Influence of 8-hydroxyquinoline on properties of anodic coatings obtained by micro arc oxidation on AZ91 magnesium alloys

Journal of Alloys and Compounds 539 (2012) 249–255

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Influence of 8-hydroxyquinoline on properties of anodic coatings obtained by micro arc oxidation on AZ91 magnesium alloys R.F. Zhang a,b, S.F. Zhang a,b,⇑, N. Yang b, L.J. Yao b, F.X. He b, Y.P. Zhou b, X. Xu b, L. Chang b, S.J. Bai b a b

Jiangxi Key Laboratory of Surface Engineering, Jiangxi Science and Technology Normal University, Nanchang 330013, China School of Material Science and Engineering, Jiangxi Science and Technology Normal University, Nanchang 330013, China

a r t i c l e

i n f o

Article history: Received 18 March 2012 Received in revised form 27 April 2012 Accepted 28 April 2012 Available online 8 May 2012 Keywords: Magnesium alloys Micro arc oxidation Electrolyte 8-Hydroxyquinoline

a b s t r a c t The influence of 8-hydroxyquinoline (8-HQ) on formation and properties of anodic coatings obtained by micro arc oxidation (MAO) on AZ91 magnesium alloys was studied by scanning electron microscope (SEM), energy dispersive spectrometry (EDS), Fourier transform infrared (FT-IR) spectroscopy and potentiodynamic polarization tests. The results demonstrate that 8-HQ can decrease the solution conductivity, take part in the coating formation and change the coating color. By developing anodic coatings with increasing thickness, insoluble Mg(HQ)2 and small pore size, 8-HQ improves the corrosion resistance of the anodized magnesium alloys. The coating shows the best corrosion resistance in the solution of 10 g/L NaOH and 18 g/L Na2SiO3 with 2 g/L 8-HQ. Ó 2012 Published by Elsevier B.V.

1. Introduction Micro arc oxidation (MAO), developed under the traditional oxidation, is an effective method to improve the corrosion and wear resistance of magnesium alloys. The coating properties depend on several factors, such as the composition of the substrate [1,2], electric parameters [2,3], the concentrations and compositions of the electrolytes [2,4–6]. Among these factors, the used electrolytes play an important role in determining the coating property. Due to the health and environmental pressure, some environmentally friendly processes have been developed in alkaline solutions mainly containing inorganic electrolytes such as silicate [1,4–17], aluminate [2,5,6] and borate [4,5,7,11,15,18]. At present, some organic additives, for example, triethanolamine [4], glycerol [9,10,12], benzotriazole [11], surfactants [12], ethylene glycol [13,14], citrate [15,18,19], sodium acetate [20] and phytic acid [21] have been used in MAO on magnesium alloys. 8-Hydroxyquinoline (8-HQ), one of the nitrogen-heterocyclic aromatic hydrocarbons with moderate toxicity in rats (oral LD50 1200 mg/kg) [22], is widely used in many areas. For instance, 8-HQ is used as a dye [23], a masking agent [24], a metal inhibitor toward the corrosion of steel [25], copper [26], aluminum [27] and magnesium [28,29]. The inhibitory mechanism of 8-HQ has been investigated and the results show that the formation of complex chelating compounds ⇑ Corresponding author. Address: P.O. Box 124, Jiangxi Key Laboratory of Surface Engineering, Jiangxi Science and Technology Normal University, Nanchang 330013, Jiangxi Province, China. Tel./fax: +86 791 83831266. E-mail address: [email protected] (S.F. Zhang). 0925-8388/$ - see front matter Ó 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jallcom.2012.04.120

