1 Introduction

Magnesium-based alloys posses an attractive combination of low density, high strength/weight ratio and good castability. Due to these properties, they are being considered as a suitable substitute of ferrous alloys/aluminium in automotive and aerospace applications [1–3]. Unfortunately, they are highly susceptible to corrosion, particularly in humid air and chloride bearing wet environment. Presence of lowest electrode potential than other metals is another problem with the Mg alloys for the occurrence of galvanic corrosion. These problems have limited their use in various applications. To control their corrosion, development of an effective corrosion resistance coating can be a viable solution. Earlier several types of coating techniques like anodizing, electrochemical plating, chemical conversion, hybrid organic coating etc. have been applied on Mg and their alloys [4–15]. However, each method is having their own advantages and disadvantages. For e.g., uneven distribution of current density in the plating bath results in non uniform plating of complex shaped structure in electrochemical plating [6].

Among various developed coatings, rather recently developed sol–gel process based ceramics coatings are found to be more effective. Sol–gel route of coating development is mainly consists of the formation of ceramic oxide films through successive reactions of hydrolysis and condensation of the precursors [16, 17]. Coatings developed through this technique offer many advantages, i.e. better adhesion, low surface preparation and processing temperatures, use of relatively simple and inexpensive equipment. In addition, coating can be applied easily using deep coating technique of any complex shaped substrate. Initially, researchers were concerned in the development of sol–gel process based coatings on steels substrate. This resulted the development of coatings of ceramics oxides such as SiO2, ZrO2, SiO2/TiO2, ZrO2/Y2O3, ZrO2/CeO2, Al2O3 etc. on the different types of steel and stainless steel substrates [18–27]. Singh et al. [28–30] observed a significant improvement in the corrosion and pitting resistances of sol–gel alumina coated low carbon steel, 9Cr 1Mo ferritic steel and 304 SS in highly corrosive NaCl solution. In comparison, relatively few study has been reported in the development of sol–gel based ceramic oxides coating on Mg alloys [31–36]. Phani et al. [35] developed sol–gel ZrO2 and ZrO2–CeO2 coatings on Mg based AZ91D and AZ31alloys. They found that both coated alloys showed superior corrosion performance in salt spray test. In the recent investigation, sol–gel based composite organic coatings deposited on Mg–Zn–Ca alloy used in clinical application [36]. As per our knowledge, none of investigation reports the development of sol–gel coatings particularly alumina coating on Mg metal.

In views of above statement, present investigation was aimed to employ development of sol–gel alumina coating on Mg metal and evaluate their corrosion 3.5 % NaCl solution. Observed results indicates that sol–gel-derived Al2O3 coating on Mg metal reduces their corrosion rate an order of magnitude lower than uncoated Mg in 3.5 % NaCl solution.

2 Experimental

2.1 Specimen preparation

Specimens used for coating developments, were cut from Mg sheet (~99 % purity) of 3 mm × 10 mm × 10 mm of dimension. After polishing with different grades of emery paper, specimens were degreased properly with trichloroethylene solution.

2.2 Synthesis of boehmite sol and coating deposition

Boehmite (AlOOH) sol was prepared by mixing Al-isopropoxide and water in their 1:100 molar ratio. The suspension was refluxed and stirred vigorously at 85 °C for 4–5 h until it converted into a clear sol. Details of synthesis of boehmite sol is available elsewhere [27–29]. Viscosity of the sol was measured as 1.58 mPa s using a Brookfield R/S plus rheostat at shear rate of 600 s−1. The synthesized boehmite sol was then utilized for coating development. Coating was applied at prepared specimens using dip coating technique of a constant withdrawal speed of 1 cm/s (approx). Finally, air dried coated specimens were heated at 250 °C in oven for 3 h.

2.3 Microstructural and compositional analysis

Microstructural and compositional analysis of the coatings was made with the help of Field Emission Scanning Electron Microscope (FE-SEM) (NOVA Nano SEM 430) equipped with EDS. The Presence of functional groups in the coatings was analyzed using FTIR (Model Nicolet 5700) in a frequency range of 700–4,000 cm−1.

2.4 Electrochemical measurements

Open circuit potential (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were carried out using a conventional three electrodes (a saturated calomel electrode (SCE) as reference electrode, platinum plate as counter electrode and test specimens as working electrode) system bearing electrochemical cell. All the electrochemical measurements were made in 3.5 % NaCl solution at room temperature (30 ± 2 °C) using computer controlled Solatron 1280 Z corrosion system equipped with Corrware and Zplot softwares. Tafel plots were obtained by carrying out potentiodynamic polarization at a constant scan rate of 1 mV/s at ± 150 mV from OCP. Impedance analysis were made in a frequency range of 20 kHz to 0.01 Hz at OCP by applying a sine potential signal of 10 mV. Different parameters related to potentiodynamic polarization and impedance measurements were derived by curve fitting method using Corrview and Zview software, respectively. Each experiment has been repeated two to three times to confirm reproducibility of the results.

