High efficiency corrosion inhibitor 8-hydroxyquinoline and its synergistic effect with sodium dodecylbenzenesulphonate on AZ91D magnesium alloy

Corrosion Science 52 (2010) 1603–1609

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High efficiency corrosion inhibitor 8-hydroxyquinoline and its synergistic effect with sodium dodecylbenzenesulphonate on AZ91D magnesium alloy H. Gao, Q. Li *, Y. Dai, F. Luo, H.X. Zhang School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China

a r t i c l e

i n f o

Article history: Received 9 July 2009 Accepted 19 January 2010 Available online 28 January 2010 Keywords: A. Magnesium B. EIS B. Polarization C. Neutral inhibition

a b s t r a c t The inhibition effects of sodium dodecylbenzenesulphonate (SDBS) and 8-hydroxyquinoline (8HQ) on the corrosion of AZ91D magnesium alloy in ASTM D1384-87 corrosive solution were investigated by the electrochemical impedance spectroscopy and potentiodynamic polarization tests. For SDBS, the inhibition effect was not significant. For 8HQ, a monotonic increase in inhibition efficiency was observed as a function of the immersion time, and the component of the film was Mg(8HQ)2, which was characterized by three spectra methods. Upon mixing 8HQ and SDBS inhibitors, a synergistic inhibition behavior was observed, and a proper synergistic inhibition mechanism was proposed. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Magnesium alloys, as promising candidates to substitute aluminium alloys, have potential wide application in automobiles, electronics and aerospace industries because of their optimal weight-to-strength ratios [1,2]. AZ91D magnesium alloy is by far one of the most common magnesium alloys, which consists of two main phases, the magnesium rich a phase, and the b phase, aluminium rich Mg17Al12. Additionally, a minority of intermetallic phase containing different elements as iron, manganese and aluminium is also present. It has been noticed that a galvanic corrosion can preferentially occur at the a phase and grain boundaries in both climatic and corrosive environments. The corrosion mechanisms of magnesium alloys in chloride-containing solution were essentially investigated and reported in recent papers [3–5]. The use of inhibitors is one of the most practical methods for protecting metals or alloys from corrosion. Compared with inorganic salt corrosion inhibitors, using organic corrosion inhibitors is an effective, inexpensive and less pollution means of reducing the degradation of metals or alloys in many fields of applications, and which has been extensively investigated during the last decade [6–12]. It is generally acknowledged that the heteroatoms such as N, S and O in organic compounds show an inhibition effect toward the corrosion of iron, copper and aluminium alloys. The main role of heteroatoms in the corrosion protection is the formation of insoluble deposits on intermetallic inclusions, thus prevents local increase of pH which is responsible for the acceleration of the intermetallics dealloying. In terms of forming insoluble complex * Corresponding author. Tel.: +86 023 68252360; fax: +86 023 68367675. E-mail addresses: [email protected], [email protected] (Q. Li). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.01.033

compounds with the components of a phase and intermetallic phase of AZ91D magnesium alloy, organic compounds could be potential candidates to provide strong inhibition effect on the corrosion of AZ91D magnesium alloy. In most recent published literatures [13–15], the 8-hydroxyquinoline has been present to be a mixed type inhibitor by the formation of a complex chelate, blocking the active sites of the 2024 aluminium alloy surface. Additionally, the inhibitory action of 8-hydroxyquinoline on the copper corrosion process has been investigated and was showing the formation of the protective film composed by Cu(II)–hydroxyquinoline complexes on the copper surface and hindered the action of aggressive ions in solution, such as Cl or OH [16]. In fact, 8-hydroxyquinoline and magnesium can also form complex magnesium chelate with the reaction shown in Fig. 1. However, investigation of corrosion inhibition effect of 8hydroxyquinoline on magnesium alloys has not been reported yet. We can assume, upon the reaction in Fig. 1, some of defects would exist in the 8-hydroxyquinoline film on the surface of magnesium alloy, and Mg2+ and H+ would be also enriched in those defects. In order to give an essential protection on the film formed, the dodecylbenzenesulphonate anion could be added. The molecular structure of sodium dodecylbenzenesulphonate is shown in Fig. 1. The addition of the dodecylbenzenesulphonate anion can form an adsorption film on the surface of AZ91D magnesium alloy and make a subsequent protection on the film. Indeed, this kind of synergistic effect of corrosion inhibitors has been drawn much attention [17–21]. Shahrabi et al. [17] have observed the corrosion inhibition effects of sodium dodecylbenzenesulphonate and 2-mercaptobenzoxazole on copper in 0.5 M sulphuric acid solution and showed a good synergistic effect.

