Study of the in situ growth mechanism of Mg–Al hydrotalcite conversion film on AZ31 magnesium alloy

Corrosion Science 63 (2012) 148–158

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Study of the in situ growth mechanism of Mg–Al hydrotalcite conversion film on AZ31 magnesium alloy Jun Chen, Yingwei Song ⇑, Dayong Shan, En-Hou Han State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China

a r t i c l e

i n f o

Article history: Received 18 February 2012 Accepted 28 May 2012 Available online 7 June 2012 Keywords: A. Magnesium B. XPS B. XRD B. SEM C. Passive films

a b s t r a c t By means of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), open circuit potential (OCP) measurement and scanning electronic microscope (SEM), the in situ growth mechanism of the Mg–Al hydrotalcite film has been proposed. The composition and morphology of the films undergo a series of variations during the growth processes. The added Al compounds are a vital contribution to the formation of hydrotalcite. The film formation involves the dissolution of the AZ31 substrate, adsorption of the ions from solution, nucleation of the precursor, followed by the dissolution of Al3+, exchanging of OH by CO32 and growth of the hydrotalcite film. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Hydrotalcite-like compounds (HTs), or so-called layered double hydroxides (LDHs), possess a special layered structure, which is similar to that of brucite, Mg(OH)2 [1]. They are generally represented by the formula [M2+1xM3+x(OH)2]x+(An)x/nmH2O, where M2+ and M3+ represent the divalent and trivalent cations, respectively, An is the interlayer anion which is replaceable by other anions, n is the charge of the interlayer anion, x is the M3+/(M2++M3+) mol ratio and m is the number of the associated water molecules [2,3]. Recently, extensive studies [4–16] have been focused on the potential applications of HTs as films to protect the metals. Many different synthesis methods for HTs have been developed. The most common method is co-precipitation. However, this method is time consuming (6–168 h), poorly crystallized, and produces large amounts of wastes [17–21]. The in situ growth technique looks as a promising alternative method due to its simplicity and versatility. Moreover, the adhesion between the film and substrate is much stronger in the case of the in situ growth method than that of the deposition methods because of the presence of chemical bonding [9]. It is greatly advantageous for the applications of the in situ growth method to fabricate protective coatings [11–13,20,22]. The in situ growth process has been widely developed on pure aluminum, Al–based alloys, as well as anodic aluminum oxide substrate to form Li–Al, Mg–Al, Ni–Al or Zn–Al HTs films [20,23,24]. Also, Liu et al. [25] observed that HT films can be grown on the

⇑ Corresponding author. Tel.: +86 24 23915897; fax: +86 24 23894149. E-mail address: [email protected] (Y. Song). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.05.022

Zn-covered stainless steel. The HTs are usually formed in an alkaline solution. So it is easy to in situ grow the HTs films on the Al or Zn-based alloys because aluminum and zinc are active in the alkaline solutions whose pH values exceed 11, which can provide the source of M2+ or M3+ to form the HTs films. However, it is difficult to grow an HT layer on the Mg alloys, as magnesium is passive in alkaline solutions. Nevertheless, a good example of the in situ growth process on Mg alloy comes from the work of Uan et al. [11–13] who fabricated an Mg–Al HT film on AZ91D by immersing the sample in a carbonic acid solution. The Mg substrate is corroded in this solution, which provides the source of Mg2+ and Al3+ for Mg–Al HT. In addition, the hydrogen evolution reaction during the corrosion of Mg substrate results in the original carbonic acid solution reaching an alkaline condition for the growth of the Mg–Al HT film. However, this technology is only available for the Al-rich Mg alloys which can provide enough Al for the formation of the Mg–Al HT. In our previous work, an Mg–Al HT conversion film has been obtained on the low Al concentration magnesium alloy AZ31 after some modification and improvement of the two-step method [22]. Furthermore, the authors have mentioned a preliminary analysis of this film transformation procedure. However, the formation mechanism of the in situ growth in this case is not completely clear. Until now, there are only a few reports on the formation mechanism of the Mg–Al HT [26–31]. However, these opinions are different and have not reached a uniform understanding. The hypotheses have been proposed as follows: (1) The formation of HTs is on the basis of Mg(OH)2. Eliseev et al. [26] believed that magnesium hydroxides precipitate as a layered structure, whereas aluminum hydroxides produce

