Role of Substrates in the Corrosion Behaviors of Micro-Arc Oxidation Coatings on Magnesium Alloys

Round 1

Reviewer 1 Report

 

The XRD results of MAO treated AZ31 and AZ91 Mg alloys are given in Fig.3. The more detailed explanation of the obtained peaks should be included. It would be useful to add the XRD patterns of the Mg alloys before the treatment. It would give the information about the structural changes in the alloys. The authors should include the XRD results of AZ31 and AZ91after the 48 h immersion in 0.1M NaCl solution. May be it would be nice to present some results of MAO coatings after corrosion test. It will allow indicating the structural changes during the corrosion test. The surface images of the MAO coatings (see fig. 2) illustrates that the coatings are very porous. That was the porosity of those coatings? It could have an effect on the corrosion resistance of the coatings. Does the authors obtained similar porosity values for the both alloys?   The conclusions should be improved by indicating the main obtained values (such as time of corrosion resistance and etc.) The sentence “The MAO coatings on Mg alloys AZ31 and AZ91 were found to possess similar porous structure and phase composition” should be improved. The introduction part is written well. However more accurate information regarding the structural changes, used coatings or Mg substrates composition, type of solution on the corrosion resistance should be indicated. The authors have published article which is related to given research. Severe shot peening (SP) was performed on the AZ31 and AZ91 Mg alloys to obtain a nanostructured surface layer. The microstructure, micro-hardness and corrosion behavior of AZ31 and AZ91 Mg alloys after SP treatment were studied in the article Journal of Alloys and Compounds 770 (2019) 500-506. It would be useful to include this article in the reference list.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript compares corrosion performance of micro-arc oxidation (MAO) coatings on two Mg alloys, AZ31 and AZ91. The authors prepared identical MAO coatings on both substrates and assessed their corrosion resistance based on electrochemical and immersion experiments. Differences in performances of both coatings are analysed in detail. The paper is well structured and written in a clear and easily understandable way. The conclusions are supported by experimental data, which are well documented. Although the paper does not bring any significantly new technical or scientific data, I recommend it for publication in Metals.

My only essential comment is linked to the use of the term „filiform corrosion“. According to the ISO 8044 standard, filiform corrosion is defined as a „type of corrosion proceeding under coating materials on metals in the form of threads, generally starting from bare edges or from local damage to the coating“. In the case shown in Figure 5a, there is no coating applied on the AZ31 substrate. Thus, the mechanism is obviously completely different. In the presented case, there is no active anode travelling under a coating and a cathodic tail, which are features typical for filiform corrosion. Real filaments are clearly separated and seldom crossing each other. Please, find another term to describe the morphology. This is particularly important for conclusions (line 284).

Further minor remarks follow:

Figure 6b, c, e, f: These are probably SEM images. If so, mention it in the figure caption. Line 182 and further: „two coatings“ is incorrect English expression. You can write „both coatings“ or „the coatings“, or „the two coatings“. Line 236. „The corrosion properties of MAO coating highly depended on the characteristics of coating and Mg substrate“ & „Hence, the different corrosion features of two coatings were mainly due to the difference of Mg substrates.“ Coating characteristics were not varied at all in this study. For the Mg substrate characteristics, there was a difference but I do not think that „highly“ is an appropriate adjective here. The difference was rather small.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf