Review Reports - Microbiologically Influenced Corrosion on Turbine Blades in a Hydroelectric Power Plant

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

In my opinion, your article is interesting and well written. Due to some inaccuracies and some my suggestions for improvement I recommend the major revision according to the following comments:

1. The last paragraph of the "Introduction" part should contain more detailed description of the subject and the goals of this study.
Beginning of the chapter "Results and Discussion": In my opinion, for a better idea, it would be useful to add a photo/sketch of the tested water turbine and mark the areas covered by biofilm.
Page 4, lines 148-149: Missing description of the hardness measurement (number and locations of indentations when measuring hardness, photo/sketch?...)
Page 5, line 155: I propose to add an explanation of the term “diatom”.
Page 5, line 170: The author’s estimation of the time (“few months”) should be explained.
Figure 4d is unreadable.
Page 7, lines 200-202: The statement “However, we must consider that concentrations of ions dissolved in water change beneath the biofilm, especially Cl^- levels, as the biofilm acts similarly to a crevice.” should be supported by a reference.
“Conclusions” chapter could contain some proposals for improvement of turbine blades corrosion resistance in the current ecological situation.

Author Response

In my opinion, your article is interesting and well written. Due to some inaccuracies and some my suggestions for improvement I recommend the major revision according to the following comments:

First of all, we would like to express our gratitude for reviewing the paper, your comments have helped us improve the manuscript.

The last paragraph of the "Introduction" part should contain more detailed description of the subject and the goals of this study.

Text added: “The goal was to assess the damage done to the turbine blades, and propose the cause of the damage. The findings will be used in order to avoid and mitigate the identified degradation mechanisms in the future operation of the hydroelectric power plant.”

2.Beginning of the chapter "Results and Discussion": In my opinion, for a better idea, it would be useful to add a photo/sketch of the tested water turbine and mark the areas covered by biofilm.

-new figure added and appropriate text. “Figure 1 shows the sketch of the Kaplan turbine (Figure 1a), the turbine blades from below, covered in biofilm (Figure 1b), and locations of the NDT testing (Figure 1c). Yellow circles in Figure 1c mark the location of 3D putty replicas, while red circles show the locations of XRF and hardness measurements. The diameter of the entire turbine is 4900 mm, and the blade thickness is from 250 to 80 mm.”

3. Page 4, lines 148-149: Missing description of the hardness measurement (number and locations of indentations when measuring hardness, photo/sketch?...)

-locations are now marked on figure 1.

4. Page 5, line 155: I propose to add an explanation of the term “diatom”.

-explanation added: Diatoms are a major group of microscopic, mostly single-celled algae found in freshwater as well as marine environments

5. Page 5, line 170: The author’s estimation of the time (“few months”) should be explained.

-explanation added: a few (2-3) months

6. Figure 4d is unreadable.

-Figure 4 has been edited.

7. Page 7, lines 200-202: The statement “However, we must consider that concentrations of ions dissolved in water change beneath the biofilm, especially Cl^- levels, as the biofilm acts similarly to a crevice.” should be supported by a reference.

-References added to support statement

Liao, J.; Fukui, H.; Urakami, T.; Morisaki, H. Effect of Biofilm on Ennoblement and Localized Corrosion of Stainless Steel in Fresh Dam-Water. Corros. Sci. 2010, 52, 1393–1403, doi:10.1016/j.corsci.2010.01.012.
Zhang, Z.; Li, Z.; Wu, F.; Xia, J.; Huang, K.; Zhang, B.; Wu, J. A Comparison Study of Crevice Corrosion on Typical Stainless Steels under Biofouling and Artificial Configurations. Npj Mater. Degrad. 2022, 6, doi:10.1038/s41529-022-00301-w.
Wu, J.; Chen, Z.; Li, G.; Teng, K.; Ge, L.; Chen, Y.; Li, L.; Qu, Q. Visualisation and Quantification of Biofilm-Substrate Interface Microenvironments Based on a Fungal-Bacterial Interaction Model: An in-Depth Investigation into Microbially Mediated Corrosion Processes. Corros. Sci. 2025, 246, doi:10.1016/j.corsci.2025.112725.

8. “Conclusions” chapter could contain some proposals for improvement of turbine blades corrosion resistance in the current ecological situation.

