Preparation of corrosion-resistant surface of magnesium alloy and its performance study

The susceptibility of magnesium alloys to corrosion has hindered the wide application of magnesium alloys. A natural green method was proposed in this study to prepare a corrosion-resistant film layer whose main substance is MgO on the magnesium alloy substrate by a two-step hydrothermal method combining water steaming and annealing treatment. The morphology, composition, structure and corrosion resistance of the coating were investigated by SEM, FTIR, XRD and electrochemical methods. The results showed that the prepared coating has excellent corrosion resistance, reducing corrosion current by about four orders of magnitude compared with the bare Mg alloy substrate, and the corrosion potential and corrosion rate are also increased in different degree.


Introduction
Known as a green material for 21st century, Magnesium and its alloys have many advantages such as light weight, low density and high strength [1] and so are widely used in electronics, automotive, aerospace and other fields [2][3][4].However, the poor corrosion resistance of magnesium alloys has largely limited their mass application in industry [5].In this context, many various surface treatment methods have been developed to improve the corrosion resistance of magnesium alloys [6,7], such as micro arc oxidation [8,9], magnetron sputtering [10,11], chemical coating treatment [12] and so on.However, in addition to expensive expenses [13,14] and cumbersome fabrication process [15], these methods also create some problems such as the need for special equipment [16,17] and the contaminated waste liquid [18].Therefore, it is of great practical significance to find a method to prepare a green anticorrosive protective film for magnesium alloy surface to improve the corrosion resistance of magnesium alloys while reducing the environmental impact.Heated to a certain pressure and temperature in the reactor, water molecules will gain some activity, which can react with some active metals.Through the chemical reactions, a hydroxide film layer can be directly formed on the alloy surface [19].The film layer can prevent corrosion, avoiding the corrosion of the metal substrate by external media to a certain extent.Therefore, many researchers have used water as one of the raw materials for corrosion-resistant treatment of magnesium alloys.Tacikowski et al [20] firstly coated TiN-Ti-Al composite titanium nitride layer on the surface of magnesium alloy by PVD treatment, and then cured the composite titanium nitride layer by steam treatment, and successfully prepared a corrosion and friction resistant protective film layer on the surface of magnesium alloy.The film prepared by this method has good corrosion and wear resistance, and steam curing also improves the corrosion resistance of composite coatings compared to hydrothermal curing.Huang et al [21] prepared a layer of ceramic coating containing CeO 2 on Al-Si-Mg alloy using curing with the aid of steam.The samples prepared with the best experimental parameters were tested by immersion, and the coating still had excellent protection after 360 h of immersion.Qiu et al [22] successfully prepared a corrosion-resistant film layer on the surface of magnesium alloy samples by immersing the steam-treated samples in a mixed solution of silane and cerium nitrate.The silane can react with the steam coating to improve the performance of the composite coating, while also possessing a certain self-healing function.The impedance modulus of the sample is improved by three orders of magnitude compared to the magnesium alloy substrate.However, in the above methods, in addition to using steam as supplementary treatment, other treatments still use expensive equipment or chemicals that have an impact on the environment.
Heat treatment is one of the common treatments for industrial metals.Different heat treatment temperatures can have different effects on the samples, and proper heat treatment can improve the mechanical properties and corrosion resistance of magnesium alloys.So far many scholars have studied the heat treatment of magnesium alloys [23,24].Solid solution treatment and aging treatment are two popular heat treatment methods.Elambharathi et al [25] studied the mechanical properties and corrosion resistance of the AZ series Mg alloy under two heat treatments.The results showed that grain refinement of the Mg alloy occurred after the heat treatment, and the β-phase of the Mg alloy was dispersed in the α-phase and the corrosion resistance was improved.Xie et al [26] carried out heat treatment on the cast magnesium alloy at 400 °C, 450 °C and 500 °C, respectively.It was found that the surface coating of the prepared samples became thinner and more uneven as the temperature increased, and the decreased density led to a degradation of corrosion resistance, proving that the high temperature could reduce the corrosion resistance of the magnesium alloy instead.Song et al [27] investigated the effect of heat treatment duration on the corrosion resistance of the samples at 160 °C.The results showed that the corrosion resistance of the samples was reduced during the first 45 h of heat treatment.In the first 45 h of heat treatment, the corrosion resistance of the sample increased with time.However, when the heat treatment exceeded 45 h, the corrosion resistance of the samples decreased.This is mainly because in the first 45 h the increasing volume fraction of β-phase could act as a barrier, which in turn improved the corrosion resistance of the substrate.However, when the time exceeded 45 h, the precipitation of the β-phase magnesium alloy caused the decline of Al element in the α-phase magnesium alloy, ultimately accelerating the corrosion rate due to due to the decrease of Al content.Therefore, suitable temperature and experiment length during heat treatment can facilitate the grain refinement in magnesium alloy, thus improving the corrosion resistance of magnesium alloy.
In this study, a two-step method combining steaming and heat treatment was employed to develop a corrosion-resistant film layer on the surface of magnesium alloy.The experiment uses only deionized water for vaporization and no other chemical reagents are used subsequently.This method does not require expensive equipment and has low preparation costs, making it economical, pollution-free, and efficient.

