoupling and dedicated grounding scheme is not grounded in enough site-specific engineering data.

Response:
We understand this concern and have clarified the status of the mitigation proposal throughout the revised manuscript. The mitigation system is now presented explicitly as a proposed engineering concept and validation-oriented workflow, not as a fully optimized or already validated final design. The revised text states clearly that post-installation validation is still required and that the proposed mitigation logic should be understood as a practical framework for implementation and verification. In addition, Section 5 was strengthened through the inclusion of a mitigation and validation workflow figure, which makes explicit that the proposed solution must be followed by baseline monitoring, installation, post-installation verification, and performance evaluation.

Comment 6

The presentation of the Figures can be improved. Figure 1 takes up an entire page and needs to be adjusted. It is suggested to keep only one of Figures 3 and 4.

Response:
We agree that the presentation of the figures required improvement. Figure 1 has been revised in order to improve readability and overall layout. With regard to Figures 3 and 4, we respectfully retained both because they serve different functions in the manuscript. Figure 3 documents the field measurement configuration, whereas Figure 4 presents the representative mixed AC/DC waveform used in the electrical interpretation of the case. To address the Reviewer’s concern, the role of each figure was clarified in the text and captions, and the overall figure presentation was improved.

Comment 7

There is an issue with the chapter numbering; after Section 3.3, it changes to 4.3.

Response:
Thank you for noting this error. The section numbering was corrected in the revised manuscript. The current version now follows a consistent structure throughout the paper.

Comment 8

The text for the result analysis is too much. A flowchart is recommended in Section 5.

Response:
We appreciate this suggestion and have implemented it. A flowchart was added in Section 5 in order to summarize the proposed mitigation and validation logic of the study. This addition improves readability and makes the practical sequence of diagnosis, critical-location identification, mitigation design, installation, verification, and follow-up easier to follow. The text was also revised in several places to improve concision and reduce repetition.

Comment 9

There are so many references in the Abstract.

Response:
We agree. The Abstract was fully revised and the references were removed in order to make it more concise, clearer, and more consistent with the Reviewer’s recommendation. The revised abstract now focuses on the essential context, measurements, findings, and practical implication of the study without bibliographic overload.

Final response

We thank the Reviewer once again for the detailed and valuable comments. We believe that the revised manuscript is now more cautious in interpretation, clearer in scope, improved in presentation, and more explicit regarding both its limitations and its practical contribution as a field-based case study.

Reviewer 3 Report

Comments and Suggestions for Authors

The novelty of this manuscript lies in the fact that it presents a real field case showing how, in a mixed steel/polyethylene urban gas distribution pipeline, an environment adjacent to an electrified railway can affect the electrical disturbance and corrosion risk of the buried steel transition section. In particular, the manuscript presents 24 h pipe-to-soil potential, 24 h AC pipe-to-soil voltage, a mixed AC/DC waveform, and coating insulation resistance at approximately eight months after installation, and it further connects these observations to a practical mitigation concept. In this respect, the study clearly contains elements that may be of interest to practical readers.
Another strength is that the authors do not frame the issue merely as a general discussion of corrosion. Instead, they explain the problem by bringing together the actual field configuration--namely the mixed layout, grounded aboveground steel, buried transitions, and railway coupling--as a single integrity problem. This type of manuscript is different from a laboratory-based paper on coating development, but it may still serve as a useful case for practitioners involved in pipeline operation, inspection, and design by prompting them to consider which sections may be vulnerable under real service conditions.
That said, the novelty is closer to case-specific applied novelty than to broader academic novelty. AC corrosion itself has already been well discussed in ISO 18086, the review by Brenna and co-workers, and AMPP/NACE guidance documents, and it is also well established that the likelihood of AC corrosion cannot be determined by voltage alone. Therefore, the contribution of this manuscript is not the discovery of a new mechanism or the proposal of a new coating concept, but rather a case report in which existing principles are applied to a specific field pipeline system.
For this reason, given the nature of the manuscript, I would recommend transfer, and I respectfully ask for the authors' understanding in this regard.

