Corrosion behavior of GeO2 and Sc2O3 Coatings on AZ31 Alloy

In this work, GeO2 (germanium dioxide) and Sc2O3 (scandium trioxide) were developed as coatings on AZ31 alloy using polymer binder. The coatings were characterized using X-ray crystallography procedure (XRD), infrared spectrum of absorption or emission of a solid procedure (FTIR), Raman spectroscopy procedure, surface examination by FESEM. The corrosion studies were analyzed using a three electrode system in 3.5% NaCl electrolyte. The bare AZ31 alloy showed open circuit potential (Ecorr) of -1.7 V (SCE) and the corrosion current density (icorr) of 3.4 x 10 -4 mA/cm2, while the Sc2O3 coated AZ31 alloy exhibited Ecorr of -1.4 V (SCE) and the icorr of 5.4 x 10 -9 mA/cm2 and while the GeO2 coated AZ31 alloy exhibited Ecorr of -1.3 V (SCE) and the icorr 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.


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][2][3][4] . Many attempts executed and reported to reduce the corrosion susceptibility of Mg alloys in aqueous medium applications [5][6][7][8][9][10][11][12] . Surface coatings demonstrated enhanced performance for magnesium alloys [13][14][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, GeO 2 (germanium dioxide) and Sc 2 O 3 (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/cm 2 , while the Sc 2 O 3 coated AZ31 alloy exhibited E corr of -1.4 V (SCE) and the i corr of 5.4 x 10 -9 mA/cm 2 and while the GeO 2 coated AZ31 alloy exhibited E corr of -1.3 V (SCE) and the icorr of 2.59 x 10 -9 mA/cm 2 . The results reveal that the GeO 2 coated AZ31 alloy demonstrated higher corrosion resistance than of bare AZ31 alloy and Sc 2 O 3 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.

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 GeO 2 and Sc 2 O 3 were taken in 3 mg weight (respectively for each electrode) and added with 10 mL PEG-ethanol binder solution at 50 o C and stirred for 4 h until homogeneous mixture formed. Then the mixed slurry was drop casted on AZ31 alloy and dried for 24 hours.

Electrochemical experiments
Corrosion behavior of uncoated AZ31, Sc 2 O 3 and GeO 2 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.

XRd analysis
The phase purity and crystallinity of uncoated AZ31, Sc 2 O 3 coated AZ31 alloy and GeO 2 coated AZ31 alloy was characterized by XRD analysis and presented in Figures 1-3.

Fig. 1. X-ray diffraction pattern of AZ31 alloy
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][17][18][19] . The base AZ31 alloy peaks demonstrated very low intensity peaks in figure, due to the thick GeO 2 .

FESEM analysis
The surface morphology of Sc 2 O 3 and GeO 2 are presented in Fig. 4 and 5, respectively. Both the scandium oxide and germanium oxide powders show their individual particles with irregular shapes or morphology.

FTIR analysis
The functional groups of Sc 2 O 3 and GeO 2 were analyzed by Fourier Transform Infrared Spectroscopy (FTIR) and presented in Fig. 6 and 7, respectively. Similarly, Fig. 7 shows the FTIR spectrum of GeO 2 compound, where it is noticed that the peaks at 498, 560, 878, 3021 and 3670 cm -1 . The peaks at 498, 560 cm -1 corresponds to the V 4 vibration mode in GeO 4 tetrahedra 29,30 . A strong absorption peak at 878 cm -1 demonstrates the V 3 vibration mode, which is the distorted tetrahedral structure of GeO 2 . The band at 3021 and 3670 cm -1 are attributed to olefinic and hydroxyl stretching.

Raman analysis
The Raman spectrum of Sc 2 O 3 compound is presented in Fig. 8. The Raman modes observed at 194, 320, 538, 420, 495, and 525 cm -1 . The major vibration band at 420 cm -1 can be assigned to totally symmetric Ag and Fg-type modes of octahedra of ScO 6 [31][32][33] . The spectrum of GeO 2 is presented in Fig. 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 GeO 2 and weak peak at 328 cm -1 shows the Ge 34,35 . Moreover, the disorder-induced peak (1350 cm -1 ) and graphite peak (1580 cm -1 ), which are caused by C when TaC + C phase is formed Moreover, the disorder-induced peak (1350 cm -1 ) and graphite peak (1580 cm -1 ), which are caused by C when TaC + C phase is formed Moreover, the disorder-induced peak (1350 cm -1 ) and graphite peak (1580 cm -1 ), which are caused by C when TaC + C phase is formed Electrochemical analysis Corrosion behavior of uncoated AZ31, Sc 2 O 3 and GeO 2 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. The linear polarization studies of uncoated AZ31 and Sc 2 O 3 coated and GeO 2 coated AZ31 alloys are shown in Fig. 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 icorr of 3.4 x 10 -4 mA/cm 2 , Sc 2 O 3 coated AZ31 alloy exhibited icorr of 5.4 x 10 -9 mA/cm 2 and GeO 2 coated AZ31 alloy exhibited icorr of 2.59 x 10 -9 mA/cm 2 . The results shows that GeO 2 coated AZ31 alloy demonstrated better performance. The electrochemical impedance curves of bare AZ31 and Sc 2 O 3 coated and GeO 2 coated AZ31 alloys in NaCl (3.5%) electrolyte is shown Fig. 12 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 GeO 2 coated AZ31 alloy showed the corrosion potential of ~ -1.3 V (SCE) and corrosion current density of 2.59 x 10 -9 mA/ cm 2 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.

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The GeO 2 (germanium dioxide) and Sc 2 O 3 (scandium trioxide) were developed as coatings on AZ31 alloy using polymer binder.

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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 icorr of 3.4 x 10 -4 mA/cm 2 , while the Sc 2 O 3 coated AZ31 alloy exhibited E corr of -1.4 V (SCE) and icorr of 5.4 x 10 -9 mA/cm 2 and while the GeO 2 coated AZ31 alloy exhibited E corr of -1.3 V (SCE) and icorr of 2.59 x 10 -9 mA/cm 2 .

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The results reveal that the GeO 2 coated AZ31 alloy demonstrated higher corrosion resistance than of bare and Sc 2 O 3 coated AZ31 alloy.