J. Chen et al. / Corrosion Science 63 (2012) 148–158

amorphous filamentary agglomerates at the first stage. Then the crystallization of the HT occurred, and the aluminum atoms diffused into Mg(OH)2. (2) The formation of HTs is based on the presence of Al(OH)3. Ma et al. [27] pointed out that the region with gibbsite–like structure was formed in the initial stage of hydrolysis. They proposed a ‘‘gibbsite-based substitution-filling model’’ to present the structure of Mg–Al LDHs, in which all the Al3+ cations located at the octahedral sites and surrounded by [Mg(OH)6] octahedrons. Both the above assumptions are in accordance with the result that the co-precipitation of aluminum and magnesium hydroxides occurs without the formation of polynuclear hydroxo complexes but agglomerates containing either aluminum hydroxides or magnesium hydroxides formed in the first stage [28]. Furthermore, there is a little research about the nucleation and growth process of HTs formed on metal by two-step in situ crystallization technique, only Lin et al. [31], who analyzed the M2+/M3+ mol ratios during the different post treatment periods and the changes in the coordinate state between Al3+ and OH groups of the precursor layer upon the HT film, but no nucleation information of the precursor film has been mentioned. However, the questions, such as the status of the Al3+ and Mg2+, and how is the precursor transformed into Mg–Al HT, have not been clearly addressed yet. It is important to clarify the details of the transformation of the precursor into Mg–Al HT. Hence, the aim of the present work is to obtain further information on the formation mechanism of the current two-step in situ growth method, In order to realize this aim, the specific components and microstructures of the films for different film formation periods are illustrated step by step to disclose the initial nucleation status, transformation and growth processes. The understanding of the HT film growth mechanism can help to provide a means of controlling the crystallization and improving the properties of the film.

149

A2 pretreatment solution for 30 min, and then was immersed into the B2 post treatment solution for 20 min. 2.2. Characterization The chemical composition of the films was probed using an ESCALAB 250 X-ray photoelectron spectroscopy (XPS) with Al Ka radiation (1486.6 eV). The power was 150 W, with a pass energy of 50.0 eV and a step size of 0.1 eV. Depth profiling was performed under 2 keV Ar-ion sputtering. The binding energy (BE) of the adventitious C 1s peak (284.6 eV) is commonly used to correct the spectra. However, for the spectra after sputtering, this correction is not appropriate because the adventitious hydrocarbons were sputtered off. Thus, the energy value of the element Mg in Mg 1s spectrum without sputtering was first corrected according to the C 1s signal as 1303.2 eV. Then, the other components were defined relative to the peak of element Mg. The data were analyzed with Xpspeak 4.1 software. The films were scraped from the samples and prepared as the finely pressed powder for the XRD examination. The XRD analysis was carried out using a Philips PW1700 diffractometer with Cu target (k = 0.154 nm) and the patterns were analyzed with MDI Jade 5.0 software. The morphologies of the films were observed using a Philips XL30 environmental scanning electronic microscope (ESEM). The open circuit potential (OCP) measurement was carried out using an EG&G potentiostat model 273. A classical three-electrode system was applied. The sample, a saturated calomel electrode (SCE) and a platinum plate were used as working electrode, reference electrode and auxiliary electrode, respectively. The OCP of the AZ31 Mg alloy exposed to the A2 pretreatment solution and the A230 min treatment precursor film exposed to the post treatment solution were investigated by an E–T curve with a sampling frequency of 5 s point1. The samples were mounted using epoxy resin with an exposure surface area of 20  25 mm in contact with the solutions.