Experimental details
The surface of AZ91D magnesium alloy substrate was first polished using SiC sandpapers with grit size of 320, 600, 1000, 1500 and 2000 mesh, respectively.The polished magnesium alloy substrate was rinsed with deionized water for 1 min to wash off the surface residue.Then the substrate was cleaned by ultrasonic in anhydrous ethanol and acetone for 5 min, and dried after the cleaning.
The polished magnesium alloy substrate was transferred to the PTFE liner.20 ml of deionized water was added to the liner as the steam source, and the magnesium alloy was racked in the middle of the liner, about 5 cm away from the deionized water below.Then the liner was placed in a stainless steel reactor and heated to 150 °Cin an electric thermostatic oven and then insulated for 5 h.After that the reactor was removed.When the reactor was free cooling to room temperature, the sample was removed from the liner.The steam-treated sample was placed in a crucible and put together in a Muffle furnace while the holding temperature was set at 345 °C, 355 °C, 365 °C and 375 °C for 90 min, respectively.When the insulation was completed, let the crucible cool with the furnace.After the temperature dropped to room temperature, the magnesium alloy sample was removed from the crucible.The experimental procedure is shown in figure 1.

Sample characterization
The surface morphology of the samples was characterized using a field emission scanning electron microscope (SEM, KYKY-EM6900, KYKY, Beijing, China).The crystal microstructure of the samples was analyzed by x-ray diffractometry (XRD, D8 Advance, Bruker AXS, Karlsruhe, Germany).The chemical composition and elemental valence of the samples were measured and analyzed by x-ray photoelectron spectroscopy (XPS, Escalab 250Xi,Thermo scientific , Waltham ,America ).The chemical composition of the samples was measured by Fourier transform infrared spectroscopy (FTIR, Nicolet 460, Thermo Fisher, Waltham, America).All electrochemical corrosion tests on the samples were performed using a CS2350H electrochemical workstation (WuHan corrtest instruments Corp, Ltd, WuHan, China) with a conventional three-electrode measurement system, with a platinum auxiliary electrode and an Ag/AgCl (saturated KCl) electrode as the reference electrode, and a working area of the test sample of 1 cm 2 was used as the working electrode.Before the electrochemical test, the test sample was subjected to an open-circuit potential test for 30 min to reduce the effect of surface instability on the electrochemical test.The test was performed at room temperature, and the parameters for the polarization curve test were set as follows: scanning speed: 1 mv s −1 , the test potential: −0.5v ∼ 1.5v.The parameters for the impedance spectrum test were like this: testing range: 1 × 10-2 Hz ∼ 1 × 105 Hz, AC amplitude: 5 mV.The test results were fitted with the equivalent circuit model and analyzed by the software.

