Round 1

Reviewer 1 Report

In present form the article can not be published and must be reject.

General remarks:

the English is very poor a chemical composition of the alloy after laser treatment should be determined an EDX analysis can not be use for quantitative analysis of light elements like O corrosion produsts of Mg alloys are well written in literarure I don't belive in +/-0,0001 V reproductibility during corrosion measurements electrochemical corrosion measurements were conducted without tempereture control (should be at 37 C deg) y axe - should be log current denity x axe - why from -0,9 to -1,5 V? Should be from -1,5 to -0,9 fig 6 - SE or BSE mode?

Author Response

The English is very poor.

We have carefully revised the grammar, structure, and tense of the paper, and polished every sentence of the essay. All the revised contents are presented in red text in the article.

A chemical composition of the alloy after laser treatment should be determined.

We analyzed the chemical composition of the original sample by XRD results. The magnesium alloy ZK60 consisted of matrix phase Mg and second phase MgZn₂. After laser treatment, the number and position of XRD diffraction peaks did not change significantly. We can think that the laser treatment does not alter the chemical composition of the magnesium alloy ZK60. This is because plastic deformation mainly utilizes the mechanical effect of the laser. In the water constraining layer, the amount of heat reaching the surface of the material is small.

An EDX analysis can not be used for quantitative analysis of light elements like O.

Although EDS does not allow accurate quantitative analysis of some lightweight elements, EDS can perform semi-quantitative analysis of some light elements such as oxygen. Since the oxygen element exists in the form of a compound, the EDS obtained in a vacuum environment is the oxygen content in the oxidation product, and the result still has a certain reference value. Based on your suggestion, we do not focus on the analysis of oxygen content in EDS results. We made the following changes in 3.4.2: the presence of oxygen and magnesium on the surface of all samples can confirm that the corrosion products are mostly Mg oxides. Compared with the untreated sample, the content of Mg on the corrosion surface of the three specimens after laser treatment was low, indicating that LSP improved the corrosion resistance of the sample in SBF. Thus, the oxidation product of magnesium was reduced.

Corrosion products of Mg alloys are well written in literarure.

We performed XRD component detection on the corroded sample and showed the XRD pattern in Figure 13(b). The results indicate that Mg, Mg(OH)₂, MgCl₂, and MgO are present in the corrosion product. Mg(OH)₂ is mainly derived from the reaction of Mg with water, MgCl₂ is derived from the reaction of Cl ions with Mg(OH)₂, and MgO is derived from the reaction of magnesium and oxygen in the matrix.

I don't believe in +/-0,0001 V reproductibility during corrosion measurements electrochemical corrosion measurements were conducted without tempereture control (should be at 37 C deg).

Based on your comments, we re-electrochemically tested the four samples at 37 degrees, and carefully corrected the errors to ensure the reproducibility of the experiment. The test results are as follows: the corrosion voltages of untreated, 1.19 GW/cm2, 1.99 GW/cm2, and 2.79 GW/cm2 are-1.3884±0.04V, -1.2256±0.03V, -1.1707±0.05V, and-1.1094±0.03V, respectively. The maximum offset of corrosion potential is 217.7mV. While the corrosion current density equal to1.378×10-5A/cm2,1.267×10-5A/cm2,1.23×10-5A/cm2, and 1.196×10-5A/cm2, respectively. The increase in laser power density resulted in a maximum increase in corrosion potential of 20.1% and a maximum reduction in corrosion current density of 13.2%.

y axe - should be log current density.

We discuss and listen to your comments to change the y-axis label in Figure 14 to log (current density/ A/cm²).

x axe - why from -0,9 to -1,5 V? Should be from -1,5 to -0,9.

At 37 degrees, we repeated several electrochemical experiments. In order to demonstrate the complete polarization curve, we determined the voltage of the X-axis from -0.9V to -1.6V. However, this interval is not fixed, and it is necessary to determine the appropriate range based on the position of the potential.

Fig 6 - SE or BSE mode?

Figure 6 is a metallographic picture taken with an optical microscope (OM).

Reviewer 2 Report

1.The XRD results are given in Fig. 13. However it is not indicated the laser power density of the sample. The XRD results demonstrated that the characteristic peak intensities and peak positions were changed after laser modification. The variation of the laser power density will lead to the structural changes or induces additional strain in the magnesium alloy ZK60. It would be useful to present XRD data of all samples after modification and add some comments about the obtained results. The XRD data of the samples after the corrosion test would give an additional information about the structural changes and formation of new phases on the surface and bulk.

