Volume 2 Paper 2
Amorphous Zirconia Films as Adhesion Promoter for Organic Coating
F. J. Rodriguez, L. Fedrizzi, R. Di Maggio and S. Rossi
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JCSE Volume 2 Paper 2
Submitted 13th September 1999, published for public review 27th October 1999
Amorphous Zirconia Films as Adhesion Promoter for Organic Coating
F. J. Rodriguez�, L. Fedrizzi*, R. Di Maggio and S. Rossi
Dipartimento di Ingegneria dei Materiali, Universit� degli Studi di Trento,
Via Mesiano, 77, 38050 Trento, Italy, mailto2('labaci','ing.unitn.it')
* Dipartimento di ICMMPM, Universit� di Roma I "La Sapienza",
Via Eudossiana, Roma, Italy
� Depto. Ingenieria Metalurgica, Facultad de Quimica, U.N.A.M.
04510 Mexico D.F., Mexico, mailto2('fxavier','servidor.unam.mx')
The two main features of a protective coating are the adhesion and the
anticorrosive protection. In order to improve both, chemical pre-treatments
have been used. The use of chromate was very popular, but recently they have
been highly restricted because of their toxicity, so that chromate-free
pre-treatments have been developed and tested. An interesting alternative
seems to be the deposition on the metallic surface of thin layers of zirconia
by sol-gel process. In this study thin films of amorphous zirconia on low
carbon steel sheets have been obtained by the dip-coating technique. The
behaviour of the zirconia films as promoters of adhesion was evaluated
measuring detachment of cross-scratched paint coated samples after Salt Fog
Chamber Test. In addition, the determination of adhesion of organic coatings
was performed by swelling in N-methyl pyrrolidone. According to the results,
the samples pretreated by zirconia layers showed a good performance, in
comparison with commercial chemical treatments (tricationic phosphate and iron
phosphate). The protective properties of the zirconia films were found
strongly depending on the process parameters, e.g. concentration of the
precursors solutions and chelating agents, which rule the thickness and the
organic residuals amount of the amorphous zirconia. In order to assess the
behaviour of these sol-gel systems, preliminary tests using Electrochemical
Impedance Spectroscopy were carried out too.
§2 Keywords: Organic coatings, Zirconia film, Adhesion, Pre-treatment.
The most important properties of a protective coating are the anticorrosive
action  and the adhesion to the substrate. The coatings act as barriers to
aggressive chemical species, resulting also active against corrosion because
of the chromate or phosphate present as pigments. However the coating
protection fails, if the paint does not adhere to the substrate adequately [1,
2]. At the aim of improving adhesion properties of organic coatings on steel,
chemical pre-treatments have been used for a long. Among these chromate and
phosphate pre-treatments showed to be quite useful. However, by law, many
countries have forbidden the use of toxic pre-treatments (chromate-like), in
spite of their good anticorrosive properties. Because of that, industry is
deeply concerned about the development of a chemical process able to
substitute chromate pre-treatments . On steel it has been employed iron or
zinc or tricationic (Zn, Mn, Fe) phosphate . The use of phosphate has shown
good results on galvanised steel. So, it improves the adhesion of the paint
film and the anticorrosive protection. But phosphates are not the best option
in ecological terms. Other chromate-free pre-treatments have been developed
and tested. The choice of the chemical conversion treatment must be based on
several criteria: economical, ecological, technical, etc. Some of them have a
chromate-like behaviour, but are not economically attractive. The application
of zirconium oxide as a pre-treatment on steel appears a promising compromise.
On the other hand, ZrO2-CeO2 and other oxides films, put
down on stainless steel by sol-gel techniques, resulted protective against dry
and wet corrosion [5,6]. Sol-gel is a process to obtain films of metallic
oxides in very mild conditions, but the reproducibility of thickness and the
quality of them depend on several parameters affecting the precursors solution
. As an example the presence of complexing agent has already been shown to
be important in improving the stability of solution of metal alkoxides, very
moisture sensitive compounds [8-10]. In this study it was evaluated if the use
of two different complexing reagents and the control of the hydrolysis allow
the formation of ZrO2 films suitable as pretreatments promoting
adhesion of organic coatings. The zirconia applied by sol-gel on low carbon
steel as an adhesion promoter instead of chromate and phosphate looks
promising, however a technological application cannot disregard the evaluation
of economical advantages.
