Tsaneva, the University S. Kozhukharov, M. Ivanova, J. Gerwann, M. Schem, 231-238
             G..Journal ofV. Kozhukharov,of Chemical Technology and Metallurgy, 43, 2, 2008, T. Schmidt

                    OF ALUMINUM ALLOY AND STEEL
                         G. Tsaneva1, V. Kozhukharov1, S. Kozhukharov1, M. Ivanova1,

                                       J. Gerwann2, M. Schem2, T. Schmidt2

  University of Chemical Technology and Metallurgy                                           Received 05 February 2008
  8, Kl. Okhridsky Blvd., Sofia – 1756, Bulgaria                                                Accepted 10 April 2008
  Leibniz-Institut fuer Neue Materialien,
  Saarbruecken – Germany
  Im Stadtwald, Gebäude 34A; 66123 Saarbrücken


        Functional, nanocomposite, sol-gel based hybrid coatings were the object of a study in respect to their corrosion-
protective capabilities of aluminum alloy (AA2024) and steel (DC01). Commercially available coating system consisting of
primer and top coat has been applied on the sol-gel coatings as an additional protection. Amorphous ZrO2 (5nm) and
crystalline CeO2 (10nm) nanoparticles were embedded in the sol-gel matrices with solid contents between 8wt% to 20wt%.
The thicknesses of the prepared three single sol-gel coatings and the four top-coated coating systems varied between 13
to 19 µm and 40 to 78 µm, respectively. The coating systems were determined by means of electrochemical, hit and
vibration tests as well as under corrosive conditions in a salt-spray chamber. Selected mechanical properties have been
investigated, as well. The nano-hybrid coatings obtained showed good mechanical and corrosion protective capabilities in
the cases of presence as well as in of absence of an industrial topcoat system.
        Key words: nanocomposite, hybrid coatings, corrosion protection, aluminum, steel, salt-spray, electrochemical
tests, mechanical tests

INTRODUCTION                                                   mately mixed polymer systems, formed through hydroly-
                                                               sis and condensation of organically modified silanes with
        Nowadays the sol-gel approach for preparing            traditional alkoxide precursors producing silica based
oxide protective films (coatings) has emerged as a ver-        network structure with organic moieties dispersed
satile method. This is an outcome of the need for devel-       throughout [3-5]. Ormosil films are interesting for the
opment of environmentally friendly coatings, which can         development of corrosion protective coatings, because
replace the chromate containing coatings, considered           they blend the mechanical and chemical characteristics
to be carcinogenic and thus hazardous pollutants [1].          of the comprising organic and inorganic networks. The
        Thin films of sol-gel oxide coatings have found        inorganic components impart durability, scratch resis-
use as protective materials on metals and other sub-           tance, and improved adhesion to metal substrates; the
strates [2]. Organically modified silicates (Ormosils) are     organic components contribute increased flexibility,
hybrid organic-inorganic materials composed of inti-           density, and functional compatibility with organic poly-

