C

Sa

b

c

a

ARRAA

KMZSCS

1

ptaiMpaea

samstacmots

0d

Electrochimica Acta 55 (2009) 131–141

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

omplex anticorrosion coating for ZK30 magnesium alloy

.V. Lamakaa,∗, G. Knörnschildb, D.V. Snihirovaa, M.G. Tarybaa, M.L. Zheludkevichc, M.G.S. Ferreiraa,c

Instituto Superior Técnico, UTL, ICEMS, Av. Rovisco Pais, 1049-001 Lisbon, PortugalRio Grande do Sul Federal University, 90040-06 Porto Alegre, BrazilUniversity of Aveiro, CICECO, Dep. Ceramics and Glass Eng., 3810-193 Aveiro, Portugal

r t i c l e i n f o

rticle history:eceived 25 June 2009eceived in revised form 13 August 2009ccepted 13 August 2009vailable online 21 August 2009

a b s t r a c t

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 anodizedlayer forming an additional transient oxide–sol–gel interlayer.

eywords:agnesium alloy

K30ol–gel coatingorrosion inhibitors

The thickness of this complex protective coating is about 3.7–7.0 �m. A blank oxide–sol–gel coatingsystem or one doped with Ce3+ ions proved to be effective corrosion protection for the magnesium alloypreventing 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 byscanning electron microscopy (SEM). The corrosion behaviour of the ZK30 substrates pre-treated with

testT), an

park anodizing the complex coating waselectrode technique (SVE

. Introduction

Magnesium-based alloys have a number of advantageoushysical- and mechanical-properties, which make them an attrac-ive choice for many industrial applications. These materialsre used when the low weight of the product is of significantmportance. Apart from extensive use in automotive industry,

g-based alloys are utilized in the production of parts for com-uters and other portable devices, aircraft, military, recreationalnd orthopaedic equipments, diving gear, and sports goods. How-ver, one of the main reasons limiting larger use of light magnesiumlloys is their high susceptibility to corrosion.

One of the approaches to corrosion protection of magne-ium recently discussed in the literature is based on high-voltagenodization of the metal surface [1–3]. The concept and process oficroarc oxidation, also known as plasma electrolytic oxidation or

park anodizing, were patented in USA in the 1990s [4,5]. Accordingo the literature, anodized layers obtained by the sparking processre generally described as relatively thick (up to 100 �m), hard,eramic-like coatings comprised of a dense inner ceramic-like layer

ostly containing Mg(OH)2 and an outer porous layer comprised

f MgO. For a better understanding of the structure and proper-ies of the anodized layers on Mg-based alloys readers may refer toeveral recently published works [1–7].

∗ Corresponding author. Tel.: +351 218 417 996; fax: +351 218 419 771.E-mail address: sviatlana.lamaka@ist.utl.pt (S.V. Lamaka).

013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.08.018

ed by electrochemical impedance spectroscopy (EIS), scanning vibratingd scanning ion-selective electrode techniques (SIET).

© 2009 Elsevier Ltd. All rights reserved.

Irregular pores that appear in the oxide layer in the course ofanodization form the pathways for corrosive species impairing thetotal protective effect of anodization. It is possible to seal the poresin order to improve the protective properties conferred by the oxidefilm. Recent achievements in sol–gel technology allow successfulformulation of durable hybrid organic–inorganic sol–gel coatings,which are used as anticorrosion pre-treatments for aluminium-and magnesium alloys [8–17]. Hybrid sol–gel coatings assure goodadhesion of the organic paint to the metal substrate and by com-bining active- and passive-protection provide an additional densebarrier against corrosive species. The sol–gel route offers versa-tile ways to synthesize effective coatings with desired properties.Functionality is achieved by varying experimental parameters suchas chemical structure, composition, and ratio of precursors andcomplexing agents, the rate and conditions of hydrolysis, synthesismedia, embedding of additional active species (e.g. encapsulated-or directly introduced corrosion inhibitors), aging and curing con-ditions and deposition procedure. However, only a few attemptsto reinforce anodized layers on magnesium-based alloys by sol–gelfilm have been reported recently [18,19].

The major objective of this work was to use a porous layer ofmagnesium oxide formed by spark anodizing as an additional inter-mediate dense barrier and reservoir of corrosion inhibitors placed

between the metal and the sol–gel coating.

We describe a complex anticorrosion multilayer coating sys-tem for the magnesium alloy ZK30 consisting of environmentallyfriendly corrosion inhibitors for magnesium alloys combined witha thin hybrid sol–gel coating without decreasing the barrier

1 himic

ptoa

2

2

btPe(wt

togmfwwu

2

1cu1ctpl

hpt

2

lwgwo(tanoirtrcmpfids

32 S.V. Lamaka et al. / Electroc

roperties of the coating or deactivation of the inhibitors. Forhis purpose, an intermediate porous layer of magnesium oxidebtained by spark anodizing was placed under the sol–gel coatingnd served as a reservoir of corrosion inhibitors.

