Microstructure and corrosion behaviors of AZ31 alloy with an amorphous-crystallin nano-composite film

Magnesium (Mg) alloy has drawn considerable attention for lightweight structural and functional materials, whereas its corrosion resistance still requires to be enhanced. A new strategy for corrosion resistance has been proposed as making an amorphous-crystalline nano-composite film on Mg alloys. The film as the composition as Al2O3/GaN with a thickness of 20 nm was prepared on AZ31 Mg alloy by atomic layer deposition. Grazing incidence x-ray diffraction, scanning electron microscopy equipped with energy-dispersive spectroscopy, transmission electron microscopy, x-ray photoelectron spectroscopy, and nano indentation tester have been used to characterize the film in details. It is verified the sample has an amorphous/crystalline/Mg interface structure, and a surface with homogeneous elemental distribution and higher hardness. Neutral salt spray test shows the film changes the corroded mode from pitting corrosion to uniform corrosion. Furthermore, electrochemical measurements indicate that the film would raise Ecorr (ΔEcorr = +0.295 V), drop icorr (about 1/10 times), and make electrical equivalent circuits change from Rs (CPE Rct (RL L)) to Rs (CRf) (CPE Rct (RL L)). All evaluations show that better corrosion resistance has been by inducing the amorphous-crystalline nano film. The amorphous layer in the film would make a more homogeneous Cl− distribution in the surface and act as a barrier to block the penetration of corrosion medium in the early stage. During corrosion, the interface between the layers in the film could retard the corrosion crack propagating further. The film would be favorate to form a denser corrosion product layer finally. A more uniform and lower corrosion occurs for AZ31 Mg alloy with this nano-composite film.


Introduction
Magnesium (Mg) alloys exhibit lots features such as low density, high specific strength and biodegradability. It has been attracting attentions in various potential applications for lightweight equipment, electronics industries and biomedical fields [1,2]. Nevertheless, the poor corrosion resistance due to the low electrode potential and non-compact natural oxidation passive film [3,4], is still the major drawback [5,6]. Surface modification is a main strategy to protect Mg alloys from corrosion [7][8][9]. Generally, the surface modifications can provide a barrier to avoid Mg alloy contacting with the corrosive medium. It has been reported that the composite film would exhibit better protective capability than just mono film because it can develop synergetic effect. For instances, hydroxyapatite/bio-glass composite coating (about 1 μm) on Ti-6Al-4V alloy, has better bio-activity and higher adhesive strength than mono hydroxyapatite coating [10]. The micro-arc oxidation layer sealed with poly composite layer (about 70 μm) on Mg alloy, would has significantly improvement in corrosion resistance and less stress corrosion cracking, compared with mono micro-arc oxidation coating [7]. The Al 2 O 3 -CeO 2 composite coating (about 29 μm) on 7075 aluminum alloy, has the about 1/10 3 i corr as that of mono Al 2 O 3 film [11]. Would the composite film in nanometer still have a significant protective effect for Mg alloy? Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Herein, an amorphous-crystallin nano-composite film are fabricated by atomic layer deposition (ALD, it can deposit continuous ultra-thin films which are conformal and pinhole-free on any kind matrix owing to the sequential, self-limiting and self-saturating surface chemical reacting process) [12][13][14][15]. The amorphous layer is designed as Al 2 O 3 , which has been applied for Mg alloy with satisfied results [16,17]. The composition of crystalline layer is GaN, which is a common protective film for metals and semiconductor [18]. This amorphous-crystallin nano-composite film on Mg alloy is firstly characterized carefully in morphology, composition, thickness, ion valence, interface structure, and then mechanical properties are investigated. Finally, the corrosion behaviour is studied by neutral salt spray test and electrochemical measurement. The corrosion mechanism for Mg alloy with this nano-composite film is deduced too.
2. Experiment 2.1. Material preparation AZ31 Mg alloy sheets (3.04 wt.% Al, 0.94 wt.% Zn, 0.44 wt.% Mn and balanced Mg) in a size of 25×25 mm 2 were applied as the substrates. Before film deposition, the AZ31 Mg alloy substrates were grounded mechanically with SiC sandpapers from P200 progressively up to P1200 and polished by diamond with 0.05 μm. Then the AZ31 Mg alloy were ultrasonically cleaned with acetone and arided by N 2 atmosphere. The bare substrates were named as SB.
The substrates were loaded into the ALD reactor, heating via a resistive heater under N 2 ambient (10 mbar). The heating temperature was measured by a thermocouple near the wafer and controlled by a PID controller. The deposition was conducted in a flow reactor at a pressure about 10 −2 mbar. GaN film was deposited by 100 cycles at 400°C (each cycle was consisted of Ga (CH 3 ) 3 (0.02 s)→N 2 purge (60 s)→NH 3 (0.02s)→N 2 purge (60 s), 1 Å/cycle). Directly after GaN film deposition, Al 2 O 3 film was prepared by another 100 cycles at 250°C (each cycle contains Al (CH 3 ) 3 (0.02 s)→N 2 purge (60 s)→H 2 O (0.02 s)→N 2 purge (60 s), 1 Å/cycle). The thickness of film can be precisely regulated by controlling the ALD deposition cycles. After ALD-deposited GaN/Al 2 O 3 film, the AZ31 Mg alloy substrates were named as SF.

