1 Introduction

The comprehensive properties of Mg such as similar Young’s modulus to that of bone, non-toxicity, etc., make them an unavoidable material in the biomedical field [1,2,3,4,5]. The possible usage of Magnesium as an implant in various fields including cardiovascular, musculoskeletal ligature, vascular stents, orthopaedics and bio-batteries for pacemakers was recommended widely [6]. It does not necessitate revision surgery thereby reducing the duration of physical disability [7, 8]. The high reactivity and rapid corrosion of Mg alloy in the physiological fluid hinder the usage in biomedical field due to the increase in alkalinity on the local sites and hydrogen evolution [9, 10]. To establish better corrosion resistance, surface treatments were widely done.

Conversion coating is a widely used technique to reduce the corrosion rate [11, 12]. The green conversion coating has gained eminence in recent days due to the employment of environment-friendly non-toxic biological macromolecules [13,14,15]. The organic compounds isolated from the plant source were effective in inhibiting corrosion [16,17,18]. Terminalia chebula, king of medicine, possess various medicinal properties including antibacterial, antioxidant and anticancer activities. The extract of T. chebula consists of various bioactive compounds of tannin, phenolic acid and flavonoids [19]. Tannins and phytic acids have effectively reduced the corrosion rate of corrosion in Mg alloy [20]. Tannin/hydroxyapatite coating on AZ31 Mg alloy was prepared by chemical conversion coating to improve the bioactivity and corrosion resistance [21]. The gallic acid and hexamethylene diamine coating on Magnesium alloy were effective in reducing the corrosion rate [22]. The organic acids react with magnesium and form the corresponding magnesium compound, which is merely insoluble and prevents the metal from degradation [23]. The phenolic network on MgZn alloy was efficient in mitigating the hydrogen evolution and was quite stable in in-vivo conditions [24]. In the present work, we formulate a conversion coating with the ethanolic extract of T. chebula on AZ31 Mg alloy and the corrosion behaviour and apatite forming ability of the coating were studied.

2 Materials and Methods

2.1 Formation of Gadolinium Conversion Coating

AZ31 Magnesium alloy was obtained from Seoul National University, South Korea. The chemical composition of the AZ31 Mg alloy was Al (2.83%), Zn (0.8%), Mn (0.37%), Cu (0.002%) and remaining Mg. The metal sheets were cut into metal coupons of dimensions 15 mm × 25 mm × 1 mm. The sample surface was mechanically polished with emery sheets (600 to 1200 grits) to remove the oxide layer. Then, the samples were thoroughly washed with acetone using a sonicator. The prepared samples were soaked in 1 N sodium hydroxide solution for 20 min in an upright position to form an oxide layer on the surface.

Terminalia chebula was obtained from the local market. The ethanolic extract of T. chebula was prepared by dissolving 5 g of the powder in 100 ml of ethanol (Hayman, Premium grade ethanol 100%) and stirred for about 2 h in room temperature. Then the solution was filtered and the extract was employed for the preparation of the organic conversion coating. The prepared sample was soaked in upright position on the extract solution for 40 min at room temperature to ensure that the entire surface was covered with the extract solution, and then was washed with distilled water and placed in the desiccator.

2.2 Surface and Physicochemical Characterizations

High-Resolution Scanning Electron Microscope with Energy-Dispersive X-ray spectroscopy (SEM/EDX), FEI Quanta FEG 200 was employed to monitor the surface morphology of the coating. The coated sample was sputtered with gold for 20 s before SEM analysis. Fourier Transform Infrared spectrometer with a single reflection ATR accessory (Perkin Elmer Spectrum two, USA) was used to identify the functional groups of the coating. The samples were scanned in the range of 4000–450 cm−1 with a spectral resolution of 0.2 cm−1. The crystallographic morphology of the coated sample was analysed with grazing incidence X-ray diffraction (Empyrean, Malvern Panalytical, United Kingdom) using CuK radiation.

2.3 Electrochemical Studies

The electrochemical studies were carried out using potentiostat/galvanostat (PGSTAT model 302 N, AUTOLAB, Netherlands) connected to a personal computer with Nova 2.0 software. The three-electrode system was employed in which the sample acts as the anode, platinum as the cathode and saturated calomel electrode as the reference electrode. The preparation of simulated body fluid (SBF) was discussed in our previous literature [4] and employed as the electrolyte. The samples were placed in a specially designed corrosion cell in which the exposed surface area was 1 cm2 to perform all the electrochemical studies. The open-circuit potential of the samples was monitored for a period of 30 min by immersing in SBF. The electrochemical impedance spectroscopic studies were executed in the frequency range of 100 kHz–10 mHz with an amplitude of the sinusoidal wave of 10 mV on OCP. The obtained electrical impedance data were fitted with ZSimpwin 3.21 version software to acquire the parameters that provide the information on the interphase/electrode environment. Potentiodynamic polarization studies were done with a scan rate of 1 mV/s from 500 mV of OCP till the breakdown of the coating. The slopes of the curves (βa and βc) were obtained by drawing slopes with the aid of NOVA 2.0 software. The polarization resistance was determined by using the Stern–Geary equation [25].

