Microstructure and Corrosion Resistance of Sn-3Ag-0.5Cu-xBi Solders

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This work prepares four Bi-modified SAC305 solder alloys with Bi contents of 0, 1, 2 and 4 wt.%, and systematically investigates the regulation law of Bi addition on microstructure evolution, electrochemical corrosion performance and pitting corrosion mechanism. The combination of microstructural characterizations and electrochemical measurements forms a complete experimental scheme consistent with the research targets. However, obvious deficiencies exist including incomplete abstract summary, insufficient literature review depth in Introduction, and shallow discussion on corrosion mechanisms, which need to be comprehensively revised before further review.

Detailed Review Comments:

1. The abstract fails to adequately emphasize the core novelty of this work. Authors are advised to restructure the abstract such that the research background, experimental methodologies and key innovative findings are clearly articulated within the opening three sentences, to boost the readability and highlight the core contributions of this study.
2. The Introduction requires an expanded literature review summarizing prior research on the microstructural and property modulation effects of Bi additions to SAC solder alloys. Identified research gaps and conflicting conclusions from existing publications should be systematically outlined to distinctly underscore the novelty of the present investigation. Additionally, the reference list suffers from unbalanced citation distribution: the majority of cited works date to 2010-2020, while recent high-impact studies (2024-2026) focused on the corrosion behavior of SAC-Bi alloys are underrepresented. More literature dedicated to chloride-mediated electrochemical corrosion should be incorporated, rather than over-relying on publications covering only Bi-induced mechanical property improvements.
3. The Introduction fails to clarify practical service scenarios of solder joints. The manuscript should elaborate on the practical engineering significance of investigating corrosion resistance in Bi-modified SAC solders, to better demonstrate the industrial demand driving research on SAC-Bi alloy systems.
4. Table 3 only reports discrete electrochemical parameter values without standard deviations calculated from three parallel replicate tests, compromising the reproducibility and statistical reliability of the electrochemical datasets. All SEM micrographs should feature circled, labelled annotations highlighting critical Bi-rich precipitate zones and localized corrosion regions for enhanced visual clarity. Furthermore, inconsistent formatting and disorganized numbering of chemical reaction equations throughout the text require full standardization and revision.
5. A thorough mechanistic analysis must be integrated to rationalize the counterintuitive observation: 1 wt.% Bi fully dissolved within the Sn matrix drastically elevates corrosion kinetics, whilst discrete Bi-rich precipitates formed at 2 wt.% and 4 wt.% Bi restore the alloy’s corrosion resistance. Quantitative comparisons of galvanic potential discrepancies across multi-phase interfaces are recommended to furnish robust quantitative support for the proposed corrosion mechanism.
6. GI-XRD characterization identifies abhurite as the sole crystalline corrosion product generated on all alloy surfaces. Nevertheless, the passive scale formed on SAC305-1Bi possesses finer crystallites, elevated microstrain and poor interfacial adhesion that readily promotes scale spallation. Comprehensive discussion is required to unpack the fundamental mechanisms responsible for the divergent morphological and crystallographic features of corrosion scales across the four alloy compositions.
7. The schematic corrosion evolution diagram (Figure 10) should be annotated to denote interfacial galvanic potential differences, alongside the anodic and cathodic domains formed by micro-galvanic cells, to deliver an intuitive, complete visual interpretation of the full corrosion progression pathway.

Author Response

Dear reviewer,

 

We are thankful for your valuable comments. They helped us to improve the quality of our manuscript. Answers to issues raised are given below. The changes in the manuscript have been highlighted as tracked changes.

 

Kind regards,

 

Authors

 

REVIEWER 1

Comments and Suggestions for Authors

This work prepares four Bi-modified SAC305 solder alloys with Bi contents of 0, 1, 2 and 4 wt.%, and systematically investigates the regulation law of Bi addition on microstructure evolution, electrochemical corrosion performance and pitting corrosion mechanism. The combination of microstructural characterizations and electrochemical measurements forms a complete experimental scheme consistent with the research targets. However, obvious deficiencies exist including incomplete abstract summary, insufficient literature review depth in Introduction, and shallow discussion on corrosion mechanisms, which need to be comprehensively revised before further review.