. Experimental

.1. Materials and reagents

In this study, plates of extruded ZK30 magnesium alloy (Alu-in, Israel) were used as the metallic substrates. Apart from Mg,he ZK30 alloy is comprised of about 3 wt% Zn and 0.6 wt% Zr [20].rior to anodizing, all the magnesium coupons were polished withmery paper, sequentially using 240, 600, 1200 and 2000 gritsStruers, SiC) and deionized water. The samples were then washedith distilled water in an ultrasonic bath and dried in air at room

emperature.All the reagents used to synthesize the sol–gel films, prepare

he anodizing solutions and solutions for immersion were purissr better grade products of Sigma–Aldrich with exception of (3-lycidoxypropyl)-trimethoxysilane, which contained at least 98%ain product. KF and Al(OH)3 used for anodizing were obtained

rom Synth (Brazil). Al(OH)3 contained at least 76.5% main productith Al2O3 being the main impurity. All the reagents were usedithout further purification. The aqueous solutions were preparedsing MilliPore purified water (� > 18 M� cm).

.2. Anodizing

Anodizing was performed in an electrolyte which contained35 g/l NaOH, 34 g/l Al(OH)3, 34 g/l Na3PO4, and 34 g/l KF. A non-ommercial inhouse-made direct current/direct voltage source wassed. The applied voltage limit was 70 V, and the current density25 mA/cm2. The duration of the tests was 10 min. A typical testonsisted of two parts. The first part was growth of a galvanos-atic film until the voltage limit was reached. The second part wasotentiostatic. The current density diminished while the voltage

imit was maintained.The porous oxide was impregnated with Ce3+ ions and 8-

ydroxyquinoline (8HQ) by immersing the anodized magnesiumlates into 0.005 M aqueous solutions of Ce(NO3)3 and 8HQ, respec-ively, for 30 min.

.3. Synthesis of sol–gel coatings

The organic–inorganic films were synthesized using a control-able sol–gel route, mixing two different sols. The sol–gel coating

as composed of in situ synthesized titania nanoparticles and (3-lycidoxypropyl)-trimethoxysilane (GPTMS). Silane-based alkosolas prepared by the hydrolysis of GPTMS in iso-propanol (ratio

f 1:1 by volume) to which diluted aqueous solution of HNO3pH = 0.5) was added accompanied by constant stirring at roomemperature for 1 h. The second alkosol was produced by controlledcidic hydrolysis (pH = 0.5) of 70% iso-propanol solution of tita-ium (IV) iso-propoxide (Ti(OiPr)4) in iso-propanol in the presencef a complexing agent (acetylacetone) and ultrasonically agitat-ng the resulting solution at a temperature of 22 ± 1 ◦C. The molaratio of Ti(OiPr)4:cAc:H2O was 1:3:5. Finally, the silane-based anditania-containing alkosols were mixed together in 2:1 volumeatio, respectively. The hybrid organo-inorganic system was keptonstantly stirred and ultrasonically agitated at 22 ± 1 ◦C for one

ore hour. This formulation was then aged for 1 h at room tem-

erature and deposited on anodized ZK30 substrates. The sol–gellms on metallic substrates were produced by a dip-coating proce-ure at a withdrawal speed of 18 cm/min and exposure time in theolution of 100 s. The samples were then cured at 120 ◦C for 80 min.

a Acta 55 (2009) 131–141

All samples tested in the present study were coated with identicalsol–gel formulation.

2.4. Microscopic characterization

SEM/EDS was used for examining the microstructure and gen-eral chemical composition of the anodized layers and sol–gel filmsbefore- and after immersion tests. A semi-in-lens Hitachi SU-70 UHR Schottky (Analytical) FE-SEM microscope coupled with aBruker EDS detector was used. Electron beam energy of 15 keV wasapplied for SEM analysis and EDS mapping. The samples for cross-section analysis were prepared by embedding the treated couponsof ZK30 into epoxy resin (Buehler). To reduce the distortion of theimage due to the signal from the epoxy resin two pieces of the samesample were glued together by their anodized and sol–gel coatedsurfaces. After solidification of the epoxy resin, the samples werepolished sequentially with 220, 320, 1000, 2400 grit emery papers(Struers, SiC) in deionized water and finally with 4000 grit emerypaper in ethanol.

2.5. Electrochemical techniques

EIS. Impedance measurements were carried out to evaluate thecorrosion protection performance of the developed complex coat-ing system on ZK30 during 4-week period of immersion in 0.005 MNaCl solution or 2-week period in 0.05 M NaCl solution at neu-tral pH level (5.5–6.0). EIS measurements were recorded using aGamry FAS2 Femtostat coupled with a PCI4 Controller at opencircuit potential applying 10 mV sinusoidal perturbations in the100 kHz to 8 mHz frequency range. Per frequency decade, 7 or 12experimental points were collected during the measurements. Aconventional three-electrode cell was used and consisted of a sat-urated calomel reference electrode, a platinum wire as a counterelectrode, and the pre-treated magnesium-based alloy as workingelectrode of a surface area of 3.3 cm2. All measurements were per-formed in a Faraday cage in order to avoid any electromagneticinterference. A simplex method was employed to fit the impedanceplots using Gamry Echem Analyst software, version 5.30.

Four different types of ZK30 samples—including the blank andthe doped—with organic- or inorganic inhibitors were tested: (1)anodized alloy, ZK Anod; (2) anodized alloy sealed with the sol–gelonly, ZK Anod SG; (3) anodized alloy immersed in Ce3+ solutionand coated with the sol–gel film, ZK Anod Ce3+ SG, and lastly (4)anodized alloy immersed in 8HQ solution and sealed with thesol–gel film, ZK Anod 8HQ SG.