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tively. The slit in the solid fluorescence spectrum tests was (EX/EM) 5 nm/5 nm and PMT was 400 V. Prior to all spectrum tests, 8HQ samples were immersed in ASTM D1384-87 corrosive solution saturated with 8HQ for 72 h, dried at 100 °C for 2 h. 2.3. Electrochemical tests

Fig. 1. Reaction equation of 8-hydroxyquinoline and molecular structure of sodium dodecylbenzenesulphonate.

This work presents a corrosion inhibitor of 8-hydroxyquinoline (8HQ) with a high efficiency and its synergistic effect with the anionic surfactant sodium dodecylbenzenesulphonate (SDBS) upon AZ91D magnesium alloy corrosion in ASTM D1384-87 corrosive solution. The results were discussed on the basis of the electrochemical impedance spectroscopy and potentiodynamic polarization tests. The interaction between the inhibitors upon mixing was analyzed by calculating the synergism parameter. The components of the films were characterized by the Fourier transform infrared spectrum, solid ultraviolet absorption spectrum and solid fluorescence spectrum. Based on these results, a proper synergistic inhibition mechanism was proposed. 2. Experimental 2.1. Materials The substrate material used was an AZ91D die-cast magnesium alloy. The chemical composition of the alloy is given in Table 1. Samples embedded into epoxy resin with an exposure area of 1 cm2 were ground with SiC papers of successively finer grit down to 2000 grit. The ground samples were degreased by ethanol in an ultrasonic bath for 10 min, rinsed in distilled water, dried in air. And then samples were immersed in different solutions (i) 8HQ: an ASTM D1384-87 corrosive solution (Na2SO4, 148 mg/L, NaHCO3 138 mg/L, NaCl 165 mg/L, pH 8.2) saturated with 8HQ; (ii) SDBC: an ASTM D1384-87 corrosive solution with 1.16 mM SDBS, the concentration of SDBC was the critical micelle concentration obtained by conductance method; (iii) 8HQ + SDBC: an ASTM D1384-87 corrosive solution saturated with 8HQ for the initial time, with the last 0.5 h immersed in ASTM D1384-87 corrosive solution with 1.16 mM SDBS and saturated with 8HQ. The solutions were prepared from distilled water and analytical grade reagent chemicals. 2.2. Spectra tests The solid Fourier transform infrared (FT-IR) spectra were recorded with an IR-10300 (TENSOR-27, Germany) to analyze the microstructure of 8HQ samples. In comparison, the original 8HQ powder was also analyzed by FT-IR spectrum using the KBr pellet technique. The solid ultraviolet absorption spectra and solid fluorescence spectra of 8HQ samples were also analyzed using an UV-2550 ultraviolet spectrophotometer (Shimadzu Co. Ltd., Japan) and an F-2500 spectrofluorophotometer (Hitachi, Japan), respec-

Table 1 Chemical composition of experimental AZ91D magnesium alloy (in wt.%). Element

Mg

Al

Zn

Mn

Ni

Cu

Ca

Fe

Composition

Bal.