Microstructure characterization
The phase constitution was analyzed by Grazing incidence x-ray diffraction with the Cu-Kα radiation (GIXRD, PANalytical X'Pert PRO, incidence beam angle is 1°at 40 kV and 40 mA). Film thickness was figured out via x-ray reflectivity (XRR, PANalytical). The surface and cross-section morphology were observed using scanning electron microscopy (SEM, FEI Quanta 200F). The analysis of film interface microstructures and compositions was conducted via transmission electron microscopy (TEM, JEOL, JEM-2100) working at 200 kV and equipped with energy-dispersive x-ray (EDS, Oxford X-man) microanalysis hardware. The coated side of two SF samples were adhered together and placed in the center of 3 mm diameter copper ring for TEM observation. Before TEM observation, the samples were mechanically thinned to 30 μm firstly, and then electropolished in 10 wt.% perchloric acid at −5°C with the voltage of 20 V. Finally, the TEM observational requirement was met using ion milling. The chemical bonds were confirmed using a x-ray photoelectron spectroscopy analysis (XPS, ESCALAB25OXI, monochromatic Al Kα excitation source 1486.6 eV) along with in situ ion etching. The adventitious carbon (C1s) core level peak at 284.6 eV was applied to calibrate the binding energies (BEs) of samples.

Mechanical properties
The mechanical properties were determined by a nano indentation tester (Fischerscope, HM2000) at a constant loading rate of 10 mN s −1 with the range of 0∼100 mN.

Corrosion behavior 2.4.1. Neutral salt spray test
Neutral salt spray test was used to determine the long-term corrosion behavior of samples and the test followed ASTM B-117. After corrosion, the samples were ultrasonically cleaned with distilled water for 10 min. Then the samples were arided by cooling N 2 gas for further experiments. The corroded samples were then studied by SEM, EDS and GIXRD.

Electrochemical measurements
The corrosion behavior of the samples was studied by electrochemical workstation (CHI650D). Electrochemical measurements were carried out in 3.5 wt.% sodium chloride (NaCl) aqueous solution by applying a conventional three-electrode method, which the sample (exposing 0.5 cm 2 ), saturated calomel and platinum plate was the working, reference and counter electrodes, respectively. The scan rate of potentiodynamic polarization (PDP) curves is 1 mV s −1 from approximately −350 mV to 450 mV. The frequency range of electrochemical impedance spectra (EIS) measurements is 10 −2 ∼10 5 Hz with 10 mV the sinusoidal voltage signal amplitude. To resolve EIS plots, the Zview software (Solartron Analytical, UK) was applied to get appropriate equivalent circuit (EC) models (σ<10% and χ 2 <0.01 for the individual parameters).

Surface characterization
The SEM images and EDS mapping of SB and SF are shown in figure 1. According to figure 1(a), there are still some grinding scratches on the SB surface clearly, while SF also exhibits same surface morphology with SB ( figure 1(b)). From the inset of figure 1(b), the surface is flat and free of voids and porosity after coating, whereas the grinding scratches still can be found. Table 1 lists the chemical compositions of red rectangle marked in figure 1. It shows that the Al, O, Ga and N contents increases apparently while the Mg and Zn elements contents decreases after ALD-deposited GaN/Al 2 O 3 nano-composite film. This means that the composite film has deposited on Mg alloy successfully. Besides, the corresponding EDS mapping indicates that the Al, O, Ga and N elements distributes over the surface uniformly. So, the results show that the surface of AZ31 Mg alloy is covered evenly by the GaN/Al 2 O 3 nano-composite film. Figure 2 shows the GIXRD patterns of SB and SF. As shown in figure 2(a), SB exhibits the obvious characteristic peaks of α-Mg phase which are ascribed to AZ31 Mg alloy matrix. After ALD-deposited GaN/Al 2 O 3 film, the relative intensities of planes as (100), (002), (101) increase due to the deposition of crystallized GaN layer, and bulging near the planes as (321), (441) appear attributing to amorphous Al 2 O 3 layer. Film thickness (D) is evaluated by XRR ( figure 2(b)) and the film thickness can be calculated out by the following