Detailed Review Comments:

1.The abstract fails to adequately emphasize the core novelty of this work. Authors are advised to restructure the abstract such that the research background, experimental methodologies and key innovative findings are clearly articulated within the opening three sentences, to boost the readability and highlight the core contributions of this study.

 

Thank you for your useful comment. The abstract has been rewritten thoroughly to highlight the core novelty of the present work. The research background, experimental methodology and key findings have also been more clearly formulated to provide a better comprehension of our results.

 

2. The Introduction requires an expanded literature review summarizing prior research on the microstructural and property modulation effects of Bi additions to SAC solder alloys. Identified research gaps and conflicting conclusions from existing publications should be systematically outlined to distinctly underscore the novelty of the present investigation. Additionally, the reference list suffers from unbalanced citation distribution: the majority of cited works date to 2010-2020, while recent high-impact studies (2024-2026) focused on the corrosion behavior of SAC-Bi alloys are underrepresented. More literature dedicated to chloride-mediated electrochemical corrosion should be incorporated, rather than over-relying on publications covering only Bi-induced mechanical property improvements.

 

Thank you for your helpful suggestion. We have now clearly underlined the existing research gaps in the field of lead-free solder research and cited recent literature. Furthermore, we have now placed more focus on discussing the recent papers studying the corrosion behavior of lead-free alloys alloys and cited recent references. New works published in 2024–2026 were added, including studies on SAC305–xBi, SAC305–Ni–Sb–Bi alloys, binary Sn–Bi alloys, Sn–58Bi solder, and multicomponent Sn–Ag–Bi–Cu–Ni solders. A greater emphasis was also placed on electrochemical corrosion studies in chloride-containing environments.

 

3. The Introduction fails to clarify practical service scenarios of solder joints. The manuscript should elaborate on the practical engineering significance of investigating corrosion resistance in Bi-modified SAC solders, to better demonstrate the industrial demand driving research on SAC-Bi alloy systems.

 

Thank you for your helpful suggestion. The critical issues regarding the use of lead-free solders in harsh, industrially relevant environments have now been clearly formulated. Recent references dealing with issues of solder joint reliability have been cited. The revised introduction now addresses the effects of high relative humidity, condensation, chloride-containing dust, airborne salts, and ionic residues from manufacturing, handling, or cleaning processes. It also explains that in the presence of moisture, these contaminants can form a thin conductive electrolyte layer on the surface of an electronic assembly that promotes localized corrosion, leakage currents, and electrochemical migration between adjacent conductors. The practical significance of these processes for automotive, outdoor, industrial, marine, and power electronics applications is also emphasized. Relevant literature on the effects of NaCl contamination and climatic conditions on the reliability of printed circuit boards has also been added.

 

4.Table 3 only reports discrete electrochemical parameter values without standard deviations calculated from three parallel replicate tests, compromising the reproducibility and statistical reliability of the electrochemical datasets. All SEM micrographs should feature circled, labelled annotations highlighting critical Bi-rich precipitate zones and localized corrosion regions for enhanced visual clarity. Furthermore, inconsistent formatting and disorganized numbering of chemical reaction equations throughout the text require full standardization and revision.

 

Thank you for your comments. The experiments were carried out as triplicate measurements. Average values and standard deviations of OCP, E_corr and j_corr from three measurements are now given in Table 3. Standard deviations of OCP and j_corr are also presented in Figs. 3 and 6. The microstructural features have been annotated and highlighted in SEM images (Fig. 1a-d). Furthermore, the formatting and numbering of chemical equations have been unified in the manuscript.

 

5.A thorough mechanistic analysis must be integrated to rationalize the counterintuitive observation: 1 wt.% Bi fully dissolved within the Sn matrix drastically elevates corrosion kinetics, whilst discrete Bi-rich precipitates formed at 2 wt.% and 4 wt.% Bi restore the alloy’s corrosion resistance. Quantitative comparisons of galvanic potential discrepancies across multi-phase interfaces are recommended to furnish robust quantitative support for the proposed corrosion mechanism.