Low carbon steel was used as substrate. 7.5cm x 10 cm samples were first
degreased with organic solvents, then dipped into solutions of zirconium
tetrabutoxide in anyhdrous butanol and withdrew at constant rate of 3 cm/s,
obtaining thin films of amorphous zirconia. After the deposition, the film
underwent thermal treatments in oven at temperature between 150-250�C, in
order to densify and remove the most organic load. Repeating the deposition
run, the thickness could be increased. The chelating agents employed in this
study are acetic acid and acetylacetone, which have been selected based on
previous study [8,9]. The hydrolysis and therefore the formation of the film
was made by reacting in controlled humidity chamber (70% RH) after each
dipping, except in series 7 and 8 where this step was made using boiling
water. When the total thickness was obtained, the samples were treated at 150
or 250�C In Table I are reported the concentration of precursor, chelating
agent, number of layers, temperature and time of the thermal treatment. On
comparison, the thickness of zirconia films on glasses, prepared in the same
conditions was measured by using a profilometer Dektak 3 . The value was
around 0.60μm, but for series 4 and 5 it
increased up to about 1.0 μm. The series
9, 10 and 11 were used as reference samples. Degreased steel was coated in
order to get a blank with poor adhesion between coating and substrate.
Tricationic phosphate is a common chemical treatment, which highly improves
the adhesion of the coating to the substrate as well as corrosion resistance.
Iron phosphate has been studied because of its use on the industry due to low
§5 Table I.
Alkoxide Molarity (g/mole)
Treated in boiling water
Treated in boiling water
§6 A multilayer polyester paint coating formed by a primer and a topcoat was
applied in all cases. The average dry film thickness (DFT) was about 30 μm.
This kind of paint is usually used for electrical appliances . A part of
the coated samples were cross-scratched and tested evaluating the damage in
Salt Fog Chamber (ASTM B117). In order to assess the adhesion promoted by the
pre-treatment, quantitative measurements were made using a pull-off test
(Sebastian IV) and measurements of the swelling of the film using N-methyl
pyrrolidone . The FTIR analyses were performed by a Spectrometer BioRad
FTS 165, coupled with a BioRad UMA 250 device. In addition, Electrochemical
Impedance Spectroscopy was used in order to monitor the increase in wet area
and some other interfacial phenomena. A three-electrode cell was used,
constituted by: Working electrode: Pre-treated and painted samples Counter
electrode: Platinum sheet Reference electrode: Ag/AgCl (207 mV vs SHE) The
electrolyte was 0.3%wt. Na2SO4, because of its
characteristics of conductivity and low aggressiveness. The equipment used was
a EG&G PAR 273 potentiostat with a Schlumberger Solartron Frequency
Response Analyser 1255. The results were registered with M398 software and
treated with the EQUIVCRT software .
§7 Results and discussion
The gel structure is determined from both the concentration of the alkoxides
solution, and temperature or length of the thermal treatments. The hydrolysis
and condensation of alkoxides of zirconium, similarly to those of other
metallic species, easily take place in the solution, leading to a gel. This is
constituted of an amorphous network of metallic oxide, even if discontinuous
because of the presence of organic or hydroxylic functionalities. Being the
hydrolysis of the alkoxides of zirconium faster than condensation, the gel
could be formed slowing down the first reaction by adding complexing agent as
acetylacetone or acetic acid. Moreover higher the molarity of alkoxide in the
solution, higher the thickness of the deposited layers. The strength and the
skeletal density of the gel as deposited improves after thermal treatment. The
densification of the gel is accompanied by noticeable shrinkage, which could
also cause cracks formation. During the heating, the most of the residuals
further react leading to a more interconnected metallic oxide. In fact one of
the advantages of using sol-gel process is the mild treatment necessary to
form a ceramic layer. Anyway, a few organic residuals, especially the
residuals deriving from the complexing substances, were always present in the
film, unless the temperature of crystallization of the zirconia has been
reached during the heating in air . The IR spectra of all samples showed
the presence of not reacted organic linked to the zirconia matrix. Increasing
the maximum temperature from 150 to 250 �C the reduction of organic load is
remarkable in the spectra of the samples 1 and 3 of Figure 1.
§8 Figure 1. Spectra IR of zirconia samples 1 and 3.
