Corrosion Behavior of Inconel 625 Coating Produced by Laser Cladding

Round 1

Reviewer 1 Report

Dear authors,

you've written a very intersting paper. I have some suggestions.

Please check your English, e.g. grammar (singular vs. plural). Fig. 2 shows two images.

Can you provide the surface area of the reference electrode. I assume 1 cm².

Can you provide next to the composition of IN 625 also the one of the substrate. 

The dentritic structure of the wire coating is not as homogeneous as the powder coating, why?

Can you add a table indicating thethicknesses of he two coatings the HAZ and eventually an influenced subsurface zone. The coatings are wavy so mean values as well as max and min would be a good information.

Are there pores at the interface. It is hard to see in the figs. 3 and 4. You should indicate HAZ in fig. 4.

Due to interdiffusion withthe substrate I would expect Ni, Mo and Cr enrichment of the subsurface zone as well as Fe + Mn outward diffusion into the coating. Can you comment on that e.g. by EDS line scan analysis of the cross sections.

The HV10 value of PIn is 311 and not 259.

A high hardness value tells you nothing about the corrosion resistance of the material. This should be claryfied in 3.1. This is the same for the homogeneity of the coating. (3.3). If a material does not show protective behavior it will corrode no matter whether it is or is not homogeneous.

Is the HAZ really 2 mm thick (3.2). I would say 2 µm!

What does (FeClOH-) mean. Is this an anion. This would indicate Fe in the oxidation state + 1. Fe is either +2 or +3. (O = 2-, Cl = 1-, H = 1+)

In an acidid solution non-precious metals dissolve via H+ reduction, i.e. H2 evaporates and Me+x goes into the solvent (Ni2+, Al3+, etc.) You HCl solution with a pH of 1 is very acidic. Did you observe H2 formation?

I assume that your coating should have a thickness of several 100-100 µm on your component. If there is a metal loss of several mm per year how can such a thin coating prevent attack on the base material?

You've treated the exposed specimens with nitric acid. How could you be sure only to attack the oxide and not the metal? 

Regards

Reviewer

Author Response

Reviewer 1

 

Thank you very much to the Reviewers for their thorough and substantive checking of the text of our publication.

Below are the answers to the questions and suggestions of the reviewers:

 

1. Please check your English, e.g. grammar (singular vs. plural). Fig. 2 shows two images.

Thanks for the suggestions. I corrected grammar errors that I noticed in the text of the article.

2. Can you provide the surface area of the reference electrode. I assume 1 cm².

We used a three-electrode set for electrochemical tests. The working electrode (W) (test material) had an area of 1 cm². We used a saturated calomel electrode (SCE(KCl) as the reference electrode (Ref). The counter electrode (C)  was made of a platinum (99.8% Pt) mesh with an area of 10 cm².

3. Can you provide next to the composition of IN 625 also the one of the substrate. 

The chemical composition of the substrate (i.e. steel S235JR) is given in the text of the article (line 18, page 3).

4. The dentritic structure of the wire coating is not as homogeneous as the powder coating, why?

The answer to the question is on page 5 line 15:

However, in the case of the PIn/S coating, a more homogeneous surface of the protective layer was obtained compared to the WIn/S. Because during laser surfacing, the coating material as Inconel 625 powder was more evenly transferred to the surface of the substrate, i.e. S325JR steel.

5. Can you add a table indicating the thicknesses of he two coatings the HAZ and eventually an influenced subsurface zone. The coatings are wavy so mean values as well as max and min would be a good information.

Thanks for the suggestion. I made an additional Table 3. Thickness of the substrate - HAZ and Inconel 625 Coatings onto the S235JR. In addition, I discussed the measurement results that I have collected in the table.

6. Are there pores at the interface. It is hard to see in the figs. 3 and 4. You should indicate HAZ in fig. 4.

Inconel 625 coatings produced by laser cladding method adhere well to the substrate of S235JR steel. Moreover, I did not notice any pores or cracks in the WIn and PIn coatings.

7. Due to interdiffusion withthe substrate I would expect Ni, Mo and Cr enrichment of the subsurface zone as well as Fe + Mn outward diffusion into the coating. Can you comment on that e.g. by EDS line scan analysis of the cross sections.

During the production of WIn, and PIn coatings, small amounts of metals penetrated the substrate-coating interface. The relevant results of the SEM / EDS tests are presented in Figures 4 and 5.

8. The HV10 value of PIn is 311 and not 259.

Thanks for the hint. I corrected the noticed error in the text of the work.

9. A high hardness value tells you nothing about the corrosion resistance of the material. This should be claryfied in 3.1. This is the same for the homogeneity of the coating. (3.3). If a material does not show protective behavior it will corrode no matter whether it is or is not homogeneous.

