Peer review of: Proton irradiation-decelerated intergranular corrosion of Ni-Cr alloys in molten salt (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
This paper studies the synergies between radiation and corrosion of a Ni-Cr alloy and reports that 
proton irradiation decelerates intergranular corrosion of this alloy in molten fluoride salt at 650C. 
The authors designed an excellent experiment and make it possible to study the radiation-
corrosion synergies of a nuclear material using an ion beam facility. Further, the results they 
obtained are very interesting and the discussions are reasonable. It is a well written manuscript 
and suitable to be published in this journal. 
 
Suggestions: 
Caption of Figure 2, page 9: “Cross-sectional comparison of corrosion with and without molten salt 
irradiation on Ni-20Cr.” should be corrected as “Cross-sectional comparison of corrosion with and 
without proton irradiation on Ni-20Cr.”? 
 
 
 
Reviewer #2: 
Remarks to the Author: 
This paper describes the results of proton irradiation of a Ni-20Cr alloy in contact with FLiNaK to 
assess the effect of irradiation on corrosion of the alloy. The topic is timely and important as it 
addresses the concern that irradiation may significantly accelerate corrosion in molten salt, as it 
has been found to do with Zircaloy in light water reactors. The result here indicates that in fact, 
irradiation decelerates corrosion and the authors offer an explanation for that observation. The 
results are novel and the paper is well written and easy to follow. The statistical analysis is 
appropriate. The supplemental information is important and useful. Results are clearly presented 
and the analysis is easy to follow. Nevertheless, there are several points that the authors need to 
address, in particular the role of irradiation- and corrosion produced-defects. 
 
1) The first concern I have is that the seal between the foil and the molten salt may not be sound. 
That is, a compression seal is prone to leakage and if this occurs, it could explain the corrosion 
pattern on the Fe foil as well as salt on the vacuum side of the Ni-20Cr foil. What evidence do the 
authors have that indicates that the seal did not leak? What was the pressure in the beamline for 
this experiment? 
 
2) P. 2, Line 100: The authors state that corrosion is a balance between irradiation-increased 
corrosiveness of the salt and deceleration of corrosion due to radiation effects in the alloy. They 
then deduce that that increased corrosion with beam current density is because of increased 
corrosion attack due to increased corrosiveness of the salt under irradiation. But their mechanism 
(described later) predicts increased corrosion deceleration with beam current density, and as they 
state on line 95, this process “outperforms” the corrosiveness of the salt. So, by their own logic, 
the corrosion rate should decrease with current density, not increase. 
 
3) P. 3, line 58: This statement essentially says that the introduction of charged hydrogen ions in 
molten salt makes a more corrosive environment. But no explanation is provided as to why this 
should occur. 
 
4) P. 3, line 35 and Supplementary section: The authors argue that radiation enhanced diffusion 
accelerates the transport of Cr to the grain boundary, and that this is, in part, responsible for 
deceleration of corrosion in molten salt. However, since GB diffusion is much faster (orders of 
magnitude) than bulk diffusion, the supply of Cr to the boundary simply serves to increase its 
dissolution rate - it doesn't raise the Cr level at the boundary since it leaves by dissolution once it 
arrives. Therefore, the RED can't explain the beneficial effect of irradiation. If RED transports Cr to 
the GB, and molten salt leaches Cr, then the corrosion should be worse under irradiation than 
without it. In fact, the increase in corrosion with current density supports this. 
 
They also argue that RIS should be insignificant in their alloy at 650°C. But then on p. 1, line 71 of 
the Supplement, they argue that RED is enhanced by RIS. This statement is false on two accounts. 
First, RIS cannot enhance RED. It is the other way around. Second, on line 43, they discounted 
RIS. So they cannot discount it and then come back and claim it has an effect. 
 
Further, on line 80 they cite both RIS and corrosion-induced vacancy injection as being jointly 
responsible for the deceleration of corrosion of Ni-20Cr in molten salt. Again, having dismissed RIS 
as insignificant, it cannot now be cited as a major factor in the observed corrosion. 
 