Surface morphology
Figure 2 shows the surface microscopy of samples at 345 °C, 355 °C, 365 °C and 375 °C at 200x and 1000x magnification.It can be seen that after different annealing treatments, the sample surface coatings have different degrees of cracks, which could be caused by the evaporation of water molecules between the layers after the coatings were heated and annealed.In figures 2(a)-(b), it can be seen that the surface of the coating is distributed with lamellar microstructure agglomerates, but the unagglomerated lamellar microstructure is larger, which could be the magnesium oxide with no change in shape after dehydration of flake magnesium hydroxide.In figures 2(c)-(d), it can be seen that with the increase of temperature, the lamellar microstructure on the surface becomes smaller and thinner, which makes the coating surface relatively flat.As the water molecules in the coating evaporate further, the cracks are deeper and larger, making it easier for corrosive media to enter the cracks.In figures 2(e)-(f), it can be seen that the coverage of the thinner lamellar structure is further expanded while larger lumpy microstructures appear.The cracks on the coating surface thus become narrower and shallower, which to a certain extent improves the protection effect.In figures 2(g)-(h), it can be seen as small  lumpy microstructures reappeared on the surface, the coating surface becomes relatively uneven again, making the contact area between the corrosive medium and the surface of the coating larger, causing a decrease in corrosion resistance.

Surface composition analysis
Fourier Transform Infrared ( FT-IR) Spectrometry were performed on the surface of the magnesium alloy sample prepared at the heating temperature of 365 °C.From figure 3, it can be seen that the stretching vibration peak of -OH exists at the wave peak 3697 cm −1 , which corresponds to the -OH hydroxyl group of water adsorbed between the coating layers on the sample surface.The bending vibration peak of H-O-H from H 2 O exists at the wave peak 1640 cm −1 , which indicates that water molecules do not completely evaporate from the sample after the secondary heating by the Muffle furnace.At the wave peak 1393 cm −1 exists the asymmetric stretching vibration peak from the C-O bond in -CO 3 2− , which, combined with the XRD results, can be considered as the carbonate ion between the Mg-Al LDH layers.
Figure 5 shows the XRD pattern of the sample surface, and at 2 q diffraction peaks appear at 32.3°, 34.5°, 36.7°,47.9°, 63.3°, 68.9°, 72.7°and 81.8°, and the analysis reveals that the crystallographic indices corresponding to Mg are (100), (002), ( 101), ( 102), ( 103), ( 112), ( 004) and (104).It is known at 2 q diffraction peaks appear at 42.8°, 62.2°and 78.6°, and the crystallographic indices corresponding to MgO are found to be (200), ( 220) and (222).At 2 q it is 58.8°, but the intensity of the diffraction peak is weak.Based on the analysis it is considered to be the structural peak of Mg-Al LDH, which belongs to the carbonate anion of the Mg 1-x Al x (OH) 2 (CO 3 )x/2-nH 2 O (Mg-Al LDH) intercalation [23].The XRD results show that after the two-step treatment, some Mg elements on the alloy surface react with H, O and C elements to form MgO and a small amount of Mg-Al LDH.
The XPS analysis of the matrix and the sample was performed and the full spectrum of the XPS analysis of both is shown in figure 6.The presence of Mg, Al, O, C and N elements can be seen on the surface of the sample.
Figure 7 shows the XPS fine spectra of AZ91D magnesium alloy substrate and the prepared samples.As shown in figure 7  Based on the analysis of the results of FT-IR, XRD and XPS tests, it can be assumed that the main elements of the coating are MgO, Mg-Al LDH and some impurities containing aluminum elements.

Coating formation mechanism
The formation mechanism of the coating is briefly discussed.First of all, during the first step of treatment, the reactor is heated to 150 °C.Under such high temperature and pressure water molecules become active and decomposes into hydrogen and hydroxide ions in large quantities.The same phenomenon happens to the magnesium alloy substrate, which also becomes active in this environment and produces a large amount of  Mg 2+ .The Mg 2+ and -OH then undergo a simple chemical reaction to deposit a layer of Mg(OH) 2 coating on the surface of the substrate, accompanied by the conversion of a small amount of Mg(OH) 2 into Mg-Al LDH [28].In the subsequent heat treatment, magnesium hydroxide gradually decomposes into magnesium oxide and water at 350 °C, but before reaching the complete decomposition temperature of Mg-Al LDH, LDH only further loses moisture and generates CO 2 at the same time [29].When the annealing temperature is 365 °C, the surface of the coating is relatively flat and the cracks on the surface of the coating are narrower and shallower, which effectively hinders the direct attack of the corrosive medium on the substrate and produces certain corrosion resistance.The following is the chemical reaction equation that occurs when the coating is formed