 

Thank you for your useful suggestion. The discussion regarding the effect of 1 wt. % Bi on the corrosion behavior of the SAC305 alloys has been expanded. More focus has now been placed on the microstructural differences between the alloys. The SAC305-1Bi alloy is shown to exhibit a distinct Ag₃Sn morphology compared to the rest of the alloys.  SAC305, SAC305–2Bi, and SAC305–4Bi alloys contained thicker and more clearly developed Ag₃Sn lamellae. However, the SAC305–1Bi alloy was characterized by a considerably finer and more densely distributed populated network of thin elongated Ag₃Sn particles. These differences indicate that the addition of 1 wt.% Bi modified the solidification morphology of the Ag₃Sn-containing eutectic regions differently from the SAC305-2Bi and SAC305-4Bi alloys.

Since the SAC305–1Bi alloy contained a larger number of very thin, elongated, and partially fragmented Ag₃Sn particles densely distributed within the interdendritic regions, their electrochemical effect was different compared to the remainder of the alloys. The fragmentation of elongated Ag₃Sn into a larger number of finer particles has substantially increased the total area and density of the β-Sn/Ag₃Sn interfaces. Since Ag₃Sn is electrochemically nobler than the surrounding Sn-rich matrix, intense local micro-galvanic coupling developed at β-Sn/Ag₃Sn interfaces, with Ag₃Sn supporting cathodic reactions and the adjacent Sn-rich matrix undergoing preferential anodic dissolution. The high density of fine Ag₃Sn particles provided a high density of local galvanic interaction sites, thereby leading to a higher corrosion current recorded for this alloy (Fig. 6).

 

6. GI-XRD characterization identifies abhurite as the sole crystalline corrosion product generated on all alloy surfaces. Nevertheless, the passive scale formed on SAC305-1Bi possesses finer crystallites, elevated microstrain and poor interfacial adhesion that readily promotes scale spallation. Comprehensive discussion is required to unpack the fundamental mechanisms responsible for the divergent morphological and crystallographic features of corrosion scales across the four alloy compositions.

 

Thank you for your comment. The discussion regarding the crystallite size of the corrosion product, elevated micro-strain and resulting passive scale spallation has been extended. The interpretation of the corrosion behavior of the SAC305-1Bi alloy based on the formation of local galvanic micro-cells at the Sn/Ag₃Sn interfaces, is consistent with the observed post-corrosion morphology. The corrosion layer of the SAC305–1Bi alloy was less adherent and locally spalled off (Fig. 8). Therefore, plentiful exposed portions of the underlying substrate have been observed. The EDS mapping further indicated that the localized disruption of the layer occurred in the vicinity of Ag₃Sn-containing regions. The fine and dense network of dispersed Ag₃Sn particles promotes the simultaneous initiation of corrosion at β-Sn/Ag₃Sn interfaces (Fig. 8, 10). Coalescence of these locally attacked regions may undermine the developing corrosion product layer and reduce its mechanical support by the substrate. The morphology of the underlying substrate may also influence the nucleation and growth of abhurite. In SAC305, SAC305–2Bi, and SAC305–4Bi alloys, the lower density of Ag₃Sn lamellae may have provided fewer spatially concentrated sites of preferential Sn dissolution. Under such conditions, the corrosion product may nucleate at a lower number of locations and subsequently grow into larger plate-like crystallites and whisker-like features (Fig. 7a, c, d). By contrast, the large number of fine Ag₃Sn particles in the SAC305–1Bi alloy generates a high density of local dissolution and precipitation sites. The corrosion product may consequently undergo repeated nucleation at many closely spaced locations rather than unrestricted growth of a smaller number of existing crystallites.

 

7. The schematic corrosion evolution diagram (Figure 10) should be annotated to denote interfacial galvanic potential differences, alongside the anodic and cathodic domains formed by micro-galvanic cells, to deliver an intuitive, complete visual interpretation of the full corrosion progression pathway.