2. Experimental

3. Results

2.1. Fabrication of the Mg–Al HT film

3.1. XPS analysis of the films

The material used in this study was AZ31 Mg alloy. The surface of the samples was ground to 5000 grit SiC paper, ultrasonically cleaned in ethyl alcohol, and then dried in cold air. The films were prepared by two-step method. The carbonic acid solution was prepared by bubbling CO2 gas through 200 mL of distilled water for about 10 min at room temperature (20 ± 2 °C). It was denoted as A1 pretreatment solution with a pH value of about 4. The pretreatment solution named as A2 was based on the A1 solution with an addition of 0.5 mol L1 Na2CO3 solution saturated with Al compounds by dissolving a pure Al panel in it, and the pH was about 8. Both the pretreatment solutions of A1 and A2 were heated to 60 °C in a water bath. The post treatment solutions named as B1 and B2 were prepared by dropwise addition of 2 mol L1 NaOH solution to the A1 and A2 pretreatment solutions, respectively, until achieving the pH value of 10.5, and then were heated to 80 °C in a water bath. A continuous bubbling of CO2 gas to the pretreatment solutions was carried out during the preparation of the precursor films. The samples were immersed in the pretreatment solutions for different time to form the precursor films which were denoted as pretreatment solution-time. For example, A2-5 s means that the AZ31 Mg substrate was immersed in A2 pretreatment solutions for 5 s. The precursor films with treatment time of 30 min were further immersed into the post treatment solutions to obtain the final films which were denoted as, for example, A2-30 min/B2-20 min. This notation represented that the sample was first treated in the

Fig. 1 shows the XPS analysis of the A2-5 s treatment sample after 30 s etching. The C 1s spectrum indicates the presence of CO32 [32–34]. The spectrum of Al 2p only displays one peak, corresponding to Al. The peak of Mg 1s is resolved into three components. The peak at 1304 eV is assigned to Mg–O vibration mode. Binding energy (BE) at 1302.4 eV is attributed to magnesium hydroxyl stretching (Mg–OH), which situated at a lower BE than in the Mg(OH)2 (1302.7 eV). Combining with the presence of CO32, it indicates that Mg–OH group is linked by CO32 bonds. Also, the metal Mg from the substrate is detected at 1303.2 eV. The spectrum of O 1s is divided into three peaks, 531.8, 530.8 and 529.3 eV, which are attributed to CO32, OH and O2, respectively [33–35]. Therefore, the XPS spectra herein prove that the initial film consists of magnesium compounds, but no aluminum compounds. Fig. 2 shows the XPS depth profile for the A2-30 s treatment sample. Fig. 2a shows the atomic ratio percent vs. sputtering time curve, in which the data acquired without sputtering is not taken into account because of surface contamination. The whole curves can be divided into three regions. In region I (<150 s), the variations of the four elements are not significant, implying that the detected information is only attributed to the film. In region II, the content of O decreases quickly, while the contents of Mg and Al gradually increase from the outer to the inner layer. It indicates that the information is associated with both the film and matrix during this sputtering time. It is apparent that the chemical

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Fig. 1. XPS analysis of the A2-5 s treatment sample after 30 s etching: (a) C 1s (without etching); (b) Al 2p; (c) Mg 1s and (d) O 1s.

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Fig. 2. XPS depth profile for the A2-30 s treatment sample: (a) atomic ratio percent vs. sputtering time curve; (b) Al 2p; (c) Mg 1s and (d) O 1s.

composition shows little change with increasing sputtering time after 970 s. By comparing the high-energy resolution spectra of Al 2p, Mg 1s, and O 1s from the A2-30 s treatment film, it can be seen that the peak positions shift and the full width at half maximum

(FWHM) changes after sputtering different time. The Al 2p spectrum after 10 s etching can be only assigned to Al3+ which may correspond to aluminum containing hydroxides. As for the 280 s etching curve, the Al 2p contains two peaks, which correspond to Al simple substance and Al3+, respectively, indicating that the

