Electrochimica Acta 55 (2009) 131–141
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
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, 1049-001 Lisbon, Portugal
Rio Grande do Sul Federal University, 90040-06 Porto Alegre, Brazil
c
University of Aveiro, CICECO, Dep. Ceramics and Glass Eng., 3810-193 Aveiro, Portugal
b
a r t i c l e
i n f o
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
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 anodized
layer forming an additional transient oxide–sol–gel interlayer.
The thickness of this complex protective coating is about 3.7–7.0 ␮m. A blank oxide–sol–gel coating
system or one doped with Ce3+ 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).
© 2009 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.: +351 218 417 996; fax: +351 218 419 771.
E-mail address: sviatlana.lamaka@ist.utl.pt (S.V. Lamaka).
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.08.018
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
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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.
All samples tested in the present study were coated with identical
sol–gel formulation.
2. Experimental
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 SU70 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.
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 (3glycidoxypropyl)-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 Al2 O3 being the main impurity. All the reagents were used
without further purification. The aqueous solutions were prepared
using MilliPore purified water ( > 18 M cm).
2.2. Anodizing
Anodizing was performed in an electrolyte which contained
135 g/l NaOH, 34 g/l Al(OH)3 , 34 g/l Na3 PO4 , 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/cm2 . 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 Ce3+ ions and 8hydroxyquinoline (8HQ) by immersing the anodized magnesium
plates into 0.005 M aqueous solutions of Ce(NO3 )3 and 8HQ, respectively, for 30 min.
2.3. 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 (3glycidoxypropyl)-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 HNO3
(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:H2 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.
2.4. Microscopic characterization
2.5. 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 0.005 M
NaCl solution or 2-week period in 0.05 M NaCl solution at neutral pH level (5.5–6.0). 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 cm2 . 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 5.30.
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 Ce3+ solution
and coated with the sol–gel film, ZK Anod Ce3+ 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 Ce3+ 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 31 × 31 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
S.V. Lamaka et al. / Electrochimica Acta 55 (2009) 131–141
133
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 0.7–3.0 ␮m as assessed by
the cross-sectional SEM/EDS analysis, Fig. 3a.
3.2. 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 1907
4-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.
3.1. 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 0.5–5 ␮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), VMg /VMgO = 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 MgF2 . Fluoride ions additionally incorporated in anodized
layer may also reveal inhibiting effect in the course of corrosion
process reacting with anodically generated Mg2+ .
Fig. 2. SEM/EDS observation of porous magnesium oxide structure obtained by
spark anodizing on ZK30 alloy: plane view (a), overall EDS spectrum (b).
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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.
3.3. Immersion tests in dilute 0.005 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 0.005 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.
S.V. Lamaka et al. / Electrochimica Acta 55 (2009) 131–141
135
Fig. 6. Optical photographs of sample surfaces supplemented with pH values of bulk solution after one month of immersion in 0.005 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 TiO2 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 Ce3+ ions ZK Anod Ce3+ 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 (Zmod 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 Ce3+ 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 solu-
tion. 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.
3.4. 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 Ce3+ SG.
The evolution with time of the impedance spectra of the
ZK Anod Ce3+ 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 RSG and capacitance CSG of the sol–gel coating can be clearly distinguished from
Fig. 7. Evolution of Bode plots in 0.005 M NaCl in the course of immersion.
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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 RMIX and CMIX and
from the response of the dense oxide layer ROx and COx . 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, CDL ,
and corresponding polarization resistance, Rpolar . 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 0.005 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 RSG that occurs due to the electrolyte uptake, the modulus of complex impedance remains higher than 5 M cm2 . 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
Fig. 9. Comparison of the Nyquist plots of different samples after 2 weeks immersion
in 0.05 M NaCl solution.
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, Rsol
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 Ce3+ and 8HQ with that of the
undoped sample.
At the beginning of the immersion the hybrid coating with
the Ce3+ ions displays the highest resistance, RSG , and the presence of 8HQ the lowest values of RSG , while the undoped coating
ZK Anod SG keeps the middle position, Fig. 12a. Rapid decrease of
RSG 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
RSG 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 Ce3+ ions.
Fig. 10. Bode plots with corresponding fitting for anodized ZK30 specimens
immersed 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.
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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).
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Fig. 13. Optical image and corresponding pH mapping over sol–gel covered anodized sample of ZK30, ZK Anod Ce3+ SG. Two artificial defects are hidden by the hydrogen
bubbles in the optical image.
The resistance of the transitional layer RMIX 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 ROx
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 ROx and RMIX is sharper
for the ZK Anod Ce3+ 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. ROx and
RMIX for ZK Anod 8HQ SG are 1–2 orders of magnitude lower than
for the ZK Anod Ce3+ 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, Rpolar, which
characterizes the rate of the corrosion process. The polarization
resistance of ZK Anod Ce3+ SG while remaining between 7 and
14 M cm2 falls to 1 M cm2 for the ZK Anod SG sample. Yet
polarization resistance for both samples remained quite high compared to Rpolar for the coating system doped with 8HQ, where it
dropped 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 confirm the effective anticorrosion performance of the inhibitor-doped
complex coating ZK Anod Ce3+ 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 Ce3+ SG sample. The maps and optical images were taken after
exposure to NaCl solution. The time of exposure is indicated in each image.
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139
Fig. 15. SEM micrographs and EDS elements mapping of the artificial defect in ZK Anod Ce3+ 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
140
S.V. Lamaka et al. / Electrochimica Acta 55 (2009) 131–141
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:
2H2 O + 2e− → 2OH− + H2 ↑
Cathodic reactions
−
O2 + 2H2 O + 4e → 4OH
2+
(2)
−
Anodic reaction
Mg → Mg
Overall reaction
Mg + 2H2 O → Mg(OH)2 + H2
+ 2e
(1)
−
(3)
(4)
Formation of monovalent cation Mg+ seems also possible [32,33]:
Mg → Mg+ + e−
Anodic partial reaction
Overall reaction
+
2Mg + 2H2 O → 2Mg
2+
(5)
−
+ 2OH + H2
(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 Ce3+ SG sample favours the formation of Mg(OH)2 that
precipitates and blocks further propagation of the corrosion process and degradation of the coating. Mg2+ 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 . Ce3+ 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 Ce3+ 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 Ce3+ 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 0.005 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 0.005 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 Ce3+ ions allows immersion of
ZK30 magnesium alloys into 0.005 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 Ce3+ ions due to
additional formation of stable and insoluble cerium hydroxides. The
rapid passivation observed of both types of defects—natural and
artificial—is explained by the sharp rise of pH even in small defects,
due to the initial corrosion reaction which leads to the precipitation
of corrosion products and sealing of the defect.
Thus, the developed complex anticorrosion coating beneficially
combines increased corrosion resistance of the magnesium substrate due to the additional protective oxide layer and the greater
adhesion of thin sol–gel coating to the porous anodized surface,
while also conferring active protection properties owing to the
corrosion inhibitor securely impregnated in the porous reservoirs.
Acknowledgement
The financial support of FCT—Fundação para a Ciência e a Tecnologia, through project REDE/1509/RME/2005 and GRICES/CAPES
is gratefully acknowledged.
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