Electrochimica Acta 55 (2009) Contents lists available at ScienceDirect. Electrochimica Acta

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1 Electrochimica Acta 55 (2009) Contents lists available at ScienceDirect Electrochimica Acta journal homepage: Complex anticorrosion coating for ZK30 magnesium alloy S.V. Lamaka a,, G. Knörnschild b, D.V. Snihirova a, M.G. Taryba a, M.L. Zheludkevich c, M.G.S. Ferreira a,c a Instituto Superior Técnico, UTL, ICEMS, Av. Rovisco Pais, Lisbon, Portugal b Rio Grande do Sul Federal University, Porto Alegre, Brazil c University of Aveiro, CICECO, Dep. Ceramics and Glass Eng., Aveiro, Portugal article info abstract Article history: Received 25 June 2009 Received in revised form 13 August 2009 Accepted 13 August 2009 Available online 21 August 2009 Keywords: Magnesium alloy ZK30 Sol gel coating Corrosion inhibitors Spark anodizing This work aims at developing a new complex anticorrosion protection system for ZK30 magnesium alloy. This protective coating is based on an anodic oxide layer loaded with corrosion inhibitors in its pores, which is then sealed with a sol gel hybrid polymer. The porous oxide layer is produced by spark anodizing. The sol gel film shows good adhesion to the oxide layer as it penetrates through the pores of the anodized layer forming an additional transient oxide sol gel interlayer. The thickness of this complex protective coating is about m. A blank oxide sol gel coating system or one doped with Ce 3+ ions proved to be effective corrosion protection for the magnesium alloy preventing corrosion attack after exposure for a relatively long duration in an aggressive NaCl solution. The structure and the thickness of the anodized layer and the sol gel film were characterized by scanning electron microscopy (SEM). The corrosion behaviour of the ZK30 substrates pre-treated with the complex coating was tested by electrochemical impedance spectroscopy (EIS), scanning vibrating electrode technique (SVET), and scanning ion-selective electrode techniques (SIET) Elsevier Ltd. All rights reserved. 1. Introduction Magnesium-based alloys have a number of advantageous physical- and mechanical-properties, which make them an attractive choice for many industrial applications. These materials are used when the low weight of the product is of significant importance. Apart from extensive use in automotive industry, Mg-based alloys are utilized in the production of parts for computers and other portable devices, aircraft, military, recreational and orthopaedic equipments, diving gear, and sports goods. However, one of the main reasons limiting larger use of light magnesium alloys is their high susceptibility to corrosion. One of the approaches to corrosion protection of magnesium recently discussed in the literature is based on high-voltage anodization of the metal surface [1 3]. The concept and process of microarc oxidation, also known as plasma electrolytic oxidation or spark anodizing, were patented in USA in the 1990s [4,5]. According to the literature, anodized layers obtained by the sparking process are generally described as relatively thick (up to 100 m), hard, ceramic-like coatings comprised of a dense inner ceramic-like layer mostly containing Mg(OH) 2 and an outer porous layer comprised of MgO. For a better understanding of the structure and properties of the anodized layers on Mg-based alloys readers may refer to several recently published works [1 7]. Corresponding author. Tel.: ; fax: address: sviatlana.lamaka@ist.utl.pt (S.V. Lamaka). Irregular pores that appear in the oxide layer in the course of anodization form the pathways for corrosive species impairing the total protective effect of anodization. It is possible to seal the pores in order to improve the protective properties conferred by the oxide film. Recent achievements in sol gel technology allow successful formulation of durable hybrid organic inorganic sol gel coatings, which are used as anticorrosion pre-treatments for aluminiumand magnesium alloys [8 17]. Hybrid sol gel coatings assure good adhesion of the organic paint to the metal substrate and by combining active- and passive-protection provide an additional dense barrier against corrosive species. The sol gel route offers versatile ways to synthesize effective coatings with desired properties. Functionality is achieved by varying experimental parameters such as chemical structure, composition, and ratio of precursors and complexing agents, the rate and conditions of hydrolysis, synthesis media, embedding of additional active species (e.g. encapsulatedor directly introduced corrosion inhibitors), aging and curing conditions and deposition procedure. However, only a few attempts to reinforce anodized layers on magnesium-based alloys by sol gel film have been reported recently [18,19]. The major objective of this work was to use a porous layer of magnesium oxide formed by spark anodizing as an additional intermediate dense barrier and reservoir of corrosion inhibitors placed between the metal and the sol gel coating. We describe a complex anticorrosion multilayer coating system for the magnesium alloy ZK30 consisting of environmentally friendly corrosion inhibitors for magnesium alloys combined with a thin hybrid sol gel coating without decreasing the barrier /$ see front matter 2009 Elsevier Ltd. All rights reserved. doi: /j.electacta