                     Journal of the University of Chemical Technology and Metallurgy, 43, 2, 2008

mer paint system. Hybrid films may be tailored to have             hydrolysis, 0.1 M hydrochloric acid was added as a cata-
exceptional durability and adhesion to metals, while               lyst. CeO2 nanoparticles (Aldrich) or ZrO2 nanoparticles
providing a dense, flexible barrier to the permeation of           (INM production) were added to the solution at concen-
water and corrosion agents [1].                                    trations of 20 wt. %, calculated in respect to the solid
       The aim of the present paper is to investigate              material. For the third solution, the organic substructure
corrosion protective capabilities of sol-gel prepared              of the hybrid material, consisting of 2,2’-bis-(4-
hybrid coatings and such combined with additional com-             hydroxyphenyl)-propane (BPA), was dissolved in an or-
mercial protective coating systems, by means of me-                ganic solvent. Finally, after completing the hydrolysis, the
chanical and salt-spray tests and electrochemical cor-             three solutions were mixed together. Before coating depo-
rosion test.                                                       sition process on the substrates, the organic cross linking
                                                                   was commenced by addition of 1-methylimidazole to start
EXPERIMENTAL                                                       the organic polymerisation; the detailed procedure has been
                                                                   described elsewhere [6]. These sol-gel coatings were syn-
Materials, reagents and sol-gel preparation                        thesized at the Leibniz Institut für Neue Materialien gem.
                                                                   GmbH, Saarbrucken, Germany.
        An inorganic-organic hybrid sol was synthesized                    Four-layered coatings architecture has been de-
by mixing and hydrolysing three solutions that previously          posited as is schematically shown in Fig. 1. The first
had been prepared separately. For the first solution,              layer was deposited by sol-gel dip- coating process, with
tetraethoxysilane (TEOS) and methyltriethoxysilane                 composition including epoxysilane, methyltriethoxsy-
(MTEOS) and SiO2 nanoparticles, as components of the               silane and tetraethoxysilane with variation in the wt%
inorganic network, were mixed and hydrolysed by con-               of CeO2 and ZrO2 nanoparticles from 8 to 20wt%. In
centrated hydrochloric acid (HCl). In the second solu-             addition, one thin layer of pyrolytic silica was added to
tion, for creation of the organic substructure and cross           improve the adhesion of the commercially available
linking with the inorganic network, 3-                             primer and finally a top-coat paint system EADS(R) has
glycidoxypropyltrimethoxysilane (GPTS) was used. To assist         been applied for additional protection.

  Fig. 1. Schematic view of four layers coating system
  1- EADS top-coat (thickness 20-30 µm), 2- EADS primer coat (thickness 20-30µm), 3- SiO2 layer (50-100nm); 4- sol-gel coating (5-
  13µm); 5- metal substrate

             G.. Tsaneva, V. Kozhukharov, S. Kozhukharov, M. Ivanova, J. Gerwann, M. Schem, T. Schmidt

                 Table 1. Initial conditions of testing on hit and vibrations.

                   No    Type of the     Dimension         Standard              Conditions of testing
                            test                           methods
                   1     Sinusoidal          Hz         EN 60068-2-6      Temperature 19,8 ± 2oC
                         vibrations                     method Fc         Humidity          38 ± 2%
                                                                          from 10 to 60 Hz,      g=2
                                                                          from 60 to 500 Hz,     g=1
                                                                          from 500 to 2000 Hz, g = 0.5
                                                                          30 min for each vibration.
                                                                          Measurements carried out on
                                                                          vertical axis.
                   2         Hit              g         EN 60068-2-27     Temperature       18 ± 2oC
                                                        method Ea         Humidity           36 ± 2%
                                                                          10g – 2000 hits
                                                                          15g – 500 hits
                                                                          75g – 500 hits
                                                                          Measurements carried out on
                                                                          vertical axis.