Author Response

The novelty of this manuscript lies in the fact that it presents a real field case showing how, in a mixed steel/polyethylene urban gas distribution pipeline, an environment adjacent to an electrified railway can affect the electrical disturbance and corrosion risk of the buried steel transition section...

Response:
We sincerely thank the Reviewer for the careful reading of the manuscript and for the constructive and balanced assessment. We particularly appreciate the recognition that the manuscript presents a real field case of practical relevance and that it frames the investigated configuration—mixed steel/polyethylene layout, grounded aboveground steel, buried transitions, and railway coupling—as an integrated integrity problem rather than as a purely general corrosion discussion.

We also appreciate the Reviewer’s observation regarding the nature of the novelty. We agree that the contribution of the present work is primarily case-specific and applied, rather than the proposal of a new corrosion mechanism or a new coating concept. In response to this point, the manuscript was revised to clarify this positioning more explicitly. The revised version now presents the paper as a field-based case study in which established AC-interference and corrosion principles are applied to a specific mixed urban gas pipeline system under realistic service conditions. Several formulations that could have suggested a broader mechanistic claim were moderated accordingly.

We also revised the manuscript to make its practical scope clearer by:

• moderating statements that were too strong regarding corrosion proof;
• clarifying that the interpretation is risk-oriented rather than deterministic;
• explicitly stating the limitations of the study, including the single-case basis, the 24 h monitoring duration, and the absence of direct wall-loss validation; and
• presenting the mitigation strategy as a proposed and validation-oriented engineering concept, rather than a fully confirmed solution.

We respectfully acknowledge the Reviewer’s recommendation regarding transfer. At the same time, we hope that, in its revised form, the manuscript may still be considered suitable for the present journal and Special Issue because its principal contribution lies in the monitoring, interpretation, and mitigation planning of a realistic buried urban infrastructure case, which we believe aligns well with the practical and field-oriented scope of the issue.

We thank the Reviewer again for the thoughtful comments, which helped us improve the manuscript substantially.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised version of the manuscript is clearly improved compared to the previous submission. The authors have addressed the main comments.

The title has been corrected and now follows standard scientific practice. Figure 1 has also been significantly improved and is now much clearer and easier to interpret. The overall presentation is better and the interpretation is more balanced than before.

The addition of information on soil conditions is appreciated and improves the technical context of the study.

There are still a few minor points. The proposed mitigation strategy is not validated with post-installation data, so this should be clearly emphasized as a proposed or conceptual approach. In addition, the manuscript could still be slightly shortened in some sections to improve readability..

Overall, the manuscript is now in a good shape and can be accepted after minor revision.

Author Response

We would like to thank the Reviewer for the careful evaluation of our manuscript and for the constructive comments provided. We are pleased that the revised version is considered significantly improved and closer to publication quality.

Regarding the remaining minor comments:

1. Mitigation strategy (conceptual vs. validated approach)
   We agree with the Reviewer that the proposed mitigation strategy should not be interpreted as a validated solution. In the revised manuscript, we have explicitly clarified that the proposed approach represents a conceptual engineering solution derived from the present field observations and has not yet been validated by post-installation measurements. This clarification has been introduced in Section 5 (Mitigation Strategy and Validation Logic) and further emphasized in Section 6.3 (Study limitations).

2. Soil conditions and environmental context
Following the Reviewer’s suggestion, we have strengthened the description of the local soil conditions. In addition to the compositional analysis, we have included the measured soil pH (5.5) and clarified the implications for electrochemical behavior, while also explicitly stating the limitations due to the absence of direct resistivity and full soil characterization data (Sections 2.2 and 6.3).

3. Clarity and readability of the manuscript
The manuscript has been carefully revised to improve clarity and readability. Several sentences have been simplified, and minor reductions of repetitive statements have been performed in the discussion sections.

We believe that these revisions fully address the Reviewer’s remaining concerns and have further improved the technical clarity and balance of the manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript has been revised thoroughly and can be accepted in its present form. 