2 132 S.V. Lamaka et al. / Electrochimica Acta 55 (2009) properties of the coating or deactivation of the inhibitors. For this purpose, an intermediate porous layer of magnesium oxide obtained by spark anodizing was placed under the sol gel coating and served as a reservoir of corrosion inhibitors. 2. Experimental 2.1. Materials and reagents In this study, plates of extruded ZK30 magnesium alloy (Alubin, Israel) were used as the metallic substrates. Apart from Mg, the ZK30 alloy is comprised of about 3 wt% Zn and 0.6 wt% Zr [20]. Prior to anodizing, all the magnesium coupons were polished with emery paper, sequentially using 240, 600, 1200 and 2000 grits (Struers, SiC) and deionized water. The samples were then washed with distilled water in an ultrasonic bath and dried in air at room temperature. All the reagents used to synthesize the sol gel films, prepare the anodizing solutions and solutions for immersion were puriss or better grade products of Sigma Aldrich with exception of (3- glycidoxypropyl)-trimethoxysilane, which contained at least 98% main product. KF and Al(OH) 3 used for anodizing were obtained from Synth (Brazil). Al(OH) 3 contained at least 76.5% main product with Al 2 O 3 being the main impurity. All the reagents were used without further purification. The aqueous solutions were prepared using MilliPore purified water ( >18M cm) Anodizing Anodizing was performed in an electrolyte which contained 135 g/l NaOH, 34 g/l Al(OH) 3, 34 g/l Na 3 PO 4, and 34 g/l KF. A noncommercial inhouse-made direct current/direct voltage source was used. The applied voltage limit was 70 V, and the current density 125 ma/cm 2. The duration of the tests was 10 min. A typical test consisted of two parts. The first part was growth of a galvanostatic film until the voltage limit was reached. The second part was potentiostatic. The current density diminished while the voltage limit was maintained. The porous oxide was impregnated with Ce 3+ ions and 8- hydroxyquinoline (8HQ) by immersing the anodized magnesium plates into M aqueous solutions of Ce(NO 3 ) 3 and 8HQ, respectively, for 30 min Synthesis of sol gel coatings The organic inorganic films were synthesized using a controllable sol gel route, mixing two different sols. The sol gel coating was composed of in situ synthesized titania nanoparticles and (3- glycidoxypropyl)-trimethoxysilane (GPTMS). Silane-based alkosol was prepared by the hydrolysis of GPTMS in iso-propanol (ratio of 1:1 by volume) to which diluted aqueous solution of HNO 3 (ph = 0.5) was added accompanied by constant stirring at room temperature for 1 h. The second alkosol was produced by controlled acidic hydrolysis (ph = 0.5) of 70% iso-propanol solution of titanium (IV) iso-propoxide (Ti(OiPr) 4 ) in iso-propanol in the presence of a complexing agent (acetylacetone) and ultrasonically agitating the resulting solution at a temperature of 22 ± 1 C. The molar ratio of Ti(OiPr) 4 :cac:h 2 O was 1:3:5. Finally, the silane-based and titania-containing alkosols were mixed together in 2:1 volume ratio, respectively. The hybrid organo-inorganic system was kept constantly stirred and ultrasonically agitated at 22 ± 1 C for one more hour. This formulation was then aged for 1 h at room temperature and deposited on anodized ZK30 substrates. The sol gel films on metallic substrates were produced by a dip-coating procedure at a withdrawal speed of 18 cm/min and exposure time in the solution of 100 s. The samples were then cured at 120 C for 80 min. All samples tested in the present study were coated with identical sol gel formulation Microscopic characterization SEM/EDS was used for examining the microstructure and general chemical composition of the anodized layers and sol gel films before- and after immersion tests. A semi-in-lens Hitachi SU- 70 UHR Schottky (Analytical) FE-SEM microscope coupled with a Bruker EDS detector was used. Electron beam energy of 15 kev was applied for SEM analysis and EDS mapping. The samples for crosssection analysis were prepared by embedding the treated coupons of ZK30 into epoxy resin (Buehler). To reduce the