Surface treatment, coating deposition and curing pro-         cal tests, namely on hit and vibration, were performed.
cedure                                                        In every following cycle the conditions of testing were
                                                              accelerated. The surface morphology of the tested samples
        Aircraft Al-alloy AA2024 and mild steel DC01          was observed by reflective optical microscopy. The tests
have been used as substrates for single sol-gel coating       were performed according to EN 60068-2-6 standard
deposition. In the case of mild steel the substrates were     method Fc. In Table 1 are presented the initial condi-
degreased with acetone and cleaned with an industrial         tions of testing on hit and vibrations, respectively.
metal detergent (Gardoclean 360 from Henkel, Germany).
The aluminium samples were degreased with acetone,                   Salt-spray corrosion resistance tests
cleaned with Metaclean T2001 (Chemie Vertrieb                        The tests in neutral salt spray (NSS) have been
Hannover, Germany), and afterwards etched in an alka-         executed according to ISO 9227/ASTM 117 standard
line cleaner (Turco Liquid Aluminetch Nr. 2 from Turco        requirements. It was preformed first on single layered
Chemie, Germany). Afterwards the substrates were              sol-gel coatings deposited on both AA2024 and DC01
desmutted with Turco Liquid Smutgo NC. The follow-            substrates. Corrosion media, according to the standard
ing labelling of the coatings are used: Coating 1- coating    requirements, was salt-fog, obtained by pulverization
without nanoparticles, with thickness 19 µm; Coating 2-       of a 5% water solution of NaCl, pH = 6.5-7.2. The tem-
Coating with CeO2 (with use of 20 wt.% nanoscale CeO2),       perature in the camera working space was sustained in
thickness value of 13 µm and Coating 3- Coating with          the range of 35±2oC.
ZrO2 (with use of 20 wt.% nanoscale ZrO2),with thick-                The edges of the samples have been isolated with
ness 14 µm. All coatings were deposited by dip-coating        sticker for galvanic purposes (Polyken, USA). After-
process and cured at 120 °C for 4 h.                          wards, the samples were put on racks made of glass,
        The four layer coatings were deposited only on        under angle of 20o ± 5o in respect to the vertical, exactly
AA2024 substrate. The samples were thermally treated          following the standard description.
at 120oC from 1 to 4 hours. The measured thickness of                It is necessary to underlain that according to the
the coatings varied from 40 to 78µm.                          standard requirements, salt-fog feeding was controlled
                                                              by measuring of the quantity of collected condensate in
       Mechanical tests                                       2 cylinders (horizontal area of collection – 80cm2, for
       The mechanical tests were carried out on single        each cylinder). The average quantity from both cylin-
layered sol-gel coatings, only. Three cycles of mechani-      ders, collected for 24 hours, was from 1 to 2 ml/h. The

                    Journal of the University of Chemical Technology and Metallurgy, 43, 2, 2008

duration of the first tests in NSS, was 4 – 24 hours               protective paint have been object of polarization resis-
cycles i.e. 96 hours in total. Another test at the same            tance, Rp (Ù.cm2) and density of corrosion current, Icorr
conditions was performed for 7 days (168 hours) on 4               (A/cm 2) measurements, respectively. Samples were
layered samples on AA2024 substrate. An “X” scratch                treated in 5% NaCl water solution. Polarization curves
on the surface, was made diagonally before test execu-             were obtained in Tafel’s coordinates.
tion. In every 24 hour’s cycle in SST chamber, the
samples were examined with respect to the adhesion by                      Impedance Spectroscopy measurements
means of a tape test (tape – Tesa fix ¹ 5338), accord-
ing to ISO 2819 requirements.                                              The impedance spectra were obtained on both
                                                                   AA2024 and DC01 substrate samples by a frequency
       Electrochemical measurements                                response analyzer FRA-2 unit - PG30/2-AUTOLAB.
                                                                   Measurement conditions are frequency band from
       The electrochemical measurements have been ac-              0.001Hz to 30MHz in a dynamic range of more than
complished by potentiostat / galvanostat PG 30/2-                  100dB. The minimum sensitivity was 10ìV. Further
AUTOLAB, using Ag/AgCl referent electrode (Metrohm).               impedance spectroscopy measurements of both four and
The corrosion media was 5% water solution of NaCl, pH              single layered coatings on AA2024 were made in open
= 7 ± 0.2 at a room temperature 20 ± 2 °C. Corrosion               circuit potential (OCP) and frequency in the range from
potential (Ecorr) was obtained after minimum two mea-              5 mHz to 1 MHz in 5% NaCl water solution.
surements of each sample. The time for measurements
was 1000 seconds for every single test. Stationary polar-                  Microscopic observations
ization (anodic, EA at iA=15 A/dm3) was defined with
minimum of 3 measurements of each sample.                                Additionally, microscopic observations of the
       It is necessary to mark that both sol-gel coatings          surface morphology were executed by using of both
with and without top and primer coatings of corrosion              Atomic Force Microscopy and Optical Microscopy. In

                   Fig. 2. Surface topology of the metal substrates obtained by AFM and OM.