8.77

0.74

0.18

0.001

0.001

<0.01

<0.001

Electrochemical tests were employed to evaluate the inhibition effects of different inhibitors. The electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests were performed by a CHI660C system (Chenhua Co., China) and a PS-268B system (Beijing Zhongfu Corrosion and Protection Co. Ltd., China), respectively. A three-electrode cell was employed in those tests. The samples, as working electrode, each included an electrical connection wire which was initially attached the surface of samples with the conducting glue, and the pretreatment of the samples was described in the materials section. A KCl-saturated calomel electrode (SCE) and a platinum sheet were used as reference and counter electrodes, respectively. Prior to each electrochemical measurement, a stabilization period of 30 min was applied. All of the tests were operated in the air, without oxygen excluded. In electrochemical impedance spectroscopy tests, the measuring frequency range was from 105 Hz to 10 mHz, with an ac amplitude of 5 mV. The electrochemical impedance data were fitted with the ZSimpWin software using non-linear least square fit techniques. In the potentiodynamic polarization tests, the potential was scanned from 1.7 to 1.0 V (vs. SCE) with a scanning rate of 1 mV/s. At least three samples were measured for each recipe. All of tests were performed at 18 ± 2 °C. The corrosion inhibition efficiency gz (%) is defined as follows:

gz ¼

Rsum  Rsum0  100 Rsum

ð1Þ

where Rsum0 and Rsum are values of the total resistances (X cm2) obtained in absence and presence of inhibitors from EIS tests. The corrosion inhibition efficiency gp (%) is defined as follows:

gp ¼

icorr0  icorr  100 icorr0

ð2Þ

where icorr0 and icorr are the corrosion current densities (A/cm2) obtained in absence and presence of inhibitors from potentiodynamic polarization tests. 3. Results and discussion 3.1. Characterization of films prepared by 8HQ Fig. 2 presents the FT-IR spectra of films prepared by 8HQ on the surface of AZ91D magnesium alloy (Fig. 2a) and the original 8HQ powder (Fig. 2b). In Fig. 2a, the bands at 1498, 1465 cm1 are associated with the stretching vibration of aromatic CAC, CAN skeletal, respectively. The bands at 1381 and 1322 cm1 are attributed to the in plane bending vibration and deformation vibration of OAH, respectively. The peaks at around 1106 cm1 are ascribed to the stretching vibration of CAO. The band at around 2389 cm1 represents the stretching and bending vibrations of C@O bond of absorbed CO2 in the air [16]. Compared with Fig. 2b, the increase in the energy of the intermolecular hydrogen bonds from about 3100 to 3400 cm1 (in Fig. 2a) is indicating a polymeric association between 8HQ molecules. Moreover, the stretching vibration of aromatic CAH at around 3040 cm1 is weakened. In addition, intensity changes also occur in main bands when films form on magnesium alloy surface. These FT-IR measurements indicated the direct bonding between Mg atoms and

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Fig. 4. Solid fluorescence (a) emission spectrum of Mg(8HQ)2 film and 8HQ powder and (b) excitation spectrum of Mg(8HQ)2 film.

Fig. 2. FT-IR spectra obtained on (a) 8HQ samples after 72 h immersion and (b) the original 8HQ powder.

8HQ molecules via O and N atoms, and the formation of the Mg(8HQ)2 film [16,22]. The original 8HQ powder and Mg(8HQ)2 film were further examined by a solid UV–vis absorption spectroscopy, and the results are illustrated in Fig. 3. In the UV–vis spectrum of the original 8HQ powder, high energy band in the 220–250 nm region is probably due to the p ? p* transition, and the strong band at 250– 425 nm should be assigned to the n ? p* charge transition in quinoline ring [23]. For the solid UV–vis spectrum of the Mg(8HQ)2 film, it is similar to the UV–vis spectrum of the 8HQ power in shape, the absorption band of Mg(8HQ)2 at 220–275 nm is attributed to p ? p* transition in the large p-conjugated system. The complex also exhibits a strong band at 275–480 nm, which should be assigned to the n ? p* charge transition and metal-to-ligand charge transfer (MLCT) transition [23]. Notably, the UV–vis spectrum of the Mg(8HQ)2 film results in the red shift of solid UV–vis bands, this might be attributed to the coordination of ligands and

Fig. 3. Solid UV–vis absorption spectra of the original 8HQ powder and 8HQ samples after 72 h immersion.

