Oxygen-free deposition of ZrO2 sol±gel ®lms on mild steel for corrosion
protection in acid medium
F. PERDOMO L.
Departamento de Materiales, Universidad del Valle, Cali, Colombia
L. A. AVACA
Instituto de Quõmica de Sao Carlos, USP, C. P. 780, 13560-970 Sao Carlos-SP, Brazil
Â Ä Ä
M. A. AEGERTER
Instituto de Fõsica de Sao Carlos, USP, Sao Carlos-SP, Brazil
Â Ä Ä
P. DE LIMA-NETO
Departamento de Quõmica Analõtica e Fõsico-Quõmica, UFC, Centro de Ciencias, Fortaleza-CE, Brazil
Â Â Â Â
Previous work from this laboratory the Instituto de 30 mm 3 15 mm 3 1 mm size were mechanically
Quõmica de Sao Carlos has shown the effectiveness cut from large foils and polished with successively
of sol±gel-derived ceramic coatings for the protec- ®ner grades of emery paper. Then, the samples were
tion of stainless steel against corrosion in H2 SO4 degreased ultrasonically in acetone, cleaned with
[1±5] and NaCl solutions [6, 7]. The coatings used distilled water and dried in air.
were ZrO2, SiO2, TiO2 ±SiO2 and Al2 O3 ±SiO2. In all Zirconium isopropoxide (Zr(OC3 H7 )4 ) diluted in
the cases, a large degree of protection was achieved isopropanol was used as the source of zirconia. The
with corrosion rates in H2 SO4 diminishing by a preparation procedure of the precursor solution and
factor of almost 6 for tests at room temperature the coating deposition technique have been fully
[1±3] and 8.5 at 50 8C [4, 5]. Among the coatings, described elsewhere [5, 6]. The sol±gel ®lms were
ZrO2 was most ef®cient and, at the same time, the densi®ed for 2 h in a quartz-tube furnace with an
most easily prepared. argon atmosphere. The temperature was increased at
However, for practical applications it is important a rate of 5 8C min 1 up to the desired ®nal value
to study the deposition of ceramic coatings on mild (500±800 8C).
steel (MS) substrates for the protection of this The physical characterisation of the ®lms was
widely used material. Meanwhile, early attempts to carried out at different stages of the densi®cation
deposit these ®lms on MS surfaces following the process using infrared (IR) spectroscopy, scanning
conventional procedure resulted in discontinuous and electron microscopy (SEM) and energy-dispersive X-
non-adherent coatings. This was proved to be due to
the simultaneous growth of iron oxides from the
In the meantime, one of the useful characteristics of
the sol±gel method is related to the metal alkoxides
used as precursors. These compounds contain a metal
atom directly linked to suf®cient oxygen atoms to
Transmittance (arbitrary units)
produce the desired oxide without participation of the
atmosphere. Therefore, it should be possible in
principle to apply this technique for coating formation
onto oxygen-reactive metal surfaces.
The aim of this work is to describe the conditions
for ZrO2 ®lm formation on MS surfaces in the (b)
absence of oxygen to avoid chemical oxidation of
the substrate and to promote adherent and protective
®lms. The ZrO2 coatings were deposited by dip
coating using a sol preparation involving sonocata-
lysis [1±6]. The ®lms were prepared through
hydrolysis and polymerization of metal alkoxide (c)
solutions. This was followed by conversion to an
oxide layer by heat treatment in a furnace with an 4000 3500 3000 2500 2000 1500 1000 500
argon atmosphere. Wavenumber (cm 1)
The substrate used was MS of composition Figure 1 IR spectra for zirconia ®lms deposited on MS: (a) after initial
99.440 wt % Fe, 0.350 wt % Mn, 0.110 wt % Si, drying at 25 8C; (b) after heat treatment in an argon atmosphere at
0.023 wt % S and 0.018 wt % P. Samples of 700 8C; (c) after heat treatment in an argon atmosphere at 800 8C.
ray analysis (EDXA). These techniques allowed us to ing electrodes were the coated and the uncoated MS
follow the decomposition of the organic material, to plates. A 273 PAR potentiostat linked to a micro-
observe the surface and the cross-section of the computer for data acquisition and handling through
coated samples and to con®rm the presence of Zr in the 352 PAR corrosion measurements software was
the deposited ®lms, respectively. used for the electrochemical experiments.
The IR spectra were obtained using a Bomem Fig. 1 shows the IR spectra of the coated samples
Fourier transform spectrophotometer in the 400± before and after densi®cation for 2 h at 700 and
4000 cm 1 range by re¯ection at an incident angle 800 8C in an argon atmosphere. In the spectrum of
of 308. The morphology of the surface was examined the dried sol±gel ®lm (Fig. 1a) a large band can be
using a Zeiss DSM 960 microscope. The chemical observed at 3500 cm 1 corresponding to the OH
identi®cation of the elements in the ®lm was made group of water. Furthermore, two well-de®ned bands
using a Link Analytical QX-2000 instrument. at 1453 cm 1 and at 1578 cm 1 corresponding to the
Electrochemical measurements were carried out by asymmetric stretching of the Zr±O±C bond from the
potentiodynamic polarization curves at 1 mV s 1 in metallic precursor and one band at 660 cm 1
deareated 0.5 M H2 SO4 (Merck p.a.) aqueous (Milli- corresponding to the symmetric vibration of the
Q) solutions at 25 8C. An electrochemical cell with a Zr±O±Zr bond [8, 9] are also observed.