( )
In which λ is x-ray wavelength, θ c is the critical angle of total reflection, θ m,m+1 is constructive interference diffraction peaks of incident angle. GaN/Al 2 O 3 nano-composite film thickness can be calculated out as 20 nm. Figure 3 shows TEM image and EDS results for the cross-section of SF. TEM image reveals that the GaN/Al 2 O 3 composite film is well bonded on the surface without any peeling or voids. Mg, Al, O, Ga and N elements is observed distinctly by EDS mapping in figure 3(b), indicating that the composition of nanocomposite film is same as the designed one.
XPS with ion etching is performed to determine the composition and structure of the nano-composite film interface (figures 4(a)-(c)).
The XPS survey spectrum of SF consists of several BE (binding energy) peaks belongs to electronic states as Mg2p, O1s, N1s, Al2p, and Ga3d (49.8 eV, 532.0 eV, 397.3 eV, 73.9 eV, 18.1 eV, figure 4(a)). High resolution XPS spectra of N 1s has a typical peak located at 397.3 eV which is origin from the N-Ga bonding [21][22][23][24]. For Ga3d spectrum, there are two peaks located at 19.9 eV and 18.1 eV, the former Ga3d 5/2 peak ascribed to GaN [24], the latter Ga3d 5/2 peak ascribes to metal Ga [25]. O1s spectrum has 534.8 eV and 531.7 eV, which ascribe O1s to adsorbed O 2 and Al 2 O 3 , respectively [26,27]. Al 2p spectrum has a peak at 73.9 eV related to Al2p 3/2 in Al 2 O 3 [28][29][30]. After ion etching 12 nm ( figure 4(b), Al 2 O 3 layer had been stripped and GaN layer exposure), the peaks assigned to O1s, Al2p 3/2 in Al 2 O 3 decrease, the peaks of N1s, Ga 3d 3/2 in GaN increase, and the peaks of O1s in O 2 and Al2p 3/2 in metal Al [27] increase, comparing with SF result (figure 4(a)). After ion etching 25 nm   From the above characterizations, it is self-evident that a GaN/Al 2 O 3 nano-composite film was deposited on the surface of AZ31 Mg alloy successfully by ALD. The film, consisting of amorphous Al 2 O 3 upper-layer and crystallin GaN sub-layer, covers the surface with excellent conformity and uniformity, and attaches well to the Mg alloy substrate.

Mechanical properties
The nanohardness, Young's modulus and stiffness of the SB and SF is studied by Nano indentation tester. Figure 5(a) shows the typical load-depth curves of samples after instrumented indentation testing. The curve of SF is fluent without any zigzag fluctuation and disconnection, which indicates that GaN/Al 2 O 3 nano-composite film is smooth and no-cracking during the loading process. The results of nano indentation tester ( figure 5(b)) show the GaN/Al 2 O 3 nano-composite film improves the nanohardness and Young's modulus whereas the stiffness decreases [31,32].