metal ions, and the formation of additional five-member rings, which increasing the p ? p* conjugated length, accordingly increasing conjugated degree of the polymeric complex and reducing the energy gap between p and p* molecular orbital of the ligand [24]. The results of solid fluorescence emission spectrum and excitation spectrum tests are listed in Fig. 4a and b, respectively. The solid fluorescence emission spectrum (Fig. 4a) of the polymeric complex Mg(8HQ)2 displays a maximum emission wavelength kem,max at 500 nm with an excitation wavelength at 290 nm. But for original 8HQ powder, kem,max is at 471 nm with the same excitation wavelength and the fluorescence emission peak is very weak. In respect that the introduction of metal ions enhanced the conformational rigidity in the molecule structure and reduced the non-radiative decay of the MLCT excited state, the polymeric complex was apt to emit fluorescence [24]. The band gaps (Eopt g ) of the Mg(8HQ)2 complex and 8HQ powder could be estimated from the onset absorption (UVonset) with Eopt g ðeVÞ ¼ hc=k (h = 6.626  1034 J s, c = 3  108 m/s, 1 eV = 1.602  1019 J), and the results are 2.92 and 3.10 eV, respectively. This also indicated that Mg(8HQ)2 was apt to emit fluorescence. The excitation spectrum of the Mg(8HQ)2 film is shown in Fig. 4b, compared this spectrum with the UV–vis absorption spectrum of the Mg(8HQ)2 film in Fig. 3, it is obvious that the bands are both between 220 and 480 nm, which indicated the absorption of excited light energy

Fig. 5. Nyquist plots of substrate and SDBC samples for immersion time up to 72 h.

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Fig. 6. Equivalent circuit models for substrate and samples after 72 h immersion (a) substrate and (b) SDBC samples.

by (8HQ)2Mg molecule, causing the transition from ground state to excited state and then emitting the fluorescence of the ligand. All of spectrum tests indicated that Mg2+ on the surface of substrate could direct bonding with 8HQ molecules via O and N atoms and form the Mg(8HQ)2 film on the substrate surface. This insoluble complex compounds might provide high inhibition effect on the corrosion of AZ91D magnesium alloy, and tests using electrochemical methods were in good agreement with those spectrum tests shown in the following sections.

3.2. Electrochemical impedance spectroscopy tests To provide the information on the inhibition effect, electrochemical impedance spectroscopy was well suited for monitoring in situ any perturbation by an inhibitor according to the electrochemical processes in the interface of metal/corrodent. The influence of SDBC on the adsorption of magnesium alloy surface, high corrosion inhibition effect of 8HQ and the synergistic effect of the two have been studied. In order to check the reproducibility of the results, at least three samples were measured for each film and the values of low frequency impedance of three parallel measurements all converged within 25%. Fig. 5 shows Nyquist plots of bare substrate and SDBC samples. It is well known that the semicircle diameter of the Nyquist plot is proportional to the anticorrosion performance. Larger diameter reveals better anticorrosion performance. In contrast with bare substrate, the SDBC samples had better anticorrosion performance with prolonging immersion time until 48 h but with one exception, after 5 h immersion the diameters of the two semicircles both decrease to initial value of substrate, which could be attributed to the incomplete adsorption of SDBC and the corrosion of the substrate. It is also noted that after 72 h immersion the anticorrosion performance no longer increases and almost remains the same value as 48 h immersion. To further investigate the information of the films, fitting the equivalent circuits to the EIS data was a useful tool. Taking into account the similar phenomenon in Nyquist plots in Fig. 5, which all consist of one high frequency capacitance loop and one low frequency capacitance loop, the same equivalent circuit was proposed for magnesium alloy substrate and SDBC samples, as shown in Fig. 6a and b. For substrate, in Fig. 6a, the high frequency capacitance loop is attributed to the formation of the MgO passivation film on the surface of the alloy explained with the solution resistance Rs, the film resistance Rfi and the constant phase element CPEfi. The low frequency capacitive loop is due to the polarization resistance Rpo and the double layer capacitance CPEdl. For SDBC samples, in Fig. 6b, the physical interpretation of electric components Rfi and CPEfi is slightly different from the one given to the substrate, Rfi and CPEfi can be attributed to the formation of the

Fig. 7. Bode plots of 8HQ samples for immersion time up to 72 h.

Fig. 8. Bode plots of 8HQ + SDBC samples for immersion time up to 72 h.

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Fig. 9. Equivalent circuit models for substrate and samples after 72 h immersion (a) 8HQ samples and (b) 8HQ + SDBC samples.

