Volume 6 Paper 57
Effect of Drying Temperature on Chromate Conversion Coatings on Zinc
X. Zhang, C. van den Bos, W.G. Sloof, H. Terryn, A. Hovestad and J.H.W. de Wit
Keywords: Chromate conversion coating, Corrosion, Electrochemical impedance, XPS, Zinc
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JCSE Volume 6 Paper 57
Submitted 6th July 2003
of Drying Temperature on Chromate Conversion Coatings on Zinc
X. Zhang1, C. van den Bos1, W.G.
Sloof1, H. Terryn2, A. Hovestad3 and J.H.W. de
1Department of Materials
Science and Technology, Delft University of Technology, Rotterdamseweg 137,
2628 AL Delft, The Netherlands
Institute for Metals Research, Rotterdamseweg 137, P.O. Box 5008, 2600 GA
Delft, The Netherlands
3TNO Institute of
Industrial Technology, P.O. Box 6235, 5600 HE Eindhoven, The Netherlands
effect of drying temperature on chromate conversion coatings on zinc has been
studied using electrochemical impedance spectroscopy (EIS) and potentiodynamic
measurements combined with FTIR and XPS surface analytical techniques. The
chromate coatings were generated on pure zinc specimens in a solution (pH 1.2)
containing dichromate anions and sulphuric acid at room temperature. The
coatings were dried at three different temperatures: 60, 110 and 210 �C. The
results show that the drying temperature not only affects the morphology of the
layer, but also changes the chromium oxidation states in the layer. The
coatings dried at 60 �C showed passivity, but the coatings dried at 110 �C had
the fewest cracks and the highest corrosion resistance. Drying at higher
temperatures (210 �C) degrades the chromate coatings by widening the cracks and
reducing soluble Cr(VI) in the chromate layer. The thermal reduction of Cr(VI),
detected by XPS, is probably responsible for the decrease of the Cr(VI) content
of the coating.
Keywords: Chromate conversion coating, Corrosion, Electrochemical
impedance, XPS, Zinc
conversion coatings (CCCs) have been used for protecting metals against
corrosion for many years. However, the toxic nature of Cr(VI) species
necessitates a search for alternatives . Although a lot of research
has been done and many valuable results have been obtained, the mechanism by
which CCCs inhibit the corrosion of metals is still not fully understood [2-5]. An understanding of the
protective mechanism of CCCs may be useful in the search for alternatives.
§3 It is necessary to dry chromate
coatings after the chromating process, because in the wet condition the
coatings are liable to mechanical damage. Furthermore, the slow removal of
water from the coating at ambient temperature due to evaporation can result in
pore formation and poor adhesion to the underlying zinc substrate . It has been reported that
drying at elevated temperatures (above 50 �C) can result in the formation of
brittle, cracked chromate coatings on zinc, which provide less effective
corrosion protection . For magnesium substrates,
however, it has been reported that heating can improve the protection provided
by chromate coatings . A progressive decrease in the
Cr(VI) content of chromate coatings on aluminum alloys with increasing heating
temperature was reported by Laget et al.. For 2024 aluminum alloy, they
found that this fall in the Cr(VI) content was associated with a significant
decrease in the corrosion resistance of the coating. For 1100 aluminum alloy,
on the other hand, no significant change in the corrosion resistance was
observed. Clearly, the effect of the drying (or heat treatment) temperature on
the corrosion protection provided by chromate conversion coatings is
complicated, and is strongly dependent on the nature of the metal substrate.
§4 In this paper, the surface structure and
electrochemical characteristics of chromate coatings on zinc, after drying at
different temperatures, have been investigated in order to understand the
influence of the drying temperature on the corrosion performance of chromate
coatings in a solution containing chloride.
2.1�� Conversion coating
sheets (99.95% Zn) were cut into 7.5�2.5 cm specimens for the Fourier Transform
Infrared spectroscopy (FTIR) analysis, 2.0�1.6 cm specimens for the X-ray
photoelectron spectroscopy (XPS) analysis and 2.0�2.0 cm specimens for
electrochemical measurements. All the specimens were polished using 1 µm diamond grains as the
final polishing step. The polished specimens were cleaned in acetone and
ethanol for 2 minutes, sequentially. The surfaces of specimens were activated
in 0.25 % HNO3 for 30 seconds and rinsed in deionized water before
the chromating treatment was performed. Chromate conversion coating was carried
out in a bath containing 200 g/l Na2Cr2O7 + 10
g/l H2SO4 (pH 1.2) for 10 seconds at room temperature.