$$R_{p} = \frac{{\beta_{a} \times \beta_{c} }}{{2.3 i_{corr} \left( {\beta_{a} + \beta_{c} } \right)}},$$
(1)

where βa and βc are slopes of anodic and cathodic parts of the polarization plots in V/dec; Rp is polarization resistance in kΩ.

2.4 Immersion Studies

The organic conversion-coated sample was immersed in the SBF solution for 7 days at 37 °C. The SBF solution was changed on alternate days. HR-SEM/EDX and ATR-IR were performed to ensure apatite formation on the surface.

3 Results and Discussions

3.1 Topographical Analysis

Figure 1 shows the SEM image of herbal extract conversion coating on Mg alloy. The coating displayed the thallus-like morphology. The cracks are seen on the surface due to dehydration and surface shrinkage. The cross section of the organic conversion coating showed that the thickness of the coating was ~ 14.5 µm. The elemental analysis shows the uniform distribution of carbon, nitrogen, oxygen and magnesium.

Fig. 1
figure 1

SEM images (a), cross section (b), EDX profile (c) and mapping (d) of organic conversion coating

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The functional groups of the ethanolic extract of T.chebula and conversion coating formed on AZ31 Mg alloy are given in Fig. 2. The ethanol extract of T. chebula showed a broad peak at 3340 cm−1 corresponding to the hydroxyl group (–OH) [27]. The saturated –CH stretching of the polyol is seen at 2911 cm−1 [28]. The phenolic compounds are observed at 1320, 1232 and 1174 cm−1. The characteristic peaks of amines are seen at 1605 and 1523 cm−1. The peaks at 1711 cm−1 signify the presence of the carboxylic acid (–C = O) group and the glycosidic linkages are at 1049 cm−1 [29]. The significant peak shift from 1523 to 1580 cm−1 and 1605 to 1655 cm−1 indicates the coating formed on the surface [30]. The tannins are weak acids that effectively react with the metal and form a coating on the surface. The tannins present in the extract of T. chebula have the ability to chelate with Mg+ ions by bonding with –OH groups of the aromatic rings; thereby developing a highly cross-linked passive layer of hydroxybenzoic acid magnesium [26]31.

Fig. 2
figure 2

The chemical interaction on the organic conversion coating formation determined using ATR-IR spectra of T. chebula extract and organic conversion coating

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The X-ray diffraction (XRD) analysis of the conversion coating on Mg alloy is shown in Fig. 3. The International centre for diffraction data (ICDD) card no. 01-079-6692 matches with the base metal magnesium. The crystal cubic structure of MgO showed peaks at 36.9° and 78.3°, which matches with the ICDD card no. 045-0946 [32]. The broad peak around 20° indicates the formation of coating in the amorphous state.

Fig. 3
figure 3

The crystallographic pattern of the organic conversion coating

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3.2 Electrochemical Studies

Figure 4 provides potentiodynamic polarization plot of the bare and coated sample and the corresponding data are provided in Table 1. The coated sample was compared with the bare reported in our previous work [33]. The potentiodynamic polarization curve was analysed with NOVA 2.0 software and the corrosion current (icorr), corrosion potential (Ecorr), polarization resistance (Rp) and corrosion rate (CR). The corrosion rate of the coating (0.012 mm/yr) has considerably declined in comparison to the bare (0.962 mm/yr). The anodic current density was lower than the cathodic current density for both coated and bare. A positive shift in the corrosion potential was observed in the coated sample than bare. The oxide layer on the surface of Mg alloy breaks down at the potential of − 1.42 V, whereas the coating breaks down at 0.59 V. The coating can withstand a large potential range indicating that they have the ability to withstand the dynamic potential generated during the bone remodelling [34,35,36]. The protection efficiency was calculated and found to be 98.75%. The results indicate that the penetration of the physiological fluids was hindered by the passive layer formed on the surface [37].

Fig. 4
figure 4

Potentiodynamic polarization curve of the bare and organic conversion coating in SBF solution

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Table 1 Potentiodynamic polarization data of bare and organic conversion coating sample
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Figure 5 shows the Nyquist and Bode plots obtained from EIS studies. There is a correlation between the capacitance loop in the high-frequency region and corrosion rate. The capacitive semicircle diameter is proportional to the corrosion resistance of the material. The semicircle diameter in the high-frequency region of the coating was bigger than the bare indicating that the corrosion rate was slowed down by the passive coating layer.

