Degradation Analysis of Thin Mg-xAg Wires Using X-ray Near-Field Holotomography

Round 1

Reviewer 1 Report

The paper deals with a very interesting topic and has a great potential to have an impact on future research of degradation of biodegradable materials.

I have several points, which can enhance  the quality of this nice paper:

1) The sentence "However, during the recrystallization treatment of Mg-6Ag, the 420°C was not exceeded for a sufficient time" (in rows 150 and 151, Results section) should be reformulated. The annealing temperature was 400 °C, so 420 °C was not exceeded, therefore, only a part of precipitates could be dissolved. In my oppinion, in this case, the annealing at 400 °C could even get the material closer to the equilibrium state (compared to the as-drawn state).

 

 

Author Response

Thank you for this important comment. We have adopted the suggestion and changed the sentence to: “The annealing temperature was 400 °C, so 420 °C was not exceeded and precipitates were not dissolved completely. In this state the phase Mg4Ag precipitates are thermodynamically stable in the phase diagram (Figure 1 (c)).”

Reviewer 2 Report

1、The degradation rate of recrystallized Mg-6Ag first increases and then decreases. The aforementioned SEM shows that the recrystallized Mg-6Ag has silver precipitates. Is the increase in degradation rate due to the effect of silver precipitation, and why does it decrease afterwards？

2、The degradation rate of Mg-6Ag first increases and then decreases, and the pitting factor decreases with time. As mentioned later, the smaller pitting factor is related to the larger degradation rate, so should the trend of the pitting factor increase first and then reduce。

3、Higher Ag content will lead to higher pitting corrosion tendency, is it contradictory to higher Ag content, higher degradation rate and smaller pitting factor?

4、Figure 3 shows the effect of the material cluster, namely the silver precipitation, on Mg-2Ag

5、T4 treatment and ECAP, no analysis of Mg-6Ag

6、Which materials are used for degradation experiments

Author Response

Thank you for the questions to help clarify the manscript. Please find the response and edits in the following:

1. The initial increase in degradation rate is mostly due to the formation of a Mg-OH layer that passivates the metallic material underneath and therefore decreases the degradation rate. The precipitates are in the volume of the sample and increase the degradation rate over the whole period of time. Additionally, it needs to be stated that only one sample was imaged after 1 day of immersion, so while the results show the afore-mentioned trend, these need to be considered critically.
2. As the overall correlation shows that a smaller degradation rate correlates to a higher pitting factor and vice versa, the degradation rate trend would suggest that the pitting factor should first decrease and then increase. This is the case when considering the median pitting factor for all time points, although the minimum median is at 3 days of degradation instead of 2. This can be attributed to the high variance of the data. 

3. Mg-6Ag (Figure 4) shows lower degradation and a higher pitting factor. Larger pitting factors can be associated with lower degradation rates, while the lowest pitting factors are associated with larger degradation rates (Figure 5). Therefore we find - in contrast to the literature - that higher Ag content led to a lower degradation rate, which is in agreement to the tendency for a higher pitting factor according to the correlation. As stated in the manuscript, the contradictory findings to the literature may be attributed to differences in the material microstructure.

4. Figure 3 is meant to provide a visual assessment of the image data obtained from holotomography and the segmented residual metal. In our opinion, no effects of the material cluster on Mg-2Ag can be derived from this single image. Moreover, it cannot be assessed whether the cluster was present prior to degradation or is a product of the degradation process.

5. Estrin et al. [15] used ECAP on Mg-2Ag and Mg-4Ag samples and found precipitations more pronounced in Mg-4Ag. These precipitates are suggested to be the reason for higher degradation rates observed in their work. We analysed in this work Mg-2Ag and Mg-6Ag each with favored precipitation by the recrystallization treatment and dissolved with T4 annealing.

6. Wire pieces of Mg-2Ag and Mg-6Ag in the recrystallized and annealed state were used for the degradation experiment. The “In vitro degradation” subsection was edited for clarification.