SVET and SIET. Commercial equipment manufactured by Appli-cable 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 ofthe complex protective coating doped with Ce3+ ions, artificialdefects of 100 �m size and larger were created on the surface ofthe sample before immersion. Periodical measurements were takenduring its exposure to 0.05 M NaCl neutral solution. The scannedarea was about 1.5 mm × 1.5 mm. The local currents and H+ activ-ities were mapped on a 31 × 31 grid, which generated 961 datapoints. The details of the SVET and SIET set-up and procedures arereported elsewhere [21].

Briefly, the vibrating electrode of the SVET was an insulated Pt–Irprobe (Microprobe Inc., USA) with Pt black deposited on the spheri-cal 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 probewas 124 Hz.

Localized pH measurements were recorded using pH-selective glass-capillary microelectrodes. The silanized glass

S.V. Lamaka et al. / Electrochimic

Fta

mab4s2wm(waf

3

iiptoTaa

3

orVcaoeamiio

aacilp

SEM images of the cross-section and EDS element mapping ofthe same area of ZK Anod SG sample are presented in Fig. 5. Thethickness of the sol–gel layer is in the range 3–4 �m. Apart fromthe ZK30 magnesium substrate, the sol–gel layer, and the epoxy

ig. 1. Schematic representation of the complex protective coating that compriseshe layer of magnesium oxide, inhibitor adsorbed in the pores of the anodized layernd sealed with sol–gel film.

icropipettes were back-filled with the inner filling solutionnd tip-filled with selective ionophore-based oil-like mem-rane. The ion-selective membrane consisted of 6 wt% ETH 1907-nonadecylpyridine, 12 mol% (relative to ionophore) potas-ium tetrakis(4-chlorophenyl)borate, and membrane solvent-nitrophenyloctyl ether. All reagents for pH-selective membraneere Selectophore grade products of Fluka. The pH-selectiveicroelectrodes were calibrated using commercially available

Fluka) and homemade pH buffers. The linear range of pH responseas 2–10, the Nernstian slope was 54.8 ± 0.7 mV/dec. The local

ctivities of H+ were mapped 30 �m above the surface. The timeor acquisition for each SIET data point was 3 s.

. Results and discussion

The schematic representation of the complex protection coat-ng system developed is shown in Fig. 1. The porous oxide layerncreases the corrosion resistance of the magnesium substrate androvides the reservoirs for the corrosion inhibitor, which impedeshe corrosion process when the corrosive media penetrates thexide layer through the microdefects present in the sol–gel film.he oxide layer also serves to enhance considerably the surfacerea of the substrate coming into contact with the sol–gel coatingnd results in better adhesion.

.1. Anodized layers

Fig. 2a presents the plane view of the anodized layer obtainedn ZK30 alloy by spark anodizing. Breakdown of the oxide filmesults in the formation of pores of diameters in the range 0.5–5 �m.oltage transients during anodizing show an abrupt change fromonventional anodizing to the prevalence of sparking conditions atbout 55 V. An irregular porous layer uniformly covers the surfacef the alloy. The distribution of the pores on the surface is influ-nced by the alloy’s microstructure [6]. Furthermore, the cracksre clearly visible in the anodized layer. Presumably dielectric- andechanical breakdown mechanisms may cause sparking depend-

ng on the conditions of film formation [6]. Mechanical breakdownn the MgO layer can be attributed to the molar volume mismatchf Mg and MgO (Pilling Bedworth rule), VMg/VMgO = 0.81 [22].

The elements of the anodizing electrolyte, namely Al, P and Fre detected in the oxide layer besides the Mg, Zn and Zr whichre the main elements of the alloy, Fig. 2b. Fluoride increase anti-

orrosion stability of the anodized layer due to incorporation ofnsoluble MgF2. Fluoride ions additionally incorporated in anodizedayer may also reveal inhibiting effect in the course of corrosionrocess reacting with anodically generated Mg2+.

a Acta 55 (2009) 131–141 133

Cross-sectional SEM/EDS images in Fig. 3a–c reveal that theanodized film is composed of two layers, a thin inner barrier layerand an outer randomly porous layer. Both oxide layers appear tobe enriched with aluminium, Fig. 3b and c. The thickness of theentire anodized film was in the range 0.7–3.0 �m as assessed bythe cross-sectional SEM/EDS analysis, Fig. 3a.

3.2. Sol–gel coatings

Hybrid organic–inorganic sol–gel coatings are environmentallyfriendly pre-treatments for aluminium- and magnesium alloys andhave been extensively studied over the last few years [9]. Theproperties of such coatings are reported in our previous papers[14,15,23]. Before its application on metallic substrates, the sol–gelsolution is homogenous and transparent and is light-yellow incolour. The viscosity of these hybrid mixed sols remains in the range8–22 cP for 2 weeks [23].

The SEM image (Fig. 4) shows the plane view of the depositedsol–gel coating. Neither cracks nor pores are visible in the coating at700 times magnification. The white spots seen in the EDS analysisimage are dust specks mostly composed of carbon.