distortion of the image due to the signal from the epoxy resin two pieces of the same sample were glued together by their anodized and sol gel coated surfaces. After solidification of the epoxy resin, the samples were polished sequentially with 220, 320, 1000, 2400 grit emery papers (Struers, SiC) in deionized water and finally with 4000 grit emery paper in ethanol Electrochemical techniques EIS. Impedance measurements were carried out to evaluate the corrosion protection performance of the developed complex coating system on ZK30 during 4-week period of immersion in M NaCl solution or 2-week period in 0.05 M NaCl solution at neutral ph level ( ). EIS measurements were recorded using a Gamry FAS2 Femtostat coupled with a PCI4 Controller at open circuit potential applying 10 mv sinusoidal perturbations in the 100 khz to 8 mhz frequency range. Per frequency decade, 7 or 12 experimental points were collected during the measurements. A conventional three-electrode cell was used and consisted of a saturated calomel reference electrode, a platinum wire as a counter electrode, and the pre-treated magnesium-based alloy as working electrode of a surface area of 3.3 cm 2. All measurements were performed in a Faraday cage in order to avoid any electromagnetic interference. A simplex method was employed to fit the impedance plots using Gamry Echem Analyst software, version Four different types of ZK30 samples including the blank and the doped with organic- or inorganic inhibitors were tested: (1) anodized alloy, ZK Anod; (2) anodized alloy sealed with the sol gel only, ZK Anod SG; (3) anodized alloy immersed in Ce 3+ solution and coated with the sol gel film, ZK Anod Ce 3+ SG, and lastly (4) anodized alloy immersed in 8HQ solution and sealed with the sol gel film, ZK Anod 8HQ SG. SVET and SIET. Commercial equipment manufactured by Applicable Electronics controlled by the ASET Program (Sciencewares) was used to perform the Scanning Vibrating Electrode Technique (SVET) measurements and the Scanning Ion-selective Electrode (SIET) study. To evaluate the corrosion inhibition performance of the complex protective coating doped with Ce 3+ ions, artificial defects of 100 m size and larger were created on the surface of the sample before immersion. Periodical measurements were taken during its exposure to 0.05 M NaCl neutral solution. The scanned area was about 1.5 mm 1.5 mm. The local currents and H + activities were mapped on a grid, which generated 961 data points. The details of the SVET and SIET set-up and procedures are reported elsewhere [21]. Briefly, the vibrating electrode of the SVET was an insulated Pt Ir probe (Microprobe Inc., USA) with Pt black deposited on the spherical tip of 10 m diameter. The probe was located 150 m above the surface and vibrated in the perpendicular direction to the surface (Z) with amplitude of 20 m. The vibration frequency of the probe was 124 Hz. Localized ph measurements were recorded using phselective glass-capillary microelectrodes. The silanized glass

3 S.V. Lamaka et al. / Electrochimica Acta 55 (2009) Cross-sectional SEM/EDS images in Fig. 3a c reveal that the anodized film is composed of two layers, a thin inner barrier layer and an outer randomly porous layer. Both oxide layers appear to be enriched with aluminium, Fig. 3b and c. The thickness of the entire anodized film was in the range m as assessed by the cross-sectional SEM/EDS analysis, Fig. 3a Sol gel coatings Fig. 1. Schematic representation of the complex protective coating that comprises the layer of magnesium oxide, inhibitor adsorbed in the pores of the anodized layer and sealed with sol gel film. micropipettes were back-filled with the inner filling solution and tip-filled with selective ionophore-based oil-like membrane. The ion-selective membrane consisted of 6 wt% ETH nonadecylpyridine, 12 mol% (relative to ionophore) potassium tetrakis(4-chlorophenyl)borate, and membrane solvent 2-nitrophenyloctyl ether. All reagents for ph-selective membrane were Selectophore grade products of Fluka. The ph-selective microelectrodes were calibrated using commercially available (Fluka) and homemade ph buffers. The linear range of ph response was 2 10, the Nernstian slope was 54.8 ± 0.7 mv/dec. The local activities of H + were mapped 30 m above the surface. The time for acquisition for each SIET data point was 3 s. Hybrid organic inorganic sol gel coatings are environmentally friendly pre-treatments for aluminium- and magnesium alloys and have been extensively studied over the last few years [9]. The properties of such coatings are reported in our previous papers [14,15,23]. Before its application on metallic substrates, the sol gel solution is homogenous and transparent and is light-yellow in