such as Cu(HQ)2, Al(HQ)3 and Mg(HQ)2 on metal surface plays an essential role in the inhibition of metal corrosion [26–29]. Inhibiting action is also related to the adsorption of 8-HQ molecules on the surface, avoiding the adsorption of aggressive ions, such as Cl or OH [25,26,29]. In view of insolubility and stability of Mg(HQ)2 in water solution, it is speculated that 8-HQ may be helpful for improving the coating property obtained by MAO on magnesium alloys. However, to the best of our knowledge, there is no report on this area. In this paper, the influence of 8-HQ on formation and property of anodic coatings was systematically studied. 2. Experimental An AZ91 magnesium alloy ingot with a nominal composition of 9.0 wt.% Al, 1.0 wt.% Zn and balance Mg was chosen as the substrate. Samples for MAO treatment were masked with sealant leaving an area of 5 cm  6 cm exposed, with a 3 mm diameter hole drilled for connection with the anode by a screw. The equipment for MAO consisted of a MAOI-50C power supply (Chengdu Pulsetech Electrical Co., Ltd, China), a stainless steel barrel and a stirring and cooling system that controlled the solution temperature below 40 °C. In a base solution of 10 g/L NaOH, different concentrations of 8-HQ and 18 g/L Na2SiO3 were added to form five solutions and they were separately called as 5Q, 18Si, 2Q-18Si, 5Q-18Si and 8Q-18Si. Electrolyte compositions and operating parameters are listed in Table 1. Chemical reagents were weighed by BP211D electronic scales (precision 0.01 mg, Sartorius, Germany) and the solution conductivity was measured by a DDS-307W Microprocessor Conductivity Meter. Surface and cross-sectional morphologies of the anodized samples were observed by a RIGMA field emission scanning electron microscope (FE-SEM) after they were rinsed with distilled water, dried in a cool air stream and coated with gold. Chemical compositions of anodic films were determined by energy dispersive spectrometry (EDS) in the SEM. The coating structure was analyzed using a D8 advance X-ray diffractometer with Cu Ka radiation. Anodic coatings obtained by MAO were scrapped off from magnesium alloys with a razor [30] and were characterized by a Nicolet 460 Fourier transform

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Solution

Electrolyte constituents

Operating conditions

5Q 18Si 2Q-18Si

10 g/L NaOH and 5 g/L 8-HQ 10 g/L NaOH and 18 g/L Na2SiO3 10 g/L NaOH, 18 g/L Na2SiO3 and 2 g/L 8-HQ 10 g/L NaOH, 18 g/L Na2SiO3 and 5 g/L 8-HQ 10 g/L NaOH, 18 g/L Na2SiO3 and 8 g/L 8-HQ

Temperature 15–40 °C, current density 40 mA/cm2, frequency 2000 Hz, duty cycle 20%, anodizing time 3 min

5Q-18Si 8Q-18Si

infrared (FT-IR) spectrometer in the range of 400–4000 cm1. In comparison, the original 8-HQ powder was also analyzed by KBr pellet technique. Potentiodynamic polarization tests were measured in 3.5 wt.% NaCl solution using a CHI760C Electrochemical Workstation. A classical three-electrode cell was used with platinum as the counter electrode and a saturated calomel electrode as the reference electrode. The quiet time was 1200 s and scan was conducted with a constant rate of 0.001 V/s from initial potential of 1.8 V vs. SCE towards more noble direction until the film breakdown occurred.

3. Results

Voltage (V)

Table 1 Electrolyte constituents and operating conditions for MAO on magnesium alloys.

340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0

10 g/L NaOH and 5 g/L 8-HQ 10 g/L NaOH and 18 g/L Na2SiO3 10 g/L NaOH, 18 g/L Na2SiO3 and 5 g/L 8-HQ 0

20

40

60

80

100 120 Time (s)

140

160

180

200

Fig. 2. Variations of voltage with time during MAO treatment in the solutions of 5Q;18Si and 5Q-18Si under current density 40 mA/cm2, frequency 2000 Hz, duty cycle 20% and anodizing time 3 min.