Fig. 2. SEM/EDS observation of porous magnesium oxide structure obtained byspark anodizing on ZK30 alloy: plane view (a), overall EDS spectrum (b).

134 S.V. Lamaka et al. / Electrochimica Acta 55 (2009) 131–141

Fig. 3. Cross-section SEM (a) and EDS (b and c) imaging of blank anodized ZK30sample.

Fig. 5. (a) SEM image of cross-section view of anodized specimen coated with sol–gel filmlayer (white arrows), barrier oxide layer (black arrows), mixed oxide-sol–gel layer, the la

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 visi-ble: these are (1) thin barrier layer of magnesium oxide formed onthe surface of metallic magnesium (indicated by white arrows inFig. 5) and (2) transition layer composed by porous oxide and thesol–gel film (between black arrows). The latter forms as a resultof the sol–gel that flows into the outer porous part of the anodicfilm. Interpenetration of the sol–gel and anodized layers results inperfect adhesion of the sol–gel film.

The unique protective properties of thin sol–gel coatings origi-nate from the formation of stable Si–O–Me bonds, which preventthe corrosive medium access to the substrate surface and therebydelaying the setting in of corrosion reactions. Incorporating anyintermediate layer between the substrate and sol–gel coating isliable to weaken the adhesion of the sol–gel films as shown inRef. [24]. However, the approach adopted here and in our previouspapers [25,26] of building up the porous reservoirs with a highlydeveloped surface helps overcome this limitation. Apparently, thepresence of inhibitors in the pores of the oxide does not affect theadhesiveness of sol–gel film to the anodized surface.

3.3. Immersion tests in dilute 0.005 M NaCl solution

To test the anticorrosion protective performance of developed

complex coating the first batch of samples was immersed indilute neutral 0.005 M NaCl solution. Comparative optical pho-tographs of the samples’ surface after 4 weeks of immersion arereproduced in Fig. 6. pH values of NaCl solution in the electrochem-ical cells recorded after immersion tests complement the optical

; (b) EDS mapping of the same zone showing Mg-based substrate, the inner oxideyer of sol–gel coating and the epoxy mount.

S.V. Lamaka et al. / Electrochimica Acta 55 (2009) 131–141 135

F k soluf

pbZwtatdosciwdtpdsotb

nMd

imcacpdpmsHasap

oaswcprwt

ZK Anod Ce SG sample in the course of the immersion test isshown in Fig. 10. At the beginning of the immersion test, theimpedance spectra of anodized ZK samples sealed with the sol–gelfilm showed three time constants. The resistance RSG and capaci-tance CSG of the sol–gel coating can be clearly distinguished from

ig. 6. Optical photographs of sample surfaces supplemented with pH values of bulor impedance tests was 18 mm.

hotographs. Although no intensive release of hydrogen bub-les was observed visually the blank anodized ZK30 specimen,K Anod, and the anodized sample impregnated by 8HQ sealedith sol–gel film, ZK Anod 8HQ SG, become of darker colour with

ime, Fig. 6a and b. The pH of the bulk solution after immersionltered into the alkaline range, up to 7.7. It should be stated thathe 8HQ doped sol–gel film was rough in appearance and lookedamaged immediately after deposition of the film. Apparently, therganic inhibitor chemically interacts with the components of theol–gel film. This leads to the disruption of the polymerization pro-ess, decomposition of the coating, as well as deactivation of thenhibitor. An interesting observation, as reported in our previous

orks, was that this effect was not visible when 8HQ was dopedirectly onto the same sol–gel composition [27,28]. On the con-rary, the presence of inhibitors even improved the anticorrosionrotective performance of the coating. However, when 8HQ waseposited on a developed surface of nanostructured TiO2 and sub-equently covered with a similar sol–gel coating the sol–gel wasbserved to have decomposed (data not published). Most probablyhis effect is related to the rapid decomposition of 8HQ catalyzedy the developed oxide surface.

Neither signs of corrosion attack nor any indication of delami-ation of the sol–gel film are visible on the surface of the anodizedg sealed with sol–gel ZK Anod SG or where the same system was

oped with Ce3+ ions ZK Anod Ce3+ SG, Fig. 6c and d.The visual observations complemented the electrochemical

mpedance spectroscopy tests. EIS provides a quantitative esti-ation of coating degradation and the rate of emergence of

orrosion processes. The Bode plots of selected impedance spectrare presented in Fig. 7. The resistive response at low frequenciesorresponds to the polarization resistance for the ZK Anod sam-le. A rise in polarization resistance over the 4 weeks of immersionuring the tests can be ascribed to the densifying and sealing of theorous oxide layer due to conversion of MgO to Mg(OH)2, in whicholar volume is larger than that of MgO [22] and which is more

table thermodynamically in aqueous solutions than MgO [29].owever, an increase of pH up to 9.9 measured in the bulk solutionfter 1 month of immersion suggests that the electrochemical dis-olution of magnesium and the accompanying cathodic reactionslso take place resulting in the formation of additional corrosionroducts (see Section 3.5).

