The Effect of Crystallization and Phase Transformation on the Mechanical and Electrochemical Corrosion Properties of Ni-P Coatings

Round 1

Reviewer 1 Report

The manuscript is well written and structured. The methods described clearly.

In my opinion, this work could be published after minor revision:

1. The observed degradation of the barrier properties of the MP coating is explained in the manuscript by appearance of the comet-like structures in its morphological structure. The proposed mechanism requires a more detailed description. If these defects appear due to increase of P content, then why they disappear (or not) in HP coating?
2. Why there are no signs of comet-like structures in Fig 1?
3. At what content of P appears such a defect?
4. Why it has such form?

Author Response

Dear reviewer,

Thank you for the factual comments that will lead to the improvement of the manuscript. I firmly hope that I have sufficiently responded to all your comments. All corrections are summarized below and are highlighted in the revised manuscript.

Kind regards

Buchtík Martin

Author Response File: Author Response.pdf

Reviewer 2 Report

It is a fairly prepared manuscript with proper experimental approaches. The results are clearly described, but some supporting data are missing. The discussion needs improvement. I recommend a revision before publication. Please refer to the following comments and questions. 

• Was the solution deaerated or not for the corrosion test? It affects the corrosion potential largely. In addition, how was the specimen surface finished before the electrochemical tests? Passivation or oxidation of the AZ91 alloy, in particular, needs to be clear. 
• It was stated that the sizes of the nodules did not change and that cracking occurred during the heat-treatment (p.4, from the 159th line). However the supporting data, that is, the surface and cross-section images after the heat treatment are not presented. The data in [27] is far from being good evidence. The images and composition analyses after heat treatment are highly necessary to show the grain size, cracks or defects, diffusion of elements, peeling or deformation of the coating layer. 
• Fig. 6 implies that the crystal growth mechanism of Ni and that of Ni₃P is totally different, as the temperature dependence is distinguished.  The authors may employ a growth model from literature and explain these specific growth behaviors. 
• Fig. 6 and Fig. 8 worth being analyzed together. Considering the general relationship between the microstructure and the hardness, what is the authors' opinion about the following fact? 1) The crystal size of Ni may affect the hardness at a temperature below 300 'C. 2) Although the dependence of crystalline size on the temperature at 300~400 'C is different from that at the temperature range higher than 400 'C, the microhardness does not show such discontinuity. 
• Is it confirmed that any elemental diffusion did not occur across the interface between the matrix alloy and the coating? Any compositional change will make a significant difference in the corrosion properties.
•  The "comet-like defect" in Figure 10 need to be confirmed further. What are the chemical composition and the phase? What can make such a particle on the surface? It may be just a contaminant particle introduced during the sample preparation.  Did you have a similar case reported previously? 
• Please provide supporting data for galvanic corrosion. If galvanic corrosion is the exact cause of the poor corrosion resistance, the corrosion would proceed preferentially at the bottom of cracks. 
• There are several errors in font or subscripts. 

Author Response

Dear reviewer,

Thank you for the factual comments that will lead to the improvement of the manuscript. I firmly hope that I have sufficiently responded to all your comments. All corrections are summarized below and are highlighted in the revised manuscript.

Kind regards

Buchtík Martin

Author Response File: Author Response.pdf

Reviewer 3 Report

Comment in attached file.

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

Thank you for the factual comments that will lead to the improvement of the manuscript. I firmly hope that I have sufficiently responded to all your comments. All corrections are summarized below and are highlighted in the revised manuscript.

Kind regards

Buchtík Martin

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The manuscript has been revised and the questions of the reviewer were answered well.