After the conversion treatment, the specimens were rinsed in deionized water.
After drying in flowing air, these specimens were divided into three groups and
dried in an oven at 60, 110 or 210�C for 30 minutes,
2.2�� Morphology analysis
surface morphology of the chromated specimens was investigated by means of SEM.
The width of the microcracks was also measured using SEM.
2.3�� FTIR and XPS analyses
§7 The compositions of the
chromate coatings were analyzed by FTIR. The reflection
absorbance IR spectra were obtained on the NexusTM spectrometer from
Nicolet using OMNIC software. The specimen was laid on a flat sample support.
The angle of incidence was 84 degrees. The detector used was a liquid nitrogen
cooled MCT-B detector. The spectra were recorded in the 4000 to 400 cm-1
range with a resolution of 4 cm-1.
§8 The composition of the chromate layer was
also analysed by XPS. The XPS analysis was carried out with a PHI 5400 ESCA
using 400 Watt Mg Kα radiation (1253.6 eV). This instrument is
equipped with a Spherical Capacitor Analyser (SCA) operating with a constant
pass energy value. The energy scale of the spectrometer was calibrated
according to the method described by Anthony et al.. Overview spectra were obtained in the range of 0 � 1100 eV with an
analyser pass energy of 71.55 eV. The intensities of Cr 2p, O 1s, C 1s, S 2p
and Zn 2p photoelectron lines were recorded separately with an analyser pass
energy of 35.75 eV. The electrons emitted from the specimens were detected at
an angle of 45� with respect to
the specimen surface. The C 1s peak (284.8 eV) was used as a reference to
correct for electrostatic charging. The X-ray satellites, present in all
measured spectra as a consequence of the non-monochromatic nature of the
incident X-ray beam, were removed using the relative height and displacements
with respect to the height and position of the Mg Kα. In order to assess the relative
amounts of the species constituting the photoelectron lines, curve fitting was
performed with symmetrical Gaussian-Lorentzian peaks after smoothing of the
curve and Shirley-type subtraction of the background. The number of components
to be fitted to any particular spectrum was determined by choosing the fit with
the minimum reduced chi-squared value (χ2/n), where n is
number of degrees of freedom. Cr 2p spectra were fitted for chromium in the
form of Cr2O3 (576.3 � 0.2 eV), Cr(OH)3 or CrOOH (577.4 � 0.2 eV) and Cr(VI) (579.2 � 0.2 eV)
in the chromate layer [10-12].
2.4�� Open-circuit potential and polarization measurements
potential (OCP) and potentiodynamic polarization measurements were done in a
cell containing a platinum counter electrode and a reference electrode
(saturated calomel electrode: Eo ≈ 0.241 VNHE).
Specimens were immersed in 0.01 M NaCl solution (pH 6) for 1 hour to establish
a relatively steady open-circuit potential (OCP). Polarization measurements
were then obtained by scanning the potential first from OCP to �0.25 V versus
OCP, then in the opposite direction to OCP and ending at +0.25V versus OCP. The
scan rate was 0.167 mV/s.
§10 Electrochemical impedance spectroscopy
(EIS) measurements were carried out in 0.01 M NaCl solution after immersion for
0.5 hours at open circuit. The counter electrode was a flat circular platinum
net, parallel to the surface of the zinc specimens. The reference electrode was
an Ag-AgCl/Cl- (saturated KCl) electrode (Eo ≈ 0.197 VNHE ≈ -0.044 VSCE). The
impedance response was analyzed using a Solartron 1255 frequency response
analyzer coupled with a Solartron 1287 electrochemical interface in the
frequency range of 60 kHz � 0.1 Hz with 10 mV a.c. amplitude versus the OCP.