colour. The viscosity of these hybrid mixed sols remains in the range 8 22 cp for 2 weeks [23]. The SEM image (Fig. 4) shows the plane view of the deposited sol gel coating. Neither cracks nor pores are visible in the coating at 700 times magnification. The white spots seen in the EDS analysis image are dust specks mostly composed of carbon. SEM images of the cross-section and EDS element mapping of the same area of ZK Anod SG sample are presented in Fig. 5. The thickness of the sol gel layer is in the range 3 4 m. Apart from the ZK30 magnesium substrate, the sol gel layer, and the epoxy 3. Results and discussion The schematic representation of the complex protection coating system developed is shown in Fig. 1. The porous oxide layer increases the corrosion resistance of the magnesium substrate and provides the reservoirs for the corrosion inhibitor, which impedes the corrosion process when the corrosive media penetrates the oxide layer through the microdefects present in the sol gel film. The oxide layer also serves to enhance considerably the surface area of the substrate coming into contact with the sol gel coating and results in better adhesion Anodized layers Fig. 2a presents the plane view of the anodized layer obtained on ZK30 alloy by spark anodizing. Breakdown of the oxide film results in the formation of pores of diameters in the range m. Voltage transients during anodizing show an abrupt change from conventional anodizing to the prevalence of sparking conditions at about 55 V. An irregular porous layer uniformly covers the surface of the alloy. The distribution of the pores on the surface is influenced by the alloy s microstructure [6]. Furthermore, the cracks are clearly visible in the anodized layer. Presumably dielectric- and mechanical breakdown mechanisms may cause sparking depending on the conditions of film formation [6]. Mechanical breakdown in the MgO layer can be attributed to the molar volume mismatch of Mg and MgO (Pilling Bedworth rule), V Mg /V MgO = 0.81 [22]. The elements of the anodizing electrolyte, namely Al, P and F are detected in the oxide layer besides the Mg, Zn and Zr which are the main elements of the alloy, Fig. 2b. Fluoride increase anticorrosion stability of the anodized layer due to incorporation of insoluble MgF 2. Fluoride ions additionally incorporated in anodized layer may also reveal inhibiting effect in the course of corrosion process reacting with anodically generated Mg 2+. Fig. 2. SEM/EDS observation of porous magnesium oxide structure obtained by spark anodizing on ZK30 alloy: plane view (a), overall EDS spectrum (b).

4 134 S.V. Lamaka et al. / Electrochimica Acta 55 (2009) Fig. 4. SEM image of the anodized sample coated with sol gel film, plane view. glue used for mounting, two more structural layers are clearly visible: these are (1) thin barrier layer of magnesium oxide formed on the surface of metallic magnesium (indicated by white arrows in Fig. 5) and (2) transition layer composed by porous oxide and the sol gel film (between black arrows). The latter forms as a result of the sol gel that flows into the outer porous part of the anodic film. Interpenetration of the sol gel and anodized layers results in perfect adhesion of the sol gel film. The unique protective properties of thin sol gel coatings originate from the formation of stable Si O Me bonds, which prevent the corrosive medium access to the substrate surface and thereby delaying the setting in of corrosion reactions. Incorporating any intermediate layer between the substrate and sol gel coating is liable to weaken the adhesion of the sol gel films as shown in Ref. [24]. However, the approach adopted here and in our previous papers [25,26] of building up the porous reservoirs with a highly developed surface helps overcome this limitation. Apparently, the presence of inhibitors in the pores of the oxide does not affect the adhesiveness of sol gel film to the anodized surface Immersion tests in dilute M NaCl solution Fig. 3. Cross-section SEM (a) and EDS (b and c) imaging of blank anodized ZK30 sample. To test the anticorrosion protective performance of developed complex coating the first batch of samples was immersed in dilute neutral M NaCl solution. Comparative optical photographs of the samples surface after 4 weeks of immersion are reproduced in Fig. 6. ph values of NaCl solution in the electrochemical cells recorded after immersion tests complement the optical Fig. 5. (a) SEM image of cross-section view of anodized specimen coated with sol gel film; (b) EDS mapping of the same zone showing Mg-based substrate, the inner oxide layer (white arrows), barrier oxide layer (black arrows), mixed oxide-sol gel layer, the layer of sol gel coating and the epoxy mount.