Fig. 5
figure 5

Corrosion behaviour of the organic conversion coating was determined from Nyquist (a), Bode impedance (b) and phase angle (c). The corresponding equivalent circuit (d) of bare and coating

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The obtained EIS spectra were matched with electrochemical equivalent circuits (EEC) fitted with the electrochemical equivalent circuits with the chi-square value of 10–4 to 10–3 and the acquired data are given in Table 2. Rs illustrates the solution resistance; In bare, R1, R2 and RL signify the resistance of the natural oxide layer, substrate and inductor resistance. The CPE1 and CPE2 are the constant phase angle element of the natural oxide layer and substrate, respectively. L represents the inductance on the substrate. In coating, R1 and R2 is the resistance offered by the passive coating layer and the substrate. The corresponding constant phase angle elements are represented as CPE1 and CPE2.

Table 2 Electrochemical impedance data of bare and coating obtained by fitting with the equivalent electrochemical circuits
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By combining Nyquist and Bode plots, the bare exhibited two capacitive loops and an inductive loop. The first capacitive semicircle of the bare was formed as a result of charge transfer in the partially formed oxide/hydroxide layer on the substrate and electrolyte, while the second capacitive loop was attributed to the mass transport relaxation of the oxide/hydroxide layer. The inductive loop was obtained as a result of adsorption/desorption of Mg+ species on the surface which initiates pitting corrosion [38]. The first capacitive loop of the coating in the high-frequency region reflects the electrical double layer and the second capacitive loop in the mid- and low-frequency region signifying the resistive behaviour of the coating. The bare showed a resistance value of 2.588 KΩ indicating their poor resistance in the SBF solution, whereas the coating showed a higher resistance of 405.5 KΩ. The chelation of Mg2+ ions by the organic compounds on the conversion coating impedes the ionic movement from the coating and also from the solution by forming a barrier [30].

In the Bode phase angle plot, the bare showed an initial phase angle of 23° in the high-frequency region and reaches a maximum of 54° in the mid-frequency region and drops down to − 4° in the low-frequency region. The negative phase angle in the low frequency signifies the rapid dissolution of Mg from the bare. In the coating, the phase angle begins at 46° and attains 67° in the mid-frequency; then reaches 19° in the low-frequency region. The disappearance of the inductive loop was seen on the coating. The phytate and tannate complex on the coating surface prevents the release of Mg2+ ions from the substrate through the electrostatic interactions [26, 39]. Therefore, the corrosion behaviour of the material was improved.

3.3 Apatite Forming Ability

The organic conversion coating was immersed in SBF for 7 days. The surface morphology analysed using SEM/EDX and the functional group by ATR-IR analysis are given in Fig. 6. The clustered aggregates of nanospherical apatite deposition are seen on the surface (given as in Fig. 6a, b). The existence of carbon, oxygen, sodium, calcium, phosphorous and magnesium was identified indicating the apatite forming the ability of the organic conversion coating.

Fig. 6
figure 6

Apatite formation on the organic conversion coating after immersion in SBF for 7 days—SEM image (a), EDX profile (b) and ATR-IR spectrum (c)

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The possible apatite formation mechanisms are (i) the glycosidic linkages on the conversion coating of the Mg surface indicate the adsorption of sugar molecules during the coating formation. When the coating is placed in SBF, the glucose converts into gluconic acid, which in turn recruits the Ca2+ ions [15]. (ii) The conversion coating formed on the surface consists of polyphenolic compounds including tannates and phytates, which chelate with the Ca2+ ions. The recruited Ca2+ binds with the negatively charged phosphate ions in a co-ordinate fashion leading to the deposition of apatite [40]. From the ATR-IR spectrum (Fig. 6c), the 7-day immersion sample of the coating exhibited the peaks at 3318, 1645 and 1340 attributed to hydroxyl group, amines and phenolic compounds. The two new peaks apart from the organic coating peaks at 1020 and 524 cm−1 correspond to the γ3 and γ4 vibrations of the phosphate [41]. Hence, it is confirmed that the coating has the ability to support the apatite formation.

4 Conclusion

The organic conversion coating was formulated with the extract of T. chebula on AZ31 Mg alloy. The following observations and conclusions were derived from the work.

  • The conversion coating formed the thallus-like morphology. The coating surface showed the presence of amines, phenols, carboxylic and hydroxyl groups along with glycosidic linkages confirming the formation of the conversion coating in the amorphous state.

  • The coating was effective in enhancing the corrosion resistance in ‘limited’ in-vitro conditions. Also, the coating has the ability to support the apatite formation on the surface by chelating the ions from the SBF solution.

Overall, the coating with the herbal extract can be a simple and effective method that can be employed in bio-implants.