3.1�� Morphology of
chromate conversion coatings
§11 Figs. 1, 2 and 3 show the SEM images for
chromate-coated specimens dried at different temperatures, 60, 110 and 210 �C, respectively.
§12 All the chromated specimens showed
microcracks in the coatings. The width of the cracks, for a coating dried at 60
�C, was about 90 nm. For a coating dried at 110 �C, it was about 110 nm and for a coating dried at 210 �C, it was 300 nm. However, concerning the crack density, i.e.
the length of crack per unit area, the coating dried at 110 �C showed the lowest crack density. The same crack densities were
also observed under an optical microscope (no optical micrographs are presented
here), so the formation of the microcracks cannot be attributed to the vacuum
conditions inside the scanning electron microscope.
§14 Fig. 1.
SEM image for a chromate coating dried in an oven at 60�C for 30 minutes.
§16 Fig. 2.
SEM image for a chromate coating dried in an oven at 110�C for 30 minutes.
§18 Fig. 3.
SEM image for a chromate coating dried in an oven at 210�C for 30 minutes.
reduction of the Cr(VI) in the coatings
§19 Fig. 4 shows the vibration spectra for the
chromated pure zinc samples dried at 60, 110 and 210 �C. Almost all the peaks in these spectra become shorter with the
increase of the drying temperature, except for the peaks in the range of 607 ~
§20 The vibration bands around 607 and 677 cm-1
were attributed to Cr2O3. The bands between 860 and 948
cm-1 were attributed to the Cr2O72- or
CrO42- anions due to the ν1 and ν3 vibrations, respectively [13;14]. The band at 1126 cm-1 was attributed to
SO42- . The broad absorption band observed between 3000 and 3620 cm-1
was attributed to water or water of hydration, and the H-O-H bending motion at
1620 cm-1 was also seen . The peaks at 1060 and 1426 cm-1 may also be attributed to the
bending vibration of water coordinated to the Cr2O3�2H2O
§21 Fig. 4 shows that the area underneath the
broad peak at 3440 cm-1 is smaller for the chromate coatings dried
at the higher temperatures (110 and 210 �C), suggesting that the dehydration of the coating was significant
for these samples. Concerning the Cr(VI) species, it is important to observe
that, for the chromate coating dried at 210 �C, the peak at 948 cm-1 becomes lower and narrower while
the peak near 677 cm-1 becomes taller than for the sample dried at
60 �C. For the
specimen dried at 110 �C, both
peaks become lower than for the sample dried at 60 �C. These changes suggest that a certain fraction of Cr(VI) species
is transferred to Cr(III) compounds when the specimen is dried at a higher temperature.
§23 Fig. 4.
FTIR vibration spectra for three chromated pure zinc samples: (a) dried at 60 �C,
(b) 110 �C and (c) 210 �C for 30 minutes.
§25 Fig. 5.
FTIR vibration spectra obtained from a single chromated pure zinc sample
sequentially dried at: (a) 60, (b) 110 and (c) 210 �C
for 30 minutes at each temperature.
§26 Fig. 5 shows the vibration spectra obtained
from a single chromated pure zinc sample sequentially dried at 60, 110 and 210 �C for 30 minutes at each temperature. Again, dehydration with an increase
of temperature was observed. The peak at 948 cm-1 becomes shorter
and the peaks near 677 cm-1 become taller, again showing that a
certain fraction of Cr(VI) species has been transferred to Cr(III) compounds
after drying at higher temperatures.
§27 Fig. 6 shows an XPS spectrum for a chromate coating
dried at 60�C. It shows
the presence of C, O, Cr, Zn and S in the top layer of the coating. When the
drying temperature was increased, the carbon and zinc signals also increased
(the relevant figure is not presented here), while the oxygen signal decreased.
Of course, the oxidation of zinc would result in an increase of the oxygen
content. However due to the dehydration of the chromate layer the net result is
a loss of oxygen.
§28 Fig. 7 shows the Cr 2p XPS spectra for
chromate coatings dried at different temperatures. The peak at 579.2 eV is
attributed to Cr(VI). This peak becomes shorter for the coatings dried at
higher temperatures. By increasing the drying temperature from 60 to 110 and
210 �C, the ratio of
Cr(VI) to total chromium in the coating decreased from 35% to 32% and 12%,
§30 Fig. 6. XPS spectrum for a chromate coating dried at
§32 Fig. 7. Cr 2p XPS spectra for chromate coatings dried
at different temperatures.