             G.. Tsaneva, V. Kozhukharov, S. Kozhukharov, M. Ivanova, J. Gerwann, M. Schem, T. Schmidt

         Table 2. Adhesion of 4 layered coatings consisted of EADS primer and top coats.

                  Composition                                    Adhesion (yes / no)
                                          24 h.        48 h.   72 h.    96 h.    120 h.     144 h.    168 h.
                   Ref EADS                no           no      no       no        no         no        no
            Hydrophil+CeO2/EADS             no          no     yes       yes       no         no        no
                    GMT+8%                 yes          no      no       no        yes       yes       yes
                    GMT+8%                  no         yes      no       no        no         no        no
                   GMT+16%                 yes         yes     no*      no*        no*       no*       no*
                   GMT+20%                 yes         yes     yes      no*        no*       yes       yes

         * only local delamination in one cut’s part

the first case, the observation was accomplished over          samples are detected. Generally, all coated samples
square area with 49.5ìm linear size. In the second case        have shown good protective capabilities of the sol-gel
an optical microscope with refracted light was used;           coatings, although a delaminating effect of the coating
image magnification of 232 times was applied.                  from the substrate is obvious after 96h (full) cycle. The
                                                               effect was checked for both kinds of coatings: without
RESULTS AND DISCUSSIONS                                        nanoparticles, and those with ZrO2 nanoparticles incor-
                                                               porated in coatings on DC01. The best protective prop-
       Mechanical tests on hit and vibrations                  erties are achieved in the case of coatings on AA2024.
                                                               As is shown in Fig.2 the roughness of the steel substrate
        The samples after hit and vibrations testing have      is enough higher than that of the aluminum alloy but the
shown good protective capabilities of the sol-gel coat-        adhesion to the second one is better. From Fig. 2 could
ings. The behaviour was identical for both kinds of coat-      be seen that the aluminum surface possesses sharp edges
ings, with and without nano-particles incorporated. Even       and furrows between them. Namely that the surface mor-
at accelerated conditions of testing, the coatings with        phology could act as adsorptive centers due to the dis-
absence and presence of CeO2, or ZrO2 nanoparticles            equilibrium of the forces of attraction over the superfi-
on both kinds of substrates possess a durable mechani-         cial particles, which cause the superficial tension.
cal resistance. After correlation of the results obtained,             Besides, its direct influence, the roughness of the
the samples with ZrO2 nanoparticles display a good in-         metal substrates could be related with the adhesion of
dication for mechanical protection.                            the upper coatings over the sol-gel layer. That additive
                                                               indirect influence is expected because of the low thick-
       Salt-spray tests                                        ness of the sol-gel layer. Namely, that layer repeats the
                                                               profile of the substrate’s surface. In the case of the alu-
       The test results on single sol-gel coatings for 96      minum alloy there is presence of multitude of parallel
hours showed that both substrates (without coatings)           furrows which might retain the coating layers. That re-
were totally damaged still after the first 24 hours cycle.     taining is also observable on the interfaces between the
Visually white rust on AA2024 alloy and red on DC01            inner and outer coating layers.