I agree with the reviewer's suggestion. The hardness of the material and its structure are not directly related to the corrosion resistance of the material. I have made appropriate corrections to the text of the article (page 5, lines 7 and 22).

10. Is the HAZ really 2 mm thick (3.2). I would say 2 µm!

Thanks for the suggestion. Indeed the thicknesses of the substrate - HAZ and the WIn and PIn coatings were poorly measured. Therefore, I took new SEM images of the cross-sections of the tested materials (Figure 3 a), and 3 b)) and carefully measured the thicknesses of the characteristic layers for both coatings. I collected the measurement results in Table 3 and discussed in the text of the publication.

11. What does (FeClOH-) mean. Is this an anion. This would indicate Fe in the oxidation state + 1. Fe is either +2 or +3. (O = 2-, Cl = 1-, H = 1+)

Thanks for the suggestion. Indeed, the electron balance of reaction (2) was badly done. Therefore, I have re-edited the substrate corrosion mechanism in a chloride corrosive environment (reactions (2) - (4), and (8), (9)).

12. In an acidid solution non-precious metals dissolve via H+ reduction, i.e. H2 evaporates and Me+x goes into the solvent (Ni2+, Al3+, etc.) You HCl solution with a pH of 1 is very acidic. Did you observe H2 formation?

Corrosion test substrate i.e. S235JR steel and Inconel 625 coatings we have carried out in an electrolyte containing 1.2 M Cl-, pH 1.5. Only during the course of the experiment, hydrogen was released in the cathode area on the surface of the working electrode, according to the reaction (1).

13. I assume that your coating should have a thickness of several 100-100 µm on your component. If there is a metal loss of several mm per year how can such a thin coating prevent attack on the base material?

The basic unit of the corrosion rate is kg / m2s. In practice, more convenient units are used, e.g. mg/cm2h, mg/dm2 / day or most often mm/year. It is worth adding that all units of the corrosion rate result from the balance of parameter units that appear in the formula that was used to calculate the corrosion rate of the tested material (in this case it is equation (7)). In laboratory practice, the corrosion rate expressed in the form of mm/year is understood as the linear spread of the corrosion nucleus over a specified time on the surface (not inside) of the tested metal.

14. You've treated the exposed specimens with nitric acid. How could you be sure only to attack the oxide and not the metal? 

Indeed, we used dilute nitric acid to remove corrosion products from the tested materials. To protect the substrate (i.e. S235JR) against dissolution, we covered it with an epoxy layer that is not soluble in corrosive environment and diluted nitric acid.

 

                                                                                                      Mieczyslaw Scendo

 

Reviewer 2 Report

The improvement of corrosion resistance of steels is a continuous concern for scientist and engineers. This paper presents results of microstructural analysis (SEM and EDS), microhardness, and corrosion tests of Inconel 625 (In) laser cladding coatings onto the (S235JR) steel (S). The coatings were produced by using wire (WIn/S) and powder (PIn/S).

A review of the published literature on the connected subject proves the originality of this research. Unfortunately, the paper does not present in an objective scientific way the results.

For this reason, I request a major revision. My suggestions are the next:

1.       The literature survey omitted a previously published paper on corrosion resistance of laser clads of Inconel 625 (see the below recommended Supplementary references).

2.       The dimensions of the multilayer cuboid specimen used for laser deposition ought to be mentioned.

3.       In line 182, the hardness of Pin/S coating is 311 HV10 and not 259 HV10.

4.       Check the indicated dimensions (in millimeters), represented in Figure 3 at a scale of 50 microns, and specified in lines 216-221.

5.       SEM images and EDS analysis results must be presented for all the studied cases of both (WIn/S) and powder (PIn/S) coatings, e.g., Figures 3, 4, and 5.

6.       In Figure 8a, b, and c, the corroded zones have to be indicated.

7.       The last two phrases from the Conclusions section are not connected and must be reformulated.

Supplementary references:

1.       https://cyberleninka.org/article/n/260218.pdf

Author Response

Reviewer 2

 

Thank you very much to the Reviewers for their thorough and substantive checking of the text of our publication.

Below are the answers to the questions and suggestions of the reviewers:

1.       The literature survey omitted a previously published paper on corrosion resistance of laser clads of Inconel 625 (see the below recommended Supplementary references).

In fact, we previously published an article: K. Staszewska, M. Scendo, Mechanism and Kinetics Oxidation of Inconel 617 and 625 Alloys, Technical Issues, 1 (2016) 82-89. However, we have not cited this article in the present publication because its content is different from the subject of the work currently being edited.

