Corrosion behavior of GeO2 and Sc2O3 Coatings on AZ31 Alloy

Introduction

Magnesium alloys find many applications in space and aerospace applications due to high strength and low density. Even though, they suffer more corrosion due to the more negative electromotive force in the series ^1-4. Many attempts executed and reported to reduce the corrosion susceptibility of Mg alloys in aqueous medium applications ^5-12. Surface coatings demonstrated enhanced performance for magnesium alloys ^13-15. Recently, Wu et al fabricated diamond like DLC/AlN/Al coating on AZ31 using sputtering method and demonstrated a noble shift in the corrosion potential of AZ31 alloy in 3.5% NaCl solutions ¹³. Graphene based coatings were developed by Han et al by anodically oxidized and more noble shift around -1.15 V (SCE) in in 3.5 % NaCl electrolyte ¹⁴.  WC coatings also developed by plasma electrolytic oxidation on AZ31 alloy and the author’s demonstrated positive shift in corrosion potential ¹⁵. However, there is much scope to improve the resistance against corrosion of Mg alloys for surface coatings by tuning the porosity and with suitable binders.

In this work, GeO₂ (germanium dioxide) and Sc₂O₃ (scandium trioxide) were developed as coatings on AZ31 alloy using polymer binder. The bare AZ31 alloy showed open circuit potential (E_corr) of -1.7 V (SCE) and the corrosion current density (i_corr) of 3.4 x 10^-4 mA/cm², while the GeO₂ coated AZ31 alloy exhibited E_corr of -1.4 V (SCE) and the i_corr of 5.4 x 10^-9 mA/cm² and while the GeO₂ coated AZ31 alloy exhibited E_corr of -1.3 V (SCE) and the i_corr of 2.59 x 10^-9 mA/cm². The results reveal that the GeO₂ coated AZ31 alloy demonstrated higher corrosion resistance than of bare AZ31 alloy and Sc₂O₃ coated AZ31 alloy. The novelty of this work is on the fabrication of in-organic materials like oxides coatings formulation on the AZ31 alloy using organic binder, which showed effective role on the corrosion resistance enhancement of AZ31 alloy. The implications of this work may pave new pathways for Mg alloys corrosion resistance improvement.

Materials and Methods

Chemicals

Germanium (IV) oxide (GeO₂, 99.99%, CAS No: 1310-53-8) and Scandium(III) oxide (Sc₂O₃, 99.995%, CAS No: 12060-08-1), Poly(ethylene glycol) (PEG₃, Bioultra 8000, CAS No: 25322-68-3), were purchased from Merck.

Sample preparation

The AZ31 alloy sheet was procured and cut into 11 x 11 x 5 mm dimension using wire EDM. Further, the samples were undergone metallurgical polishing before the application of coating on AZ31 alloy. Metallurgical polishing was employed using 200 to 1200 SiC grit polishing papers and finally treatment with 0.5 microns diamond paste cloth polishing. The samples were then washed to remove the polishing impurities and dried at room temperature.

Electrode Preparation

The oxide powders of GeO₂ and Sc₂O₃ were taken in 3 mg weight (respectively for each electrode) and added with 10 ml PEG-ethanol binder solution at 50 ºC and stirred for 4 h until homogeneous mixture formed. Then the mixed slurry was drop casted on AZ31 alloy and dried for 24 h. 

Electrochemical experiments

Corrosion behavior of uncoated AZ31, Sc₂O₃ and GeO₂ coatings over AZ31 alloy was studied in 3.5% NaCl medium with an assembly consisting of three electrode system, where Pt as counter electrode, AZ31 alloy samples with and without coatings were as working electrodes and SCE as reference electrode.

Results and Discussion

XRD analysis

The phase purity and crystallinity of uncoated AZ31, Sc₂O₃ coated AZ31 alloy and GeO₂ coated AZ31 alloy was characterized by XRD analysis and presented in Figures1-3.

Figure 1: X-ray diffraction pattern of AZ31 alloy Click here to View figure

The pure AZ31 alloy X-ray diffraction peaks matched with the standard JCPDS file no 35-0821, demonstrating hexagonal crystal structure belonging P63/mmc for Mg alloy (a=b=3.2094 Å and c=5.211Å). This result is also well matched with reported literature ^16-19.

