Hydrothermal synthesis of ZnTa 2 O 6 , ZnNb 2 O 6 , MgTa 2 O 6 and MgNb 2 O 6 pseudo-binary oxide nanomaterials with anticorrosive properties

. ZnTa 2 O 6 , ZnNb 2 O 6 , MgTa 2 O 6 and MgNb 2 O 6 pseudo-binary oxide nanomaterials were synthesized through the hydrothermal method at 250 ° C. Obtained materials were characterized by X-ray diffraction, UV-VIS measurements, ﬁ eld emission-scanning electron microscopy and atomic force microscopy techniques. XRD results show that the single phases of ZnTa 2 O 6 , ZnNb 2 O 6 , MgTa 2 O 6 and MgNb 2 O 6 pseudo-binary oxides nanomaterials were obtained, no thermal treatment being required. The values for the optical band gap of each material are settled in the range 3.60 – 3.80 eV. The anticorrosion characteristics of the obtained compounds were also evaluated after deposition on carbon steel in 0.5 M Na 2 SO 4 media by open circuit potential measurements and potentiodynamic polarization technique with Tafel representation. The inhibition ef ﬁ ciency of pseudo-binary oxides deposited on carbon steel electrode was in the range 37 – 59.17%, promising for improvement of the anticorrosion properties. deposited deposition as and of in acid


Introduction
Corrosion is one of the most studied processes because this affects everything around us. Corrosion inhibition represents a goal due to the damages that corrosion leads to the carbon steel equipment's and installations in a lot of industries, whose remedy implies financial and also time losses. To fight against this process, there were developed methods and techniques as: plating's of the steel's parts or the usage of different organic or inorganic corrosion inhibitors À which are often used to protect materials in different environments [1][2][3][4][5][6].
The present study presents the obtaining of ZnTa 2 O 6 , ZnNb 2 O 6 , MgTa 2 O 6 and MgNb 2 O 6 pseudo-binary oxides nanomaterials by using the hydrothermal method at 250°C. Also, are presented results regarding structural, morphological, topographical and optical characterizations of the named materials, and the results regarding the corrosion inhibition efficiency for each obtained nanomaterial evaluated in 0.5 M Na 2 SO 4 media.
hydrothermal parameters (t, T), in order to achieve a successfully morphology and crystalline structure of the materials. Based also on a previously experience in obtaining anticorrosive materials by the hydrothermal method [21,22], the optimal hydrothermal parameters were chosen as it can be seen in Table 1. It was found that the anticorrosive materials ZnTa 2 O 6 , ZnNb 2 O 6 , MgTa 2 O 6 and MgNb 2 O 6 can be successfully synthesized at a 1:1 molar ratio of the used precursors for each material, at a temperature of 250°C for 12 h. The pH values of the synthesis were fixed at 13, using NaOH (97%, Merck), resulting an alkaline medium.
Further, the obtained pseudo-binary oxide nanomaterials were deposited as thin layers (using the drop casting method in acetone medium), in different combinations on carbon steel electrode disks. The carbon steel disks, with 10 mm diameter and 2 mm thick, had the following chemical composition: 93.80% Fe; 4.81% Ni; 0.51% Co; 0.005% Cu; 0.19% P and 0.01% S. Before the drop casting depositions, the carbon steel disks were polished with emery paper and thus rinsed with double distilled water and degreased with ethanol. The modified carbon steel disks were used as working electrodes in the corrosion tests.

Apparatus
The phase identification of the synthesized nanomaterials was investigated by X-ray diffraction (XRD) on a X'pert Pro MPD X-ray diffractometer with monochromatic Cu Ka (l = 1.5418 A ) on an incident radiation. For the morphological investigations regarding the obtained pseudo-binary oxides, field emission-scanning electron microscopy (SEM/EDAX) (Model INSPECT S) and atomic force microscopy (AFM) (Model NanoSurfEasyScan 2 Advanced Research) were used. The optical band gap for each pseudo-binary oxide nanomaterial was calculated by recording the difusse reflectance spectra at room temperature using an UV-VIS-NIR spectrometer Lambda 950.
A Voltalab potentiostat (Model PGZ 402) was used to perform the electrochemical measurements. The potentiostat was coupled with a three electrodes electrochemical cell comprising: a platinum wire as a counter electrode, a saturated calomel electrode as the reference electrode and the working electrodes consisting in bare or drop casting modified carbon steel disks (OL). The potentiodynamic polarization measurements were recorded by sweeping the potential from À1.3 to À0.6 V at a scan rate (n) of 1 mV/s at room temperature (23°C). Before the polarization, the open circuit potential (OCP) of the modified electrodes was monitored for 30 min. For the corrosion tests a 0.5 M Na 2 SO 4 solution was used.