Author Response

We would like to sincerely thank the Reviewer for the positive evaluation of the revised manuscript. We appreciate the time and effort invested in reviewing our work and are pleased that the manuscript is now considered suitable for publication in its present form.

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you very much for the opportunity to review this valuable manuscript. I believe that several points require further consideration in order to raise the overall quality of the paper.

Q1) The Introduction describes, in a reasonably natural way, the background for the use of mixed steel/polyethylene gas distribution pipelines in congested urban areas and the vulnerability of buried steel transitions. That said, in its current form, it does not yet make sufficiently clear what specific gap this manuscript fills in comparison with the existing literature. In particular, while the general discussion of AC interference, the vulnerability of mixed pipelines, and the influence of electrified railways is well presented, it would be helpful to state more directly what this manuscript newly demonstrates, as compared with previous field studies or modeling studies. At the end of the Introduction, please clarify more explicitly why this case is academically necessary in relation to the existing body of work. 

Q2) Throughout the manuscript, the nearby electrified railway is interpreted as the "principal source" of the recorded electrical perturbation. Based on the data currently presented, it is convincing that the railway is a highly plausible contributing factor, but the evidence does not appear sufficient to conclude, after effectively excluding other possible electrical disturbances, that it is the principal source. The proximity to the railway, the approximately 12 m parallel alignment, and the detection of AC voltage at points A-H are important observations, but they do not seem enough on their own to support source attribution at the current level of certainty. I would therefore recommend either moderating the wording in this regard or strengthening the explanation that supports the source-identification logic. 

Q3) The 24 h monitoring results presented in the manuscript are a central basis for the main argument, yet the term "weighted mean" used in Table 2 and Table 3 is not defined. Since this value is subsequently used as the basis for interpreting spatial trends and risk severity, the calculation method and its meaning should be clearly explained so that readers can interpret the numbers correctly. If the term is used without definition, the subsequent interpretation becomes unstable. Please revise the Methods section accordingly and state this point explicitly. 

Q4) The numerical inconsistency between Figure 4 and Table 3 needs to be corrected. In Table 3, the weighted mean AC pipe-to-soil voltage at point E is given as 0.95 Vrms, whereas the description of the representative waveform in Figure 4 states an AC effective value of 0.35 Vrms for point E. If these are the same physical quantity measured at the same location, the discrepancy is too large to ignore. If they correspond to different measurement definitions, different reference points, different filtering conditions, or data obtained at different times, this distinction should be made very clear both in the main text and in the figure caption. This issue directly affects the credibility of the manuscript in its present form. 

Q5) The interpretation of the pipe-to-soil potential values needs to be framed more carefully. The manuscript uses the values in Table 2 to support statements such as elevated corrosion risk or electrochemically unfavorable conditions, but it does not yet explain clearly enough under what exact conditions these potential values were measured or what limitations they carry as field measurements obtained under interference conditions. In the current form, readers may take these values as if they were direct quantitative indicators of corrosion severity. It would therefore be better to state more explicitly that these values are not direct measurements of corrosion rate, but field indicators obtained under interference conditions. 

Q6) The conversion of 0.41-1.23 Vrms to approximately 1.6-3.5 Vpp should be checked again. Since the magnitude and waveform of the electrical signal are used as important evidence in the paper, the definitions and conversions of parameters such as Vrms, Vpp, and mean DC component should be strictly accurate. At present, the lower end of the range appears somewhat awkward from a calculation standpoint, and this is not merely a minor numerical detail but a point that affects the overall credibility of the electrical interpretation. Please review the related numerical expressions once again in both the main text and the figure captions. 

