Y L Raja Mohan | S. Parameswari | P. Karpagavinayagam | S. Thanikaikarasan | C. Vedhi* | A. Cyril*
© 2023 IIETA. This article is published by IIETA and is licensed under the CC BY 4.0 license (http://creativecommons.org/licenses/by/4.0/).
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Herein, the nanoparticles of SrTiO3 perovskite oxide were synthesized using simple hydrothermal process. The synthesized perovskite oxide material was characterized by XRD and FTIR techniques. The XRD analysis reveals that the formed SrTiO3 perovskite oxide in cubic crystal phase with high phase purity. The FTIR result demonstrate the stretching and vibration bands of Ti-O and Sr-O along with O-H functional groups. The SrTiO3 perovskite oxide powder was added to silicone resin and the paste was applied on AZ31 alloy sample, which was then used for electrochemical analysis. The corrosion characteristics of SrTiO3 perovskite oxide coating over AZ31 alloy were investigated in 7% NaCl electrolyte, wherein platinum foil used as counter electrode, saturated calomel electrode (SCE) as reference and SrTiO3 perovskite oxide coated AZ31 alloy as working electrode. The results reveal that the open circuit potential of AZ31 alloy shifted from -1.57 V (SCE) to -0.95 V (SCE). The corrosion rates of bare AZ31 alloy calculated and found as 7 x 10-2 mA/cm2, while the same after coating was measured as 9 x 10-6 mA/cm2. The electrochemical impedance results also reveal that corrosion protection induced by the coating. Hence, it is summarized that the SrTiO3 perovskite oxide coating demonstrated improved corrosion resistance for AZ31 alloy.
AZ31, SrTiO3, silicone resign, corrosion, impedance
Magnesium alloys have light weight and high density, which makes them applicable in various applications such as aerospace and automobile sectors. Nevertheless, having the said advantages, the magnesium and it’s alloys suffer higher corrosion in many mediums due to the more electromotive force in series [1-4]. Hence, research and development efforts are directed to improve the corrosion resistance of Mg in various methods such as addition of allying elements and protective coatings [5-12]. The development of protective coatings on magnesium alloys found as easy and economically favorable process [13-15]. Ren et al. developed calcium deficient hydroxyapatite (CDHA) layer on AZ31 alloy, which the mass loss of AZ31 alloy is reduced in SBF solution [13]. Hanas et al. developed polycaprolactone (PCL) nano-fibrous layer coatings using electrospinning technique. The results shows that nano-fibres PCL coating showed improved corrosion resistance [14]. Zhang et al. developed Mg–Al-layered double hydroxide (LDH) coatings using both co-precipitation and hydrothermal process, wherein the LDH coatings showed nano flake structure arranged vertically, which shoed excellent corrosion resistance performance [15]. Huang et al. studied the photo-effect on the corrosion properties of SrTiO3 coatings using hot dipped process on zinc coated steel. The results shows that the SrTiO3 coatings demonstrated reducing photo-corrosion effect of zinc coated steel [16]. The Effect of SrTiO3 coatings on AZ31 alloy has not explored till the date.
2.1 Chemicals
Strontium hydroxide Sr(OH)2ꞏ8H2O was purchased Sigma Aldrich, Titanium chloride (Ti(OH)4)was purchased from Merck and Silicone resin 1805was purchased from Dow Corning.
2.2 Sample preparation
The AZ31 alloy was cut into the dimensions of 12 x 12 x 3 mm from the sheet and polished using SiC grit papersand finally with 1 and 0.5 diamond paste. In the process, the samples were exposed to 200 SiC grit paper and extended the process further up to 1200 grit paper polishing, in the end, 1- and 0.5-microns diamond paste polishing was carried out. The samples were washed with water to remove the polish impurities.