§9 Salt Fog Chamber (First Part)
The results obtained in Salt Fog Chamber test are shown in Table II. The
detachment near the scratch was measured by gently tearing off the disbonded
area with a knife. In some cases blisters were also observed. The disbondment
was not uniform along the cross scratch, so the data reported in the table are
average values. In Table II also the maximum and the minimum were shown, when
the range was too wide. These data evidenciated the adhesion properties of
zirconia films depend on several parameters. Time and temperature of thermal
treatment appear to be the most important of them. As discussed above, the
heating promotes the reduction of organic load, so that the presence of a very
few residuals seems to be advantageous to the adhesion. In order to maintain
low the amount of organic residuals when the thickness of the films increases,
the time or the temperature of the treatment have to be increased too. On the
other hand, the zirconia prepared from solution containing the acetylacetone
showed the worst behaviour among the sol-gel samples, even when they underwent
the same thermal treatment. The stronger chelating action of acetylacetone
with respect acid and its very difficult hydrolysis could account for these
findings. The acetylacetonato groups bonded to zirconium prevent the
condensation and the formation of an oxide network . At the aim of to
promote the hydrolysis, some of the samples were also hydrolysed in boiling
acidic water, but the recorded improvement was too small and not sufficient to
consider interesting the way. In fact that condition did not enhance the
condensation accordingly, and the surface of the samples appeared dusty and
not continuous, through observation with a stereoscopic microscope. Finally
the microstructure of the samples obtained from solution containing the acid
appeared more suitable to adhere to the organic coating.
§10 Table II.
Detachment after 7 days (mm)
Detachment after 21 days (mm)
0.7 (0.6 to 1)
1.2 (1 to 1.3)
0.5 (0.3 to 0.8)
2 (1.8 to 2.5)
Small blistering near the scratch
0.8 (0.5 to 1)
3.5 (3 to 4)
Total detachment after 21 days. There were
large blisters all over the samples.
0.6 (0.5 to 1)
It was difficult to detach
1.5 (1 to 2)
After 7 days, all the paint was detached
0.6 (0.2 to 1)
1.8 (1 to 2)
Different detachment depending on the
position in the cross scratch. Blistering
1.1 (0.5 to 1.8)
Disbonded area is brittle. Blistering
Blistering all over the sample
0.1 (0.1 to 0.2)
It was difficult to detach
2.5 (2 to 3)
There was blistering mainly near the
§11 It is remarkable that the series 5 along with series 1 resulted the best in
promoting adhesion. Moreover, the series 5 resulted even better than the other
at longer time of analysis. The use of a more concentrated solution of
alkoxide seems to be positive for a good performance of the zirconia film. At
first glance this result could appear odd. Anyway the presence of a great
number of residuals, in the film prepared from a concentrated solution, means
they are closer than in a layer obtained from a diluted one. That
configuration is likely more favourable to give condensation reactions,
resulting a dense and thick metallic oxide layer. A ranking of the adhesion
behaviour in Salt Spray Fog of our samples could be done. As regard the
reference samples, those without pre-treatment resulted obviously not good at
all. The use of iron phosphate as a pre-treatment shows good results in short
times of exposure (7 days), compared with the zirconia film in series 1, 5 and
2, but it is not a good option in long service life. Since now, tricationic
phosphate looks the best option .
§12 Salt Fog Chamber Test (Second Part)
The results previous discussed brought about the further study was
restricted to samples of series 1 and 5. Different sheet were coated by
zirconia and another polyester paint system (also used on domestic electric
appliences). Moreover, in order to prove without any doubt the effect of the
heat treatment, other samples have also been prepared, the features of which
are reported in Table III, along with the results of Salt Fog Chamber test.
The results were even better than those expected. All the samples with sol-gel
films applied and hydrolysed without the thermal treatment showed a
significant detachment in short-time exposure. Besides of the effect of the
new paint, it is noteworthy the better performance of the sol-gel
pre-treatment with respect the iron phosphate, even though not as good as the
tricationic posphate. Moreover the behaviour of these new series maintains
very good also after large time of exposure in Salt Fog Spray. The results of
the test of reference samples had shown once more how important is the
pre-treatment of the steel in promoting adhesion. The low corrosion and the
negligible blistering phenomenon observed for the samples 5" and 5 proved
the reactions of condensation take place fastly in a concentrated solution. So
that a thicker and stiffer layer of gel could be obtained from those
solutions, which can take more advantage from the thermal treatment already at
§13 Table III.
Solution Molarity, Tmax and time of the
Loss of adhesion (detachment from
the scratch in mm)
0.3M, 250�C, 30’
Few blisters and corrosion in the scratch
0.3M, no heat treatment
Completely covered with small blister
0.6M, 150�C, 30’
Blisters and some points of rust in the
0.6M, no heat treatment
2 – 3
Completely covered with small blister
0.6M, 250�C, 30’
Few small blisters and some points of rust in
9 – 10
Large blisters near the cross-scratch
First blistering after 27 days (small
blisters). Few points in the scratch
Large blisters near the cross-scratch.
* The comment refers to the appearance of the samples on the 27th
day or when retired.