 

Thank you for your helpful comment. The schematic of the corrosion mechanism (Fig. 10) has been annotated to indicate the individual reaction steps. The critical features, i.e., the formation of micro-galvanic cells at the Sn/Ag₃Sn phase boundaries in the SAC305-1Bi alloy and their progression, have been highlighted. In the SAC305–1Bi alloy, the refinement and fragmentation of Ag₃Sn increase the density of β-Sn/Ag₃Sn interfaces. These interfaces provide numerous sites for local micro-galvanic interactions and preferential dissolution of the adjacent Sn-rich matrix. The resulting high density of local reaction sites promotes repeated nucleation of abhurite, producing a corrosion layer with smaller coherent domains, higher microstrain, and reduced adhesion. The local spallation repeatedly exposes fresh substrate and sustains corrosion.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Regarding the introduction:

• The literature review focuses almost exclusively on Bi-containing SAC305 alloys. The authors should also discuss previous corrosion studies on Bi additions in other Sn-based lead-free solder systems, as these may provide valuable insight into the mechanisms governing corrosion behavior and help place the present work into a broader context.
• The introduction summarizes some previous findings (references 16-20) but does not critically analyze the existing literature. The authors should identify unresolved issues, inconsistencies, or conflicting observations regarding the influence of Bi on corrosion behavior, thereby providing a clearer scientific justification for the present study.

Regarding the section “2. Materials and Methods”:

• The authors stated “The electrolyte was prepared by weighing 35 g of NaCl and dissolving it in 1L of demineralized water prior to the experiment”……..This preparation seems inaccurate, it should be “The electrolyte was prepared by dissolving 35 g of NaCl in demineralized water and diluting to a total solution mass of 1000 g.”
• The authors should specify how many independent linear sweep voltammetry (LSV) measurements were conducted. Performing replicate measurements is recommended to assess the reproducibility and reliability of the electrochemical data.

Regarding the section “3. Results and discussion”:

• The authors have characterized corrosion behavior using LSV. It would be interesting to know whether electrochemical impedance spectroscopy (EIS) measurements were considered. As EIS provides information on charge transfer resistance and surface film characteristics, the authors may wish to comment on why this technique was not employed in the present work.

Author Response

Dear reviewer,

 

We are thankful for your valuable comments. They helped us to improve the quality of our manuscript. Answers to issues raised are given below. The changes in the manuscript have been highlighted as tracked changes.

 

Kind regards,

 

Authors

 

REVIEWER 2

 

Comments and Suggestions for Authors

Regarding the introduction:

• The literature review focuses almost exclusively on Bi-containing SAC305 alloys. The authors should also discuss previous corrosion studies on Bi additions in other Sn-based lead-free solder systems, as these may provide valuable insight into the mechanisms governing corrosion behavior and help place the present work into a broader context.

 

Thank you for your comment. In introduction, we have now also included other Bi-containing alloys for soldering including binary Sn-Bi alloys, ternary Sn-Zn-Bi and multicomponent alloys. Recent references have been cited.

 

• The introduction summarizes some previous findings (references 16-20) but does not critically analyze the existing literature. The authors should identify unresolved issues, inconsistencies, or conflicting observations regarding the influence of Bi on corrosion behavior, thereby providing a clearer scientific justification for the present study.

 

Thank you for your suggestion. The effect of variable Bi concentration on the corrosion resistance of various lead-free solder alloys has now been discussed, providing clear scientific motivation for the present work. Bi alloying has been reported to both improve and degrade the utility properties, depending on its concentration, distribution of Bi-rich phases, microstructural features, and corrosion environment. At the same time, a gap in research exists since previous studies of SAC305 alloys investigated only Bi additions greater than 2 wt%. The studies of lower concentrations than 2 wt. % Bi are under-represented. Therefore, the study of the SAC305-1Bi alloy is unique as it provides additional information on the effect of Bi on the corrosion behavior that was not covered in previous investigations of SAC305–xBi alloys.

 

Regarding the section “2. Materials and Methods”:

• The authors stated “The electrolyte was prepared by weighing 35 g of NaCl and dissolving it in 1L of demineralized water prior to the experiment”……..This preparation seems inaccurate, it should be “The electrolyte was prepared by dissolving 35 g of NaCl in demineralized water and diluting to a total solution mass of 1000 g.”

 

Thank you for your comment. Yes, the electrolyte was prepared by dissolving 35 g of NaCl in de-ionized water to provide a total solution mass of 1 kg. The statement has been reformulated.

 

• The authors should specify how many independent linear sweep voltammetry (LSV) measurements were conducted. Performing replicate measurements is recommended to assess the reproducibility and reliability of the electrochemical data.