§33 Fig. 8 shows the polarization curves
obtained in aerated 0.01 M NaCl solution, for three chromate-coated specimens
all dried at 60 �C. The mixed
potential moved about 120 mV in the negative direction against the OCP. All of
the specimens showed a kind of anodic passivation behaviour in a range of 160
mV. The polarization curves display a good degree of reproducibility.
§34 Fig. 9 shows the polarization curves
obtained in the same corrosive medium for coatings dried at the three different
temperatures. For all the coatings, the anodic breakdown potential was almost
the same, around �0.92 VSCE. Only for the coating dried at 60 �C did the mixed potential move in the negative direction against the
OCP. Significantly, the cathodic current recorded near the OCP was smaller for
the coatings dried at 110 �C than
for the coatings dried at the lowest temperature (60 �C). For the chromate coatings dried at the highest temperature (210 �C), the cathodic current measured near the mixed potential was
larger than for the coatings dried at 60 �C.
§36 Fig. 8. Polarization curves for three
chromate coated specimens dried at 60 �C. The potential scan rate was 0.167
Fig. 9. Polarization curves for
chromate coatings dried at different temperatures. The potential scan rate was 0.167
§39 Fig. 10. The Nyquist impedance plot
for non-chromated pure zinc in aerated 0.01 M NaCl solution (pH 6).
§41 Fig. 11. The Nyquist impedance plots for
the coatings dried at different temperatures.
12. The Bode impedance plots for the coatings dried at different temperatures.
§44 Fig. 10 shows a Nyquist impedance diagram
for a pure zinc specimen (with no chromate coating) in aerated 0.01 M NaCl
solution (pH 6). It shows that zinc corrosion is charge transfer controlled
dissolution in the NaCl solution. The charge transfer resistance is about 5.5 kΩ�cm2 in this case.
§45 Fig. 11 shows the Nyquist plots of
impedance for the coatings dried at different temperatures. For the coatings
dried at 60 and 110 �C, two
capacitive loops were observed, while for the coatings dried at 210 �C, only one capacitive loop was observed. The capacitive loop at
high frequencies is related to the chromate layer and the loop observed at
lower frequencies is related to the double layer at the zinc/electrolyte
§46 Fig. 12 shows the Bode plots of impedance
for the coatings dried at different temperatures. It shows that all the
chromate coatings have larger impedance than the bare zinc. However, among the
chromate coatings, the coating dried at 110 �C has the largest impedance and
the coating dried at 210 �C has the
lowest impedance, which is consistent with the results of the polarization
measurements. The phase versus frequency curves show that there are two time
constants for the coatings dried at 60 and 110 �C, while for the coating dried at 210 �C and the pure zinc there is only one time constant.
§47 The larger impedance
shown by the chromated specimens compared to the bare zinc can be attributed to
the barrier nature of chromate coatings. The chromium hydroxides/oxides in the
coatings hinder the access of anions such as chloride and oxygen to the metal . Furthermore, the hydrophobicity of
the chromate coatings will also hamper the access of oxygen dissolved in the solution
to the zinc.
§48 The drying process hardens chromate
coatings and increases the abrasive resistance of the surface . However, the results presented above show that different drying
temperatures have different effects on the morphology of chromate coatings on
zinc and on their corrosion properties.
§49 First, the drying process affects the
morphology of the chromate coatings. Microcracks exist in all greenish chromate
coatings on zinc when dried in air. Cracks in chromate coatings result from
internal tensile stress initiated during the chromating process and the tensile
stress increases with the thickness of the coatings . During the drying process in air, the gel-like film formed is
dehydrated and the film shrinks so that microcracks develop.