EIS test
In order to study the corrosion resistance of the samples prepared by the two-step method, electrochemical polarization curve tests are performed on the AZ91D magnesium alloy substrate and the prepared samples.The relationship between the corrosion resistance and heat treatment temperature of the samples is experimented and tested using 3.5 wt% NaCl aqueous solution as the test electrolyte to investigate the pattern between the two and to determine the optimal parameters.The corrosion polarization curves of the samples are shown in figure 8. Curve A is the polarization curve of the untreated magnesium alloy substrate in 3.5 wt% NaCl aqueous solution, curves B, C, D and E are the polarization curves of the samples in 3.5 wt% NaCl aqueous solution after steam treatment with heating temperatures of 345 °C, 355 °C, 365 °C and 375 °C, respectively.The related electrochemical test results are shown in table 1.In general, when the tested samples have higher self-corrosion potential E corr and lower selfcorrosion current I corr , the tested samples have slower corrosion rate and better corrosion resistance.It can be seen that the E corr of AZ91D magnesium alloy substrate is −1.5443V, the I corr is 5.6056 × 10 −5 A cm −2 and the corrosion rate is 1.1888 mm a −1 .Compared to the substrate, there has been a change towards the positive potential for the E corr of the prepared sample changes after the two-step treatment, which means that the corrosion resistance of the sample has been further improved.As the heat treatment temperature increases, the I corr of the sample increases, but decreases when the heat treatment temperature is 375 °C compared to that at 365 °C.In curves B, C, E and F, a sudden change in current density can be seen at different potentials, which may be related to the pitting of the coating surface, indicating that pitting occurs when the aqueous NaCl solution penetrates into the cracks on the coating surface and contacts the substrate.In contrast, the curve D curve is relatively flat, indicating no occurrence of pitting, and although E corr is not the highest, I corr has a significant increase compared to the other heating temperatures.It shows that after the heat treatment at 365 °C, the corrosion resistant film on the surface of the substrate shows better protection and stability in the aqueous solution containing Cl-ions.That is why 365 °C is chosen as the best treatment temperature, at which the E corr of the sample is 0.022398V, the I corr is 1.4288 × 10 −8 A cm −2 , and the corrosion rate is 3.0302 × 10 −4 mm a −1 .According to the electrochemical tests and the results of previous studies [24,25], it can be concluded that the corrosion resistance of the magnesium alloy has been improved with the increase of temperature in the range of 345 °C to 365 °C.This is because the microstructures of the magnesium alloy is changed after the heat treatment.The β-phase dominates which is beneficial to the improvement of the corrosion resistance of the substrate.However, when the temperature increases to 375 °C, the electron microscope images show the surface coating is less denser and uniform, similar to the experimental result of Xie et al [24], which may be the main reason for the deterioration of the corrosion resistance of the samples.Using 3.5 wt% NaCl aqueous solution as the test electrolyte, the Nyquist of AZ91D magnesium alloy substrate and the Nyquist of the sample after two-step treatment are noted as curve a and b in figure 9, respectively.In general, the larger the radius of the capacitive arc of the Nyquist plot of the test sample, the lower the corrosion rate of the test sample.Meanwhile the higher the resistance of the test sample, the stronger the ability to hinder the exchange of electrons.For metal surfaces, when the electron exchange capacity is lower, it indicates that the corrosion rate of the sample is lower, that is, the better the corrosion resistance.From figure 9, it can be seen that a loop is shown in the plot, which means that the 3.5 wt% solution is in direct contact with the surface of the magnesium alloy substrate when the test is carried out and corrosion occurs immediately   afterwards.And in figure 9(b), the curve of the magnesium alloy sample prepared by the two-step method under the best conditions shows a trend of constant increasing and no decreasing, which means that the radius of the sample's capacitive arc resistance is obviously much larger than the radius of the AZ91D magnesium alloy substrate.It indicates that the coating is well isolated from the 3.5 wt% NaCl solution during the test phase and the substrate is not eroded.It means that the corrosion resistance of the magnesium alloy samples prepared by the two-step method under optimal conditions is better than that of the AZ91D magnesium alloy substrate.This is consistent to the polarization curve results.With the extension of the test time, the curve does not show a decreasing trend, indicating that the coating performance is very stable.In order to further investigate the corrosion resistance of the samples prepared by the two-step method, electrochemical impedance spectroscopy tests were performed on the AZ91D magnesium alloy substrate and the samples prepared by the two-step method under the heating temperature of 365 °C.The curves A and B in figure 10 are the Bode plots of the AZ91D magnesium alloy substrate and the magnesium alloy sample prepared by the two-step method under the condition of 3.5 wt% NaCl aqueous solution as the test electrolyte, respectively.Normally, the higher the impedance modulus in the low frequency part, the better the corrosion resistance.As can be seen in figure 10, the curves of the sample are considerably higher than those of the magnesium alloy substrate throughout the testing phase.In the low frequency part, the modulus of the sample is about four orders of magnitude higher than that of the substrate, which indicates that the corrosion resistance of the sample has been improved after the two-step treatment, which is the same as the results of the polarization curve.As the experiment proceeds, the curve of the sample keeps decreasing slowly from the low frequency to the medium frequency part.This indicates that the sample surface corrosion protection coating is slowly dissolving, while the substrate modulus has increased, because the corrosion products in the low frequency stage is attached to the surface of the substrate, which has played a temporary protective role, but it is still inferior to the sample.At the second half of the high-frequency stage, the sample and the substrate modulus are rapidly decreasing, indicating that the dissolution rate of the sample coating accelerates, while the coating has lost its protective role for substrate surface which further corrodes.The equivalent circuit diagram is shown in figure 11.
The equivalent circuit model for the electrochemical impedance spectra of the Mg alloy substrate and the prepared sample is shown in figure 11, where RS corresponds to the solution resistance, R1 and Q1 correspond to the interlayer resistance and capacitance, respectively.L and R correspond to the inductance and the  resistance of the inductor, respectively.Figure 11(b) shows the equivalent circuit of the coating, indicating that the electrolyte solution is uniformly penetrated into the coating.R2 and Q2 correspond to the coating resistance and capacitance.As time increases, the membrane resistance and charge transfer resistance gradually decrease while the corrosion rate increases.