3.1. Effects of 8-HQ on electrolyte conductivity The conductivity of aqueous solutions measured in different solutions at 14 °C is shown in Fig. 1. The conductivity (k) of 10 g/L NaOH is 47.7 mS/cm. After 5 g/L 8-HQ and 18 g/L Na2SiO3 were separately added into the solution above, the conductivity values became 41.5 and 55.8 mS/cm, correspondingly (Fig. 1), indicating that 8-HQ can decrease the solution conductivity, while Na2SiO3 can increase it. After 2, 5 and 8 g/L 8-HQ were added into the solution of 10 g/L NaOH and 18 g/L Na2SiO3 (18Si), the conductivity values continually decreased and reached 53.7, 51.4 and 48.7 mS/cm, respectively (Fig. 1). As one kind of amphiprotic compounds, an 8-HQ molecule is ionized into an H+ and a C9H6NO in an alkaline solution. After 8-HQ is added into the alkaline solution, H+ ions from 8-HQ will react with OH ions from the alkaline solution to produce water. The ion conductivity of OH is much larger than that of water molecules. After OH ions are counteracted into water, the solution conductivity decreases. With the increase of 8-HQ concentration, more OH ions are consumed resulting in the continual decrease of the solution conductivity. 3.2. Effects of 8-HQ on coating formation Fig. 2 shows voltage variations versus time during MAO treatment in 5Q, 18Si and 5Q-18Si.

20

10

0

48.7 10 g/L NaOH, 18 g/L Na 2SiO3and 8 g/L 8-HQ

30

51.4 10 g/L NaOH, 18 g/L Na 2SiO3and 5 g/L 8-HQ

41.5 40

10 g/L NaOH and 18 g/L Na 2SiO3

50

53.7 10 g/L NaOH, 18 g/L Na 2SiO3 and 2 g/L 8-HQ

55.8

10 g/L NaOH and 5 g/L 8-HQ

Solution conductibity (mS/cm)

60

Solutions Fig. 1. The solution conductivity measured at 14 °C in different solutions.

During the first 10 s, the voltage increases rapidly with time in three solutions. After 10 s, the curve slope of voltage with time in 18Si is slightly smaller than that in 5Q-18Si but much larger than that in 5Q. After anodizing for 3 min, the final voltages treated in 5Q, 18Si and 5Q-18Si are separately 143 V, 296 V and 316 V. The MAO process is a competition between the destruction of old film and development of new film and the variation of voltage with time during the process can indicate which step dominates the process [5,7,31]. If the voltage increases with treatment time, the process is dominated by the coating formation, otherwise it is determined by the old film destruction [5,7,31]. In addition, the curve slope of voltage with time can indicate the speed of coating formation. In generally, the larger the curve slope of working voltage with time is, the faster the coating is formed on the substrate [15]. Fig. 2 indicates that 8-HQ is not an effective electrolyte as sodium silicate but can play a role in promoting coating formation, which is further verified by the appearance of the anodized specimens. The pictures of the anodized specimens obtained in three solutions are shown in Fig. 3. As shown in Fig. 3, after MAO treatment of magnesium alloys in 5Q, a discontinuous film is developed on the sample surface (Fig. 3a), which indicates that 8-HQ is not an effective coating agent. After treatment in 18Si, a concrete grey film is developed on magnesium alloys (Fig. 3b) and it becomes light blue after addition of 5 g/L 8-HQ (Fig. 3c). In the light of our previous work under the same MAO conditions as used in the paper, where we found that anodic coatings could not be developed in a solution only containing NaOH [21], it is concluded that 8-HQ can take part in the coating formation and change the coating color. 3.3. Effects of 8-HQ on morphology, composition and structure Surface morphologies of anodic coatings obtained in 5Q at two kinds of magnification are shown in Fig. 4. After treatment in 5Q, the obtained coating is not continuous but many white particles are formed on the substrate (Fig. 4a). At higher magnification, it is evident that leaf-like material is developed under the action of 8-HQ. As shown in Fig. 4b, chemical compositions on a planar area (without anodic coatings) such as Point A and on an area with the leaf-like material such as Point B are detected by EDS analysis. Point A contains 17.7% C (in at.%, the same below), 0.2% N, 9.1% O, 69.3% Mg, and 3.6% Al, showing