Te¯on sample holder that exposed only 1 cm2 of the The spectra of the densi®ed ®lms (Fig. 1b and c)
surface to the electrolyte was used. The auxiliary show the zirconium oxide formation on the MS
electrode was a Pt foil and a saturated calomel surface. The vibrations corresponding to the OH
electrode (SCE) served as the reference. The work- groups and the Zr±O±C bonds have disappeared
Figure 2 Scanning electron micrographs of MS samples coated with ZrO2 and heat treated for 2 h (a) (c) in an argon atmosphere at (a) 600 8C,
(b) 700 8C and (c) 800 8C, and (d) in an air atmosphere at 800 8C.
completely while the band at 660 cm 1 remains. The TA B L E I Corrosion parameters determined from the potentio
large vibration bands at 510 cm 1 and 390 cm 1 dynamic curves for uncoated and coated MS samples heat treated
suggest the formation of Fe2 O3 [10, 11]. The analy- for 2 h at different temperatures in an argon atmosphere: corrosion
potential, Ecorr , polarization resistance Rp and corrosion rate
sis of these results demonstrates that the oxygen
present in the alkoxide is suf®cient for the Sample Heat treatment Ecorr Rp Corrosion
densi®cation of the zirconium oxide. It also indicates temperature (mV) (Ù cm2 ) rate
that the ZrO2 ®lm is supported on a thin layer of
oxides from the substrate. In fact, ZrO2 ®lms have MS As received 494 24 1428
good adherence over iron oxide layers because they 500 532 18 1966
600 525 17 2026
effectively interact, promoting the formation of Fe± 700 536 19 1693
O±Zr bonds . 800 516 53 436
The surface morphology evolution of the ZrO2 MSaZrO2 500 539 18 2243
coatings deposited on MS and heat treated at 600 534 21 1434
different temperatures is presented in Fig. 2. It can 700 520 27 1005
800 547 109 184
be observed (Fig. 2a±c) that more compact ®lms are
obtained when the densi®cation is carried out at
higher temperatures. Thus, the ®lm heat-treated at
600 8C (Fig. 2a) presents cracks on the surface while
those densi®ed at 700 8C (Fig. 2b) and 800 8C evolution is the reaction for the cathodic branch of
(Fig. 2c) are continuous. Additionally, Fig. 2c shows all curves. The cathodic current density for the heat-
a surface ®lm with well-de®ned grain boundaries. X- treated MS is somewhat lower than that of the
ray analysis of these samples revealed the presence untreated substrate, as shown by curves b and a in
of a mixture of monoclinic and tetragonal ZrO2 Fig. 3. Additionally, the presence of the coating
structures. This agrees with previously reported (curve c) shifts the cathodic current densities
results [5, 13]. For comparison, Fig. 2d shows that towards lower values. In all cases, the slope of the
the ®lm heat treated for 2 h at 800 8C in an air Tafel region is approximately constant, suggesting
atmosphere is discontinuous and non-adherent owing that the reaction mechanism remains unchanged.
to the simultaneous growth of an iron oxide layer This indicates that the ZrO2 ®lm acts as a physical
from the substrate. EDXA of the latter sample barrier that diminishes the exposed area of the
showed that ZrO2 is mainly located at the boundaries substrate to the electrolyte. This behaviour is very
of the islands (clear regions) while the iron oxides similar to that observed for the sol±gel-derived ®lms
are at the centre. Finally, SEM observations of the on type 316L stainless steel in H2 SO4 solutions
cross-section in coated samples densi®ed at 800 8C [1±5]. On the other hand, the shape of the anodic
show a homogeneous layer with an average thickness branch of the curves in Fig. 3 indicates that the
of 0.7 ìm. coated sample has a better-de®ned passive region in
Fig. 3 shows the potentiodynamic polarization the range 0.5±1.0 V (SCE), with current density
curves of MS as received (curve a), heat treated for values lower than those for both uncoated samples.
2 h at 800 8C in an argon atmosphere (curve b) and The electrochemical parameters, obtained from the
coated with ZrO2 densi®ed under the same condi- data in Fig. 3, for the uncoated and coated samples
tions (curve c). This ®gure con®rms that the heat treated for 2 h at different temperatures in an
presence of the coating alters the electrochemical argon atmosphere are collected in Table I. It is worth
behaviour of the MS substrate, both as received and noting that a direct heat treatment of MS at 800 8C
after heat treatment without the coating. Hydrogen under those special conditions greatly improves the
corrosion resistance of this material. In addition, this
table indicates that the coated samples heat treated
up to 700 8C still have a high corrosion rate while
that with the ZrO2 densi®ed at 800 8C shows a value
7.8 times lower than measured for as-received MS.
c a b The sol±gel method proved to be appropriate for
E (V (SCE))
0.5 depositing ceramic oxide ®lms on MS substrates
using an inert atmosphere. The ®lm acts as a physical
0.0 barrier than can increase the lifetime of the substrate
by a factor of 7.8. This idea of chemical protection
by ceramic oxide ®lms can now be extended to other
reactive materials since the coatings can be generated
under oxygen-free conditions.
7 6 5 4 3 2 1
log i (mA cm
Figure 3 Potentiodynamic polarization curves reduced at 1 mV sÀ1 in
The authors thank Conselho Nacional de Desenvol-
deareated 0.5 M H2 SO4 aqueous solutions at 25 8C on MS. Curve a, as
received; curve b, heat treated at 800 8C for 2 h in an argon Â Â ËÄ
vimento Cientõ®co e Tecnologico and Coordenacao
atmosphere; curve c, coated with ZrO2 densi®ed at 800 8C for 2 h in Â
de Pessoal de Nõvel Superior±PICD, Brazil, for
an argon atmosphere. ®nancial support.
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