J. Chen et al. / Corrosion Science 63 (2012) 148–158

information contains both the film and matrix. The Mg 1s spectrum after 10 s etching is broader than that of after 970 s etching, and it can be divided into the peaks corresponding to Mg simple substance and magnesium compounds. It should be mentioned that the Al 2p and Mg 1s can only be assigned to Al and Mg simple substance and the peak of O 1s is very weak after 970 s etching, indicating that the signals are mainly attributed to the matrix. 3.2. XRD patterns of the films The XRD spectra of the AZ31 alloy matrix, precursor and final films formed by different processes are shown in Fig. 3. Fig. 3a presents the spectra of the bare alloy and pretreated films. It can be seen that the AZ31 base is composed of a-Mg matrix and the second phases of MnAl and Al11Mn4 (curve f in Fig. 3a). After pretreated in A2 solution for 30 s, the thin film is composed of Mg5(CO3)4(OH)25H2O, Al5(OH)13(CO3)5H2O and a few amount of Al(OH)3 (curve a in Fig. 3a). Then, after 5 and 10 min immersion, the components are not changed (curve b in Fig. 3a). After A2– 20 min treatment, a new component of Mg2Al(OH)7 appears in the film (curve c in Fig. 3a). Subsequently, MgAl2(OH)8xH2O appears in the final precursor film after A2-30 min treatment, while Mg5(CO3)4(OH)25H2O and Al5(OH)13(CO3)5H2O almost disappear (curve e in Fig. 3a). The A1-30 min treatment film prepared in the A1 pretreatment solution without the additional Al-containing compounds (curve d in Fig. 3a) consists of Mg2Al(OH)7 as well as a minority of Mg5(CO3)4(OH)25H2O and Al5(OH)13(CO3)5H2O. However, the compound of MgAl2(OH)8xH2O is not detected in this film, which is different from the A2-30 min treatment precursor film formed in A2 solution. The transformation of the precursor film after A2-30 min treatment into the Mg–Al HT film is analyzed based on the XRD patterns in Fig. 3b. Curve a shows that the components in the A2-30 min/B210 min treatment sample is Mg6Al2(OH)184.5H2O (at the 2h posi-

(a)

151

tions of 11.33° and 22.85°) as well as a small amount of Mg4Al2(OH)143H2O and Mg5(CO3)4(OH)25H2O. The initial HT has already been synthesized after 20 min post treatment in the B2 solution. However, the X-ray peaks show low intensity and broad peaks, which indicates that the compound in this film presents a low degree of crystallization (curve b in Fig. 3b). The X-ray peaks of the Mg6Al2(OH)16CO34H2O (at the 2h positions of 11.2° and 22.7°) become intense (curve c in Fig. 3b) when the immersion time was increased to 1.5 h, which indicates an increase in the crystallization and growth of the Mg–Al HT film with increasing post treatment time. The influence of the additional Al compounds in the pretreatment and post treatment solutions on the formation of the Mg– Al HT film is detected by the XRD analysis. Fig. 3c shows the XRD patterns of the samples after A1-30 min/B2-1.5 h, A1-30 min/B11.5 h, A2-30 min/B1-1.5 h treatment, respectively. It can be seen that the compositions of all the films are Mg2CO3(OH)23H2O, Mg4Al2(OH)143H2O and Al5(OH)13(CO3)5H2O. But there is no HT detected in all of the three films. 3.3. OCP measurements and SEM morphologies observation of the films In order to study the film formation process, the open circuit potential (OCP) of the AZ31 Mg alloy exposed to the A2 pretreatment solution and the A2-30 min treatment precursor film exposed to the B2 post treatment solution were investigated by E–T curves combined with the SEM morphologies observation. The OCP of the Mg substrate rapidly increases in the initial stage of 10 min immersion, corresponding to the linear growth of the precursor film. Then, the OCP of the precursor film grows very slowly and finally reaches to a stable level (Fig. 4a [22]). Fig. 5 shows the SEM micrographs of the AZ31 alloy pretreated for different time. In the initial 5 s, the surface of the AZ31 substrate is already covered

(b)

(c)

Fig. 3. XRD spectra of the AZ31 alloy matrix, precursor and final films formed by different processes (h-Mg, }-Mg2Al(OH)7, j-MgAl2(OH)8xH2O, d-Mg5(CO3)4(OH)25H2O, q-Mg6Al2(OH)16CO34H2O, #-Mg6Al2(OH)184.5H2O, -Al5(OH)13(CO3)5H2O, s-Mg4Al2(OH)143H2O, 4-Mg2CO3(OH)23H2O, w-Al(OH)3, N-MnAl, -Al11Mn4).