The resistive response (Zmod curve) of ZK Anod SG decreasednly slightly (Fig. 7) throughout 4 weeks of immersion which isvery promising result for a highly corrosion-susceptible magne-

ium alloy. The pH of the bulk solution remained neutral at 6.0,hich is also an evidence of the absence of the corrosion pro-

ess. A comprehensive analysis of fitted spectra is given in the nextart of this article. The ZK Anod Ce3+ SG sample showed similaresults. Thus, the samples where the pores of the anodized layerere sealed by the sol–gel film did not suffer corrosion attack in

he course of the 4-week immersion test in pH-neutral NaCl solu-

tion after one month of immersion in 0.005 M NaCl. The diameter of the round cell

tion. The presented results by now explicitly indicate the degree ofanticorrosion efficiency of the developed sol–gel films. However,immersion tests monitored with EIS were repeated in more con-centrated 0.05 M NaCl solution in order to establish the superioranti-corrosion properties of the developed coatings.

3.4. Immersion tests in 0.05 M NaCl solution

The visual appearance of the four samples after 2 weeks ofimmersion (Fig. 8) was found analogous to that obtained for sam-ples after the tests in dilute solution. The Nyquist plots for thesesamples were in-line with visual observations and are presentedin Fig. 9. The extent of corrosion attack is greater than in the sam-ples immersed in dilute NaCl solution. The pH values of the bulksolution measured after immersion also indicated deeper corrosioneffects in all samples. The surface of the blank anodized sam-ple ZK Anod became evenly grey. One big deep pit and a threadof the filiform corrosion are visible. Two deep pits and an areaof grey colour that looks similar to one on the blank ZK Anodsample are the outcomes of exposure to NaCl solution of 8HQ-doped sample, ZK Anod 8HQ SG. Small isolated pits appeared inthe samples with the sol–gel sealed anodized layer, ZK Anod SGand ZK Anod Ce3+ SG.

The evolution with time of the impedance spectra of the3+

Fig. 7. Evolution of Bode plots in 0.005 M NaCl in the course of immersion.

136 S.V. Lamaka et al. / Electrochimica Acta 55 (2009) 131–141

F ochemf

tfgwfid

muAittasNsto

ruea

si

ps

Fi

lowed, revealing the weak barrier properties and stability of thisfilm. Gradual change in the sol–gel film’s resistance after the first2 days of immersion is the evidence of the good barrier propertiesand stability of the blank film and of the coating with the Ce3+ ions.

ig. 8. Visual appearance of the samples and pH of solution in corresponding electror impedance tests was 18 mm.

he response of the mixed sol–gel/oxide layer RMIX and CMIX androm the response of the dense oxide layer ROx and COx. This is inood agreement with the SEM/EDS images, Fig. 5, which shows twoell defined layers: one transitional layer formed by the sol–gellm flowing into the outer pores of the anodized layer and oneense oxide layer which was formed directly on the metal surface.

The immersion in NaCl solution results in the growth oficrodefects in the sol–gel coating and anodized layer. This opens

p pathways for corrosive species to the surface of magnesium.n additional fourth time constant appears after several days of

mmersion in the low frequency region. This is ascribed to the ini-iation of the corrosion attack and is attributed to the existence ofhe double-layer capacitance at the metal/electrolyte interface, CDL,nd corresponding polarization resistance, Rpolar. This time con-tant appears along with the first pits on the samples’ surface, Fig. 8.ote that this time constant was not present for the sol–gel coated

amples when they were immersed in dilute 0.005 M NaCl solu-ion, as it results from impedance spectra, Fig. 7, and from visualbservation of the samples, Fig. 6c and d.

In spite of the rapid fall in the initial values of the sol–gel filmesistance RSG that occurs due to the electrolyte uptake, the mod-lus of complex impedance remains higher than 5 M� cm2. Thismphasizes the good barrier properties of the complex coatingsnd their stability over time.

Spectra of ZK Anod SG and ZK Anod 8HQ SG showed the

ame number of time constants but different absolute values ofmpedance modulus and phase angle.

For the quantitative estimation of the corrosion protectiveroperties of different complex coatings, experimental impedancepectra were fitted with the equivalent circuits, which simulated

ig. 9. Comparison of the Nyquist plots of different samples after 2 weeks immersionn 0.05 M NaCl solution.

ical cell after 2 weeks of immersion in 0.05 M NaCl. The diameter of the round cell

the response of the anodized alloy sealed with sol–gel layer.Schematic representation of the equivalent circuits and their physi-cal interpretation are shown in Fig. 11. In the equivalent circuit, Rsolis the resistance of the corrosive medium, namely 0.05 M NaCl solu-tion. Constant phase elements (CPE) instead of pure capacitanceswere used for fitting experimental spectra. Such modification isobligatory if the phase shift of a capacitor differs from −90◦ [30].

Fig. 12 presents the evolution of different parameters of thecoated samples obtained after the fitting of the experimentalspectra. This figure also shows comparison of anticorrosion per-formance of the coatings containing Ce3+ and 8HQ with that of theundoped sample.

At the beginning of the immersion the hybrid coating withthe Ce3+ ions displays the highest resistance, RSG, and the pres-ence of 8HQ the lowest values of RSG, while the undoped coatingZK Anod SG keeps the middle position, Fig. 12a. Rapid decrease ofRSG during the first hours of contact with chloride solution is usuallyobserved for the sol–gel coatings of this type [14,15,25]. Penetra-tion of water and chloride ions through the nano-sized pores ofthe coating is responsible for this drop in resistance. The fall inRSG of the 8HQ doped system continued during the days that fol-

Fig. 10. Bode plots with corresponding fitting for anodized ZK30 specimensimmersed in solution of Ce3+ and coated with the sol–gel film (ZK Anod Ce3+ SG).Evolution of spectra in the course of 2-week immersion test in 0.05 M NaCl.