- Means that the samples were retired because of the total detachment of the
§14 Swelling with N-methyl pyrrolidone (NMP)
Van Ooij et al  have proposed the measure of the adhesion of the paint
coating on the base of its swelling in N-methyl pyrrolidone (NMP). In fact the
swelling produces stress, which could detach the paint from substrate. This
test of adhesion of a film to metal depends on the thickness of film, the
nature of the coating, and especially the pre-treatment on substrate surface.
The results of this test are expressed as time of total detachment of the
organic film in NMP. This parameter is named NMPRT (N-methyl pyrrolidone
retention time). The test is based on the hypothesis that NMP acts on organic
coating, disregarding substrate and pre-treatment. Actually the presence of
organic residuals in the zirconia films of this study, even if negligible,
could invalidate the assumption, so that the detachment could be due both to
the stresses induced by swelling and a chemical-physical interaction between
NMP and the zirconia films. In order to clarify this effect FTIR analyses on
unpainted samples, before and after the immersion in NMP at 60�C, have been
performed. In some cases (series 1) the organic residual part in the zirconia
thin film was highly removed (Figure 2) by NMP, but in the series 5, the
difference were not detectable before and after immersion (Figure 3), probably
due to the greater thickness of these samples. It can also evidenciated the
presence of the NMP adsorbed on the films, from a signal at about 1700 cm-1
referring to C=O bonds. So, it seems that the remotion of the organic
components of the zirconia thin film by the NMP affects the results of this
§15 Figure 2. FTIR spectra of series 1 before (A) and after immersion in NMP
§16 Figure 3. FTIR spectra of series 5 before (A) and after immersion in NMP
There was no total detachment of the coating on the samples pre-treated
with phosphate, but after 2 hours we could appreciate the formation of
blisters reaching a 4M and 7M degree (according to ASTM D 714), for the iron
and tricationic phosphates, respectively. Anyway the results showed again that
the zirconia films, even if are not as good as tricationic phosphate, are
better than the degreased steel.
§17 Pull-off test
The tests carried out with a pull-off device (Sebastian IV) have not been
useful in this research, because of the type of failure. The failure mode was
decohesive in the topcoat, and we did not see the primer. As a result we just
can say that the mechanical features of the coating are poor or inferior to
the substrate adhesion.
§18 Impedance measurements
Impedance measurements were performed in order to follow the progressive
loss of adhesion between the coating and the pretreatment or substrate.
Because of the high quality of the organic paint, for all the samples only a
capacitive behaviour could be recorded, so that the coating system do not
allow us to evaluate the differences existing among the pre-treatments. At the
aim of overtaking the impasse, some defects have been introduced on the
organic coating by using a metallic pin. The size of the defects was almost
the same in all samples (≈150 μm
diameter), according to the measurements through microscopical observations.
That value was also confirmed by the analyses of the first loop in the
impedance diagrams (Figure 4), which allow to determine the area of the
defect. Generally the whole Nyquist diagrams show two semicircles at the
beginning of the experiment, and a third one after few hours (Figure 5). The
fitting of the impedance results was performed by Equivcrt software . In
the high frequency semicircle, the simulation have yielded values, which could
be associated with the properties of the coating as well as the size of the
defect. Anyway in absence of a model, the interpretation of the rest of the
diagram is not easy and still in progress. As an example, the semicircle with
a maximum frequency at 3.9 Hz in Figure 5 can be deconvolved in two
components. Provoking larger defects (300 μm) on
the coated surface, the impedance diagrams became simpler, comparing same
period (Figure 6). It can be assumed the presence of corrosion products
occludes the defect when it is small, hiding the progress of the detachment of
the coating. Nevertheless, increasing the size of defect moves the response
due to the coating out of the experimental frequency window. As a a
consequence the size of the defect was shown a relevant and important
parameter in the corrosion processes and further efforts have to be addressed
to explain both small and large defects affect impedance response.
§19 Figure 4. Detail (high frequency zone) of the Nyquist diagram of a
coated zirconia samples with an artificial defect (150 μm);
the whole diagram is showed in Figure 5.
§20 Figure 5. Nyquist diagram of a coated zirconia sample with an artificial
defect (150 μm).
§21 Figure 6. Nyquist diagram of the coated zirconia sample with an
artificial defect (300 μm).
On the base of these results, it is easy to infer that thin amorphous zirconia
films are better pre-treatment than a commercial iron phosphate, even if they
are not so good as the tricationic one. Also, the thermal treatment on forming
the zirconia film is important and a very strict control in the use of
temperature and time of treatment is a condition in order to obtain the best
performance on adhesion of the ZrO2 film. The N-methyl pyrrolidone
test for the assessment of the adhesion between organic coating and substrate
cannot provide indisputable results because of its interaction with the
organic component still present in the studied zirconia thin film.
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