Q7) The manuscript uses the coating insulation resistance results to support the statement that the "global coating condition remains relatively good," but the description of the coating itself is very limited. Aside from noting that the coating system is a cold-applied polyethylene-based insulating layer, there is not enough information to evaluate the actual integrity of the coating. Since details such as coating thickness, field application condition, inspection results immediately after installation, and any direct information related to the possibility of local defects are lacking, the coating currently appears to function more as a supporting indicator than as a core analytical subject. If the authors wish to maintain a form that is well suited to Coatings, the depth of the coating-related description and interpretation should be strengthened. , [mdpi.com], [mdpi-res.com]

Q8) The statement that "global insulation resistance does not exclude local defects" is, in itself, technically reasonable. That said, in the present manuscript, this point seems to go beyond a general engineering remark and at times reads as though local defects are actually governing the corrosion behavior in this specific case. Based on the evidence currently presented, it does not appear possible to conclude that the existence of local defects has been directly confirmed. It would therefore be advisable to distinguish more clearly between what remains a plausible possibility and what has actually been verified in this case. Otherwise, the interpretation may appear to run ahead of the data. 

Q9) The interpretation that higher AC exposure is observed in the longer metallic section is, overall, a reasonable one. Even so, if this point is to serve as a key part of the manuscript's logic, the structural information for each metallic segment should be presented more systematically, including segment length, buried length, exposed length, support structure arrangement, and proximity to grounding. At present, the overall direction is understandable, namely that the E-H section is relatively longer and more unfavorable, but it is still difficult for the reader to follow, in quantitative terms, why that geometry leads to greater coupling or greater risk. Please clarify this structural comparison more explicitly either in Figure 1 or in the main text. 

Q10) The explanation related to the soil conditions also needs improvement. The manuscript presents the oxide composition of a clay-rich yellow clayey material and connects this to moisture retention and electrochemically unfavorable conditions. Yet, when discussing corrosion risk for buried steel, the more directly relevant variables are soil resistivity, pH, moisture condition, drainage, and ionic environment. Since the authors themselves state that direct field resistivity measurements were not performed, the current soil description is meaningful at the level of background information, but it is not sufficient to serve as a strong basis for corrosion-risk interpretation. I would therefore suggest either reducing the strength of the interpretation here or stating the limitations of the present data more explicitly. 

Q11) The integrated interpretation framework in Section 3 is conceptually organized, but it occupies a somewhat large portion of a manuscript based on a single field case. As a result, the overall character of the paper becomes slightly ambiguous: it is not entirely clear whether this is primarily a field case report or a broader paper proposing a decision framework. Given that the present study is limited to a single case, it may be more natural to compress the framework so that it mainly supports the interpretation of the results. If the authors wish to maintain this framework as a central contribution, further support would be needed to show how the procedure can be validated or generalized beyond the present case. 

Q12) The mitigation strategy and validation logic presented in Section 5 also seem somewhat ahead of the current level of data. The idea of proposing polarized electrical decoupling and dedicated grounding as an engineering option is understandable, but the paper does not yet provide post-installation validation data. For that reason, it would be more appropriate to present the mitigation as a proposed practical option rather than as a rather established solution. In its current form, the manuscript risks giving proposal and validation the same weight, which blurs the boundary of what has actually been demonstrated. 

Q13) The service-life scenario and reinspection horizon proposed in Section 7 require especially careful handling. The authors explicitly state that deterministic remaining-life calculation is not possible, yet they then present relatively specific time ranges such as 2-5 years, 5-8 years, and 8-12 years. To justify such specific horizons, one would expect corresponding long-term validation or direct degradation evidence. The present dataset is meaningful at the level of screening-type interpretation, but it does not seem sufficient to support such specific time ranges. In my view, this part would be much more appropriate if it were removed or adjusted to a qualitative recommendation rather than a quantitative time horizon. 

Q14) The connection between the Results and the Discussion would also benefit from further refinement. At present, the manuscript presents the observed values and then moves rather quickly to expressions such as electrochemically unfavorable regime, localized corrosion development, and integrity-critical locations, without fully building the intermediate explanatory steps. What the reader most wants to understand is how the measured voltages and potentials lead to corrosion-risk interpretation at this level. At the moment, that connection exists in words, but the engineering argument still feels somewhat sparse. It would therefore be helpful if the manuscript more clearly separated which interpretations are directly supported by observation and which parts remain reasonable inferences based on the literature. 