S.V. Lamaka et al. / Electrochimica Acta 55 (2009) 131–141 137

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).

138 S.V. Lamaka et al. / Electrochimica Acta 55 (2009) 131–141

F ed sab

oFatsatopfmatooRf

Fe

ig. 13. Optical image and corresponding pH mapping over sol–gel covered anodizubbles in the optical image.

The resistance of the transitional layer RMIX developed by theuter porous oxide and the sol–gel film is presented in Fig. 12b.ig. 12c shows the evolution of the inner dense oxide layer ROxnd characterizes the corrosion stability of the whole system sincehis thin layer is the last barrier that remains between the corro-ive medium and the metal. The resistance of both layers droppedfter the first day of immersion, remained stable over 2–3 days andhen rose slightly. This behaviour is explained by the formationf additional magnesium hydroxide, which seals and densifies there-existing anodized layer. Note the rise of ROx and RMIX is sharperor the ZK Anod Ce3+ SG sample than for the ZK Anod SG. The for-

ation of highly insoluble cerium hydroxides that seals the porousnodized layer occurs at lower pH (about 5.5), than the forma-

ion of Mg(OH)2 that occurs at pH = 8.5 [29,31]. Thus, densificationf the anodized layer in the cerium-containing complex coatingccurs at the earlier stage of initial corrosion reactions. ROx andMIX for ZK Anod 8HQ SG are 1–2 orders of magnitude lower thanor the ZK Anod Ce3+ SG and ZK Anod SG samples, confirming the

ig. 14. Optical micrographs and SVET maps of ionic currents measured above the surxposure to NaCl solution. The time of exposure is indicated in each image.

mple of ZK30, ZK Anod Ce3+ SG. Two artificial defects are hidden by the hydrogen

presence of an adverse effect of 8HQ on the corrosion protectionproperties of the developed coatings.

The gradual degradation of protective layers results in theappearance of the fourth time constant that enables the quan-tification of the polarization resistance, Fig. 12d, Rpolar, whichcharacterizes the rate of the corrosion process. The polarizationresistance of ZK Anod Ce3+ SG while remaining between 7 and14 M� cm2 falls to 1 M� cm2 for the ZK Anod SG sample. Yetpolarization resistance for both samples remained quite high com-pared to Rpolar for the coating system doped with 8HQ, where itdropped from around 1 M to 0.1 M� cm2 after 2 weeks.

3.5. SVET measurements and localized pH mapping

To learn more about the corrosion mechanisms and to con-firm the effective anticorrosion performance of the inhibitor-dopedcomplex coating ZK Anod Ce3+ SG sample was studied by meansof the localized electrochemical techniques, SVET and SIET.

face of ZK Anod Ce3+ SG sample. The maps and optical images were taken after

S.V. Lamaka et al. / Electrochimica Acta 55 (2009) 131–141 139

F K Anoe showif

sbcrsppt

srses

ig. 15. SEM micrographs and EDS elements mapping of the artificial defect in Zlectron images of the defect and its magnified fragment; EDS elemental mappingor SEM observations.

The pH distribution was measured over the artificially damagedample’s surface. The defects were hidden under the hydrogen bub-les in Fig. 13a, which are also indicative of the intensity of theorrosion reactions. The hydrogen bubbles generated were releasedoughly every 5–10 s at the beginning of the immersion of theample with the defects. Local alkalinization of the solution up toH = 9.9 in the background of a neutral 0.05 M NaCl solution withH = 5.5 was mapped by a glass-capillary pH-selective microelec-rode.

The evolution of local current density was measured over the

ample surface with three artificial defects, Fig. 14a. Two roundelatively deep defects were intentionally needled, while a thirdhallow and wider defect was created by the glass-capillary micro-lectrode used for SIET measurements as it crushed against theample (Fig. 14a, lower left zone).

d Ce3+ SG sample after immersion and SVET/SIET studies. (a) and (d) secondaryng distribution of (b) Mg and Si (c) O; (e) overall EDS spectrum. Au was deposited

The first SVET measurement taken immediately after SIET pHmapping (Fig. 13) corresponds to 2 h of immersion of the sampleinto 0.05 NaCl solution. High anodic- and cathodic activity accom-panied by hydrogen evolution was recorded in the area of thedefects. However, the activity waned thereafter resulting in com-plete passivation of the sample after 24 h of immersion, Fig. 14c–f.This proves that the good protective properties of the coating sys-tem impeded propagation of the artificial defects. This result isunexpected since the values of local currents were very high atthe beginning of immersion.