5 S.V. Lamaka et al. / Electrochimica Acta 55 (2009) Fig. 6. Optical photographs of sample surfaces supplemented with ph values of bulk solution after one month of immersion in M NaCl. The diameter of the round cell for impedance tests was 18 mm. photographs. Although no intensive release of hydrogen bubbles was observed visually the blank anodized ZK30 specimen, ZK Anod, and the anodized sample impregnated by 8HQ sealed with sol gel film, ZK Anod 8HQ SG, become of darker colour with time, Fig. 6a and b. The ph of the bulk solution after immersion altered into the alkaline range, up to 7.7. It should be stated that the 8HQ doped sol gel film was rough in appearance and looked damaged immediately after deposition of the film. Apparently, the organic inhibitor chemically interacts with the components of the sol gel film. This leads to the disruption of the polymerization process, decomposition of the coating, as well as deactivation of the inhibitor. An interesting observation, as reported in our previous works, was that this effect was not visible when 8HQ was doped directly onto the same sol gel composition [27,28]. On the contrary, the presence of inhibitors even improved the anticorrosion protective performance of the coating. However, when 8HQ was deposited on a developed surface of nanostructured TiO 2 and subsequently covered with a similar sol gel coating the sol gel was observed to have decomposed (data not published). Most probably this effect is related to the rapid decomposition of 8HQ catalyzed by the developed oxide surface. Neither signs of corrosion attack nor any indication of delamination of the sol gel film are visible on the surface of the anodized Mg sealed with sol gel ZK Anod SG or where the same system was doped with Ce 3+ ions ZK Anod Ce 3+ SG, Fig. 6c and d. The visual observations complemented the electrochemical impedance spectroscopy tests. EIS provides a quantitative estimation of coating degradation and the rate of emergence of corrosion processes. The Bode plots of selected impedance spectra are presented in Fig. 7. The resistive response at low frequencies corresponds to the polarization resistance for the ZK Anod sample. A rise in polarization resistance over the 4 weeks of immersion during the tests can be ascribed to the densifying and sealing of the porous oxide layer due to conversion of MgO to Mg(OH) 2, in which molar volume is larger than that of MgO [22] and which is more stable thermodynamically in aqueous solutions than MgO [29]. However, an increase of ph up to 9.9 measured in the bulk solution after 1 month of immersion suggests that the electrochemical dissolution of magnesium and the accompanying cathodic reactions also take place resulting in the formation of additional corrosion products (see Section 3.5). The resistive response (Z mod curve) of ZK Anod SG decreased only slightly (Fig. 7) throughout 4 weeks of immersion which is a very promising result for a highly corrosion-susceptible magnesium alloy. The ph of the bulk solution remained neutral at 6.0, which is also an evidence of the absence of the corrosion process. A comprehensive analysis of fitted spectra is given in the next part of this article. The ZK Anod Ce 3+ SG sample showed similar results. Thus, the samples where the pores of the anodized layer were sealed by the sol gel film did not suffer corrosion attack in the course of the 4-week immersion test in ph-neutral NaCl solution. The presented results by now explicitly indicate the degree of anticorrosion efficiency of the developed sol gel films. However, immersion tests monitored with EIS were repeated in more concentrated 0.05 M NaCl solution in order to establish the superior anti-corrosion properties of the developed coatings Immersion tests in 0.05 M NaCl solution The visual appearance of the four samples after 2 weeks of immersion (Fig. 8) was found analogous to that obtained for samples after the tests in dilute solution. The Nyquist plots for these samples were in-line with visual observations and are presented in Fig. 9. The extent of corrosion attack is greater than in the samples immersed in dilute NaCl solution. The ph values of the bulk solution measured after immersion also indicated deeper corrosion effects in all samples. The surface of the blank anodized sample ZK Anod became evenly grey. One big deep pit and a thread of the filiform corrosion are visible. Two deep pits and an area of grey colour that looks similar to one on the blank ZK Anod sample are the outcomes of exposure to NaCl solution of 8HQdoped sample, ZK Anod 8HQ SG. Small isolated pits appeared in the samples with the sol gel sealed anodized layer, ZK Anod SG and ZK Anod Ce 3+ SG. The evolution with time of the impedance spectra of the ZK Anod Ce 3+ SG sample in the course of the immersion test is shown in Fig. 10. At the beginning of the immersion test, the impedance spectra of anodized ZK samples sealed with the sol gel film showed three time constants. The resistance R SG and capacitance C SG of the sol gel coating can be clearly distinguished from Fig. 7. Evolution of Bode plots in M NaCl in the course of immersion.