Q15) The Figures and Tables are generally readable, but some of the quantitative information needed to support the engineering interpretation could be presented more clearly. Figure 1 and Figure 2 are helpful for understanding the general layout, but the interpretation of the whole paper would become much easier if the actual lengths, buried transitions, grounding points, and the relationship between the support structures and the buried steel sections were shown more explicitly. Figure 5 also illustrates the overall trend, but its presentation could still be improved so that readers can grasp the spatial severity more clearly. It would be beneficial if the figures were strengthened not merely as illustrations, but as central supporting evidence for the logical development of the paper. 

Author Response

Response to Reviewer 3

We sincerely thank the Reviewer for the careful reading of the manuscript and for the detailed and constructive comments. We appreciate the Reviewer’s recognition of the value of the field case, and we have revised the manuscript carefully in order to improve its clarity, technical rigor, and balance of interpretation. Our point-by-point responses are provided below.

Q1. Clarification of the specific gap and contribution

Response:
We agree with the Reviewer that the specific contribution of the paper needed to be stated more explicitly. The end of the Introduction was revised to clarify that the manuscript does not propose a new corrosion mechanism, but rather documents a real urban field case in which a mixed steel/polyethylene pipeline configuration, grounded aboveground steel sections, buried steel transitions, and proximity to an electrified railway are evaluated together as a single integrity problem under realistic service conditions. The revised Introduction now also explains more clearly how this field-based contribution differs from generalized criteria, laboratory studies, and predictive modeling approaches.

Q2. Source attribution for the railway interference

Response:
We thank the Reviewer for this important remark. The wording throughout the manuscript was moderated. Instead of presenting the railway as an unequivocally proven principal source, the revised text now refers to it as the most plausible dominant contributing source of the recorded alternating current perturbation under the investigated site conditions. We also explicitly state that other electrical influences cannot be entirely excluded. This clarification was introduced in the Abstract, Introduction, Site Electrical Environment section, and Conclusions.

Q3. Definition of “weighted mean”

Response:
We agree that this term required clarification. The Methods section was revised to define the weighted mean as a practical representative value of the monitored 24 h behavior, used for comparison among locations under the same monitoring conditions. We also clarified that these values are intended for spatial comparison of relative severity and should not be interpreted as direct corrosion-rate measurements or as high-resolution statistical descriptors of the signal. This clarification was added in Sections 2.3.1, 2.3.2, and 2.4.

Q4. Numerical inconsistency between Figure 4 and the AC-voltage table

Response:
We appreciate the Reviewer’s attention to this issue. The manuscript was revised to distinguish clearly between the two quantities. The AC value shown in Figure 4 corresponds to a representative waveform snapshot recorded at point E for qualitative illustration of the mixed AC/DC regime, whereas the value reported in the AC-voltage table corresponds to the 24 h weighted mean alternating current voltage used for comparison among locations. This distinction is now stated explicitly in the main text and in the caption of Figure 4.

Q5. Interpretation of pipe-to-soil potential values

Response:
We agree and revised the manuscript accordingly. The text now states more explicitly that the pipe-to-soil potential values are field indicators obtained under interference conditions, not direct quantitative measurements of corrosion rate or corrosion severity. This clarification was introduced in the Methods section and reinforced in the Results section, especially in the interpretation of Table 4.

Q6. Conversion from Vrms to Vpp

Response:
Thank you for this observation. The numerical conversion was rechecked and corrected. The manuscript now states that the recorded alternating current voltages of 0.41–1.23 Vrms correspond, assuming a sinusoidal waveform, to approximately 1.2–3.5 Vpp rather than 1.6–3.5 Vpp. The relevant expressions in the text were revised accordingly.