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Fig. 3. Pictures of the anodized samples obtained in the solutions of (a) 10 g/L NaOH and 5 g/L 8-HQ, (b) 10 g/L NaOH and 18 g/L Na2SiO3 and (c) 10 g/L NaOH, 18 g/L Na2SiO3 and 5 g/L 8-HQ under current density 40 mA/cm2, frequency 2000 Hz, duty cycle 20% and anodizing time 3 min.

Fig. 4. Surface morphologies of anodic films formed in the solution of 10 g/L NaOH and 5 g/L 8-HQ: (a) lower magnification; (b) higher magnification.

basically the substrate compositions; while Point B is composed of 50.2% C, 2.4% N, 9.9% O, 35.7% Mg and 1.8% Al, demonstrating 8-HQ entrance into anodic coatings. Fig. 5 presents the FT-IR spectra of the original 8-HQ powder (Fig. 5a) and anodic coatings formed in 5Q containing 5 g/L 8-HQ on the surface of AZ91 magnesium alloys (Fig. 5b). Compared with Fig. 5a, the vibrations in Fig. 5b at 1605, 1578, 1386 and 1327 cm1 are assigned to the quinoline group of Mg(HQ)2 [32,33]. In addition, the increase in the energy of the intermolecular hydrogen bonds from about 3100–3400 cm1 (in Fig. 5b) indicates a polymeric association between 8-HQ molecules [28]. These FT-IR measurements support that the as-developed coatings obtained in 5Q on magnesium alloys are Mg(HQ)2 [28,32,33]. In order to reveal the influence of 8-HQ concentration on coating property, surface and cross-sectional morphologies obtained in 18Si with addition of 0–8 g/L 8-HQ are investigated. Fig. 6 shows surface morphologies of anodic coatings obtained in 18Si, 2Q-18Si, 5Q-18Si and 8Q-18Si. As shown in Fig. 6, all the coatings obtained in different solutions are of porous structure, but they are evidently different in micropore characteristics. On the coating obtained in the solution without 8-HQ (18Si), the pore size and distance between two adjacent pores range from 0.1 to 4 lm and 1.2 to 8 lm, respectively (Fig. 6a). After addition of 2 g/L 8-HQ into 18Si, the developed coating exhibits better uniformity (Fig. 6b) and the number of micropores per area on coating surface decreases from 0.17 to 0.12 lm2.

However, when the used 8-HQ concentration is 5 g/L or more, the obtained coatings become heterogeneous with some regions porous and others compact (Fig. 6c and d). Chemical compositions of anodic coatings obtained in solutions with different concentrations of 8-HQ are listed in Table 2. According to Table 2, the coating obtained in 18Si mainly contains 56.8% O, 27.0% Mg and 13.2% Si. After 2 g/L 8-HQ was added into 18Si, N was not detected in the coatings. It is clear that several leaf-like particles shown by arrows are developed (Fig. 6b) and their N content is about 2.0 at.%, indicating that 8-HQ indeed takes part in the coating formation but on the total area, the N content is too low to be detected. However, in the coatings formed in 5Q-18Si and 8Q-18Si, N is present and its contents are both 2.0%, which indicates that 8-HQ can change the coating compositions when its concentration is 5 g/L or more. In order to clearly show the coating character formed in 5Q18Si and 8Q-18Si, one porous region in Fig. 6c (demonstrated by a circle) is magnified and shown in Fig. 7. As shown in Fig. 7, the obtained coatings are uneven. Some regions, such as Point C, are composed of typical anodic coatings; while other regions, such as Point D, exhibits irregularly arranged lamellate structure. In addition, chemical compositions of Point C are different from those of Point D. Point C is composed of 25.6% C (in at.%, the same below), 1.0% N, 38.7% O, 23.2% Mg, 1.2% Al, 9.8% Si and 0.5% Na; while Point D contains 63.0% C, 6.8% N, 15.3% O, 10.2% Mg, 0.5% Al, 3.7% Si and 0.5% Na. Compared with

