Effect of Ultrasonic Nanocrystalline Surface Modification (UNSM) on Stress Corrosion Cracking of 304L Stainless Steel
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn this study, the authors applied ultrasonic nanocrystalline surface modificationtreatment to 304L stainless steel welds. The peened specimens were subjected to compressive residual stress up to a depth of 1 mm from the surface. The main factors and causes affecting stress corrosion cracking properties after such peening treatments were examined from the perspective of microstructure, corrosion properties, and compressive residual stress.
In my opinion, the topic is interesting and important, and the work may be of interest to the scientific community, since austenitic stainless steels are used in industry for canisters produced by welding, which are susceptible to stress corrosion cracking when exposed to a chloride environment.
The manuscript is well organized. The text is written in a way that the reader can understand. The work is sufficiently detailed and conclusive. The results are properly discussed and explained to the reader. Microscope and SEM images are of good quality. Figures and diagrams are clear. The findings are scientifically documented, sufficiently detailed and conclusive.
Conclusions reflect the research results obtained and are consistent with the arguments presented. The references are appropriate. This section contains 44 items. 12 items were published after 2020. There are no inappropriate self-citations by the authors.
Considering the merits indicated above, I believe that the work is worth publishing. The manuscript needs only one correction.
My specific comment:
1) It is generally accepted that in the description of the EIS measurement methodology, not only a frequency range of the tests is given, but also the amplitude of the a.c. perturbation signal. Please add this information in the text of the manuscript.
Author Response
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Reviewer 2 Report
Comments and Suggestions for AuthorsThe paper is interesting and well-structured. However some points need to be clarified.
- t is unclear why corrosion was also evaluated in cross-sections. What is the significance of this? Since peening is a surface treatment, it would primarily affect the surface layer and not typically impact the rest of the cross-section, apart from a thin superficial layer. Corrosion would mainly affect the exposed surface of the component. If a crack initiates and extends through the surface, when would the component/sample be considered to have failed? Should failure be determined by surface corrosion or by the extent of corrosion across the entire cross-section?
- Why was the solution de-aerated? Additionally, why was 1% NaCl chosen instead of 3.5% NaCl, which more accurately simulates seawater?
Figures 1 and 5 -: Why do the authors use straight lines to represent the crack propagation rate and total crack time? Each data point corresponds to a different sample, and bar charts would be more appropriate for representing this type of data.
Lines 281-284: In Figure 10a, it appears that no passivation occurs for the 304LB-UNSM. However, the authors claim enhanced passivation. Please clarify this interpretation. Additionally, the authors compare the current at zero voltage, but this comparison does not seem meaningful for evaluating corrosion performance. A more relevant comparison is recommended.
- Figures 11 and 14 : Are the impedance values normalized to the surface area? The surface area exposed to the solution is critical in electrochemical tests, and this should be addressed.
- The use of a simple Randles circuit for impedance fitting appears to be a simplified approach. Why didn’t the authors use the Rp values from extrapolation of the Nyquist plot, particularly the real axis in the diagram? Moreover, the Nyquist plot is typically square-shaped, and both axes should have the same scale. If this is not done, the impedance profile could be distorted, and valuable information may be lost.
-Table 5: The authors should provide details on how the Density of States (DOS) value in Table 5 was calculated. Specifically, how were Ir and Ia evaluated?
-The pitting potential for the 304LB-UNSM, as shown in Figure X, is not clear. A cyclic polarization test would provide a more reliable evaluation of the pitting potential.
-.Conclusion: The conclusion mentions a nanostructure, but no supporting evidence is presented in the results section. Additionally, the conclusion states that corrosion resistance is improved, but the polarization curves do not show this. The conclusion should be revised to more accurately reflect the key findings of the study. Does the UNSM lead do better corrosion performance?
Author Response
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Reviewer 3 Report
Comments and Suggestions for Authorsline 22 : The U-bend specimens underwent SCC testing ( In 42% 22 MgCl2 at 155℃), microstructure examination using an optical microscope (OM) and a scanning electron microscope (SEM), electron backscatter diffraction (EBSD) analysis, and compressive residual stress measurements via the hole drilling method.
Line 37: However, welded cannisters are prone ......
Line 40: Clarify what you mean by ”lifetime protection”. These need to be contained for fare more than one human lifespan. i.e. nuclear waste lifetime.
line 56: It has been noted that the refining...
line 56: Comment: it is more likely that diffusion is enhanced by grain refinement, due to the formation of a denser network of grain boundaries through which diffusion is facilitated by the disordered structure and weaker metallic bonds. Interatomic distances are only decreased by the induced compressive stresses and and not the grain refinement and shorter bond distances actually hinder the diffusion process.