6 136 S.V. Lamaka et al. / Electrochimica Acta 55 (2009) Fig. 8. Visual appearance of the samples and ph of solution in corresponding electrochemical cell after 2 weeks of immersion in 0.05 M NaCl. The diameter of the round cell for impedance tests was 18 mm. the response of the mixed sol gel/oxide layer R MIX and C MIX and from the response of the dense oxide layer R Ox and C Ox. This is in good agreement with the SEM/EDS images, Fig. 5, which shows two well defined layers: one transitional layer formed by the sol gel film flowing into the outer pores of the anodized layer and one dense oxide layer which was formed directly on the metal surface. The immersion in NaCl solution results in the growth of microdefects in the sol gel coating and anodized layer. This opens up pathways for corrosive species to the surface of magnesium. An additional fourth time constant appears after several days of immersion in the low frequency region. This is ascribed to the initiation of the corrosion attack and is attributed to the existence of the double-layer capacitance at the metal/electrolyte interface, C DL, and corresponding polarization resistance, R polar. This time constant appears along with the first pits on the samples surface, Fig. 8. Note that this time constant was not present for the sol gel coated samples when they were immersed in dilute M NaCl solution, as it results from impedance spectra, Fig. 7, and from visual observation of the samples, Fig. 6c and d. In spite of the rapid fall in the initial values of the sol gel film resistance R SG that occurs due to the electrolyte uptake, the modulus of complex impedance remains higher than 5 M cm 2. This emphasizes the good barrier properties of the complex coatings and their stability over time. Spectra of ZK Anod SG and ZK Anod 8HQ SG showed the same number of time constants but different absolute values of impedance modulus and phase angle. For the quantitative estimation of the corrosion protective properties of different complex coatings, experimental impedance spectra were fitted with the equivalent circuits, which simulated the response of the anodized alloy sealed with sol gel layer. Schematic representation of the equivalent circuits and their physical interpretation are shown in Fig. 11. In the equivalent circuit, R sol is the resistance of the corrosive medium, namely 0.05 M NaCl solution. Constant phase elements (CPE) instead of pure capacitances were used for fitting experimental spectra. Such modification is obligatory if the phase shift of a capacitor differs from 90 [30]. Fig. 12 presents the evolution of different parameters of the coated samples obtained after the fitting of the experimental spectra. This figure also shows comparison of anticorrosion performance of the coatings containing Ce 3+ and 8HQ with that of the undoped sample. At the beginning of the immersion the hybrid coating with the Ce 3+ ions displays the highest resistance, R SG, and the presence of 8HQ the lowest values of R SG, while the undoped coating ZK Anod SG keeps the middle position, Fig. 12a. Rapid decrease of R SG during the first hours of contact with chloride solution is usually observed for the sol gel coatings of this type [14,15,25]. Penetration of water and chloride ions through the nano-sized pores of the coating is responsible for this drop in resistance. The fall in R SG of the 8HQ doped system continued during the days that followed, revealing the weak barrier properties and stability of this film. Gradual change in the sol gel film s resistance after the first 2 days of immersion is the evidence of the good barrier properties and stability of the blank film and of the coating with the Ce 3+ ions. Fig. 9. Comparison of the Nyquist plots of different samples after 2 weeks immersion in 0.05 M NaCl solution. Fig. 10. Bode plots with corresponding fitting for anodized ZK30 specimens immersed in solution of Ce 3+ and coated with the sol gel film (ZK Anod Ce 3+ SG). Evolution of spectra in the course of 2-week immersion test in 0.05 M NaCl.

7 S.V. Lamaka et al. / Electrochimica Acta 55 (2009) Fig. 11. Schematic representation of the physical meaning and corresponding equivalent circuits used for fitting experimental EIS spectra at different immersion times. Fig. 12. Evolution of parameters depicted in Figs. 10 and 11 in the course of immersion in 0.05 M NaCl solution. Variation of coating resistance (a); mixed sol gel layer (b); dense oxide layer (c); and parameters of corrosion process (d).