                    Journal of the University of Chemical Technology and Metallurgy, 43, 2, 2008

       Other possible reason for the better adhesion of      tween sol-gel coating and the industrial primer and top-
the coatings on aluminum surface in comparison to the        coat coating system is not uniform for all samples. An
steel might be accepted the generation of chemical bonds     increased nanoparticles content in the coatings exam-
between the aluminum and the coating material.               ined yields improved adhesion to the industrial coating
       It is well known that almost always on the alu-       system. The largest, delaminated parts of paint - coat-
minum surface an oxide thin layer is forms and it is         ings passed test on elasticity by bending them under
always represented in normal conditions. As a result, in     angle of 180o. It was established that the tested parts
the case of the aluminum substrates the chemical bonds       kept their integrity only in the first experimental cycle.
between the polysiloxanes of the sol gel coating and the
superficial aluminum oxide could appear. Such bonds          Electrochemical and impedance spectroscopy mea-
are possible because of the partial incompleteness of        surements
the polymerization process in the sol-gel system during
its application on the metal substrate. Consequently,               Electrochemical tests
chemical bonds between the oxide surface and some
moieties of the gel matrix are possible. The effect of the            Still after the first tests for defining of corrosion
surface preparation before the coating deposition is al-     potential and stationary polarization, good protective
ready investigated [7], by comparative measurements          capabilities of sol-gel single layers with or without in-
among five different kinds of preliminary treated sub-       cluded nanoparticles in them were established. Further
strates, as follows: polishing, etching, oxide thickening,   tests for polarization resistance and density of corro-
and etching followed by oxide thickening in warm wa-         sion current on both single and four-layered samples
ter. The corresponding samples have been compared            on AA2024 showed the following behaviour: (i)-Coat-
with referent sample obtained by direct deposition over      ings with higher initial values of Rp (108-109 Ù.cm2) and
the substrate surface. Thus, further tests should be car-    low Icorr (10-10-10-12 A/cm2) lost most of their protective
ried out to determine the main reason for the different      capabilities, but still show protective behavior after 96
adhesion behavior of the coating material on different       hours in 5 % NaCl solution. These are namely four-
substrates.                                                  layered coatings with EADS primer and top coat. (ii)-
       As evidence for the excellent adhesion perfor-        Coatings with average starting values of Rp (106 Ù.cm2)
mance of the samples, which are object of the present        and Icorr (10-8 A/cm2) exhibited small variation of Rp and
study, they have not shown any indication of delamina-       Icorr during the immersion into the NaCl solution. These
tion. Exposed for 168 hours in salt-spray chamber, the       are single-layered, sol-gel coatings. Their protective
four layered specimens did not exhibit any corrosion         capabilities proved not to be as good as those of the
products outside the diagonal X-shaped scratch. This is      four-layered coatings as some pitting corrosion appeared
an indication of a good protective behavior in salt-spray    on the sample’s surface.
corrosion medium of four layered coatings with EADS                   Generally, the analysis of the experimental data
top-coat. Still after the first 24 hours in salt spray me-   shows that the chemical nature of included nanoparticles
dia, some of the samples were observed about corro-          in GMT coatings does not affect significantly the pro-
sion in the X-scratches area regarding a presence of         tective capabilities of the layers.
white corrosion products which is typical for Al alloy.
In the next 48 h. cycle the same corrosion effect was               IS measurements
obvious on the surface of all specimens. Best behavior
in salt spray media showed compositions with maxi-                  IS measurements carried out on the single lay-
mum concentration of ZrO2 and CeO2 nanoparticles (16         ered sol-gel hybrid coatings on Al alloy and the steel
and 20 wt.%, respectively) incorporated into the sol-gel     showed very good results concerning the coating corro-
coating.                                                     sion resistance. The results obtained from the IS mea-
       Table 2 present the results obtained of adhesion      surements on four-layered coatings deposited on
tests made on four layered coatings. The adhesion be-        AA2024 match to the results received by polarization

             G.. Tsaneva, V. Kozhukharov, S. Kozhukharov, M. Ivanova, J. Gerwann, M. Schem, T. Schmidt

      Table 3. Values of the polarization resistance (Rp) and corrosion current density (Icorr) obtained by
      two independent electrochemical methods.