133: to prevent cracks from developing in the area 133 where the peening was applied [6]. should this be the other way round?
line 127, the U-bend SCC electrochemical test should be part of the corrosion tests.
line 230: Figure 5 displays the SCC test results of a U-bend specimen of 304L stainless steel 230 welded by UNSM treatment. needs fixing
line 386: UNSM is a technology used for surface modification by inducing plastic deformation ...
line 388: on the steel surface to prevent fatigue induced cracking and defects ..
line 453: That argument would make perfect sense if the sample was only exposed to the stress K field, however in this case there is a synergistic affect of corrosion and stress. the higher energy levels induced by the strain in the crystal structure will augment the corrosion reaction and contribute to accelerate corrosion by supplying part of the activation energy. To complicate matters, once the crack makes it through the modified surface, it will encounter a volume of material with residual tensile stresses. These two factors limits significantly the gain in corrosion performance that can be achieved through this process.
Author Response
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Reviewer 4 Report
Comments and Suggestions for AuthorsThe study focuses on the use of 304L stainless steel in the nuclear industry, particularly for the construction of spent nuclear fuel storage canisters. Given that these canisters are required to maintain structural integrity for thousands of years, it is essential to predict their service life and enhance their corrosion behavior. Nuclear power plants, predominantly located in coastal areas, are exposed to chloride-rich environments, which increases the risk of stress corrosion cracking (SCC).
This work examines how inherent residual stresses in welded canisters impact their SCC resistance and investigates plastic deformation techniques, such as peening, to mitigate this risk. Specifically, it highlights the use of Ultra-Sonic Nano Surface Modification (UNSM), known for enhancing SCC resistance by inducing compressive residual stresses on the surface.
The study involves experimental tests conducted on 304L stainless steel, subjected to UNSM treatment on base metal, heat-affected zone (HAZ), and weld metal to induce compressive residual stresses up to a depth of 1 mm. U-bend tests were conducted under SCC conditions (42% MgCl2 at 155℃), with microstructural analysis performed using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). Residual stress measurements were taken using the hole-drilling method, and corrosion behavior was assessed through anodic polarization tests, electrochemical impedance spectroscopy (EIS), double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests, and ASTM A262 Practice C tests.
The findings indicate that the UNSM treatment refined the outermost grains of the cross-section and improved the corrosion resistance of 304L stainless steel. Additionally, the treatment resulted in longer crack initiation times, reduced crack propagation rates, and overall enhancement of SCC resistance properties.
To strengthen the introduction, it is recommended to include references to previous studies that have investigated the effectiveness of techniques similar to Ultrasonic Nanocrystal Surface Modification (UNSM) in enhancing the stress corrosion cracking (SCC) resistance of austenitic stainless steels. For instance, it would be beneficial to cite research that has examined peening treatments, such as shot peening or laser shock peening, and their impacts on the microstructure and corrosion resistance properties of 304L stainless steel or similar materials. This information not only provides a more robust context for the research but also establishes a comparative framework that can help validate the results presented in the article.
Furthermore, it would be advantageous to clearly delineate the gap in the existing literature that this study addresses. For example, if prior research has documented the effectiveness of specific treatments but has not explored the combination of UNSM with welding under specific operating conditions, this should be explicitly highlighted. By clarifying which aspects have been less investigated, the necessity of this study and its unique contribution to the field can be justified more convincingly.
The text provides a comprehensive description of the experimental results, emphasizing the impact of UNSM treatment on the crack initiation time and propagation rate in 304L stainless steel and its welded version. However, it would be advantageous to include a more in-depth interpretation of the implications of these findings for practical applications or a more detailed comparison with other surface treatment methods. This would provide a richer context and allow the reader to better understand the industrial relevance of the results.
Although the results section encompasses a variety of tests and observations, its organization could be improved by more clearly dividing the subsections. This would help the reader follow the narrative, especially in parts that combine microstructural observations with electrochemical behavior. The inclusion of more specific subheadings within the results section would provide clarity and facilitate referencing specific parts of the findings.
To enhance the understanding of the results, a more explicit analysis of the limitations of the tests conducted would be valuable, such as potential variations in other corrosive environments or variability in material properties due to UNSM treatment. This analysis would offer a more balanced perspective and help contextualize the results within a broader framework of potential service conditions.
Author Response
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Round 2
Reviewer 2 Report
Comments and Suggestions for Authors
The authors have made an effort to address all remarks; however, two issues remain to be corrected:
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In response to Comment 4, the authors stated: "the passive current density has a wide range of potentials in the passive region. Therefore, we compare the passive current density based on the current density at a specific potential. In this manuscript, the current density at 0 V(SCE) was calculated and compared." This approach is not acceptable. The passive current density, ipassi_{\text{pass}}, should be compared within the voltage range where ipassi_{\text{pass}} remains stable, rather than selecting a specific potential arbitrarily. At 0 V, the 304LW-UNSM material appears to be in the anodic dissolution region rather than in passivation. Please identify and compare ipassi_{\text{pass}} within the appropriate stable voltage range.
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The authors indicated that they calculated the DOS value. However, the DOS is the ratio between the peak current density during the activation process (forward scan) (iai_a) and the peak current density in the reactivation process (backward scan) (iri_r). There is no evidence of forward and backward scan tests. Please include at least one DL-EPR test for a sample case and specify the voltage range for the cyclic scan. The same applies to the IGS test. Please show how the rate was calculated and provide a representative image.
Author Response
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