Conclusion
In this paper, a corrosion-resistant film layer was prepared on AZ91D magnesium alloy substrate by a costeffective and non-polluting two-step preparation method.The relationship between the heating temperature and corrosion resistance of magnesium alloys by heat treatment was investigated.Compared with the substrate, the prepared sample after the second heat treatment at 365 °C showed the best corrosion resistance, with the corrosion potential decreasing −1.5443 V to 0.022398 V, while the corrosion current and corrosion rate also decreased significantly, indicating that the prepared coating has better corrosion resistance and stability.The coating achieved good corrosion resistance by using only deionized water as the only reagent, and a simple annealing treatment as a post-treatment.The method does not require any chemicals and expensive experimental equipment, causing no pollution to the environment.The prepared coating can extend the service life of magnesium alloy to a certain extent, which has great advantages in practical applications.
(a), a characteristic peak of Mg monomer appears at 1303.0 eV, indicating the presence of monomeric Mg on the surface of the magnesium alloy matrix.In figure 7(b), a peak appears at 1303.9 eV, which should be the characteristic peak of Mg 2+ .Combined with the XRD test results, it can be known as the characteristic peak of Mg element in MgO.In figure 7(c), a characteristic peak of C-O appears at 531.6 eV; in figure 7(d), another peak appears at 531.7 eV, attributing to the characteristic peak of Mg-O in MgO.The peak appears at 529.7 eV, corresponding to the characteristic peak of C-O, which is the carbonate ion in the sample

Figure 3 .
Figure 3. FT-IR test plot of the sample.

Figure 4 .
Figure 4. XRD test chart of magnesium alloy matrix.

Figure 5 .
Figure 5. XRD test chart of the sample.

Figure 8 .
Figure 8. Electrochemical polarization curves of mg alloy substrate and the prepared sample in 3.5 wt% NaCl aqueous solution.

Figure 9 .
Figure 9. Nyquist plots of substrate and sample.

Figure 10 .
Figure 10.Bode diagram of the sample of the substrate.

Figure 11 .
Figure 11.Equivalent circuit diagram (a) the Mg alloy substrate (b) the prepared sample.

Table 1 .
Electrochemical test results under different heating temperature conditions.Corrosion current (A mm −2 ) Corrosion rate (mm a −1 )