 

The experiments were carried out in triplicate. It has now been specified in the paper.

 

Regarding the section “3. Results and discussion”:

• The authors have characterized corrosion behavior using LSV. It would be interesting to know whether electrochemical impedance spectroscopy (EIS) measurements were considered. As EIS provides information on charge transfer resistance and surface film characteristics, the authors may wish to comment on why this technique was not employed in the present work.

 

Thank you for your useful comment. LSV experiments have been preferred as they can provide a complete characterization of corrosion behavior. Although EIS is a powerful non-destructive tool for studying the interfaces of corroding materials, it has several limitations. The major problem is that very often different equivalent circuit models can be used to fit the same EIS data. Therefore, resistance and capacitance data depend on the correct choice of electric model. Furthermore, instabilities in the corrosion process, such as pitting corrosion can induce dispersion in the impedance values recorded at low frequency. Unlike polarization curves, the EIS alone cannot determine changes in corrosion potentials and relative anodic and cathodic reaction kinetics caused by different alloying elements, microstructural features (e.g., secondary phases), or solutions. A major disadvantage of AC impedance measurements is that it is not possible to convert the polarization resistance obtained into a corrosion rate without prior knowledge of the Tafel slope values and the Stern–Geary coefficient. Therefore, the potentiodynamic (PD) method combined with morphological observations has been chosen in the present study to study corrosion. The PD curve includes the regions of immunity, activity, and passivity, as indicated in the manuscript (Fig. 5). Therefore, this method can provide more complete information on the corrosion behavior of SAC305-xBi alloys. The analysis of PD curves yields useful electrochemical parameters, including the corrosion current, that can be used directly to calculate the corrosion rate.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript, titled “Microstructure and corrosion resistance of Sn-3Ag-0.5Cu-xBi solders”, submitted by Halmanová et al. to MDPI Technologies presents results on the effect of Bi-content (0 - 4 wt. % range) on the microstructure and corrosion resistance of SAC305 lead-free solder. The authors prepared four samples of the SAC305-xBi solder alloys via induction melting and characterised that microstructure via SEM-EDS measurements, phase composition via GI-XRD and the corrosion behaviour via electrochemical evaluation (OCP and potentiodynamic polarisation) in a model corrosion environment - 3.5 % NaCl. The results show that at higher Bi-content (>2%) Bi-rich precipitates are formed in the solder microstructure, however these solder alloys have a generally positive shift in potential both in terms of OCP and potentiodynamic polarisation. However, by comparison the SAC305-1Bi sample shows an anomalously high corrosion current and low pitting resistance. The corrosion products were also monitored via GI-XRD post-testing and the main one was attributed to abhurite. The authors conclude that the poor performance of the SAC305-1Bi sample is due to microgalvanic interactions in the interdendritic microstructure, suggesting that the segregation of the Bi particles in the higher Bi-content alloys mitigate galvanic corrosion. 

The manuscript is generally suitable to the scope of Technologies and is of sufficient quality both in terms of presentation and English. While the novelty is not extremely high, since it is acknowledged in the Introduction that the SAC305-Bi corrosion is studied previously, the poor corrosion performance of the 1 wt.% Bi samples is interesting enough to contribute to the field. In terms of preparation and microstructure characterisation, however, I sense that a bad impression is made between the large overlap between the preparation of the SAC305 batches in their previous work (ref. 18) - and even though it is disclaimed in the text (lines 185, 206, 213) maybe some of the details on the preparation of the solder alloys (if samples were re-used only for the electrochemical characterisation) could be simplified (i.e. the preparation of the initial samples). Additionally, since a large part of the introduction explains that the corrosion behaviour of the SAC305 was previously investigated (lines 87-97) the authors need to probably outline the gap that this publication fills more explicitly (i.e. clarify why the 1, 2, and 4 wt. % were selected in comparison to the previous study and what is not resolved by them).

Apart from this I feel that the work is publishable, however, there are several large issues with the manuscript - some missing details about the electrochemical characterisation, and also replicability of the results, so I suggest major revision and recommend that the authors improve their work via the following suggestions:

1) While the description of the corrosion measurements is very lengthy (lines 123-141), some details about the electrochemical measurements are missing - the potentiostat model is not disclosed, and additionally there is no information about the volume of the corrosion cell and the temperatures at which the measurements were taken. 