2.The authors presented the elemental composition of the samples after the corrosion test. It would be useful to included the EDS results of the magnesium alloy ZK60 before anf after laser modification. The numerical values of the obtained elements should be included in the fig. 11.

3.The conclusions should be re-written by adding the numerical values and results.

Author Response

Response to Reviewer 2 Comments

 

Point 1: The XRD results are given in Fig. 13. However it is not indicated the laser power density of the sample. The XRD results demonstrated that the characteristic peak intensities and peak positions were changed after laser modification. The variation of the laser power density will lead to the structural changes or induces additional strain in the magnesium alloy ZK60. It would be useful to present XRD data of all samples after modification and add some comments about the obtained results. The XRD data of the samples after the corrosion test would give an additional information about the structural changes and formation of new phases on the surface and bulk.

Response 1: The XRD patterns of all laser parameters are shown in Figure. 13 (a), and the results are analyzed in lines 388-344. It can be seen that the composition of the sample consists of α-Mg and MgZn₂. It also indicates that no new phase transition occurred after LSP. In addition, the position and intensity of the peak changed after LSP. That is because laser shock peening produces additional strain on the material by plastic deformation, which causes an increase in dislocation density, which in turn changes the microstructure and surface residual stress field. The changes in residual stress and grain refinement in Figures 4 and Figure 6 may be responsible for the difference in diffraction peaks.

XRD analysis was carried out on the corroded surface and the results were shown in figure 13 (b). The results indicate that Mg, Mg(OH)₂, MgCl₂ and MgO are present in the corrosion product. Mg(OH)₂ is mainly derived from the reaction of Mg with water, MgCl₂ is derived from the reaction of Cl ions with Mg(OH)₂, and MgO is derived from the reaction of magnesium and oxygen in the matrix.

Point 2: The authors presented the elemental composition of the samples after the corrosion test. It would be useful to included the EDS results of the magnesium alloy ZK60 before and after laser modification. The numerical values of the obtained elements should be included in the fig. 11.

Response 2: We don't think it is necessary to do EDS analysis on the samples before and after laser modification for the following reasons. We did XRF test on the untreated samples, obtained the composition information and listed it in Table 1.XRD test was carried out on all samples before corrosion, and the results showed that LSP did not change the composition of ZK60.That is to say, if EDS detection is carried out on all samples, the obtained elements and contents will not change too much, so we think EDS of samples before and after laser modification can not be done. We take your advice and mark the content of each element after corrosion in Figure 11.

Point 3: The conclusions should be re-written by adding the numerical values and results.

Response 3: We rewrote the conclusion by adding data and results. The following is the revised conclusion.

The Effect of laser power density on surface morphology, surface roughness, residual stress, and corrosion behavior of biocompatible magnesium alloy ZK60 are investigated. The following conclusions can be summarized:

(1) Plastic deformation increases with the increase of laser power density, which increases the value of surface roughness from the initial 0.2µm to 6.11µm.

(2) LSP changes the surface residual stress field of magnesium alloy ZK60. The surface residual stress does not increase with the laser power density increases. When the laser power density is 1.19 GW/cm², 1.99 GW/cm², and 2.79 GW/cm², the surface residual stress can be increased up to 47.2MPa, 45.7MPa, 46.4MPa, respectively. Residual compressive stress is enhanced by a maximum of 1.7 times compared with the original sample. LSP can refine the size of the grains, and the average area of the surface grains decreases approximately as the power density increases. The average grains area falls from 45μm² to 17μm².

(3) The degradation rate of magnesium alloy ZK60 in the SBF solution is decreased after LSP owing to a denser passivation film induced by higher residual compressive stress and grains refinement. In terms of the total weight loss, corrosion resistance increased by 52.1%, 45.1%, and 49%, respectively.

(4) Corrosion cracks initiation originate from corrosion pitting pits due to the influence of hydrogen embrittlement and stress concentration. Modified samples can improve corrosion pitting resistance and restrain crack initiation and propagation. The increase of calcium and phosphorus deposition is beneficial to improve the biocompatibility further.

(5) Electrochemical experiments show that the corrosion potential increased from -1.3884V to -1.1094V, and the current density decreased from 1.378×10^-5A/cm² to 1.196×10^-5A/cm². Corrosion tendency decreased by 20.1% in maximum.

(6) To summarize, when the power density is 1.19 GW/cm², the magnesium alloy ZK60 can obtain superior corrosion resistance in the SBF solution.

 

Round 2

Reviewer 1 Report

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