Hot Corrosion Behavior of Co–Al–W Superalloys with Si Additions
Round 1
Reviewer 1 Report
In this work, the authors have studied the high temperature corrosion behavior of Co-9Al-9.5W-xSi (x=0; 0.1 and 0.5 at. %) alloys in molten salts. It has been found that Si additions improve the corrosion resistance. The paper is interesting; however, it is relatively short (10 pages). It lacks some important results. A major revision is recommended.
1.The microstructure and phase constitution of the alloys before corrosion should be presented in the paper.
2.The paper is submitted to journal named Coatings. However, the alloys studied were not coated prior to oxidation. You need to sufficiently explain how the paper fits the journal scope. A developed oxide scale may serve as a coating; however, it needs to be substantially explained in the introduction.
3.Powder diffraction file numbers of identified oxides should be given in the legend of Fig. 2.
4.Lines of the manuscript should be numbered to ease orientation.
5.The authors show that Si addition decreases the rate of oxidation (Fig. 1). However, a complete SiO2 scale has not been observed (Fig. 2, Fig. 5). A natural question is: What is the principal mechanism of action of Si? Explain it clearly in the manuscript.
6.Where in the scale was Si located? You should include Si EDS maps in Fig. 6 to indicate the location of this element in the scale.
7.Rate constants should be obtained from data in Fig. 1 and compared with previously studied Co-Al alloys (https://doi.org/10.1007/BF00611691, https://doi.org/10.3390/ma13143152, etc.)
8.The conclusions section is not finished. The last sentence is open.
9.The language of the paper needs to be improved. In the discussion section, several sentences are either strange or difficult to understand, e.g., The Na2SO4 can degrade the oxides (should be Na2SO4 can react with oxides), This process repeatedly repeats (should be the process repeats itself), Johnson et al. studied the high temperature properties of the Ni-based alloys with the effect of NaCl and Na2SO4 (should be Johnson et al. studied the high temperature oxidation of Ni-based alloys by NaCl and Na2SO4), etc. Personal statements like “We know that” should be avoided. The paper should be proof-read by native speaker before resubmission.
The language of the paper needs to be improved. In the discussion section, several sentences are either strange or difficult to understand, e.g., The Na2SO4 can degrade the oxides (should be Na2SO4 can react with oxides), This process repeatedly repeats (should be the process repeats itself), Johnson et al. studied the high temperature properties of the Ni-based alloys with the effect of NaCl and Na2SO4 (should be Johnson et al. studied the high temperature oxidation of Ni-based alloys by NaCl and Na2SO4), etc. Personal statements like “We know that” should be avoided. The paper should be proof-read by native speaker before resubmission.
Author Response
General comments:
In this work, the authors have studied the high temperature corrosion behavior of Co-9Al-9.5W-xSi (x=0; 0.1 and 0.5 at. %) alloys in molten salts. It has been found that Si additions improve the corrosion resistance. The paper is interesting; however, it is relatively short (10 pages). It lacks some important results. A major revision is recommended.
Reply: We have checked the paper carefully and revised, in addition, I invited a friend (her official language is English) to improve the English usage. I believe the quality of English is greatly improved. we added more figures and some investigated results according your requirements
Specific comment
1.The microstructure and phase constitution of the alloys before corrosion should be presented in the paper.
Reply: We have added Figure 1,which gives the microstructure of the experimental materials, and concludes that it primary composed of γ and γ ′phase, as well as minor Co3W presented in grain boundary. The actual composition of the alloy were given too.
2.The paper is submitted to journal named Coatings. However, the alloys studied were not coated prior to oxidation. You need to sufficiently explain how the paper fits the journal scope. A developed oxide scale may serve as a coating; however, it needs to be substantially explained in the introduction.
Reply：The first way is to form a dense oxide scale which can provide a satisfying proactive oxide. The addition of Si, an approach to adding alloying elements, is beneficial to develop a continuous compact oxide The other way is introducing to introduce a coating,such as MCrAlY and Al. If the Si bearing alloy still can not process excellent resistance to heat corrosion, then a coating is necessary. Thereby the paper is relevant to this journal.
3.Powder diffraction file numbers of identified oxides should be given in the legend of Fig. 2.
Reply: We have added PDF file number in the text.
4.Lines of the manuscript should be numbered to ease orientation.Reply: line number have been added according to your suggestion.