MgO passivation film and the absorption of SDBC on the surface of passivation film. For more complicated system, Bode plots can give more information. The Bode plots of 8HQ samples and 8HQ + SDBC samples are presented in Figs. 7 and 8, respectively. The low frequency impedance modulus Zmod is one of the parameters which can be easily used to compare corrosion resistance of different samples. A larger Zmod demonstrates a better protection performance [25]. In Fig. 7, it is shown that Zmod is monotonic increase as a function of the immersion time for 8HQ samples. While in Fig. 8, after 48 h immersion, the Zmod reaches a relative stable value and the increases of Zmod in the following immersion is not remarkable. Namely, for 8HQ + SDBC samples, the formation of the stable protection films were faster and the films were more compact than those of 8HQ samples. Figs. 7 and 8 also show the changes of phase angle for 8HQ and 8HQ + SDBC samples with different immersion time. In Fig. 7, for 8HQ samples, the middle frequency phase angle is monotonic increase as a function of the immersion time in the initial 24 h. But obviously reducing of the middle frequency phase angle occurs in 48 h immersion with the high frequency phase angle increasing at the same time. For the 72 h immersion, the changes are more significant. These phenomena can be explained as follows, the high frequency phase angle range (104–105 Hz) of the impedance spectra corresponds to the properties of an outer layer, the middle frequency range (100–104 Hz) reflects the properties of an inner barrier layer, while the low frequency range (less than 100 Hz) corresponds to the properties of the double-electrical layer information [26]. Therefore, from 24 to 72 h immersion, the high frequency phenomenon may due to the thickness increase of the outer porous layer, and the middle frequency phenomenon can be attributed to the penetration of active chloride ions and water through the defect of Mg(8HQ)2 inner barrier layer, though the whole effect induced the increase of Zmod. In Fig. 8, for 8HQ + SDBC samples, the middle frequency phase angle is wider and higher in the initially 5 h immersion, and little reducing occurs in the 24 h immersion. However, after 48 h immersion, the curves shape is quite different with one elevatory wide middle frequency phase angle and one high frequency phase angle, the low frequency phase angle is unconspicuous. All these demonstrated that the composite films served as effective barriers against corrosion electrolyte ingress in the whole immersion time. For quantitative estimation the corrosion resistance and obtain the optimal inhibition efficiency, the equivalent circuits present in Fig. 9a and b were used to model the electrochemical behavior of 8HQ and 8HQ + SDBC samples after 72 h immersion. For 8HQ samples, the equivalent circuit in Fig. 9a is established in terms of the following considerations (I) Rs refers to the solution resistance between working electrode and reference electrode; (II) Rc and Qc are due to the conductive path in the films and described as a network

Table 2 The total fitting resistances of samples after 72 h immersion according to EIS data. Samples

Rsum (kX cm2)

gz (%)

Substrate SDBC 8HQ 8HQ + SDBC

Rs + Rfi + Rpo = 3.9 Rs + Rfi + Rpo = 8.7 Rs + Rc + Rfi + Rpo = 82 Rs + Rc + Rfi = 160

– 55 95 98

of electrolyte resistance and double layer capacitance in the pores of the films, respectively [27]; (III) the Mg(8HQ)2 film resistance Rfi paralleled with the constant phase element CPEfi; (IV) polarization resistance Rpo in parallel with the double layer capacitance CPEdl. In Fig. 9b, for 8HQ + SDBC samples after 72 h immersion, the physical interpretation of the electric components is similar to the components in Fig. 9a. With the addition of SDBC, the Rc reduced, indicating a more compact film formed on the substrate, Rfi and Qfi included the inhibition effect of both 8HQ and SDBC. The main fitting parameters for EIS data obtained for the substrate and samples after 72 h immersion are listed in Table 2, a great improvement on the corrosion resistance of the substrate was obtained for 8HQ + SDBC samples with a high total resistance Rsum. Namely, the 8HQ + SDBC films acted as good barriers against the penetration of active chloride ions and water, subsequently enhancing the anticorrosion performance of AZ91D magnesium alloy. The inhibition efficiency gz calculated from Rsum reaches a considerable percentage of 98%. For studying the synergistic effect of 8HQ and SDBC, the synergism parameter s is defined as follows [17]:

s¼

1  gA  gB þ gA gB 1  gAB

ð3Þ

where gA and gB are the inhibition efficiencies obtained by compounds A and B, respectively acting alone, and gAB is the experimentally obtained inhibition efficiency for the mixture A and B. If s > 1, the effect will be synergistic, or if s < 1, antagonistic. For the selected system, the s is 1.13, which showed moderate synergism. 3.3. Potentiodynamic polarization curve tests Fig. 10 lists the polarization tests of three parallel samples of AZ91D magnesium alloy substrate in ASTM D1384-87 corrosive solution. The data were not corrected for the ohmic potential drop between the working electrode and the Luggin capillary tip. Because for a about 1 mm spacing between the working electrode and the Luggin capillary tip, the ohmic resistance in the electrolyte alone (Na2SO4, 148 mg/L, NaHCO3 138 mg/L, NaCl 165 mg/L with trace inhibitors) is low or negligible, and the solution resistance Rs modeled from the EIS data also conform this. It can be seen that

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H. Gao et al. / Corrosion Science 52 (2010) 1603–1609 Table 4 Relevant electrochemical parameters of potentiodynamic polarization curves. Samples

Ecorr (V/SCE)

Rp (kX cm2)

icorr (lA/cm2)

gp (%)

Substrate SDBC 8HQ 8HQ + SDBC

1.59 1.59 1.44 1.44

2.15 4.30 11.94 18.33

10.1 ± 0.6 5.1 ± 0.2 1.8 ± 0.1 1.2 ± 0.2

– 50 82 88

ear polarization resistance Rp were determined from the slopes of the current–potential lines in the range of ±10 mV vs. the corrosion potential Ecorr. And the corrosion current density icorr were determined by the Stern–Geary equation:

icorr ¼ B=Rp

Fig. 10. Polarization curves of the three parallel samples of the AZ91D magnesium alloy substrate.

Table 3 Corresponding fitting results of the three parallel samples of AZ91D magnesium alloy substrate. Samples

ba (mV)

bc (mV)

B (mV)

Substrate 1 Substrate 2 Substrate 3

175 163 195

248 329 263

44.6 47.4 48.7

the reproducibility is good in the three parallel measurements. The anodic and cathodic Tafel slopes were obtained from about ±100 mV vs. corrosion potential Ecorr by OriginPro 7.5 software, and the corresponding fitting results are shown in Table 3. The B value is defined as follows:

B¼

ba bc 2:3ðba þ bc Þ

ð4Þ

where ba and bc are the anodic and cathodic Tafel slopes, respectively. The B values calculated from Eq. (4) are also list in Table 3. Fig. 11 gives the polarization plots in the potential vs. current density format. Linearity in the voltammograms at high current density implies ohmic control [28]. Therefore, both ohmic control and activation control (due to the present of the defects in the films) had influence on the polarization curves. The slopes of the potential–current lines in the vicinal of the corrosion potential Ecorr were quite different for different films. So the linear polarization resistance method was introduced to analysis this system. The lin-

ð5Þ

where B is a constant which contains Tafel slope information [29,30], which is defined in Eq. (4). There is little report about the B value of AZ91D magnesium alloy in ASTM D1384-87 corrosive solution, and as a relative comparison, the B value was used the average value of 46.9 mV in all the calculation of icorr. The average values of the relevant parameters of the polarization curves are listed in Table 4. It can be seen from Fig. 11a that the anodic reaction of electrode is inhibited due to the presence of SDBC, and icorr also decreases. In Table 4, the presence of single 8HQ causes Ecorr shift to the anodic direction and icorr decrease which meant a decrease in the corrosion rate. The inhibition efficiency gp calculated from the corrosion current density reaches 82%. For SDBC + 8HQ samples in Fig. 11b, the anodic reaction of electrode is also inhibited due to the addition of SDBC, which is similar to the polarization curve of SDBC samples. And for SDBC + 8HQ samples in Table 4, icorr decreases to 1.2 lA/cm2 and Rp reaches 18.33 kX cm2. The inhibition efficiency reaches a considerable value of 88%. The errors of icorr in the polarization tests are also listed in Table 4, icorr are all converged within 15%. And due to the limitary of the evaluation method of the polarization curves, error may also introduce in the exchange of the obtained icorr to the corrosion rate of the actual system. However, as a relative comparison and a complement to the EIS test, the polarization tests confirmed that the mixture of 8HQ and SDBC as a corrosion inhibitor had the best anticorrosion performance compared with single ones. 3.4. Study of the synergistic inhibition mechanism Through the above study, a proper synergistic inhibition mechanism was proposed. The spectrum tests indicated the formation of Mg(8HQ)2 film on the substrate surface. EIS tests illustrated that Mg(8HQ)2 films were composed by the outer porous layer and