The Sc₂O₃ coated AZ31 alloy XRD pattern is shown below. The scandium oxide peaks well matched with standard JCPDS file no: 42-1463, showing the cubic crystal structure (a=b=c=9.845 Å) that belongs to la3 space group. The peaks at 18.08, 31.4, 36.4, 52.5 and 59.2º corresponds to (211), (222), (400), (440) and (611) planes of cubic Sc₂O₃. The base AZ31 alloy peaks demonstrated very low intensity peaks in figure, due to the thick coating of Sc₂O₃. This result is also well matched with reported literature ^20-22.   

Figure 2: XRD pattern for Sc2O3 coated AZ31 alloy Click here to View figure

Figure 3: XRD pattern for GeO2 coated AZ31 alloy Click here to View figure

The GeO₂ coated AZ31 alloy XRD pattern is depicted in Figure 3. Germanium oxide peaks well matched with standard JCPDS file no: 04-0497, showing the hexagonal crystal structure (a=b=4.987 Å, c=5.65Å) that belongs to P31 space group. The peaks at 20.5º, 25.96º, 38.0º, 41.8º and 66.0º corresponds to (100), (101), (102), (201), (103), (212), (302) and (310) planes of hexagonal GeO₂ [23-26]. The base AZ31 alloy peaks demonstrated very low intensity peaks in figure, due to the thick GeO₂.  

FESEM analysis

The surface morphology of Sc₂O₃ and GeO₂ are presented in Figure 4 and 5, respectively. Both the scandium oxide and germanium oxide powders show their individual particles with irregular shapes or morphology.   

Figure 4: Surface morphology of Sc2O3 coated AZ31 alloy Click here to View figure

Figure 5: Surface morphology of GeO2 coated AZ31 alloy Click here to View figure

FTIR analysis    

The functional groups of Sc₂O₃ and GeO₂ were analyzed by Fourier Transform Infrared Spectroscopy (FTIR) and presented in Figure 6 and 7, respectively.

Figure 6: FTIR spectrum of Sc2O3 compound. Click here to View figure

Figure 7: FTIR spectrum of GeO2 compound. Click here to View figure

Figure 6 shows the FTIR spectrum for Sc₂O₃ compound, where it is noticed that the peaks at 427, 636, 1533, 3043 and 3670 cm^-1. The bands at 427 and 636 cm^-1 strongly shows the typical characteristic peaks for Sc-O bands in cubic Sc₂O₃.  The peaks at 1533cm^-1 might be attributed to the presence of C=O stretching, while the 3040 cm^-1 shows the presence of olefinic compounds. The band at 3670cm^-1 shows the hydroxyl stretching ^{27, 28}.  

Similarly, Figure 7 shows the FTIR spectrum of GeO₂ compound, where it is noticed that the peaks at 498, 560, 878, 3021 and 3670 cm^-1. The peaks at 498, 560cm^-1 corresponds to the V₄ vibration mode in GeO₄ tetrahedra [29, 30]. A strong absorption peak at 878 cm^-1 demonstrates the V₃ vibration mode, which is the distorted tetrahedral structure of GeO₂. The band at 3021 and 3670 cm^-1 are attributed to olefinic and hydroxyl stretching.

Raman analysis

The Raman spectrum of Sc₂O₃ compound is presented in Figure 8. The Raman modes observed at 194, 320, 538, 420, 495, and 525 cm^-1. The major vibration band at 420cm^-1 can be assigned to totally symmetricA_g and F_g-type modes of octahedra of ScO₆^31-33.

Figure 8: Raman spectrum of Sc2O3 Click here to View figure

The spectrum of GeO₂ is presented in Figure 9. The Raman modes observed at 122, 165, 211, 262, 328, 443, 515, 591, 881 and 971 cm^-1. The strong peak 443 cm^-1 demonstrates the hexagonal GeO₂ and weak peak at 328 cm^-1 shows the Ge ^{34, 35}.  

Figure 9: Raman spectrum of GeO2 Click here to View figure

Electrochemical analysis

Corrosion behavior of uncoated AZ31, Sc₂O₃ and GeO₂ coatings over AZ31 alloy was studied in 3.5% NaCl medium with an assembly consisting of three electrode system, where Pt as counter electrode, AZ31 alloy samples with and without coatings were as working electrodes and SCE as reference electrode.

Figure 10: Open circuit potentials of uncoated AZ31, Sc2O3 coated and GeO2 coated AZ31 alloy Click here to View figure

From the figure it is noticed that the corrosion potential or equilibrium potentials of the coated samples, showed an positive shift in the potential or noble shift, which is beneficial for the stability of the alloy. The bare alloy showed -1.7 V (SCE) corrosion potential, while the Sc₂O₃ coated AZ31 alloy showed -1.4 V (SCE) and GeO₂ coated AZ31 alloy showed -1.3 V (SCE). The GeO₂ coated AZ31 alloy demonstrated more noble shift.