2.       The dimensions of the multilayer cuboid specimen used for laser deposition ought to be mentioned.

Thanks for the suggestion. For laser cladding of S235JR steel, samples with dimensions of: 320 mm x 120 mm x 5 mm were used. However, the thicknesses of the substrate - HAZ and the WIn and PIn coatings were poorly measured. Therefore, I took new SEM images of the cross-sections of the tested materials (Figure 3 a), and 3 b)) and carefully measured the thicknesses of the characteristic layers for both coatings. I collected the measurement results in Table 3 and discussed in the text of the publication.

3.      In line 182, the hardness of Pin/S coating is 311 HV10 and not 259 HV10.

Thanks for the hint. I corrected the noticed error in the text of the work.

4.       Check the indicated dimensions (in millimeters), represented in Figure 3 at a scale of 50 microns, and specified in lines 216-221.

Thanks for the suggestion. Indeed the thicknesses of the substrate - HAZ and the WIn and PIn coatings were poorly measured. Therefore, I took new SEM images of the cross-sections of the tested materials (Figure 3 a), and 3 b)) and carefully measured the thicknesses of the characteristic layers for both coatings. I collected the measurement results in Table 3 and discussed in the text of the publication.

5.       SEM images and EDS analysis results must be presented for all the studied cases of both (WIn/S) and powder (PIn/S) coatings, e.g., Figures 3, 4, and 5.

In Figure 3, I show the SEM cross-section of Inconel 625 coatings on the S235JR substrate, which were produced by laser deposition using of wire or powder. However, the results of the SEM / EDS tests are shown in Figures 4, 5 and 6 only for selected coatings, i.e. WIn or PIn. In this way, I wanted to reduce the amount of figures in the publication. Moreover, the results of the SEM / EDS tests for non-cited coatings were similar to those in the text of the publication and therefore I did not include all the measurement results.

6.       In Figure 8a, b, and c, the corroded zones have to be indicated.

Thanks for the suggestion. In Figure 8 a), b), and c) (currently Figure. 9) I have marked the corroded zones of the surfaces of the tested materials.

7. The last two phrases from the Conclusions section are not connected and must be reformulated.

Thanks for the suggestion. The  Chapter 4. Conclusions have been redrafted.

  

                                                                                                  Mieczyslaw Scendo

Reviewer 3 Report

In the article "Corrosion Behavior of Inconel 625 Coating Produced by Laser 2 Cladding" by Mieczyslaw Scendo et al., the anticorrosive properties of Inconel 625 coatings on the S235JR substrate are investigated. To obtain the coatings, the method of laser cladding with wire or powder was used. It is shown that when using the powder, the microhardness and corrosion resistance are higher. The paper presents evidence of the conclusions made. I believe that the article will be of interest to the readers of Coatings. There are some small comments that will improve the understanding of the article:

- explain why S235JR steel was used as the substrate? As far as I understand, this is structural steel, which is actively used for the construction solutions. And the laser cladding method will require processing large areas of the material. What is the heterogeneity when processing a large area?

- Figure 3 is presented only for the laser cladding method using wire. Please provide a photo of the sample cross-section when using the powder for comparison.

Author Response

Reviewer 3

 

Thank you very much to the Reviewers for their thorough and substantive checking of the text of our publication.

Below are the answers to the questions and suggestions of the reviewers:

1. Explain why S235JR steel was used as the substrate? As far as I understand, this is structural steel, which is actively used for the construction solutions. And the laser cladding method will require processing large areas of the material.

The S235JR steel belongs to the group of structural steels and is widely used in various industries. The disadvantage of this steel is its low corrosion resistance. Corrosive damage causes an increase in costs related to the operation of parts of machines and devices. In order to reduce the costs associated with corrosion, materials that are easily corrosive are covered with a protective layer, thus limiting the damaging effects of corrosion, especially in the environment of strong electrolytes. Therefore, for this purpose, we decided to investigate the anti-corrosion properties of protective coatings that can be produced by laser using Inconel 625 wire or powder onto the S235JR steel as substrate.

2. What is the heterogeneity when processing a large area?

In my opinion, the production of protective coatings using the laser surfacing method can be carried out for both small and large metal elements. In this case, it is decisive to have a cladding set that is appropriate in terms of dimensions. Indeed, in the case of the production of protective coatings on large surfaces, the problem of homogeneity of the structure may occur, which is related to the different rate of heat dissipation during the surfacing process. in this case, additional heat treatment will be necessary.

3. Figure 3 is presented only for the laser cladding method using wire. Please provide a photo of the sample cross-section when using the powder for comparison.