2) While it is mentioned that the potentiodynamic measurements were carried by sweeping at 1 mV/s from -900 to +500 mV, it is not mentioned how the starting potential was applied (was there a holding / conditioning period). Given that this is about 400 - 500  mV negative to the observed Ecorr for most samples, this could make any explanations about the passive-layer stability (such as these mentioned in lines 334-335) tricky, since the large cathodic prepolarization may modify or reduce the surface film before reaching the anodic branch. This is an issue, since no reverse scan was performed and should be discussed carefully in the text.

3) Figures 2 - to - 6 present only a single measurement / data point per sample. Given the large difference between the literature expectation (for the 1 wt.% case, as shown in Figure 6) and that this observation is the main result of the manuscript - the authors need to disclose how many samples were independently prepared, and how many measurements were taken and demonstrate that the result for the SAC305-1Bi sample is reproducible, and not an outlier due to experimental uncertainty.

4) The interpretation of the microgalvanic mechanism, based on the standard redox potentials (lines 340-357) is quite simplistic and should probably be outlined as a plausible explanation, rather than a confident one. The mixed (corrosion electrode potentials) in the NaCl electrolyte may differ in an unexpected ways, compared to the standard redox potentials, and especially for passivating metals, such as Bi. The Sn is also problematic, since depending on the pH and Cl- activity it may form a lot of different types of passivating layers. So - statements such as “Bi is nobler compared to Sn (Table 4)” (line 351) are grossly oversimplified. They authors should tone down this interpretation (given the ‘specifics’ of most people in the electrochemistry field) and be cautions about such statements.

5) There seems to be an unexplained discrepancy between the post-corrosion SAC305-1Bi samples. Figure 7b shows a very smooth area, which as the authors explain “in the case of the SAC305-1Bi alloy, a different microstructure has been observed. The crystallites were smaller and the scale was less adherent and partially spalled off (Fig. 7b). The original alloy surface has thus been exposed.” (Lines 374-378), however, the image in Figure 8 (top-left pane) shows a very different morphology similar to the ones in Figure 7 for the 0, 2, and 4 wt. % Bi. The authors need to comment on the area chosen for the analysis in Figures 7 and 8 for the 1 wt.% Bi content.

6) Regarding the GI-XRD results it is not clear how the data for lattice parameters and crystallite size / microstrain (Table 6) were obtained. Was a Rietvelt refinement conducted and which software was used. Additionally - reporting the lattice parameters with a precision of 10(-5) Å is quite overwhelming. Additionally - the crystallite size - given that the corrosion products are largely plate-like - there could be texturing - so were these valued average from multiple peaks (i.e. as in Rietvelt) or a single reflection was used and which. 

Author Response

Dear reviewer,

 

We are thankful for your valuable comments. They helped us to improve the quality of our manuscript. Answers to issues raised are given below. The changes in the manuscript have been highlighted as tracked changes.

 

Kind regards,

 

Authors

 

REVIEWER 3

 

Comments and Suggestions for Authors

The manuscript, titled “Microstructure and corrosion resistance of Sn-3Ag-0.5Cu-xBi solders”, submitted by Halmanová et al. to MDPI Technologies presents results on the effect of Bi-content (0 - 4 wt. % range) on the microstructure and corrosion resistance of SAC305 lead-free solder. The authors prepared four samples of the SAC305-xBi solder alloys via induction melting and characterised that microstructure via SEM-EDS measurements, phase composition via GI-XRD and the corrosion behaviour via electrochemical evaluation (OCP and potentiodynamic polarisation) in a model corrosion environment - 3.5 % NaCl. The results show that at higher Bi-content (>2%) Bi-rich precipitates are formed in the solder microstructure, however these solder alloys have a generally positive shift in potential both in terms of OCP and potentiodynamic polarisation. However, by comparison the SAC305-1Bi sample shows an anomalously high corrosion current and low pitting resistance. The corrosion products were also monitored via GI-XRD post-testing and the main one was attributed to abhurite. The authors conclude that the poor performance of the SAC305-1Bi sample is due to microgalvanic interactions in the interdendritic microstructure, suggesting that the segregation of the Bi particles in the higher Bi-content alloys mitigate galvanic corrosion. 