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(a)

According to the data presented in Table 3, the corrosion current density of anodic coatings formed in 18Si is 40.0 lA/cm2. After addition of 2 g/L 8-HQ, it decreases to 2.2 lA/cm2. With the increase of 8-HQ concentration, the corrosion current densities do not change greatly and they are separately 3.2 and 3.6 lA/cm2 in 5Q-18Si and 8Q-18Si. After addition of 8-HQ into 18Si, the corrosion resistance of the anodized magnesium alloys is further improved.

100 90

Transmittance /%

80 70

1578 3046

60 1093

50

4. Discussion 40 1507

30

1381 780

(b)

20 50 45

Transmittance /%

40 35 30 25

3063 3369

1605

20 1110 15

822

1578

10

1386 1327 5 4000

3500

3000

2500

2000

1500

1000

500

The corrosion behaviour is influenced by the combined effect of thickness, compactness and phase/chemical composition of the coating [6]. It is generally believed that a compact anodic coating with less defects, higher thickness and stable composition in aggressive environments would be beneficial to provide a favourable corrosion protection to magnesium alloy substrate [16]. When used in MAO on magnesium alloys, 8-HQ plays both as an additive and an inhibitor. Firstly, compared with 18Si without 8-HQ, the coatings obtained in solutions with 8-HQ are thicker, which should result in better corrosion resistance of anodic coatings when other coating properties remain constant. Secondly, in the solutions containing 8-HQ, insoluble Mg(HQ)2 is developed in coatings. Once 8-HQ is added into 18Si, H+ ions and C9H6NO anions are present, which can be verified by the conductivity changes shown in Fig. 1. During MAO process, anions in the solution such as C9H6NO, OH 2 and SiO3 move toward the anode under the electric field and enter into anodic coatings. Therefore, in the solution containing 8-HQ, the following reactions normally occur on the anode surface [28,31,34–36]:

Wavenumber /cm-1 Fig. 5. FT-IR spectra obtained on (a) the original 8-HQ powder and (b) anodic coatings formed in the solution of 10 g/L NaOH and 5 g/L 8-HQ under current density 40 mA/cm2, frequency 2000 Hz, duty cycle 20% and anodizing time 3 min.

Point C, Point D has higher N and lower Si, which indicates that the film with lamellate structure is mainly made up of Mg(HQ)2. According to cross-sectional morphologies of anodic coatings obtained in 18Si, 2Q-18Si, 5Q-18Si and 8Q-18Si shown in Fig. 8, the coatings formed in these solutions are 5.0, 6.0, 7.5 and 6.0 lm thick, respectively, indicating that 8-HQ can slightly increase the coating thickness within the range of 2–5 g/L. However, when the concentration of 8-HQ is 8 g/L, the coating thickness decreases. XRD analyses of anodic coatings formed in different solutions are shown in Fig. 9. According to the XRD patterns, the diffraction peak of Mg substrate is detected in the coatings formed in different solutions. In addition of the substrate peak, the coatings formed in 18Si and 2Q-18Si consist of MgO and Mg2SiO4. However, the coatings formed in 5Q-18Si are made up of MgO, Mg2SiO4 and MgAl2O4, while the coatings formed in 8Q-18Si consist of Mg2SiO4 and MgAl2O4. 3.4. Effects of 8-HQ on corrosion resistance Fig. 10 shows the potentiodynamic polarization curves of anodic coatings formed in 18Si, 2Q-18Si, 5Q-18Si and 8Q-18Si. The corrosion potential (Ecorr) and the corrosion current density (icorr) derived from the potentiodynamic polarization curves are summarized in Table 3. Corrosion current density is usually applied to characterize the corrosion performance of the samples. Generally speaking, the low current density indicates good corrosion resistance property.