3 Results and discussion

3.1 Elemental and microstructure analysis

Figure 1 reveals the surface topography through the coated and uncoated parts from the same specimen. The coated part can be distinguished with the presence of a thick layer as compared to uncoated part of the specimen. Elemental X-ray mapping of aluminum (Al) and oxygen (O) through the coating as depicted in Fig. 1, exhibits their a considerable presence. However, their concentration becomes very low at the uncoated part of the specimen. In case of magnesium (Mg), elemental mapping demonstrates their presence in the bulk level at the uncoated part. But their concentration becomes very less the coated part (Fig. 1). EDS analysis of Al, O, Mg elements was made to quantify their presence in the coated and uncoated parts of the substrate. EDS analysis as presented in Table 1, show a higher concentration of Al (27 wt%) and O (52 wt%) and very low concentration of Mg (<2 wt%) at the coated part. This analysis evidenced the presence of alumina in the coating.

Fig. 1
figure 1

Elemental X-ray mapping of Al, O, Mg through the alumina coated and uncoated Mg surfaces

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Table 1 EDX analysis of Mg metal and double layered alumina coated Mg surface
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Figure 2 shows a magnified view of the coated surface though the plane and the cross section. The morphology of the coating as depicted in Fig. 2b, shows jelly like appearance of the coating. In the cross sectional view (Fig. 2c), coating can be differentiated as a white layer of 2–3 µm in thickness. Coating appears amorphous in nature which is free from the cracks and porosities.

Fig. 2
figure 2

Microstructure of alumina coating through a coated and uncoated surface, b coated surface at higher magnification, c cross section of coated specimen

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3.2 FTIR analysis

Figure 3 illustrates the FTIR spectra of boehmite (aluminium oxy-hydroxide) coated surface of the substrate. Presence of a broad band in the spectral range 3,200–3,500 cm−1 is mainly due to –OH stretching of hydrogen bonded aluminium oxy-hydroxide (AlO(OH) [37, 38]. Since complete oxidation of boehmite into γ alumina occurs at 450 °C and above temperatures [38], presence of trace of unoxidized boehmite can not be ruled out. Occurrence of a small broad band at 2,100–2,000 cm−1 spectral range is mainly related to the presence of hydrogen bonded OH group [38]. This evidences the presence of water molecules in the coating. The occurrence of next small absorption peak at further low intensity at 1,384 cm−1 is due to stretching of Al–C bond that correspond to alkyl (ethoxy) group of the precursor. Presence of organic groups is reported to help in reducing the stresses and cracks in the coating [39, 40]. Origin of small peak at 1,070 cm−1 is also correspond to boehmite [38]. A steep increase of absorption at 800 cm−1 is belongs to Al–O vibration mode of Al2O3 [37, 38, 41]. Absorption related to Al–O vibration is mainly due to presence of alumina in their different phases [42, 43]. Present FTIR measurement indicates that the most of the boehmite present in the coating get oxidized into alumina after heating at 250 °C as per following reaction (1).

$$ 2\,{\text{Al(OOH)}} \to {\text{Al}}_{ 2} {\text{O}}_{ 3} + {\text{H}}_{ 2} {\text{O}} $$
(1)

Due to heating at lower temperature, trace of the unoxidized boehmite, residual precursor along with molecular water were also analyzed in the coating. However, their presence did not make any adverse effect toward the stability/adherence of the coating.

Fig. 3
figure 3

FTIR spectra of boehmite coated Mg after heating at 250 °C

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3.3 Electrochemical corrosion evaluation

3.3.1 Open circuit potential (OCP)

The steady state of OCPs measured after 30 min of exposure in 3.5 % NaCl solution, are given in Table 2. From the trend, it can be noticed that OCP of the coated specimen occurs more positive than that of the uncoated specimen. OCP of the single layered coated specimen was found more than 30 mV positive than the uncoated specimen while OCP of the double layered coated specimen increased 75 mV positive −1.515 V than OCP measured for the uncoated specimen (−1.59 V). OCP measurement suggests the presence of the alumina coating over Mg surfaces shift their equilibrium potential in noble direction.