Fig. 15 presents SEM/EDS images of the defect tested bySIET/SVET, the same defect which is depicted in the upper part ofthe optical micrographs in Figs. 13a and 14a and e. SEM/EDS mea-surements were made after SVET/SIET studies, when the defectbecame passive. The deep defect is completely covered by the

1 himic

cbooe

r

C

A

O

F

A

O

sitiZpcpcMifcbastp

oodi7

pmt9ittaqNefi

4

forpat

[[[[[

[

[

[

[

[[[

[

[

40 S.V. Lamaka et al. / Electroc

orrosion products. The remnants of the sol–gel coating damagedy needling are also present. The EDS analysis of the smaller areaf the defect covered by corrosion products reveals the presencef cerium and high amount of oxygen as well as the main alloylements, Mg, Zn and Zr.

The corrosion process can be characterized by the followingeactions:

athodic reactions 2H2O + 2e− → 2OH− + H2↑ (1)

O2 + 2H2O + 4e− → 4OH− (2)

nodic reaction Mg → Mg2+ + 2e− (3)

verall reaction Mg + 2H2O → Mg(OH)2 + H2 (4)

ormation of monovalent cation Mg+ seems also possible [32,33]:

nodic partial reaction Mg → Mg+ + e− (5)

verall reaction 2Mg+ + 2H2O → 2Mg2+ + 2OH− + H2 (6)

Silane-based sol–gel coatings are known for their low hydrolytictability in alkaline medium. The local pH of cathodic reactions ris-ng to a value of 10 could accelerate the hydrolytic decomposition ofhe sol–gel. However, this does not seem to be the case. The reasons that the sharp local increase of pH in the damaged zone of theK Anod Ce3+ SG sample favours the formation of Mg(OH)2 thatrecipitates and blocks further propagation of the corrosion pro-ess and degradation of the coating. Mg2+ cations produced as theroducts of anodic dissolution and OH− ions formed as products ofathodic reactions were consumed in the formation of additionalg(OH)2. Ce3+ ions impregnated in the anodized layer and released

n the course of anodic dissolution of the magnesium substrateacilitate the suppression of the cathodic reactions forming pre-ipitates of Ce(OH)3 and Ce(OH)4, also consuming OH− generatedy cathodic reactions [31]. The areas of the anodized magnesiumround the defects exposed due to mechanical detachment of theol–gel film can additionally be passivated by conversion of MgOo the lower density Mg(OH)2, which partially blocks the pores andrevents penetration of corrosive medium to the thin barrier layer.

This explains the relative inactivity of the local defects visiblen the surface of ZK Anod Ce3+ SG and ZK Anod SG after 2 weeksf immersion in 0.05 M NaCl, Fig. 8 (c and d). Once formed, theseefects remain small and do not grow. The pH of the bulk solution

n electrochemical cells becomes alkaline but only slightly, up to.8 and 7.3, Fig. 8c and d.

While the ZK Anod Ce3+ SG and ZK Anod SG samples wereassive during the immersion, the ZK Anod sample underwent theost significant changes among all the tested samples. The pH of

he bulk solution in the electrochemical cell of this sample rose to.9 after immersion in 0.005 M NaCl and to 10.5 after immersion

n 0.05 M NaCl. The explanation that gradual conversion of MgOo Mg(OH)2 partially seals the pores seems to be valid in this caseoo and was discussed in literature for AZ31 Mg-based alloys [15]nd anodized WE43 [3]. A continuous increase of the lower fre-uency impedance values through 4 weeks of immersion in 0.005 MaCl solution supports this assumption, Fig. 7. However, prolongedxposure to the more concentrated 0.05 M NaCl solution leads toliform corrosion, Fig. 8 a.

. Conclusions

A new approach for formulating complex anticorrosion coatingsor magnesium-based alloys has been presented. A porous layer

f magnesium oxide formed by spark anodizing increases the cor-osion resistance and serves as a reservoir of corrosion inhibitorslaced under a thin hybrid sol–gel coating. The elements of thenodizing electrolyte (Al, P, and F) are incorporated into the struc-ure of the oxide where an inner barrier layer and outer porous

[

[

[

a Acta 55 (2009) 131–141

layers can be distinguished. Interpenetration of the sol–gel andanodized layers results in perfect adhesion of the sol–gel film tothe surface.

The effectiveness of corrosion protection was verified by EIS andSVET measurements. The use of 8-hydroxyquinoline as corrosioninhibitor disrupts the integrity of the sol–gel coatings exhibitingresults similar to those of anodized magnesium without the sol–gelcoating. The anticorrosion performance of the complex coatingsconsisting of an anodized layer covered by sol–gel film as well asin the case this film is doped with Ce3+ ions allows immersion ofZK30 magnesium alloys into 0.005 M and 0.05 M neutral aqueousNaCl solution without destructive outcomes. Gradual penetrationof the aqueous corrosive solution to the anodized layer results inconversion of MgO to the Mg(OH)2 which partially blocks the poresand prevents penetration of corrosive medium to the thin barrierlayer. This effect is enhanced in the presence of Ce3+ ions due toadditional formation of stable and insoluble cerium hydroxides. Therapid passivation observed of both types of defects—natural andartificial—is explained by the sharp rise of pH even in small defects,due to the initial corrosion reaction which leads to the precipitationof corrosion products and sealing of the defect.

Thus, the developed complex anticorrosion coating beneficiallycombines increased corrosion resistance of the magnesium sub-strate due to the additional protective oxide layer and the greateradhesion of thin sol–gel coating to the porous anodized surface,while also conferring active protection properties owing to thecorrosion inhibitor securely impregnated in the porous reservoirs.