Mg ! Mg2þ þ 2e ðanodic dissolutionÞ 3þ

Al ! Al

þ 3e ðanodic dissolutionÞ





ð1Þ ð2Þ

4OH ! 2H2 O þ 2OðO2 Þ þ 4e

ð3Þ

or 2H2 O ! O2 þ 4Hþ þ 4e

ð4Þ

Mg2þ þ 2OH ! MgðOHÞ2 ðCoating formationÞ

ð5Þ

2þ



þ 3OH ! AlðOHÞ3

ð6Þ

MgðOHÞ2 ! MgO þ H2 O

ð7Þ

2AlðOHÞ3 ! Al2 O3 þ 3H2 O

ð8Þ

Al

ð9Þ

Al2 O3 þ MgO ! MgAl2 O4

ð10Þ

According to Eq. (9), Mg(HQ)2 is formed by the reaction of 8-HQ with magnesium ions [28], which was verified in Figs. 3–5. It is well known that the 8-HQ behaves as a bidentate ligand for the formation of chelate complexes with metallic ions [26]. As is expected, the production of Mg(HQ)2, in which both nitrogen and oxygen atoms donate, gives rise to a very stable five-membered ring. Indeed, Mg(HQ)2 is highly insoluble in pH range 9.4–12.7 [27]. In addition, because of excellent thermal stability and photoluminescence property, Mg(HQ)2 has been considered to be a good luminescence material [37]. In the solutions of 5Q-18Si and 8Q18Si, MgAl2O4 is formed by the reaction between magnesium oxide (MgO) and aluminum oxide (Al2O3) [36]. The reason may be that in these solutions, the final voltage is very high because of low conductivity (Fig. 2), which results in large sparks and high temperature on the sample surface [34]. Under the high temperature, magnesium aluminate spinel (MgAl2O4) is formed.

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253

Fig. 6. Surface morphologies of anodic films formed in the solutions of (a) 10 g/L NaOH and 18 g/L Na2SiO3, (b) 10 g/L NaOH, 18 g/L Na2SiO3 and 2 g/L 8-HQ, (c) 10 g/L NaOH, 18 g/L Na2SiO3 and 5 g/L 8-HQ and (d) 10 g/L NaOH, 18 g/L Na2SiO3 and 8 g/L 8-HQ under current density 40 mA/cm2, frequency 2000 Hz, duty cycle 20% and anodizing time 3 min.

Table 2 Chemical compositions of anodic coatings obtained in the solutions without and with different concentrations of 8-HQ (in at.%). Solution

C

N

O

Mg

Al

Si

Na

18Si 2Q-18Si 5Q-18Si 8Q-18Si

– 5.9 46.0 48.6

– – 2.0 2.0

56.8 54.8 30.0 26.4

27.0 25.3 14.5 15.5

1.4 2.2 1.4 1.2

13.2 10.6 5.7 5.8

1.6 1.2 0.4 0.5

Fig. 7. Surface morphology of the porous region in Fig. 6c.