2.3 Electrode preparation
The synthesized SrTiO3 nano powder (1g) was mixed with silicone resin (10 ml) and mixed well to prepare paste (semi viscous). The paste was applied to the polished AZ31 alloy by doctor blade method and dried. The coated samples were then mounted with copper wire for electrochemical studies.
2.4 Electrochemical experiments
The electrochemical studies involving open circuit potential, polarization and impedance analysis were carried out on bare AZ31 alloy and SrTiO3 perovskite oxide coated AZ31 alloy. For this, a three-electrode system was used in which platinum foil as counter electrode, saturated calomel as reference electrode and AZ31 alloy (with and without coating) used as working electrode in 7% NaCl medium.
3.1 X-ray diffraction analysis
In order to understand the crystallography of bare AZ31 and SrTiO3 perovskite oxide coated AZ 31 alloy, X-ray diffraction technique was employed over samples and the patterns were recorded with Cu K α of 1.5418 Å.
The XRD pattern of bare AZ31 alloy is shown in Figure 1. The figure shows the sharp peaks indicating high crystalline nature of Mg alloy. It is also noticed that the preferential growth in (101) plane. The peaks are well matched with standard fileJCPDS file no 35-0821, which shows the hexagonal crystal structure a=b=3.2094 Å and c=5.211Å and belongs to P63/mmc space group [17-20].
Figure 1. XRD pattern of bare AZ31 alloy
Figure 2. XRD pattern of SrTiO3 perovskite oxide coated AZ 31 alloy
Figure 2 demonstrates the high crystallinity for SrTiO3 perovskite material. The peals are well matched with standard file JCPDF # 05-0634, which shows the cubic perovskite crystal structure (a = b = c = 3.90 Å) belongs to Pm3m (221) space group [21-24].
3.2 Surface morphology
The surface morphology of SrTiO3 perovskite oxide is shown in Figures 3-5. It is noticed that the nano particles are well distributed. The shape of the particles are irregular and the size is varied among them.
Figure 3. Surface morphology of SrTiO3 perovskite oxide
The higher magnified images showing the surface morphology of SrTiO3 perovskite oxide shows the distinct features, which includes nano sized particles. The crystal growth shows there is no much effect of surfactant in the synthesis of SrTiO3 perovskite oxide.
Figure 4. Surface morphology of SrTiO3 perovskite oxide at higher magnification
Figure 5. Surface morphology of SrTiO3 perovskite oxide at higher magnification
As mentioned in the XRD analysis, the powders are free from impurities and the figures show the shape and size of particles, which corresponds to purely SrTiO3 perovskite oxide. In many applications, the nano sized particles provide better performance, however, in the current study, the nano sized particles provided high dense packing in the coatings.
3.3 HRTEM analysis
The morphological images acquired through high resolution transmission electron microscope images is shown in Figure 6.
Figure 6. HRTEM images of SrTiO3 perovskite oxide
Figure 7. SAED pattern of SrTiO3perovskite oxide
Figure 6 reveals that the nanoparticles are clubbed together and showed as micro-sized particle. However, the individual particles are clearly demonstrating the nanosize nature in high resolution transmission electron microscopic analysis. The SrTiO3 perovskite oxide powder was sonicated and deposited on the Cu grid and examined the morphological features in HRTEM. The Figure 7 reveals the polycrystalline nature of SrTiO3 perovskite oxide.
3.4 FTIR analysis
The FTIR spectrum of SrTiO3 perovskite oxideis shown in Figure 8. The peaks are noticed at 450, 580, 760, 1456 and 3440 cm-1.
Figure 8. FTIR spectrum of SrTiO3 perovskite oxide
The peak at 450 cm-1 is obtained due to the stretching mode of Ti-O. The bands at 1450 is correspond to the vibration C-O in -CO32- due to presence trace SrCO3 [22-27]. The band at 3440 cm-1 represents the O-H group which is resulted from physically adsorbed water.