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Fig. 4. Potential vs. time curve during: (a) the formation of the precursor film in the A2 pretreatment solution and (b) the transformation of the A2-30 min treatment precursor film into the hydrotalcite film in the B2 post treatment solution [22].

with a thin but uniform and compact film. The higher resolution image of the sample shows that the film consists of fine, vermicular-like, but not well crystals particles (Fig. 5c). The number and size of the particles slowly increase with increasing immersion time. Due to the increase of the film thickness, cracking of the film has occurred after 20 min (Fig. 5d). Subsequently, the number and size of the cracks increase with increasing immersion time. The low resolution SEM images of the post treatment films obtained after different reaction durations are shown in Fig. 6. The post treatment time was chosen according to the evolution of the potential as a function of the immersion time in the B2 post treatment solution (Fig. 4b [22]). It is revealed from the observation of the SEM images that the number of the micro-cracks on the post treatment films undergoes a series of changes, and all those changes are in good accordance with the evolution of the potential. The potential decreases slightly at the beginning of region I and the cracks increases at 1.5 min. Because the Al in the AZ31 substrate is dissolved through the cracks in the precursor film and the hydrogen evolution will rupture the film. Subsequently, the dissolved Al3+ can form a new film and just seal the cracks. Thus, the potential increases slightly in region II and the cracks decrease at 3 min. Then it is followed by a steep fall of the OCP in region III and dramatic increase in the number of the cracks at 15 min, which are caused by the dissolution of the Al compounds in the formed film. The XRD result that Mg2Al(OH)7 and MgAl2(OH)8xH2O in the precursor film are transformed into the relatively lower Al content compound of Mg6Al2(OH)184.5H2O also indicates the dissolution of the Al compounds. Finally, the formation and growth of the Mg–Al hydrotalcite film occur. Thus, the potential increases fast and the number of cracks dramatically decreases. The results further confirm the analysis in our previous work [22]. In addition, the changes of film microstructure with increasing post treatment time are visible from the higher resolution of the films (Fig. 7). Before 8 min, the high resolution morphologies of the post treated films are similar to the A2-30 min/B2-8 min treatment film. Thus the similar images are not shown. It can be observed that the A2-30 min/B2-8 min treatment film exhibits a similar microscopic morphology to the precursor films. The sheet particles interlace each other just like a nest (Fig. 7a). After 15 min, the sheet particles become thicker and a film with a typical Mg–Al HT structure is observed (Fig. 7b). The blade–like flakes grow with increasing immersion time and completely cover the Mg substrate after post treatment for 1.5 h (Fig. 7c). The XRD results display that the different microstructure corresponds to different composition. The main constituent of both the A2-30 min/ B2-10 min film and the precursor film is magnesium aluminum hydrates. Thus their microstructures are similar. After post treatment for 15 min, the film morphology is changed into HT structure.

Correspondingly, the Mg6Al2(OH)16CO34H2O is detected in the A2-30 min/B2-20 min treatment film. It can be seen that the morphology changes are consistent with the XRD results. 4. Discussion 4.1. The formation of the Mg–Al HT film A further analysis of the growth mechanism of the HT film has been proposed on the basis of the detailed composition analysis and ordered distribution of Mg2+ and Al3+ in the layer by XPS analysis, XRD patterns, E–T curves and SEM observation. The formation process of the Mg–Al HT film is very complex, and the chemical compositions and morphologies of the films undergo a series of variations as a function of time. The possible reactions are listed as follows: Firstly, the substrate is dissolved to produce Mg2+, resulting in a large increase of the OH concentration and hydrogen evolution, as proposed by Song [36]. In the anodic regions:

Mg ! Mgþ þ e þ

Mg þ H2 O ! Mg

ð1Þ 2þ



þ OH þ 1=2H2 "

ð2Þ

In the cathodic regions:

2H2 O þ 2e ! H2 " þ2OH

ð3Þ

Subsequently, magnesium compound has already deposited on the surface of the substrate at the first 5 s. Based on the analysis of the XPS spectra, it implies that there is a preferential adsorption of Mg2+. The spectrum of Al 2p in the A2-5 s film only displays one peak, corresponding to Al, while the spectrum of Mg 1s is related to magnesium compound. Under our experiment conditions of continuous bubbling of CO2 gas and high temperature of 60 °C, it tends to form Mg5(CO3)4(OH)25H2O in light of Eq. (4) in view of the fact that the dypingite (Mg5(CO3)4(OH)25H2O) can be preferentially formed under such conditions of high CO2 partial pressure [37]. In addition, the favorable formation of Mg5(CO3)4(OH)25H2O can be attributed to the lower solubility product constant Ksp. The Ksp of Mg5(CO3)4 (OH)25H2O is unknown but can be estimated from the Ksp of another magnesium hydroxyl carbonate Mg2CO3(OH)23H2O, which is reported to be 3.98  1019 [38]. So it is reasonable to assume that the Ksp of Mg5(CO3)4(OH)25H2O is smaller than that of MgCO33H2O (Ksp = 2.38  106) and Mg(OH)2 (Ksp = 5.61  1012) [39]. Thus, Mg5(CO3)4(OH)25H2O is more stable and preferentially deposits at the first 5 s.

5Mg2þ þ 2OH þ 4CO2 3 þ 5H2 O ! Mg5 ðCO3 Þ4 ðOHÞ2  5H2 O #

ð4Þ

J. Chen et al. / Corrosion Science 63 (2012) 148–158

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(a)

AlMn

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(d)

(e)

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1 µm Fig. 5. SEM micrographs of the AZ31 alloy pretreated in the A2 pretreatment solution for different time: (a) bare alloy; (b and c) A2-5 s; (d and e) A2-20 min and (f and g) A230 min.

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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

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50 µm

Fig. 6. Low resolution SEM images of the post treated films obtained in the B2 solution after different reaction time: (a) A2-30 min/B2-30 s; (b) A2-30 min/B2-1.5 min; (c) A230 min/B2-3 min; (d) A2-30 min/B2-8 min; (e) A2-30 min/B2-15 min; (f) A2-30 min/B2-30 min; (g) A2-30 min/B2-1 h and (h) A2-30 min/B2-1.5 h.

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(b)

(a)

1 µm

1 µm

(c)

1 µm Fig. 7. Higher resolution SEM images of the post treated films obtained in the B2 solution after different reaction time: (a) A2-30 min/B2-8 min; (b) A2-30 min/B2-15 min and (c) A2-30 min/B2-1.5 h.

As it is mentioned above that Mg is first dissolved in the pretreatment solution, resulting in an increase of the pH value at the interface of the substrate and solution. Then, due to the high OH concentration and high temperature of 60 °C, the Al on the surface of the AZ31 substrate is dissolved to form Al3+. The dissolved Al3+ and the extra addition of Al3+ in the pretreatment solution can react with CO32 and OH after 30 s to form Al5(OH)13(CO3)5H2O:

5Al

3þ

þ 13OH þ CO2 3 þ 5H2 O ! Al5 ðOHÞ13 ðCO3 Þ  5H2 O #

ð5Þ

Then, the thickening deposition film hinders the transportation of electrons and ions. Thus the electrochemical reactions slow down and the concentration of the Mg2+ and OH originating from the dissolution of the AZ31 substrate tends to reduce. Hence, the pH value at the interface of the film and solution may be changed from alkaline to weakly acidic due to the continuous bubbling of CO2 gas to the pretreatment solution. It is known that Mg5(CO3)4(OH)25H2O is a metastable hydrous carbonate and can be written as 4MgCO3Mg(OH)25H2O. According to Eq. (6) and (7), the driving force gibbs free energy Dr GH m for Eq. (6) is about 29.5 kJ mol1, which is more negative than that for Eq. (7), +  15.67 kJ mol1 (The Df GH m values of MgCO3, Mg(OH)2, H , OH , 2+  Mg , HCO3 and H2O is 1012.1, 833.6, 0, 157.29, 454.8, 586.8 and 237.18 kJ mol1, respectively). Thus, CO32 in Mg5(CO3)4(OH)25H2O is more easily dissolved to form Mg(OH)2 under the weakly acidic condition. Simultaneously, a part of the Mg2+ ions in Mg(OH)2 is replaced by Al3+ ions to form Mg2Al(OH)7. The replacement phenomenon also exists in the work of Frost et al. They found that a significant amount of Fe3+ has replaced Mg2+ in the dypingite structure, which is related at least in chemical formula to the hydrotalcite or layered double hydroxide minerals [37]. Thus, the replacement of some of Mg2+ by Al3+ to form Mg2Al(OH)7 is also probable.