Thirdly, as an inhibitor, 8-HQ can decrease the pore size of anodic coatings by adsorption on the anode. Anodic coatings obtained by MAO show a porous structure and the mechanism is studied and speculated by some researchers [12,34]. Guo and An [12] have pointed out that the porous structure partly results from intensive oxygen evolution (Eqs. (2) and (3)). In the MAO process, once sparking occurs, a lot of oxygen bubbles are immediately adsorbed on the anode surface and periodically released from the coating surface. The diameter and the adsorption intensity of oxygen bubbles have a critical effect on the surface morphology of ceramic coatings [11,12]. After organic additives are added into the solution, they can be adsorbed at the anode/electrolyte interface and decrease the interfacial tension of gas–liquid and solid–liquid [9,11,12]. According to Young’s equation, the contact angle at the gas/solid interface decreases; the bubbles decrease in the diameter and easily release from the coating surface, forming a ceramic coating with small pore size [9,11,12]. As reported before, 8-HQ molecules can be adsorbed on metal surface [25,26]. By adsorption of 8-HQ molecules on magnesium surface, the ceramic coatings with lower porosity (Fig. 6b) or smaller pore size (Fig. 6c and d) were fabricated in solutions with 8-HQ. Under the coaction of coating thickness, pore size and composition, the corrosion resistance of anodic coatings formed in solutions with 8-HQ increases. However, when the concentration of 8-HQ is 5 g/L or more, the coatings become heterogeneous in some regions (Figs. 6c,d and 7). These loose regions, where anodic coatings cannot suppress the transfer of Cl ions between the coating and solution [17], may become the center where the initial corrosion of base metal takes place. Therefore, the corrosion resistance of anodic coatings obtained in 5Q-18Si and 8Q-18Si becomes worse than that of 2Q-18Si.

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Fig. 8. Cross-sectional morphologies of anodic films formed in the solutions of (a) 10 g/L NaOH and 18 g/L Na2SiO3, (b) 10 g/L NaOH, 18 g/L Na2SiO3 and 2 g/L 8-HQ, (c) 10 g/L NaOH, 18 g/L Na2SiO3 and 5 g/L 8-HQ and (d) 10 g/L NaOH, 18 g/L Na2SiO3 and 8 g/L 8-HQ under current density 40 mA/cm2, frequency 2000 Hz, duty cycle 20% and anodizing time 3 min.

-3.5

10 g/L NaOH and 18 g/L Na2SiO3 10 g/L NaOH, 18 g/L Na2SiO3 and 2 g/L 8-HQ

-4.5

2

log(Current density/(A/cm ))

-4.0

-5.0 -5.5 -6.0 -6.5 -7.0 -7.5

10 g/L NaOH, 18 g/L Na2SiO3 and 5 g/L 8-HQ

-8.0

10 g/L NaOH, 18 g/L Na2SiO3 and 8 g/L 8-HQ

-1.85 -1.80 -1.75 -1.70 -1.65 -1.60 -1.55 -1.50 -1.45 -1.40 -1.35 -1.30 Potential/V vs.SCE Fig. 9. XRD patterns of anodic coatings anodized in solutions without and with different concentrations of 8-HQ under current density 40 mA/cm2, frequency 2000 Hz, duty cycle 20% and anodizing time 3 min.

5. Conclusions In the paper, the influence of 8-HQ on formation and property of anodic coatings was studied and some conclusions are achieved: (1) As one kind of organic additives, 8-HQ can decrease the solution conductivity, promote the coating formation and change the coating color.

Fig. 10. Polarization curves of anodic coatings formed in the solution of 18Si with addition of 0–8 g/L 8-HQ.

Table 3 Results of potentiodynamic polarization curves of anodic coatings obtained in the solutions without and with different concentrations of 8-HQ. Solution

Ecorr (V vs. SCE)

Icorr (lA/cm2)

18Si 2Q-18Si 5Q-18Si 8Q-18Si

1.528 1.480 1.457 1.474

40.0 2.2 3.2 3.6

R.F. Zhang et al. / Journal of Alloys and Compounds 539 (2012) 249–255

(2) By taking parting in the coating formation and adsorption on the anode, 8-HQ has great effects on thickness, chemical composition and surface morphology of anodic coatings. In solutions containing 8-HQ, the coatings contain insoluble Mg(HQ)2 with decreasing pore size. (3) Potentiodynamic polarization curves show that 8-HQ can further improve the corrosion resistance of the anodized magnesium and the best corrosion resistance is achieved in the solution of 10 g/L NaOH and 18 g/L Na2SiO3 containing 2 g/L 8-HQ.

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