Thanks for the suggestion. In Figure 3, I have quoted SEM cross-section of Inconel 625 coatings onto the S235JR substrate produced by laser cladding method with use of wire or powder. Moreover, the thicknesses of the substrate - HAZ and the WIn and PIn coatings were poorly measured. Therefore, I took new SEM images of the cross-sections of the tested materials (Figure 3 a), and 3 b)) and carefully measured the thicknesses of the characteristic layers for both coatings. I collected the measurement results in Table 3 and discussed in the text of the publication.

  

                                                                                                   Mieczyslaw Scendo

Reviewer 4 Report

There are some serious issues for this paper.

1) This paper is hard to review because that most of the figures are in low resolution.

2) A lot of format issues exist. The authors didn't write this article in a scientifically rigorous way.

3) Please use a table to describe the cladding parameters for each case.

4) A line profile is needed to describe the particle size distribution.

5) How many samples are repeated for the hardness test? 

6) Line profile should be provided for EDS spot scan.

7) A EDS line scan should be conducted across the interface between substrate and coating.

8) No scale bar for the EDS element distribution in Figure 5.

9)  How many samples repeated for the potentiodynamic polarization data in Figure 6?

10) Figure 8 is ugly looking, please replace it.

Author Response

Reviewer 4

 

Thank you very much to the Reviewers for their thorough and substantive checking of the text of our publication.

Below are the answers to the questions and suggestions of the reviewers:

 

1) This paper is hard to review because that most of the figures are in low resolution.

Thanks for the suggestion. In the current version of the article, I have replaced all the figures and replaced them with better quality figures.

2) A lot of format issues exist. The authors didn't write this article in a scientifically rigorous way.

Thanks for the suggestions. I edited a large part of the article from the beginning. I improved the format of the drawings and added new drawings. The text of the work was completed in order to correctly describe the purpose of the research.

3) Please use a table to describe the cladding parameters for each case.

Thanks for the suggestion. In Figure 3, I have quoted SEM cross-section of Inconel 625 coatings onto the S235JR substrate produced by laser cladding method with use of wire or powder. Moreover, the thicknesses of the substrate - HAZ and the WIn and PIn coatings were poorly measured. Therefore, I took new SEM images of the cross-sections of the tested materials (Figure 3 a), and 3 b)) and carefully measured the thicknesses of the characteristic layers for both coatings. I collected the measurement results in Table 3 and discussed in the text of the publication.

4) A line profile is needed to describe the particle size distribution.

Unfortunately, I do not have the line profile size distribution of Inconel 625 powder. Therefore, in the publication we only mentioned as Figure 1. Morphology of Inconel 625 powder.

5) How many samples are repeated for the hardness test? 

For laboratory tests, we received three samples of each type, i.e .WIn/S, and PIn/S, from the manufacturer. We made ten hardness measurements for each sample.

6) Line profile should be provided for EDS spot scan.

As suggested by the reviewer, in Figure 5, we cited the results of the SEM micrograph, and EDS of PIn / S (we obtained similar results for WIn coating) onto the S23JR substrate, and the results of point X-ray microanalysis of the chemical composition of tested material. Whereas, Figure 4. concerns microanalysis of cross-section of PIn coating with the line scanning.

7) A EDS line scan should be conducted across the interface between substrate and coating.

As suggested by the reviewer, we made additional measurements, and the results are presented in Figure 4. X-ray microanalysis of the cross-section of the Inconel 625 coating with linear scanning from the substrate, i.e. S235JR steel through the partition zone and PIn coating.

8) No scale bar for the EDS element distribution in Figure 5.

Thank you for the information. In Figure 5 (now Figure 6), for the distribution map of elements i.e. Ni, Cr, Nb and Mo I introduced the appropriate scale.

9)  How many samples repeated for the potentiodynamic polarization data in Figure 6?

We used three samples for the tests, i.e. S235JR steel as a substrate and with a protective coating, which was produced using Inconel 625 of wire or powder. We did three corrosion tests for each sample. Therefore, in the publication we present the averaged test results from three measurements for each sample.

10) Figure 8 is ugly looking, please replace it.

Unfortunately, I do not have a nicer photo. In fact, the cited photo (Figure 8) shows quite well the surface condition of the tested samples after five hours of exposure to an aggressive chloride environment. To improve the clarity of the drawing, I marked areas of the corroded surface of the specimens on it.

  

                                                                                                     Mieczyslaw Scendo

Round 2

Reviewer 2 Report

The main concerns, regarding the thickness values of the coatings in the cross-section SEM images,  were solved by the authors. For the rest of my suggestions, I have received pertinent explanations.

In conclusion, I recommend this paper for publication.

Reviewer 4 Report

I still feel the resolution of the figures in the pdf manuscript is pretty low, the authors should double-check the original file. 

If a line profile is not available, how can you know the size distribution data is corrected or not?