Their understanding of the role of irradiation enhancement of Ni and Cr supply to the grain 
boundary is also incorrect. In a Ni-Cr alloy, Cr diffuses preferentially against the vacancy flux. Ni 
diffuses easier via interstitials than does Cr, so the net result is Ni enrichment and Cr depletion at 
grain boundaries. The reason RIS is minimal at very high temperature is that the thermal vacancy 
concentration is such that a concentration gradient cannot be supported. RED does not change the 
preference of Cr to deplete and Ni to enrich. The injection of vacancies at the grain boundary can 
alter the direction of flow, but for that to happen, it would have to “overpower” the vacancy supply 
from irradiation. The authors have not done an adequate assessment of this situation to make 
conclusions on the role of vacancy injection due to corrosion. So their conclusions on its effect are 
really speculation. 
 
 
 
Reviewer #3: 
Remarks to the Author: 
I do not think the paper can be published in the journal. 
On page 1 
1. Line 33 “ radiation damage always results in negative effects”, this statement is not right, some 
experiments have shown that radiation can have some positive effects on corrosion, even for 
neutron radiation 
2. Line 52 “making corrosion in molten salt different from aqueous corrosion” actually, both types 
of corrosion in molten salt and aqueous solution are electrochemical process including anodic 
dissolution and cathodic reduction, the corrosion mechanisms are the same. 
3. Line 53 “ the predominant… , is selective dissolution of Cr“ Actually, selective dissolution of Cr is 
just one type of the corrosion, which depends on the corrosion potential of the molten salt. if the 
corrosion potential is above the Ni dissolution potential, Ni can be dissolved very faster than Cr. 
Corrosion not only depends on the thermodynamic potential but also depends on the dissolution 
kinetics when the corrosion potential is above its dissolution potential 
4. Line 69-71, “in liquid lead…oxidation was the primary corrosion mechanism”. Actually, in liquid 
lead, the major corrosion mechanism is physical dissolution, not oxidation, the oxygen control 
technology is applied in liquid lead to reduce the corrosion by stopping the physical dissolution 
 
On Page 3: 
1) Eq.1 seems not right, the unit on the left and right are not the same, the authors need to 
double check the reference and actually understand what the equation is in the reference [25] 
2) The authors think that it is EuF3 that leads to corrosion. I highly doubt it. I read through the 
reference cited, the original reference was proposed to use Eu3+/Eu2+ couple to mitigate 
corrosion by decreasing corrosion potential. The author needs to make at least a very simple 
thermodynamic calculation to show that the standard potential of Eu3+/Eu2+ is more positive 
than Cr2+/Cr, if it is more positive, which means it is possible that it is Eu3+ that leads to 
corrosion, or it is impossible 
3) The impurity of the salt is unknown, especially, the oxygen level and the moisture lever, both 
oxygen and moisture can be the corrosion sources. Therefore, the oxygen level and moisture level 
at which they prepared the salt must be given. 

4) Figure 4 is not complete. The authors need to know that the molten salt corrosion includes two 
reactions: anodic dissolution/oxidation and cathodic reduction. The corrosion of Cr is through a 
reaction Cr-2e->Cr2+, there must be a species that can be reduced to “eat” electron released by 
Cr dissolution. The species must be diffuse in the salt to reach the corrosion surface, the second 
reaction was completely neglected in Figure 4. I do not think the author really understand the 
corrosion mechanisms by molten salt. 
Another important factor that will lead to corrosion was not addressed in the document. The 
author used proton (H+) which is completely stopped in the salt and accumulate in the salt. H+ is 
a corrosive species in salt because H(+) +e ->H2 which can be reduction reaction that may lead to 
corrosion, which is not related to radiation. It is because the author artificially introduced corrosive 
species into their system. Therefore, H(+) influence on corrosion during experiments needs to 
addressed and discussed.