Reviewer 3 Report

This MS reported a new NFHT technology to detect the degradation rate of magnesium alloy, which provides new method to assess the degredation behavior of biodegradable metal. However, there are some questions :

1, Fig.3 shows the slice tomographic detected by the technology. In comparison with fig.3a, the red-dash line outline the residual metal, the gold color in Fig.3c shows the residual metal detected by NFHT. Very significant difference was found between two images. Why? If it is true, does it mean that NFHT can not give an accurate image result?

2, In Fig.4, the degradation rate detected by NFHT ranges in a very large range, from 0 to 5mm/year. Why? Was it caused by NFHT technology or samples?

3, For image analyses technology, the accuracy  is very important. How about the accuracy of this technology? How did you get this?

4, it is necessary to do more experiments or more samples to prove the feasibility of this technology.

5, Is it possible to be used in vivo detection of the the degradation? Do you have experimental results? please show. 

Author Response

Please find the response to your comments listed below:

1. Thank you for highlighting the lack of clarity in the image. In fact, Fig 3a and 3c are showing different depictions of the same sample obtained using NFHT. Fig 3a is the actual image following image reconstruction, which is then processed and segmented to enable a quantitative analysis. Based on the segmentation, surface renderings were created, which are shown in Fig 3b and c. Figure 3c was slightly rotated within the image plane with respect to 3a, which may have resulted in the apparent difference. We have adjusted the rotation for better visualisation. The observed difference does not relate to the accuracy of NFHT. Additionally, we have added some text in the image caption to clarify the matter.

2. The wide variability in the sample degradation rate is due to the strong and inhomogeneous degradation of the material itself. The inhomogeneity may be accentuated by NFHT due to the small field of views that are investigated, but Figure A2 shows that the intra-sample variance is high too, therefore we conclude that it is in fact the material that leads to the variable degradation rate. We have added a sentence in line 249 to clarify this fact.

3. For NFHT, the magnification and therefore the voxel size and resolution are related to the focal spot size that is created using the Fresnel zone plates, as well as the defocus distances used. In the current setup the focal spot size is 95 nm according to Flenner et al. [24], thus limiting the image resolution to approx. 100 nm. However, since the image voxel size after binning is 352 nm, this determines the spatial accuracy of the method. We have added a sentence in line 124 to highlight this important fact.

4. The general feasibility of NFHT has been established by P. Cloetens at ESRF and T. Salditt at Göttingen University. We have recently published a study showing that NFHT can be used to analyse the ZK60 precipitate structure leading to the same conclusions as other more established techniques in materials characterization (SEM, SAXS) [1]. While the present study is the first time that NFHT is used for the assessment of Mg degradation, the image data is similar to other X-ray tomographic techniques, so that the technique’s feasibility is ensured. We have performed 40 experiments, which is a very high number of experiments for NFHT, due to the long imaging times required and the necessity for beamtime at a synchrotron radiation source. Nevertheless, we generally agree with the reviewer that an even higher number of experiments would enable a better assessment of the material behaviour, due to the high variability in the data, as outlined in line 247/248.

   [1] B. Zeller-Plumhoff, A. Robisch, D. Pelliccia, E. Longo, H. Slominska, A. Hermann, M. Krenkel, M. Storm, Y. Estrin, R. Willumeit-Römer, T. Salditt, D. Orlov, Nanotomographic evaluation of precipitate structure evolution in a Mg-Zn-Zr alloy during plastic deformation, Scientific Reports 10, 16101 (2020). doi: 10.1038/s41598-020-72964-x

5. Thank you for this interesting question. Unfortunately, the detection in vivo using this technique is not possible, due to the X-ray energy used which limits the transmission through body tissue and the limited field of view. However, the imaging of explants following sacrifice of animals would be possible. At the current stage the presented wires have not yet been implanted into animals.

Round 2

Reviewer 2 Report

The manuscript can be accepted for publication after carefully checking the English spelling and grammar.