Corrosion behaviour 3.3.1. Neutral salt spray test
To evaluate the long-term corrosion resistance of the GaN/Al 2 O 3 film, the neutral salt spray test is carried out. Figure 6 shows the surface SEM images after corrosion. Figure 6(a) exhibits that the SB suffers from severe pitting corrosion and there are obvious pits and cracks distribute on the surface. According to previous study [1], the cracks mainly results from the volumetric expansion by the accumulation of corrosion products on the  surface of matrix. As for SF, it exhibits different surface corrosion morphology from SB, having less pits and cracks ( figure 6(b)). So, the corrosion of matrix can become milder and uniform by ALD-deposited GaN/Al 2 O 3 film. Figure 7 exhibits the cross-sectional SEM images and EDS mapping of SB and SF after salt spray test. As can be seen in figure 7(a), SB exhibits irregular and thick corroded layer that made of oxides and hydroxides (as shown in the elemental mapping images) [33], and the corroded pits of SB shows deep depth with many cracks. After ALD-deposited GaN/Al 2 O 3 film, the sample is subjected to a milder and more uniform corrosion. Only a thin corroded layer forms on the surface and remains unaltered in depth ( figure 7(b)). Thus, the GaN/Al 2 O 3 film can obviously promote the corrosion resistance of AZ31 Mg alloy. Figure 8 is the GIXRD patterns of surface for samples after salt spray test. As it can be seen, the main characteristic peaks of samples are still the peaks of α-Mg while the peaks of corrosion products (MgAl 2 O 4 and Mg (OH) 2 ) can also be detected. However, the peak intensities of the corrosion products decrease after GaN/Al 2 O 3 film covering, which indicating that GaN/Al 2 O 3 film has good corrosion resistance.  These results indicate that the uniform distribution of GaN/Al 2 O 3 nano-composite film leads to uniform corrosion pattern without pitting corrosion. Besides, it can furthermore conduce to promote the corrosion resistance of matrix.

Electrochemical test
PDP curves and corrosion data are shown in figure 9 and table 2. After ALD-deposited GaN/Al 2 O 3 film, the corrosion potential (E corr ) and corrosion current density (i corr ) of SB increases from −1.621 V to −1.326 V and decreases from 6.171×10 -5 A·cm −2 to 1.457×10 -6 A·cm −2 , respectively. These results imply that the tendency of corrosion initiation for AZ31 Mg alloy reduces and the corrosion rate also becomes slower apparently [34]. It can be seen initially that GaN/Al 2 O 3 composite film exhibits good corrosion resistance. All these results show that the condense GaN/Al 2 O 3 film can largely hinder Cl − permeation and improve corrosion resistance [35,36]. In addition, the polarization resistance (R p ) is calculated using simplified Stern-Geary relationship to evaluate the corrosion rate, as shown in follow (equation (2)) [37]:  In which β a is the anode Tafel slope and β c is the cathodic Tafel slope. R p value increases from 6.06×10 5 to 2.18×10 7 Ω·cm 2 . In addition, the corrosion rate can be figured out via the Faraday's law by equation (3) as [38]: In which A is the atomic weight of the metal, ρ is the density, n is the number of electrons exchanging in the dissolution reaction, and F is the Faraday constant (26.801 A·h/mol). The corrosion rates obtained from the method could be ranked in the increasing order: SF (0.03 mm y −1 )<SB (1.41 mm y −1 ). A −1 l −1 l −1 these results presented clearly that the corrosion resistance of SB is improved evidently after ALD-deposited GaN/Al 2 O 3 composite film.
The corrosion behavior of SB and SF was further studied by EIS. As the figure 10(a) shown, it can be found that both samples consist of capacitive and inductive loops, but SB only has one capacitive loop and SF has two capacitive loops. Generally, the capacitive loop can be attributed to the charge transfer reaction and layer effects. The inductive loop can be assigned to the physical adsorption processes and pitting corrosion of Mg matrix [8,39]. Compared with the curves between SB and SF, the diameter of the curves in medium frequency range increases remarkably after coating in figure 11(a), and Bode plots shows the impedance modulus (|Z|) value is higher at low frequency range in figure 10(b), which indicates a better corrosion protection for GaN/Al 2 O 3 film [5,40].
After ALD-deposited GaN/Al 2 O 3 film, it can be seen in figure 10(c) that two well defined time constants arise in the frequency-phase angle diagram, which is coherent to the two capacitive loops in figure 11(a). It also indicates that the GaN/Al 2 O 3 composite film induce another corroded mode to Mg alloy [35]. Table 2. Data for potentiodynamic polarization testing in 3.5 wt.% NaCl aqueous solution.   In addition, to more accurately interpret EIS plots, the equivalent circuits (ECs) was done to figure out the the electrochemical properties and physicochemical process during corrosion. By fitting EIS plots in figure 10(a), ECs and individual parameter values have been obtained, shown in figure 11 and table 3. The solution resistance and the charge transfer resistance are described by the R s and R ct , respectively. The dissolution of AZ31 Mg alloy and corroded production during corrosion is described by R L and inductance (L). R s /R ct and C show the resistance and capacitive reactance, respectively [8]. Besides, the nonideal resistive and capacitive behavior of the samples are described by the constant phase elements (CPE) , which is described by the formula [35,41]: Where Y CPE is CPE constant, n is empirical exponent of CPE. CPE is the pure capacitive when n=1, and CPE is the pure resistance while n=0. The corrosion mode changes from R s (CPE R ct (R L L)) for SB to R s (CR f ) (CPE R ct (R L L)) for SF, in figure 12. New parameters as (CR f ) add to ECs after GaN/Al 2 O 3 composite film coating. R ct value also raises as 118.20 Ω·cm 2 for SB and 174.00 Ω·cm 2 for SF. All these results indicate that the corrosion resistance of matrix can be improved effectively by ALD-deposited GaN/Al 2 O 3 film [42]. From the results in section 3.3, it is obvious that the GaN/Al 2 O 3 composite film make a more uniform and slower corrosion for Mg alloy.