Figure 11: Linear polarization studies of uncoated AZ31 and Sc2O3 coated and GeO2 coated AZ31 alloy Click here to View figure

The linear polarization studies of uncoated AZ31 and Sc₂O₃ coated and GeO₂ coated AZ31 alloys are shown in Figure 11. From the figure, it is noticed that bare AZ31 demonstrated higher corrosion susceptibility than coated samples. The corrosion rate details are as follows, the bare alloys exhibited the i_corr of 3.4 x 10^-4 mA/cm², Sc₂O₃ coated AZ31 alloy exhibited i_corr of 5.4 x 10^-9 mA/cm² and GeO₂ coated AZ31 alloy exhibited i_corr of 2.59 x 10^-9 mA/cm². The results shows that GeO₂ coated AZ31 alloy demonstrated better performance. The electrochemical impedancecurves of bare AZ31 and Sc₂O₃ coated and GeO₂ coated AZ31 alloys in NaCl (3.5 %)electrolyte is shown Figure 12. The EIS studies were studied in the frequency range of 1MHz to 100 mHz at open circuit potential.

Figure 12: EIS curves of AZ31 and Sc2O3 and GeO2 coated AZ31 alloys Click here to View figure

Table 1: Electrochemical polarization data of AZ31 and Sc₂O₃ and GeO₂ coated AZ31 alloys.

SNo	Electrode	Ecorr (V vs SCE)	Icorr (mA/cm2)
01	Bare AZ31 alloy	-1.7	3.4 x 10-4
02	Sc2O3 coated AZ31 alloy	-1.4	5.4 x 10-9
03	GeO2 coated AZ31 alloys	-1.3	2.59 x 10-9

 

The EIS studies shows that bare AZ31 alloy showed very less corrosion resistance, while the Sc₂O₃ coated AZ31 alloy showed intermediate performance and GeO₂ coated AZ31 alloy showed superior corrosion resistance. 

White et al fabricated TiO₂ coating over AZ31 alloy using plasma electrolytic oxidation (PEO), where the coating demonstrated enhanced corrosion protection to Mg alloy ³⁶. The coating showed the noble shift in corrosion potential up to -1.4 V (SCE) in 3.5 % NaCl electrolyte. Similarly, Chen et al developed MgO, MgAl₂O₄ and MgSiO₃ composed coating through micro arc oxidation process and demonstrated that the ceramic coated sample showed corrosion potential of ~1.5 V in 3.5% NaCl medium ³⁷. Tan et al developed Ca-P coatings on AZ31 Mg alloy via chemical deposition and noticed that Ca-P coating dramatically decreased the corrosion rates and improved corrosion resistance. The authors demonstrated the corrosion potential up to -1.5 V (SCE) in 3.5% NaCl medium ³⁸. In this work, the GeO₂ coated AZ31 alloy showed the corrosion potential of ~ -1.3 V (SCE) and corrosion current density of 2.59 x 10^-9mA/cm² in 3.5% NaCl medium. This work demonstrated enhanced corrosion protection for AZ31 alloy with proposed coatings in comparison with literature and paves new pathway for the corrosion protection improvement of magnesium alloys.

Conclusion

The GeO₂ (germanium dioxide) and Sc₂O₃ (scandium trioxide) were developed as coatings on AZ31 alloy using polymer binder.

The corrosion studies were analyzed using a three electrode system in 3.5% NaCl electrolyte. The bare AZ31 alloy showed open circuit potential (E_corr) of -1.7 V (SCE) and i_corrof 3.4 x 10^-4 mA/cm²,

while the Sc₂O₃ coated AZ31 alloy exhibited E_corrof -1.4 V (SCE) and i_corr of 5.4 x 10^-9 mA/cm² and while the GeO₂ coated AZ31 alloy exhibited E_corr of -1.3 V (SCE) and i_corr of 2.59 x 10^-9 mA/cm². The results reveal that the GeO₂ coated AZ31 alloy demonstrated higher corrosion resistance than of bare and Sc₂O₃ coated AZ31 alloy.

Acknowledgement

The authors would like to thank Department of Industrial Chemistry, Alagappa University, Karaikudi, India and the Post graduate & Research Department of Chemistry, Raja Doraisingam Government Arts College, Sivaganga, for providing the academic support and research support.

Conflict of Interest

The authors declare that the current work has not conflict interest.

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