MgCO3 þ Hþ ! Mg2þ þ HCO3

ð6Þ

MgðOHÞ2 þ Hþ ! Mg2þ þ OH þ H2 O

ð7Þ

Subsequently, decomposition also occurs to Al5(OH)13 (CO3)5H2O as Mg5(CO3)4(OH)25H2O, which is metastable and is changed into Al(OH)3. At the same time, a part of the Al3+ ions in Al(OH)3 is replaced by Mg2+ ions to form Al-rich compound MgAl2(OH)8xH2O. It can be found that Mg(OH)2 or Al(OH)3 does not exist as a simple substance, but mix with each other to form LDH in the precursor film. In LDH all the cations coordinate octahedrally with hydroxyl groups by a common bridge type to form a closely packed network structure [30], so LDH is more stable than simple Mg(OH)2 or Al(OH)3. Then, when the precursor film is immersed into an alkaline post treatment solution of pH 10.5, both the Mg2Al(OH)7 and MgAl2(OH)8xH2O leach out Al3+ as Eq. (8) and (9) due to the low chemical stability of Al in an alkaline condition, and then they are transformed into Mg6Al2(OH)184.5H2O with an Mg/Al mol ratio of 3. It has been reported by Yang et al. that the 3:1 structure has the greatest stability and therefore has a larger quantity of interlayer molecules in the 3:1 HT [40]. Furthermore, it is in accordance with the Lin’s research, in which the authors pointed out that the precursor film was not transformed to crystalline Mg–Al–Zn LDH until the Al3+ content in the precursor film was reduced [31].

3Mg2 AlðOHÞ7 þ OH þ 4:5H2 O ! Mg6 Al2 ðOHÞ18  4:5H2 O þ AlðOHÞ4 ð8Þ

6MgAl2 ðOHÞ8  xH2 O þ 10OH ! Mg6 Al2 ðOHÞ18  4:5H2 O þ 10AlðOHÞ4 þ xH2 O

ð9Þ

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Finally, the OH in the interlayer of Mg6Al2(OH)184.5H2O is exchanged by the ions of CO32 contained in the post treatment solution to form the typical HT of Mg6A12(OH)16CO34H2O, as Eq. (10) due to the fact that the carbonate ions CO32 have an exceptionally high affinity to the LDHs [41].

Mg6 Al2 ðOHÞ18  4:5H2 O þ CO2 3 ! Mg6 Al2 ðOHÞ16 CO3  4H2 O þ 2OH þ 0:5H2 O

ð10Þ

4.2. The influence of the additional Al compounds on the formation of Mg–Al HT film The additional Al compounds are very important for the formation of Mg–Al HT. When either of the pretreatment solution or post treatment solution is not added with extra Al compounds, the HT film cannot be formed. When the pretreatment solution is not added with extra Al compounds, whatever the post treatment solution is added with extra Al compounds or not, it cannot form HT. Lin et al. [31] suggested that the precursor should first leach out of amphoteric Al3+, which was originally randomly coordinated with surrounding OH in the precursor but coordinated octahedrally with OH groups in the crystalline conversion coating. Herein, there is no Al–rich compound MgAl2(OH)8xH2O in the A1-30 min treatment precursor film, but only Mg2Al(OH)7 leaches out of Al3+. It will cause the shortage of Al3+, which affects the transformation of a double layered structure with an ordering distribution of Mg2+ and Al3+. Therefore, the small amount of Mg2Al(OH)7 can be only changed into Mg2CO3(OH)23H2O and Al5(OH)13(CO3)5H2O. In addition, there is also no HT in the A2-30 min/B1-1.5 h treatment film. The probable reason is that most of the Al3+ contained in the precursor film is migrated into the B1 solution due to the high