Structural and morphological properties
The X-ray diffraction patterns recorded at room temperature in the 2u range of 10-80°are presented in The morphology of the resulting pseudo-binary oxide nanomaterials (as powders, before the depositions) and the  formation of the agglomerates are represented in Figure 2. It is to be mentioned that for the pseudo-binary oxide nanomaterials which contain Ta, are preserved cubic shapes beside irregular shapes ( Fig. 2a and 2c, while in the morphology of the pseudo-binary oxides with Nb content, the accicular shapes of the agglomerates is preserved (Fig. 2b and 2d). From the EDAX images for ZnTa 2 O 6 , ZnNb 2 O 6 , MgTa 2 O 6 and MgNb 2 O 6 nanomaterials (Fig. 3) it can be observed that only the specific lines for Zn, Mg, Ta, Nb and O are presented. Topographic analysis for the surface of the obtained nanomaterials was perfomed using atomic force microscopy. The recorded images are presented for each nanomaterial in Figure 4a-d.
To calculate the surface roughness for each sample of material, the NanoSurf EasyScan 2 computer program and the following equations were used [23]: for the average roughness and for the mean square root roughness, where N and M represent the number of the crystal axes x and y respectively; z represents the average height of crystallites; x k and y l are the maximum and minimum deviations from the average crystallite.
In Table 2 are presented the calculated values for the measured areas of the obtained nanomaterials. The measurements were taken in the non contact mode using a scan size of 1 mm Â 1 mm. The measured area at the surface for each material was 1.30 pm 2 .

Optical properties
Using the Kubelka-Munk equations [24,25], the absorbance was calculated for each obtained pseudo-binary oxide nanomaterial. From the absorption spectra (Fig. 5) can be observed the maximum absorption peak for each sample as it follows: 311 nm for ZnTa 2 O 6 , 311 nm for ZnNb 2 O 6 ,   {(k/s) hn} 2 versus hn, where k denotes absorption coefficient, s is scattering coefficient and hn is the photon energy, the optical band gaps were estimated for the obtained materials as it follows: E g (ZnTa 2 O 6 ) = 3.6 eV, E g (ZnNb 2 O 6 ) = 3.72 eV, E g (MgTa 2 O 6 ) = 3.8 eV and E g (MgNb 2 O 6 ) = 3.76 eV.

Polarisation curves
In Figure 6 are represented the Tafel plots of the investigated OL electrodes recorded after 30 min open circuit potential (OCP) in 0.5 M Na 2 SO 4 solution. The slopes were determined in the Tafel region of the anodic and cathodic curves before and after the corrosion potential (U).
As it can be seen in Table 3, where the calculated parameters from the Tafel plots are summarized, the corrosion potential (E corr ) of the OL electrode is À0.916 and the corresponding corrosion current density (i corr ) is 24.08 mA/cm 2 . The polarization curves were shifted towards the region of lower corrosion current densities in thepresence of ZnNb 2 O 6 and MgTaO 6 and the polarization curves shifted towards the region of higher corrosion current densities in the presence of ZnTa 2 O 6 and MgNb 2 O 6 .  The inhibition efficiencies (IE%) were calculated based on equation (3) where i 0 corr and i corr are the corrosion current densities in the absence and in the presence of the pseudo-binary oxide nanomaterials deposited as thin layers on carbon steel electrodes.
In the case of ZnTa 2 O 6 , obtained through the hydrothermal method, the IE% of 56.27% is higher than the reported IE% of 48.61% for ZnTa 2 O 6 obtained through the solid state method [10], while in the case of the ZnNb 2 O 6 obtained through the hydrothermal method, the IE% of 37% is lower than for the ZnNb 2 O 6 obtained through the solid state method (52.70%) [10].
The polarization resistance (Rp) increases from 1.53 kV cm 2 for bare OL to 2.36 kV cm 2 for MgNb 2 O 6 ,while in the case of ZnNb 2 O 6 the value of Rp is decreasing to 1.31 kV cm 2 which also reflects in the IE which is only 37%.
Analyzing the evolution of open circuit potential (OCP) with time for the investigated electrodes (Fig. 7), it can be seen that an exposure time of 30 min leads to a shift in free potential towards more negative values. Comparing the OCP profiles, it can be observed that in almost 20 min, the profile of the untreated electrode presents a decrease in potential until it reaches at the same value as for: ZnNb 2 O 6 , MgTaO 6 and MgNb 2 O 6 .

Conclusion
ZnTa 2 O 6 , ZnNb 2 O 6 , MgTa 2 O 6 and MgNb 2 O 6 pseudobinary oxide nanomaterials were obtained through the hydrothermal synthesis method at 250°C for 12 h. XRD results reveal that the single phase of the obtained pseudobinary oxide nanomaterials can be obtained through the hydrothermal synthesis at a pH value of 13. The optical band gaps were estimated from the diffuse reflectance spectrum of each materials to be in the range 3.6-3.8 eV. The inhibition efficiency for the obtained materials was calculated and for ZnTa 2 O 6, MgTa 2 O 6 and MgNb 2 O 6 nanomaterials were obtained values of IE% over 50%.
Taking into consideration that the tested materials   containing Zn and Mg in combination with Ta and Nb did not completely satisfied our expectations, we believe that a further approach using materials containing Mn in combination with Ta and Nb will add a benefit to the efficiency of corrosion, as it was already reported in [27,28].