The manuscript is generally suitable to the scope of Technologies and is of sufficient quality both in terms of presentation and English. While the novelty is not extremely high, since it is acknowledged in the Introduction that the SAC305-Bi corrosion is studied previously, the poor corrosion performance of the 1 wt.% Bi samples is interesting enough to contribute to the field. In terms of preparation and microstructure characterisation, however, I sense that a bad impression is made between the large overlap between the preparation of the SAC305 batches in their previous work (ref. 18) - and even though it is disclaimed in the text (lines 185, 206, 213) maybe some of the details on the preparation of the solder alloys (if samples were re-used only for the electrochemical characterisation) could be simplified (i.e. the preparation of the initial samples). Additionally, since a large part of the introduction explains that the corrosion behaviour of the SAC305 was previously investigated (lines 87-97) the authors need to probably outline the gap that this publication fills more explicitly (i.e. clarify why the 1, 2, and 4 wt. % were selected in comparison to the previous study and what is not resolved by them).

Apart from this I feel that the work is publishable, however, there are several large issues with the manuscript - some missing details about the electrochemical characterisation, and also replicability of the results, so I suggest major revision and recommend that the authors improve their work via the following suggestions:

1) While the description of the corrosion measurements is very lengthy (lines 123-141), some details about the electrochemical measurements are missing - the potentiostat model is not disclosed, and additionally there is no information about the volume of the corrosion cell and the temperatures at which the measurements were taken. 

 

Thank you for your useful comment. The potentiostat model (PGU 10 V-1A-IMP-S, Jaissle Electronic Ltd., Waiblingen, Germany), corrosion cell volume (0.5 L) and laboratory temperature (22 °C) have been specified in the manuscript.

 

2) While it is mentioned that the potentiodynamic measurements were carried by sweeping at 1 mV/s from -900 to +500 mV, it is not mentioned how the starting potential was applied (was there a holding / conditioning period). Given that this is about 400 - 500  mV negative to the observed Ecorr for most samples, this could make any explanations about the passive-layer stability (such as these mentioned in lines 334-335) tricky, since the large cathodic prepolarization may modify or reduce the surface film before reaching the anodic branch. This is an issue, since no reverse scan was performed and should be discussed carefully in the text.

 

We agree that a larger sweeping window in the cathodic region may eventually slightly modify the surface layer because of more intense cathodic processes. However, this sweeping range was used to obtain a sufficiently large linear Tafel region. This sweeping window was required to obtain a linear region for at least one decade of current (Fig. 4). It should also be noted that scanning 400-500 mV away from the corrosion potential is not unusual as it has been used in numerous publications before. See, e.g., https://doi.org/10.1016/j.matchar.2025.115442,  https://doi.org/10.1007/s10854-026-16703-4,   https://doi.org/10.1016/j.corsci.2018.08.041, etc. We have applied it here to record a sufficiently large linear Tafel region required for a reliable determination of corrosion current.

 

3) Figures 2 - to - 6 present only a single measurement / data point per sample. Given the large difference between the literature expectation (for the 1 wt.% case, as shown in Figure 6) and that this observation is the main result of the manuscript - the authors need to disclose how many samples were independently prepared, and how many measurements were taken and demonstrate that the result for the SAC305-1Bi sample is reproducible, and not an outlier due to experimental uncertainty.

 

Three samples of each alloy composition were prepared to study the microstructural features and corrosion behavior. The corrosion experiments were carried out in triplicates, each time with a freshly prepared alloy surface. The results given in Table 3 are reported as average from three measurements. The standard deviations of OCP and j_corr are also given in Figs. 3 and 6.

 

4) The interpretation of the microgalvanic mechanism, based on the standard redox potentials (lines 340-357) is quite simplistic and should probably be outlined as a plausible explanation, rather than a confident one. The mixed (corrosion electrode potentials) in the NaCl electrolyte may differ in an unexpected ways, compared to the standard redox potentials, and especially for passivating metals, such as Bi. The Sn is also problematic, since depending on the pH and Cl- activity it may form a lot of different types of passivating layers. So - statements such as “Bi is nobler compared to Sn (Table 4)” (line 351) are grossly oversimplified. They authors should tone down this interpretation (given the ‘specifics’ of most people in the electrochemistry field) and be cautions about such statements.