Q7. Strengthening the coating-related description

Response:
We agree that the coating-related description needed to be strengthened. Section 2.3.4 was substantially expanded. The revised manuscript now describes the coating system as a cold-applied polyethylene-based POLIBAND system with a two-layer structure consisting of an inner adhesive layer and an outer polyethylene insulating layer. We also included manufacturer-declared characteristics such as dielectric strength, volume resistivity, water absorption, adhesion, and compliance with ISO 21809 and DIN 30670, together with the field insulation-resistance results measured after approximately eight months of burial. A dedicated table summarizing both manufacturer data and field performance indicators was added to support the coating-related interpretation more explicitly.

Q8. Local defects: plausible vs. verified

Response:
We appreciate this distinction and revised the manuscript to reflect it more carefully. The text now states that localized coating defects remain a plausible but not directly verified contributing factor. We no longer present local defects as if they had been demonstrated to govern corrosion behavior in this specific case. This distinction was clarified both in the coating section and in the interpretation of the insulation-resistance results.

Q9. Structural comparison of the metallic sections

Response:
We agree that this comparison needed to be presented more systematically. The manuscript now includes a dedicated table summarizing the main structural characteristics of the metallic sections, including total length, buried length, exposed length, and the main structural features relevant to interpretation. Figure 1 was also improved to show the principal metallic segment lengths, buried transition zones, and support locations more explicitly. These additions were intended to make the structural basis of the interpretation, particularly for the E–H section, much clearer.

Q10. Soil-related interpretation

Response:
We agree with the Reviewer that the soil-related interpretation should remain cautious. The revised manuscript now treats the oxide composition and measured soil pH as background information rather than as a strong independent basis for corrosion-risk assessment. At the same time, the limitations are now stated more explicitly: direct field resistivity, drainage, moisture, and ionic-content measurements were not available. These clarifications were introduced in Sections 2.2 and 6.3.

Q11. Scope of the integrated interpretation framework

Response:
We appreciate this observation. In the revised manuscript, the role of Section 3 was clarified so that it supports the interpretation of the field results rather than appearing as a broadly generalized decision framework detached from the case study. The manuscript now more clearly positions the framework as a structured interpretation logic derived from and applied to the present case, rather than as a universally validated standalone procedure.

Q12. Mitigation strategy as proposed practical option

Response:
We agree. The mitigation strategy and validation logic were revised to emphasize that they represent a proposed practical engineering option, not an already demonstrated solution. The revised Section 5 now explicitly states that the proposed approach has not yet been validated by post-installation measurements and should be regarded as a validation-oriented mitigation concept requiring follow-up verification under actual service conditions.

Q13. Service-life scenario and reinspection horizons

Response:
We agree that the earlier formulation was too specific for the available dataset. Section 7 was revised so that the prognosis is now expressed in qualitative integrity-management terms rather than through fixed numerical time horizons. The revised version retains the scenario-based logic, but the recommendations are now framed as relatively early, planned, or longer reinspection approaches depending on exposure and mitigation status, without presenting precise unsupported time intervals.

Q14. Connection between Results and Discussion

Response:
We appreciate this comment and revised the manuscript to clarify more explicitly the distinction between directly observed field results and literature-based engineering inferences. In particular, the integrated results summary was revised to state more clearly that the non-uniform electrical and electrochemical exposure is directly supported by the measurements, whereas the corrosion-risk interpretation remains inferential in the absence of excavation or wall-loss data. This distinction was also reinforced in the Discussion section.

Q15. Strengthening the figures and tables as supporting evidence

Response:
We agree. The figures and tables were revised and strengthened so that they function not merely as illustrations but as central supporting elements of the engineering interpretation. Figure 1 was improved to show the main metallic segment lengths, buried transition zones, and support locations more explicitly. Figure 2 was clarified to better show the pipeline position within the railway-influenced corridor. A structural comparison table was added in Section 2.1, and the presentation of the coating-related and monitoring-related tables was also strengthened. These changes were intended to improve the readability and evidential value of the figures and tables throughout the manuscript.

Final response

We once again thank the Reviewer for the detailed and valuable comments. We believe that the revisions introduced in response to these observations have significantly improved the clarity, balance, and technical rigor of the manuscript.