Fig. 11. Potential vs. current density plots for (a) substrate, SDBC and (b) 8HQ and 8HQ + SDBC samples after 72 h immersion.

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the inner compact layer. With prolonging the immersion time for 8HQ samples, the thickness of outer porous layer increased, at the same time corrosion occurred on the substrate, thus Mg2+ and H+ would enrich in the defects of the outer porous layer. With the addition of SDBC, Mg2+ and H+ would act as bridges, and dodecylbenzenesulphonate negative ions would easily adsorb on the defects of the outer porous layer, thus increased the thickness of the inner compact layer and offered effective synergistic protection effect with 8HQ on AZ91D magnesium alloy. Both EIS tests and polarization tests confirmed the synergistic inhibition effect of SDBS and 8HQ. 4. Conclusions The inhibition effects of SDBS and 8HQ on the corrosion of AZ91D magnesium alloy in ASTM D1384-87 corrosive solution were investigated by electrochemical impedance spectroscopy and potentiodynamic polarization tests. For SDBS, the inhibition effect was limited, while 8HQ could effectively protect magnesium alloy from corrosion, and the spectrum tests showed that the component of the film was Mg(8HQ)2. Upon mixing 8HQ and SDBS inhibitors, the synergistic inhibition behavior was observed and the inhibition efficiency can reach a considerable value of 98% according to EIS tests, which provided effective protection for AZ91D magnesium alloy. Furthermore, the synergistic inhibition mechanism was studied and a proper synergistic inhibition mechanism was proposed. Acknowledgements The authors thank the supports of the Natural Science Foundation of Chongqing, China (CSTC. 2005BB4055) and High-Tech Cultivation Program of Southwest Normal University (No. XSGX06). References [1] J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys—a critical review, J. Alloys Compd. 336 (2002) 88–113. [2] K. Funatani, Emerging technology in surface modification of light metals, Surf. Coat. Technol. 133–134 (2000) 264–272. [3] G. Ballerini, U. Bardi, R. Bignucolo, G. Ceraolo, About some corrosion mechanisms of AZ91D magnesium alloy, Corros. Sci. 47 (2005) 2173–2184. [4] M.C. Zhao, M. Liu, G.L. Song, A. Atrens, Influence of microstructure on corrosion of as-cast ZE41, Adv. Eng. Mater. 10 (2008) 104–111. [5] P.L. Bonora, M. Andrei, A. Eliezer, E.M. Gutman, Corrosion behaviour of stressed magnesium alloys, Corros. Sci. 44 (2002) 729–749. [6] M.L. Zheludkevich, K.A. Yasakau, S.K. Poznyak, M.G.S. Ferreira, Triazole and thiazole derivatives as corrosion inhibitors for AA2024 aluminium alloy, Corros. Sci. 47 (2005) 3368–3383. [7] G. Moretti, F. Guidi, G. Grion, Tryptamine as a green iron corrosion inhibitor in 0.5 M deaerated sulphuric acid, Corros. Sci. 46 (2004) 387–403. [8] W.J. Guo, S.H. Chen, Y.Y. Feng, C.J. Yang, Investigations of triphenyl phosphate and bis-(2-ethylhexyl) phosphate self-assembled films on iron surface using electrochemical methods, Fourier transform infrared spectroscopy, and molecular simulations, J. Phys. Chem. C 111 (2007) 3109–3115.

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