 

Thank you for your useful comment. We agree that the interpretation of the micro-galvanic mechanism based on standard redox potentials of constituent elements represents a first approximation. Local electrochemical techniques, e.g., scanning Kelvin probe force microscopy, could provide more details on the electrochemical activity of the individual microstructural constituents. Unfortunately, we don’t have these techniques in our lab. We have therefore rewritten the discussion of corrosion behavior in accordance with your suggestions. We have now provided the difference in standard redox potentials of constituent elements as a plausible explanation only.

 

5) There seems to be an unexplained discrepancy between the post-corrosion SAC305-1Bi samples. Figure 7b shows a very smooth area, which as the authors explain “in the case of the SAC305-1Bi alloy, a different microstructure has been observed. The crystallites were smaller and the scale was less adherent and partially spalled off (Fig. 7b). The original alloy surface has thus been exposed.” (Lines 374-378), however, the image in Figure 8 (top-left pane) shows a very different morphology similar to the ones in Figure 7 for the 0, 2, and 4 wt. % Bi. The authors need to comment on the area chosen for the analysis in Figures 7 and 8 for the 1 wt.% Bi content.

 

We apologize for providing wrong Fig. 8 in the previous version of the manuscript. It represented a different alloy (SAC305). This figure has now been replaced with a correct one.  It represents the microstructural features and EDS element mapping of the corrosion product

Fig. 8 Microstructure and EDS element mapping of the post-corroded SAC305-1Bi alloy

 

6) Regarding the GI-XRD results it is not clear how the data for lattice parameters and crystallite size / microstrain (Table 6) were obtained. Was a Rietvelt refinement conducted and which software was used. Additionally - reporting the lattice parameters with a precision of 10(-5) Å is quite overwhelming. Additionally - the crystallite size - given that the corrosion products are largely plate-like - there could be texturing - so were these valued average from multiple peaks (i.e. as in Rietvelt) or a single reflection was used and which. 

 

Thank you for your comment. Yes, the Rietveld refinement was used to describe the experimental GI-XRD peaks. Phase identification and Rietveld refinement were performed using Malvern PANalytical X´Pert HighScore Plus software, version 5.1. Initial identification was based on comparing experimental diffraction patterns with the ICDD database. Only entries containing complete structural data, including atomic positions, and showing good agreement with the experimental pattern were used for subsequent refinement. Instrument broadening was determined from the x-ray diffraction pattern of LaB6, NIST Standard Reference Material 660c, collected in GI-XRD configuration.

Lattice parameters, coherent diffraction domain size, and microstrain were obtained from the full-pattern Rietveld refinement. Preferred crystallographic orientation was treated using Spherical Harmonics. Details regarding the software used, database, structural models, profile function, and refined parameters have been added to the revised manuscript.

We also agree that the original number of decimal places for the lattice parameters implied unrealistic precision. The values Table 6 have therefore been rounded in accordance with their experimental uncertainty.

Given the plate-like morphology of abhurite, the potential influence of preferred orientation has also been addressed in the revised text. Crystallite size values are now designated as average coherent diffraction domain sizes obtained by full-profile analysis, rather than direct dimensions of the plates observed via SEM. The interpretation of differences in domain size and microstrain has been moderated accordingly.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors The authors have revised the manuscript well and it can be accepted for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

I would like to thank the authors for their efforts to improve the manuscript and I believe that currently it is of a sufficient quality for a publication. I still have a minor issue with the misunderstanding on why the initial application of a -900 mV potential in the -900 to +500 mV sweep window. My point here is not that the sweep window is unusual, nor problematic, but I was just asking on - what the starting potential was (i.e. whether that the -900 mV was applied immediately, since this would affect the starting conditions, especially with passivating films, or if the authors started at OCP and then sweeped towards the initial -900 mV). 

Nevertheless, given that the replication data is provided and that the manuscript is sufficiently improved, I believe that it is publishable and can be accepted in its current state in Technologies.