chemical activation of Al in an alkaline of pH 10.5 post treatment solution without Al compounds. Whereas, the Al compounds in the precursor film will be dissolved less in the B2 solution with supersaturation of Al compounds. Interestingly, it is found that if the post treatment films do not form HT, they all contain artinite Mg2CO3(OH)23H2O, but no Mg5(CO3)4(OH)25H2O which exists in the precursor films. Because the pretreatment and post treatment solutions have different pH values, the more OH will form the lower Mg/OH ratio species. Furthermore, there is also no Mg(OH)2 in the films. It is considered to be related to the relative lower solubility product constant Ksp of Mg2CO3(OH)23H2O [38,39]. 4.3. The model for the formation mechanism of Mg–Al HT film on AZ31 alloy In this process, the above analysis implies that there exist two models of the precursor formation mechanism: (1) the conventional Mg(OH)2-based substitution model, magnesium hydroxide precipitates firstly, and then a part of the Mg2+ ions are replaced by Al3+ ions to form Mg2Al(OH)7. (2) Al(OH)3-based substitutionfilling model as Ma et al. proposed [30], aluminum hydroxide precipitates preferentially, Mg2+ substitutes Al3+ to form MgAl2(OH)8xH2O. It demonstrates that the precursor particles are formed by the partial replacement, and the metals present as a mixed hydroxides rather than in separate component. Therefore, a possible mechanism of in situ growth of HT film on the surface of the AZ31 alloy substrate can be proposed as shown in Fig. 8. Firstly, the AZ31 substrate is dissolved to give Mg2+, meantime the enriched Mg2+ is combined with OH and CO32 to form Mg5(CO3)4(OH)25H2O, which is deposited on the surface of Mg substrate. Then the further dissolution of the Al in the substrate and the transportation of Al3+, OH and CO32 from the solution to

Fig. 8. Mechanism of the in situ growth of hydrotalcite film on the surface of the AZ31 alloy substrate (the treatment is as followers: AZ31 alloy is first pretreated in the A2 solution with a continuous bubbling of CO2 gas at a temperature of 60 °C for 30 min to form a precursor film and then post treated in the B2 solution at 80 °C for 1.5 h).

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the substrate surface occur to form Al5(OH)13(CO3)5H2O. Subsequently, the basic carbonates decompose to generate Mg(OH)2 and Al(OH)3, respectively and one after another, which are partially replaced to form two kinds of LDHs, Mg2Al(OH)7 and MgAl2(OH)8xH2O as the precursor film. After that, when the precursor film is immersed into the post treatment solution, the Al-rich compounds Mg2Al(OH)7 and MgAl2(OH)8xH2O leach out Al3+ to form Mg6Al2(OH)184.5H2O accompanying with thinning and rupture of the precursor film. Finally, the OH in the interlayer of Mg6Al2(OH)184.5H2O is exchanged by CO32 to form Mg6Al2(OH)16CO34H2O. After that, the Mg–Al HT particles grow and crystal gradually. Meantime, the Mg2+, Al3+, CO32 and OH in the post treatment solution can also precipitate to form HT crystal grains under such conditions of high pH value and temperature. Thus, the cracks can be sealed and the film becomes compact and uniform during the subsequent immersion process. The sealing of cracks can also be proven by the cross-sectional images of the films which were reported in our previous work [22]. 5. Conclusions (1) The components and morphologies of the films undergo a series of variations during the growth processes. The number of the micro-cracks on the post treatment films changes in accordance with the evolution of the potential. (2) In current process, the additional Al compounds are very important for the formation of Mg–Al hydrotalcite. When either of the pretreatment solution or post treatment solution is not added with extra Al compounds, the hydrotalcite film cannot be formed because of the shortage of the Al content. (3) A possible mechanism of the in situ growth of hydrotalcite film on the AZ31 alloy is proposed as follows: (i) dissolution of the AZ31 substrate and deposition of Mg5(CO3)4 (OH)25H2O and Al5(OH)13(CO3)5H2O; (ii) decomposition of the carbonates to form hydroxides, replacing of Mg2+ or Al3+ to form the precursor of hydrotalcite, Mg2Al(OH)7 and MgAl2(OH)8xH2O; (iii) dissolution of the Al3+ from the precursor to form Mg6Al2(OH)184.5H2O, thinning and rupture of the precursor film; and (iv) exchanging of OH by CO32 to form Mg6Al2(OH)16CO34H2O, growth of the film and sealing the cracks.

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