Oxygen-free deposition of ZrO2 sol±gel films on mild steel for

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					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
                                                                     Ã
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
    Â          Ä
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
substrate.
   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)




                                                                                               (a)
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.

                                                                                                                                      295
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.

296
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
                                                                                     (8C)                                   (MPY)
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 [12].                                                                     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
               1.5
                                                                       that with the ZrO2 densi®ed at 800 8C shows a value
               1.0
                                                                       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
               0.5
                                                                       by ceramic oxide ®lms can now be extended to other
                                                                       reactive materials since the coatings can be generated
               1.0
                                                                       under oxygen-free conditions.
                     7   6   5           4       3   2       1
                                                2)
                                 log i (mA cm
                                                                       Acknowledgements
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.
                                                                                                                                     297
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