Table 2 Different electrochemical parameters derived by Tafel extrapolation
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3.3.2 Tafel plots

Tafel plots were obtained by carrying partial potentiodynamic polarization in the potential range of ±150 mV from the OCP. Tafel plots as given in Fig. 4, were recorded for all the thee types of studied specimen (Mg alone, Mg single layered alumina coated and double layered alumina coated) in 3.5 % NaCl solution. Different parameters like corrosion current density (Icorr), cathodic (βc) and anodic (βa) Tafel constants obtained from the Tafel plots, are summarized in Table 2. Icorr was derived after extrapolation of anodic and cathodic polarization curves. Icorr is overall current flow as a result of onset of the anodic (oxidation) and cathodic (reduction) reactions. According to measured results as presented in Table 2, Icorr of Mg metal (uncoated) occurs more than two times higher than single layered coated specimen while Icorr of the double layered coated specimen becomes one order of magnitude lower than Mg metal. Measurement of comparatively a higher values of βa and βc of the coated specimens as compared to uncoated specimen suggests the developed alumina coating control both, anodic and cathodic reactions. Occurrence of an order of magnitude lower Icorr for double layered alumina coated specimen than uncoated specimen indicates that double layered coating impart better corrosion protection.

Fig. 4
figure 4

Tafel plots for Mg alone and single and double layered alumina coated Mg surface in 3.5 % NaCl solution

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3.3.3 Electrochemical impedance spectroscopy (EIS)

Electrochemical impedance spectroscopy measurements were performed for all the three types of studied specimens at their OCP. The observed EIS spectra in their Nyquist plot form, are shown in Fig. 5. From the EIS diagrams, it can be seen that all the specimens show the occurrence of a depressed semi circles with presence of capacitive loop. The radius of the capacitive semicircle increase with the increase of coated layer. It occurs minimum for Mg metal while maximum for double layered coated surface. Another interesting point to be noted here that Mg metal and single layered alumina coated specimen show the existence of inductive loop (just above the real axis) in the lower frequency region. Though, the occurrence of inductive behavior is much more pronounced in Mg metal. In case of double layered alumina coated specimen, inductive behavior is insignificant. The occurrence of such behavior in the Nyquist diagram is the consequence of adsorption of anions [44]. It may therefore be inferred that the adsorption of Cl− ions occurs with very fast rate at the Mg metal as compared to alumina coated surfaces.

Fig. 5
figure 5

EIS (Nyquist plots) Mg alone and single and double layered alumina coated Mg metal in 3.5 % NaCl solution

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Different parameters like charge transfer resistance (Rct), electrical double layer capacitance (Cdl) and solution resistance (Rs) derived by curve fitting method, are given in Table 2. Because of the occurrence of single semi circle in the Nyquist plots, an equivalent circuit consisting of resistor connected in series to a parallely connected resistor and capacitor has been used for data analysis. Measurement of more than two times higher Rct values of double layered alumina coated Mg (4,250 Ωcm2) than Mg metal alone (2,175 Ωcm2) indicates the effectiveness of alumina in increasing of corrosion resistances of magnesium. A higher Rct ultimately responsible for the reduction in diffusion process through the coating/metal interfaces. Single layered alumina coated Mg has also shown a significant increase (~25 %) in their Rct values as compared to Mg metal. A fast rate of diffusion of oxidized Mg2+ ions at the metal oxide/solution interface is the reason for the occurrence of a lowest Rct value for the Mg metal. Measurement of a higher value of Cdl for the Mg metal as compared to coated specimens is due to an increase of dielectric constant. Penetration of the electrolyte through cracks and pores increase the dielectric constant of the interfaces [45]. This suggest that presence of porous oxide layer at the Mg surfaces increases diffusion process. The measured solution resistance as given in Table 2, also in accordance with charge transfer resistance measurement. A comparatively higher value of Rs for double layered alumina coated specimen as compared to Mg alone indicate that former one posses a better resistant surfaces. Present EIS results are in good agreement with the corrosion rate measured by the Tafel plots.

4 Conclusions

  1. 1.

    Sol–gel based alumina coatings developed at the Mg surface reduces their corrosion rate an order of magnitude lower in 3.5 % NaCl solution.

  2. 2.

    EDX, X-ray mapping and FTIR analysis evidence the presence of Al2O3 layer at the coated Mg surface

  3. 3.

    Microstructural studies of the coated surface suggests the presence of cracks free amorphous alumina layer of around 2–3 µm in thickness.

  4. 4.

    EIS analysis indicates alumina coated layer acts as barrier layer between metal/solution interfaces that control the diffusion phenomena up to maximum level.