§50 The chromate coatings dried at 210 �C have wider microcracks than the coatings dried at 60 and 110 �C. The corrosion current for the specimens dried at 210 �C is larger and the impedance is smaller than for the specimens
dried at 60 and 110 �C. This
means that drying at higher temperature (210 �C), as also suggested in literature , causes significant degradation of the chromate coatings. The
microcracks are the weak places where corrosion can easily start . The wider and deeper the cracks, the smaller the corrosion
resistance will be. From the polarization measurements, the chromated specimens
dried at 60 and 110 �C both
have a lower corrosion current density. The impedance measurements also showed
that the impedance is the largest for the specimens dried at 110 �C. This may be related to the observation that there are fewer
microcracks in the coatings of the specimens dried at 110 �C, although the width of the cracks in these coatings was slightly
larger than the width of the cracks in the coatings dried at 60 �C (see Figs. 1 and 2).
§51 The drying process also affects the Cr(VI)
species in the coatings. A progressive decrease of the Cr(VI)/total Cr ratio
with temperature was observed by Laget et al.  for aluminum alloys. However, it has also been reported that the
loss of �self-healing� effect of the chromates is mainly due to dehydration,
which makes the Cr(VI)� species
immobile or insoluble, rather than to the reduction of Cr(VI) in the coating [2;5]. Another observation reported in literature is that the thermal
effect on the corrosion resistance of chromate coatings depends on the
substrate. Laget et al. attributed the fact that heating treatments do
not affect the corrosion resistance of chromate coatings on Al-1100 alloy to
the good corrosion resistance of the relatively pure aluminum substrate . Gallaccio et al. observed that, in salt-fog tests, heating
chromate coatings on electroplated zinc to above 75 �C resulted in marked damage, but conversely that heating chromate
coatings on magnesium improves the protection provided by the coatings .
§52 In our case, Fourier Transform Infrared
Spectrometry and XPS analyses of the chromate coatings have shown that the
relative ratio of Cr(VI)-O species to Cr(III)-O compounds decreases after
drying at higher temperatures. It means that the drying process does indeed
cause Cr(VI) to be reduced to Cr(III). The polarisation curves obtained on the
chromate coatings dried at 60 �C suggest
that a kind of anodic passivation behaviour exists in a range of 160 mV, and
all of the chromated specimens, independent of the drying temperature, showed
approximately the same anodic breakdown potential, �0.92 VSCE. All
of the chromated specimens showed a more negative corrosion potential than the
uncoated zinc in the 0.01 M NaCl solution, suggesting that this kind of
conversion coating has a cathodic inhibitive effect on the corrosion of zinc:
the corrosion rate of zinc is controlled by the cathodic reaction. For the
coatings dried at 110 �C, the
ratio of Cr(VI)/total Cr did decrease with respect to the ratio for the
coatings dried at 60 �C, but
this reduction was small and was not significant enough to have a negative
effect on the corrosion behaviour of the coated specimens.
to Mansfeld, the high electrochemical impedance values of chromate conversion
coatings are only of secondary importance, and the corrosion protection is
mainly provided by mobile Cr(VI), which inhibits the initiation and propagation
of pits (for example during salt spray tests) . Buchheit et
al. reported a correlation between salt spray test results and
electrochemical impedance measurements, but chromate conversion coatings can pass salt spray tests
with comparatively low coating resistance values , again indicating that active
protection resulting from the presence of mobile Cr(VI) in the coating is
probably more important.
results presented in this paper show that the temperature at which chromate
coatings on zinc are dried affects not only the coating morphology, but also
changes the chromium oxidation states in the coating. All of the chromate
coatings investigated in this study showed microcracks in the layer. The width
of these cracks increased with the drying temperature. The coatings dried at 60
�C showed passivity, but the
corrosion resistance was not the highest. Drying at a moderate temperature (110
�C) appeared to increase the
corrosion resistance with respect to drying at 60 �C. However, drying at higher
temperatures (210 �C) degraded the
chromate coatings by widening the cracks and reducing the mobility of the
soluble Cr(VI) species. The thermal reduction of Cr(VI), detected by FTIR and
XPS, is probably responsible for the decrease of the Cr(VI) content of the
layer. Thus, drying makes chromate coatings hard, but on the other hand, it
reduces the self-healing effect of the soluble chromates.
research was supported by the Dutch Ministry of Economic Affairs
(innovation-directed research program Milieutechnologie/Zware Metalen, project
number IZW 98102).
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