8 138 S.V. Lamaka et al. / Electrochimica Acta 55 (2009) Fig. 13. Optical image and corresponding ph mapping over sol gel covered anodized sample of ZK30, ZK Anod Ce 3+ SG. Two artificial defects are hidden by the hydrogen bubbles in the optical image. The resistance of the transitional layer R MIX developed by the outer porous oxide and the sol gel film is presented in Fig. 12b. Fig. 12c shows the evolution of the inner dense oxide layer R Ox and characterizes the corrosion stability of the whole system since this thin layer is the last barrier that remains between the corrosive medium and the metal. The resistance of both layers dropped after the first day of immersion, remained stable over 2 3 days and then rose slightly. This behaviour is explained by the formation of additional magnesium hydroxide, which seals and densifies the pre-existing anodized layer. Note the rise of R Ox and R MIX is sharper for the ZK Anod Ce 3+ SG sample than for the ZK Anod SG. The formation of highly insoluble cerium hydroxides that seals the porous anodized layer occurs at lower ph (about 5.5), than the formation of Mg(OH) 2 that occurs at ph = 8.5 [29,31]. Thus, densification of the anodized layer in the cerium-containing complex coating occurs at the earlier stage of initial corrosion reactions. R Ox and R MIX for ZK Anod 8HQ SG are 1 2 orders of magnitude lower than for the ZK Anod Ce 3+ SG and ZK Anod SG samples, confirming the presence of an adverse effect of 8HQ on the corrosion protection properties of the developed coatings. The gradual degradation of protective layers results in the appearance of the fourth time constant that enables the quantification of the polarization resistance, Fig. 12d, R polar, which characterizes the rate of the corrosion process. The polarization resistance of ZK Anod Ce 3+ SG while remaining between 7 and 14 M cm 2 falls to 1 M cm 2 for the ZK Anod SG sample. Yet polarization resistance for both samples remained quite high compared to R polar for the coating system doped with 8HQ, where it dropped from around 1 M to 0.1 M cm 2 after 2 weeks SVET measurements and localized ph mapping To learn more about the corrosion mechanisms and to confirm the effective anticorrosion performance of the inhibitor-doped complex coating ZK Anod Ce 3+ SG sample was studied by means of the localized electrochemical techniques, SVET and SIET. Fig. 14. Optical micrographs and SVET maps of ionic currents measured above the surface of ZK Anod Ce 3+ SG sample. The maps and optical images were taken after exposure to NaCl solution. The time of exposure is indicated in each image.

9 S.V. Lamaka et al. / Electrochimica Acta 55 (2009) Fig. 15. SEM micrographs and EDS elements mapping of the artificial defect in ZK Anod Ce 3+ SG sample after immersion and SVET/SIET studies. (a) and (d) secondary electron images of the defect and its magnified fragment; EDS elemental mapping showing distribution of (b) Mg and Si (c) O; (e) overall EDS spectrum. Au was deposited for SEM observations. The ph distribution was measured over the artificially damaged sample s surface. The defects were hidden under the hydrogen bubbles in Fig. 13a, which are also indicative of the intensity of the corrosion reactions. The hydrogen bubbles generated were released roughly every 5 10 s at the beginning of the immersion of the sample with the defects. Local alkalinization of the solution up to ph = 9.9 in the background of a neutral 0.05 M NaCl solution with ph = 5.5 was mapped by a glass-capillary ph-selective microelectrode. The evolution of local current density was measured over the sample surface with three artificial defects, Fig. 14a. Two round relatively deep defects were intentionally needled, while a third shallow and wider defect was created by the glass-capillary microelectrode used for SIET measurements as it crushed against the sample (Fig. 14a, lower left zone). The first SVET measurement taken immediately after SIET ph mapping (Fig. 13) corresponds to 2 h of immersion of the sample into 0.05 NaCl solution. High anodic- and cathodic activity accompanied by hydrogen evolution was recorded in the area of the defects. However, the activity waned thereafter resulting in complete passivation of the sample after 24 h of immersion, Fig. 14c f. This proves that the good protective properties of the coating system impeded propagation of the artificial defects. This result is unexpected since the values of local currents were very high at the beginning of immersion. Fig. 15 presents SEM/EDS images of the defect tested by SIET/SVET, the same defect which is depicted in the upper part of the optical micrographs in Figs. 13a and 14a and e. SEM/EDS measurements were made after SVET/SIET studies, when the defect became passive. The deep defect is completely covered by the