- The authors show that Si addition decreases the rate of oxidation (Fig. 1). However, a complete SiO2 scale has not been observed (Fig. 2, Fig. 5). A natural question is: What is the principal mechanism of action of Si? Explain it clearly in the manuscript.
Reply: The observation of cross section of oxide shows no SiO2 scale, and EDS mapping of Si also indicates the lack of continuous SiO2. The beneficial effect of Si addition may derive from the promotion of a continuous dense Al2O3 scale.
6.Where in the scale was Si located? You should include Si EDS maps in Fig. 6 to indicate the location of this element in the scale.
Reply: The mapping of Si have been attached in the Figure. Si is concentrated in the outerlayer and intermediate layer of the oxide scale.
7.Rate constants should be obtained from data in Fig. 1 and compared with previously studied Co-Al alloys (https://doi.org/10.1007/BF00611691, https://doi.org/10.3390/ma13143152, etc.)
Reply: the rate constant have been counted and compared with data reported by above mentioned article. The constant for Si-free alloy is1.81X10-8g2m-4s -2, and it drop to 4.2X10-9g2m-4s -2 when the Si content is 0.5%.
8.The conclusions section is not finished. The last sentence is open.Reply: we modified the last sentence. 9.The language of the paper needs to be improved. In the discussion section, several sentences are either strange or difficult to understand, e.g., The Na2SO4 can degrade the oxides (should be Na2SO4 can react with oxides), This process repeatedly repeats (should be the process repeats itself), Johnson et al. studied the high temperature properties of the Ni-based alloys with the effect of NaCl and Na2SO4 (should be Johnson et al. studied the high temperature oxidation of Ni-based alloys by NaCl and Na2SO4), etc. Personal statements like “We know that” should be avoided. The paper should be proof-read by native speaker before resubmission.Reply: we have invited a professional teacher to revise the paper, now the language was greatly improved.
Reviewer 2 Report
The results shown in Figure 1 needs to show the reproducibility of the data with bars.
The style of writing should be improved. There are too long paragraphs.
The contrast in Figure 6 (b,c,e,f,h,l) is not good enough and should be improved.
The text of the manuscript needs correction once there are too many grammar errors such as the first phrase of the abstract reproduced here: "The hot corrosion behavior of Co-9Al-9.5W-xSi (where x=0%, 0.1%, 0.5%, atomic fraction) alloys in a salt mixture of 75 % Na2SO4+25% NaCl (wt.%) at 900°C were investigated"
Author Response
1.The results shown in Figure 1 needs to show the reproducibility of the data with bars.
Reply: Figure1(Now Figure 2) have added with error bars.
- The style of writing should be improved. There are too long paragraphs.
Reply: the paper have been revised greatly.
- The contrast in Figure 6 (b,c,e,f,h,l) is not good enough and should be improved.
Reply: The contrast and brightness is adjusted in Figure 6(now Fig.7). But for the deplete layer for specific element, some darkness still presents, although it is improved.
- The text of the manuscript needs correction once there are too many grammar errors such as the first phrase of the abstract reproduced here: "The hot corrosion behavior of Co-9Al-9.5W-xSi (where x=0%, 0.1%, 0.5%, atomic fraction) alloys in a salt mixture of 75 % Na2SO4+25% NaCl (wt.%) at 900°C were investigated"
Reply: The paper have been revised greatly in English writing .
Reviewer 3 Report
The reviewed paper describes an interesting study of the Co-Al-W superalloy in a corrosive environment at high temperatures. Many interesting measurement techniques were used in the research. The research results are supported by a thorough literature analysis. The conclusions of the research are useful and clearly formulated. The only drawback of the work is Figure 6 - it is difficult to read too dark colors.
Author Response
The reviewed paper describes an interesting study of the Co-Al-W superalloy in a corrosive environment at high temperatures. Many interesting measurement techniques were used in the research. The research results are supported by a thorough literature analysis. The conclusions of the research are useful and clearly formulated. The only drawback of the work is Figure 6 - it is difficult to read too dark colors.
Reply: The contrast and brightness of the figurehave been adjusted in Figure6(now Fig.7), now it is much better. But for the concentration of specific element deviate greatly, some darkness still exists, indicates the depletion of specific element.
Round 2
Reviewer 1 Report
The authors answered my previous comments. The manuscript has been improved. It can be accepted for publication.
Author Response
The authors answered my previous comments. The manuscript has been improved. It can be accepted for publication.
Reply: We have checked the paper carefully and revised again, with the help an English. I believe the quality of English is greatly improved.