3.5 Electrochemical analysis
The open circuit potentials of AZ31 and SrTiO3 perovskite oxide coated AZ31 alloy are studied with respect to time and presented in Figures 9 and 10, respectively. It is noticed that the open circuit potential of AZ31 alloy is -1.64 V (SCE) and the same SrTiO3 perovskite oxide coated AZ31 alloy is -0.95 V (SCE). The noble side shift in potential is clearly noticed, which indicate the enhancement in corrosion protection.
Figure 9. Open circuit potentials of bare AZ31 alloy
The OCP was highly unstable due to the evolution of hydrogen gas on the surface reaction of pure Mg alloy. However, the presented values are smoothened and represented in Figure 9.
Figure 10. Open circuit potentials of SrTiO3 perovskite oxide coated AZ31 alloy
The SrTiO3 perovskite oxide coated AZ31 alloy shows the corrosion potential around -0.95 V (SCE). This feature clearly indicate that the perovskite coating is beneficial as the corrosion potential shifted to more noble direction, which shows the higher corrosion protection induced due to the coating.
The corrosion rates of bare AZ31 alloy and SrTiO3coated AZ31 alloy were studied and the corrosion behavior is shown in Figure 11.
Figure 11. Tafel plot of bare AZ31 alloy
From the Figure 11, it is noticed that corrosion potential is noticed that bare AZ31 demonstrated the corrosion rate 3.4 x 10-4 mA/cm2. The corrosion rates were calculated using the software and interpretated. Both the anodic and cathodic curves indicate that Mg alloy involved in chemical reaction with the electrolyte and produced hydrogen cathodically and some notable passivation observed in anodic region.
Further, the corrosion rate of SrTiO3 perovskite oxide coated AZ31 alloy measured from the Tafel plot which is shown in Figure 12. The calculated corrosion rate of coatings is found as 9 x 10-6 mA/cm2.
Figure 12. Tafel plot of SrTiO3 perovskite oxide coated AZ31 alloy
From the Figure, it is noticed that corrosion potential is noticed that SrTiO3 perovskite oxide coated AZ31 demonstrated the corrosion rate 9 x 10-6 mA/cm2. There is significant reduction in the corrosion rate after applying the SrTiO3 perovskite oxide coating. The shift of Ecorr into noble direction implies that enhanced corrosion protection. The perovskite coating shoed less corrosion rates in the NaCl medium.
In addition to this, the electrochemical impedance curves of bare AZ31 and SrTiO3 perovskite oxide coated AZ31 alloyare shown Figures13 and 14, respectively. The Nyquist results show that the corrosion resistance is increased for the SrTiO3 perovskite oxide coated AZ31 alloy in comparison with bare AZ31 alloy.
Figure 13. Electrochemical impedance curves of bare AZ31 alloy
The impedance curve as Nyquist plot is noticed in the images 13. The Nyquist curve shows the full half circle. The curve was fit with randles equivalent circuit model as shown below.
Figure 14. EIS-Nyquist plots of bare AZ31 and SrTiO3 perovskite oxide coated AZ31 alloy
The Nyquist plot of SrTiO3 perovskite oxide coated AZ31 alloy showed half circle and best fit to randles equivalent circuit model. The resistance measurements indicate that the bare AZ31 alloy showed very less corrosion resistance, while theSrTiO3 perovskite oxide coated AZ31 alloy showed superior corrosion resistance.
The nanoparticles of SrTiO3 perovskite oxide was synthesized using hydrothermal route.
The SrTiO3 perovskite oxide-basedcoatings were developed on AZ31 alloy using silicone resin.
The bare AZ31 alloy showed corrosion current densityas 3.4 x 10-4mA/cm2 , while the same for SrTiO3 perovskite oxide coated AZ31 alloy noticed as 9 x10-6 A/cm2.
The results reveal that the SrTiO3 perovskite oxide coated AZ31 alloy demonstrated higher corrosion resistance than that of bare AZ31 alloy.
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