                        Coating                    Chronovoltammetry                 Impedance spectroscopy
       No             composition         Rp     (Ùcm2)       Icorr  (Acm-2)              Rp    (Ùcm2)
                                         t = 0h t = 96h     t = 0h     t = 96h          t = 0h     t = 96h
              1      2                      3        4         5          6                7          8
       1     S1     Ref                     -        -          -          -               -           -
       2     S2  Ref EADS               4.10 107 4.76 105 2.40 10-10 4.46 10-8       3.98 107    2.85 105
       3      Hydrophyl + CeO2
             S3                         1.73 106 3.53 106 1.38 10-8 1.61 10-9        4.79 105    2.62 106
       4      Hydrophyl + CeO2
          S4                            9.63 108 1.15 107 5.98 10-11 1.87 10-8       6.48 108      8.11 106
       5 S5 GMT 8% ZrO2 1h              1.95 106 5.25 106 2.70 10-9     4.43 10-9    2.30 105      1.94 106
       6 S6 GMT 8% ZrO2 1h              4.22 109 1.83 107 1.19 10-8     2.43 10-10   2.37 109      1.40 107
       7 S7 GMT 8% ZrO2 4h              1.10 106 5.83 105 4.01 10-9     1.08 10-7    2.10 105      1.72 104
       8 S8 GMT 8% ZrO2 4h              1.76 109 5.01 106 1.13 10-11 1.49 10-8       3.30 109      4.21 106
       9 S9 GMT 16% ZrO2 4h             9.72 106 4.77 105 1.96 10-8     3.39 10-8    7.17 106      6.79 103
       10     GMT 16% ZrO2 4h
          S10                           4.22 109 1.44 107 2.19 10-12 6.94 10-9       1.66 109      1.17 107
       11 S11 GMT 20% CeO2 4h           1.83 106 3.44 106 1.07 10-8 4.42 10-9        2.53 105      1.84 104
       12 S12 GMT 20% CeO2 4h           2.43 109 5.37 106 6.27 10-12 2.10 10-9       2.01 109      8.46 106

curves. Good correlation between both kinds of elec-         dress the optimization of the adhesion between the
trochemical measurements is achieved only in the case        sol-gel layer and the industrial coating system for a
of the four-layered samples showing high values of Rp.       better corrosion protection and determination of the
        The spectra of the single-layered samples is com-    optimal value of the nanoparticles content in the sol-
plicated, due to the appearance of a new time constant       gel coatings, as well.
in impedance spectrums, which is probably related to
the electrochemical reactions taking place at the metal/     Acknowledgements
coating interface of the sample [8, 9].                              The support from the EC in the frame of 6-th RTD
        The values of Rp and Icorr, obtained by two inde-    programme (EC- Contract NMP3-CT-2005-011783) is
pendent electrochemical methods, are summarised and          greatly acknowledged.
presented in Table 3.
                                                             1. I. Suzuki, Corrosion- Resistant Coatings Technol-
       The tests conducted gave an indication that sol-           ogy, Marcel Dekker ed., N. Y., 1989, p85.
gel prepared single hybrid coatings and four-layered         2. M. Guglielmi, J. Sol-Gel Sci. Tecn. 8 1997, 443-449
protective coatings consisting of the hybrid coating         3. T. Iwamoto and J.D. Mackenzie, J. Mater. Sci. 30
material, a thin bond layer and an industrial top-coat            1995, 2566-2570
twice deposited, exhibit efficient barrier properties.       4. J. Wen and G.L. Wilkes, Chem. Mater. 8 1996, 1667-1681
There was established a contributing effect regarding        5. R. Kasemann and H. Schmidt, New J. Chem. 18 1994,
the corrosion protection of AA2024 alloy and mild                 1117-1123
steel against aggressive attacks and mechanical dam-         6. H. Schmidt, G. Jonschker, and S. Langenfeld
ages of the environment. Further experiments will ad-             “Verfahren zum Schutz eines metallischen Substrates

                   Journal of the University of Chemical Technology and Metallurgy, 43, 2, 2008

    vor Korrosion” (Inst Neue Materialien), Pat. DE        8. A. M. Bond, R. G. Compton and D.A. Fiedler, Elec-
    19813709 (27.03.1998).                                     troanalytical methods, Edit. F. Scholz., 2006, p75.
7. A. S. Hamdy and D.P. Butt, Surf. and Coat. Tech. 201    9. B. B. Damaskin, O. A. Petriy and G. A. Tsyrlina,
    2006, 401-407.                                             Electrochemistry, Ed. „Chemistry“ 2001, p 220.


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