Discussion
AZ31 Mg alloy exhibits severe corrosion, mainly caused by galvanic effect between α-Mg matrix and the noble second phase [33,43]. During corrosion, the Cl − non-homogeneous distribution causing by non-uniform microstructure would accelerate the non-uniformly corrosion [44]. Exposed in chloride-containing media, Cl − would concentrate on inhomogeneity area such as scratches and protruding second phase particles for SB, while the distribution of Cl − would be more uniform due to the covering of GaN/Al 2 O 3 composite film for SF (figures 12(a1) and (b1), based on figure 1). At the beginning of corrosion, the corrosion of matrix would be accelerated as serving as anode while the noble second phase hardly corrodes [43,45]. The main corrosion products are magnesium oxide and hydroxide. Cracks in corrosion product layer may be produced by the cooperative effects of hydrogen pressure and expansion stress due to the formation of magnesium hydride during corrosion [46]. However, GaN/Al 2 O 3 composite film can act as a physical barrier and corrode homogeneously, and the corrosion possess in early stage would be more uniform and slower than SB (figures 12(a2) and (b2), based on figures 9∼11) [47][48][49]. As the time goes on, the corrosion product layer of SB become thicker and the cracks propagate in layer. Furthermore, the noble second phase fall down to form pits when the matrix is dissolved around them for SB ( figure 12(a3), based on figures 6∼7) [45]. As for SF, the corrosion product layer become thicker too bu t the interfaces (in the composite film itself and between film and matrix) would retard the crack propagating in layer (figure 12(b3), based on figures 6∼7) [44,50]. Finally, the aggressive Cl − would assemble in the cracks and pits more serious, which would result further severe pitting corrosion. A fragile corrosion product layer is formed on the surface of SB, which cannot prevent further corrosion. However, a relative tight corrosion product layer exhibit better protect effect for SF (figures 12 (a4) and (b4)).
The amorphous-crystallin composite film can make a more homogeneous Cl − distribution in the surface and act as a barrier to block the penetration of corrosion medium in the early stage. During corrosion, the film can retard the corrosion crack propagating further, and form a denser corrosion product layer finally. A more uniform and lower corrosion occurs for AZ31 Mg alloy with the nano-composite film.

Conclusions
An amorphous-crystallin nano-composite film as GaN/Al 2 O 3 has been deposited on the surface of AZ31 Mg alloy by atomic layer deposition (ALD). The composite film on Mg alloy are characterized firstly (such as morphology, composition, thickness, etc), then corrosion behavior is evaluated by neutral salt spray test and electrochemical measurement. The conclusions are deduced as follows: (1) a compact GaN/Al 2 O 3 nano-composite film could be made on Mg alloy by ALD with a thickness of 20 nm, and the film exhibits uniform coverage and makes the element distribution in surface more homogeneous. This nano-composite film consists of amorphous Al 2 O 3 upper layer and crystallin GaN sublayer. Moreover, it is well attached to the Mg alloy substrate.
(2) This nano-composite film can increase the hardness and Young's modulus of Mg alloy whereas the stiffness decreases.
(3) The salt spray test indicates that this nano-composite film introduces a uniform corrosion mode for AZ31 Mg alloy.
(4) The electrochemical test shows that GaN/Al 2 O 3 nano-composite film improves the corrosion resistance of AZ31 Mg alloy significantly. It makes an increase in corrosion potential from −1.621 V to −1.326 V and a decrease in corrosion current density from 6.171×10 -5 A·cm −2 to 1.457×10 -6 A·cm −2 by PDP test.