-proposals added. The resistance of stainless steel to MIC could be improved by increasing the Cr, Ni, Mo, W, and N concentrations in steel, as they increase resistance to chlorine attack.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors present an analysis of corrosion damage observed on turbine blades made of G-
X4CrNi13-4 martensitic stainless steel at the Brežice Hydroelectric Power Plant on the Sava
River in Slovenia. The topic is timely and potentially interesting to readers. However, similar
studies concerning corrosion and cavitation damage of hydraulic turbine blades, including
components made of G-X4CrNi13-4 and other 13Cr–4Ni martensitic stainless steels, have
already been reported. Therefore, the novelty of the present study appears moderate and
should be more clearly demonstrated in comparison with the existing literature. The
manuscript is appropriately structured and cites relevant literature. However, several
shortcomings should be addressed, as outlined below:
1. The reference [1] should have more details, see
https://www.mdpi.com/authors/references.
2. Chapter 2. Materials and Methods. The authors should describe the Materials and Methods
section in greater detail. In particular, the sampling procedure should be clearly specified. The
statement “A small surface of the blade…” alone is insufficient (line 99). The dimensions of the
steel turbine blades should also be provided, including the material thickness.
3. Lines 124-126. This statement by the authors must be described more precisely: “Analysis
of the damage distribution indicated no statistically significant preferential sites; the
occurrence of the pits appeared to be stochastically distributed across the blade
surfaces.” What statistical calculations were made?
4. Lines 147-148. Where were the hardness measurements? Near the correction place or in
the no-damage zone?
5. Chapter 3. Results and Discussion. The size and location of the corrosion pits on the
turbine blades should be specified. For example, the affected areas could be marked and
dimensioned in Figures 4 and 5.
6. The microstructure of the blade material should be described in more detail, including the
grain size and the presence of martensite, δ-ferrite, carbides, and non-metallic inclusions.

Author Response

Thank you for the comments we have improved our paper accordingly.

The reference [1] should have more details, see
https://www.mdpi.com/authors/references.

-reference was corrected.
2. Chapter 2. Materials and Methods. The authors should describe the Materials and Methods
section in greater detail. In particular, the sampling procedure should be clearly specified. The
statement “A small surface of the blade…” alone is insufficient (line 99). The dimensions of the
steel turbine blades should also be provided, including the material thickness.

-additonal figure was added, the required information was added. “Figure 1 shows the sketch of the Kaplan turbine (Figure 1a), the turbine blades from below, covered in biofilm (Figure 1b), and locations of the NDT testing (Figure 1c). Yellow circles in Figure 1c mark the location of 3D putty replicas, while red circles show the locations of XRF and hardness measurements. The diameter of the entire turbine is 4900 mm, and the blade thickness is from 250 to 80 mm.”
3. Lines 124-126. This statement by the authors must be described more precisely: “Analysis
of the damage distribution indicated no statistically significant preferential sites; the
occurrence of the pits appeared to be stochastically distributed across the blade
surfaces.” What statistical calculations were made?

-The sentence is somewhat exaggerated, and we thank you for pointing out the issue. It was not our intention to mislead, but it was merely an unfortunate use of words. No statistics were made on the distribution of corrosion damage. The original meaning was, that we did not see a pattern and that the damage was randomly distributed, done by visual analysis. We have corrected the text accordingly.
4. Lines 147-148. Where were the hardness measurements? Near the correction place or in
the no-damage zone?

-The hardness tests were made on places with no damage to check if the material had the correct hardness values.
5. Chapter 3. Results and Discussion. The size and location of the corrosion pits on the
turbine blades should be specified. For example, the affected areas could be marked and
dimensioned in Figures 4 and 5.

-the figures have been edited, the dimensions of the corrosiopn pitts have been added. The location of the pit replicas is shown in figure 1, as well as sites for hardness and chemical composition measurements.
6. The microstructure of the blade material should be described in more detail, including the
grain size and the presence of martensite, δ-ferrite, carbides, and non-metallic inclusions.

-text added: The steel microstructure consists mainly of martensite, but there are some smaller δ-ferrite islands. Some minor non-metallic inclusions were visible, they had globular shape. This would indicate silicates or modified alumina non-metallic inclusions, but because the microstructure is cast, there is no elongation of deformable inclusions during rolling. The absence of typical non-metallic inclusion shapes makes the type and subsequent chemical composition harder to identify with optical light mcroscopy.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors, I recommend to accept your article.

Reviewer 2 Report

Comments and Suggestions for Authors

I accept the manuscript in its present form. I am satisfied with the answers of the authors.