Acknowledgement

The financial support of FCT—Fundacão para a Ciência e a Tec-nologia, through project REDE/1509/RME/2005 and GRICES/CAPESis gratefully acknowledged.

References

[1] Y. Mitzutani, S.J. Kim, R. Ichino, M. Okido, Surf. Coat. Technol. 170 (2003) 143.[2] F.A. Bonilla, A. Berkani, Y. Liu, P. Skeldon, G.E. Thompson, H. Habazaki, K.

Shimizu, C. John, K. Stevens, J. Electrochem. Soc. 149 (2002) B4.[3] S.J. Xia, R. Yue, R.G. Rateick, V.I. Birss, J. Electrochem. Soc. 151 (2004) B179.[4] D.E. Bartak, B.E. Lemieux, E.R. Woolsey, USA patent 5 264 113, 1993.[5] E.L. Schmeling, B. Roschenbleck, M.H. Weidemann, USA patent 4 978 432, 1990.[6] D. Peixoto Barbosa, G. Knornschild, Surf. Coat. Technol. 203 (2009) 1629.[7] D. Peixoto Barbosa, G. Knornschild, H.P. Strunk, Mater. Res. 6–1 (2003) 103.[8] M. Guglielmi, J. Sol–Gel Sci. Technol. 8 (1997) 443.[9] D. Wang, G.P. Bierwagen, Prog. Org. Coat. 64 (2009) 327.10] M. Fir, B. Orel, A.S. Vuk, A. Vilcnik, R. Jese, V. Francetic, Langmuir 23 (2007) 5505.11] A. Atkinson, D.L. Segal, J. Sol–Gel Sci. Technol. 13 (1998) 133.12] N.N. Voevodin, J.W. Kurdziel, R. Mantz, Surf. Coat. Technol. 201 (2006) 1080.13] A. Conde, A. Duran, M. de Damborenea, Prog. Org. Coat. 46 (2003) 288.14] S.K. Poznyak, M.L. Zheludkevich, D. Raps, F. Gammel, K.A. Yasakau, M.G.S. Fer-

reira, Prog. Org. Coat. 62 (2008) 226.15] S.V. Lamaka, M.F. Montemor, A.F. Galio, M.L. Zheludkevich, C. Trindade, L.F.

Dick, M.G.S. Ferreira, Electrochim. Acta 53 (2008) 4773.16] X.F. Yang, D.E. Tallman, V.J. Gelling, G.P. Bierwagen, L.S. Kasten, J. Berg, Surf.

Coat. Technol. 140 (2001) 44.17] A.N. Khramov, V.N. Balbyshev, L.S. Kasten, R.A. Mantz, Thin Solid Films 514

(2006) 174.18] A.L.K. Tan, A.M. Soutar, I.F. Annergren, Y.N. Liu, Surf. Coat. Technol. 198 (2005)

478.19] J. Guo, L. Wang, S.C. Wang, J. Liang, Q. Xue, F. Yan, J. Mater. Sci. 44 (2009) 1998.20] MatWeb: Material Property Data. http://www.matweb.com/2009.21] S.V. Lamaka, O.V. Karavai, A.A. Bastos, M.L. Zheludkevich, M.G.S. Ferreira, Elec-

trochem. Commun. 10 (2008) 259.22] J.A. Dean (Ed.), Lange’s Handbook of Chemistry, 12th ed., McGraw-Hill Book

Comp, 1979.23] D. Shchukin, M. Zheludkevich, K. Yasakau, S. Lamaka, H. Möhwald, M.G.S. Fer-

reira, Adv. Mater. 18 (2006) 1672.

24] S.V. Lamaka, D.G. Shchukin, D.V. Andreeva, M.L. Zheludkevich, H. Möhwald,

M.G.S. Ferreira, Adv. Funct. Mater. 18 (2008) 3137.25] S.V. Lamaka, M.L. Zheludkevich, K.A. Yasakau, M.F. Montemor, P. Cecilio, M.G.S.

Ferreira, Electrochem. Commun. 8 (2006) 421.26] S.V. Lamaka, M.L. Zheludkevich, K.A. Yasakau, R. Serra, S.K. Poznyak, M.G.S.

Ferreira, Prog. Org. Coat. 58 (2007) 127.

himic

[

[

[

S.V. Lamaka et al. / Electroc

27] K.A. Yasakau, M.L. Zheludkevich, O. Karavai, M.G.S. Ferreira, Prog. Org. Coat. 63(2008) 352.

28] A.F. Galio, S.V. Lamaka, M.L. Zheludkevich, L.F.P. Dick, I.L. Müller, M.G.S. Ferreira,Surf. Coat. Technol., in press.

29] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE,Houston, 1974.

[[

[[

a Acta 55 (2009) 131–141 141

30] C.S. Hsu, F. Mansfeld, Corrosion 57 (2001) 747.31] K.A. Yasakau, M.L. Zheludkevich, S.V. Lamaka, M.G.S. Ferreira, J. Phys. Chem. B

110 (2006) 5515.32] G. Song, A. Atrens, Adv. Eng. Mater. 9 (2007) 177.33] N.D. Tomashov, Theory of Corrosion and Protection of Metals, The Macmillan

Company, New York, 1966.