10 140 S.V. Lamaka et al. / Electrochimica Acta 55 (2009) corrosion products. The remnants of the sol gel coating damaged by needling are also present. The EDS analysis of the smaller area of the defect covered by corrosion products reveals the presence of cerium and high amount of oxygen as well as the main alloy elements, Mg, Zn and Zr. The corrosion process can be characterized by the following reactions: Cathodic reactions 2H 2 O + 2e 2OH + H 2 (1) O 2 + 2H 2 O + 4e 4OH (2) Anodic reaction Mg Mg e (3) Overall reaction Mg + 2H 2 O Mg(OH) 2 + H 2 (4) Formation of monovalent cation Mg + seems also possible [32,33]: Anodic partial reaction Mg Mg + + e (5) Overall reaction 2Mg + + 2H 2 O 2Mg OH + H 2 (6) Silane-based sol gel coatings are known for their low hydrolytic stability in alkaline medium. The local ph of cathodic reactions rising to a value of 10 could accelerate the hydrolytic decomposition of the sol gel. However, this does not seem to be the case. The reason is that the sharp local increase of ph in the damaged zone of the ZK Anod Ce 3+ SG sample favours the formation of Mg(OH) 2 that precipitates and blocks further propagation of the corrosion process and degradation of the coating. Mg 2+ cations produced as the products of anodic dissolution and OH ions formed as products of cathodic reactions were consumed in the formation of additional Mg(OH) 2.Ce 3+ ions impregnated in the anodized layer and released in the course of anodic dissolution of the magnesium substrate facilitate the suppression of the cathodic reactions forming precipitates of Ce(OH) 3 and Ce(OH) 4, also consuming OH generated by cathodic reactions [31]. The areas of the anodized magnesium around the defects exposed due to mechanical detachment of the sol gel film can additionally be passivated by conversion of MgO to the lower density Mg(OH) 2, which partially blocks the pores and prevents penetration of corrosive medium to the thin barrier layer. This explains the relative inactivity of the local defects visible on the surface of ZK Anod Ce 3+ SG and ZK Anod SG after 2 weeks of immersion in 0.05 M NaCl, Fig. 8 (c and d). Once formed, these defects remain small and do not grow. The ph of the bulk solution in electrochemical cells becomes alkaline but only slightly, up to 7.8 and 7.3, Fig. 8c and d. While the ZK Anod Ce 3+ SG and ZK Anod SG samples were passive during the immersion, the ZK Anod sample underwent the most significant changes among all the tested samples. The ph of the bulk solution in the electrochemical cell of this sample rose to 9.9 after immersion in M NaCl and to 10.5 after immersion in 0.05 M NaCl. The explanation that gradual conversion of MgO to Mg(OH) 2 partially seals the pores seems to be valid in this case too and was discussed in literature for AZ31 Mg-based alloys [15] and anodized WE43 [3]. A continuous increase of the lower frequency impedance values through 4 weeks of immersion in M NaCl solution supports this assumption, Fig. 7. However, prolonged exposure to the more concentrated 0.05 M NaCl solution leads to filiform corrosion, Fig. 8 a. 4. Conclusions A new approach for formulating complex anticorrosion coatings for magnesium-based alloys has been presented. A porous layer of magnesium oxide formed by spark anodizing increases the corrosion resistance and serves as a reservoir of corrosion inhibitors placed under a thin hybrid sol gel coating. The elements of the anodizing electrolyte (Al, P, and F) are incorporated into the structure of the oxide where an inner barrier layer and outer porous layers can be distinguished. Interpenetration of the sol gel and anodized layers results in perfect adhesion of the sol gel film to the surface. The effectiveness of corrosion protection was verified by EIS and SVET measurements. The use of 8-hydroxyquinoline as corrosion inhibitor disrupts the integrity of the sol gel coatings exhibiting results similar to those of anodized magnesium without the sol gel coating. The anticorrosion performance of the complex coatings consisting of an anodized layer covered by sol gel film as well as in the case this film is doped with Ce 3+ ions allows immersion of ZK30 magnesium alloys into M and 0.05 M neutral aqueous NaCl solution without destructive outcomes. Gradual penetration of the aqueous corrosive solution to the anodized layer results in conversion of MgO to the Mg(OH) 2 which partially blocks the pores and prevents penetration of corrosive medium to the thin barrier layer. This effect is enhanced in the presence of Ce 3+ ions due to additional formation of stable and insoluble cerium hydroxides. 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