Materials Sciences and Applications, 2010, 1, 202-209
doi:10.4236/msa.2010.14032 Published Online October 2010 (
Copyright © 2010 SciRes. MSA
Anti-Corrosion Performance of Cr+6-Free
Passivating Layers Applied on Electrogalvanized
Célia Regina Tomachuk1, Alejandro Ramón Di Sarli2, Cecilia Inés Elsner2
1Energy and Nuclear Research Institute, IPEN/CNEN-SP, CCTM, Av. Prof. Lineu Prestes, São Paulo, Brazil; 2CIDEPINT: Research and
Development Center in Paint Technology (CICPBA-CONICET), Av.52 s/n entre 121 y 122. CP. B1900AYB, La Plata-Argentina.
Received April 29th, 2010; revised July 22nd, 2010; accepted August 4th, 2010.
Hexavalent chromium-based passivation treatments have been successfully used as promoters of conversion coatings
for many years. Their effectiveness is without question although there are many problems with regard to their environ-
mental suitability. Hexavalent chromium compounds are carcinogenic and toxic. These problems have lead researchers
to evaluate other potential systems, with lower toxicity, to ascertain if they can replace chromates as effective passiva-
tors. Researchers have proposed several alternative passivation treatments; these are processes based on molybdates,
permanganates, titanates, rare earth metal and Cr3+ (considered to be non-carcinogenic) compounds. In this work, zinc
coatings obtained from free-cyanide alkaline bath and submitted to a Cr3+ based passivation treatment with different
colors were studied. The corrosion behavior was studied by polarization measurements and mainly by electrochemical
impedance spectroscopy in 0.6 N NaCl solution. Morphological observations on the coatings surface were also per-
formed. The results indicate that the green-colored Cr3+ passivated coatings have a good corrosion resistance followed
by yellow and blue-colored passivation respectively. They could be a less polluting alternative to the traditional chro-
mated coatings.
Keywords: Zinc, Conversion Treatment, Impedance Spectroscopy, Salt Spray, Corrosion
1. Introduction
Electroplated zinc coating is employed as active galvanic
protection for steel. However, as the zinc is an electro-
chemically highly reactive metal, its corrosion rate may
be also high in indoor but particularly under outdoor ex-
posure conditions [1]. For this reason, it is necessary a
post treatment in order to increase the lifetime of zinc
coatings. In current industrial practice, this treatment
consists of immersion in a chemical bath that forms a
conversion layer on plated zinc. This latter layer is a di-
electric passive layer with high corrosion resistance and
is also a better surface for paint adherence. The main
problem of traditionally used post treatments is the pres-
ence of Cr6+ salts, considered carcinogenic substances
which usage is forbidden by European norms [2]. Re-
sponding to increasingly more rigorous environmental
protection activities, recent years have shown progressive
advances in order to reduce the use of environmen-
tally-hazardous materials. In line with this purpose, the
development of various kinds of chromate-free coated
steel sheets, to be used in food, automotive, appliances,
etc. industries, is being extensively explored all over the
world. In this sense, the most common transitional alter-
native to Cr6+ is Cr3+, which is used since the mid 1970’s
[3-9]. According to Fonte et al. [10], the Cr3+ conversion
layer formed in a bath containing transition metal ions
such us Co2+, Ni2+ and Fe2+ showed higher corrosion re-
sistance than those formed in a bath without transition
metal ions. This finding was confirmed by Tomachuk et al.
Molybdates, tungstates, permanganates and vanadates,
including chromium like elements, were the first chemi-
cal elements tried as hexavalent chromium substitutes
[13-17]. Recently many alternative coatings were devel-
oped based on zirconium and titanium salts [18-20], co-
balt salts [21,22], organic polymers [23,24] and rare earth
salts [25]. However, preparation and corrosion behavior
of these coatings is not clear and their practical usage is
In order to find an alternative treatment to Cr6+ con-
Anti-Corrosion Performance of Cr+6-Free Passivating Layers Applied on Electrogalvanized
Copyright © 2010 SciRes. MSA
version coating, several treatments that present a good
anti-corrosive behavior, a high benefit/cost relation and,
mainly, low environmental impact are still to be devel-
oped. Usually, the corrosion behavior of coatings is eva-
luated using traditional tests such as Salt Spray [26],
Kesternich test [27], saturated humidity [28]. However
the authors consider important the application of elec-
trochemical methods to obtain fast information about the
corrosion reactions kinetics.
Among the electrochemical techniques that can be
used, the electrochemical impedance spectroscopy (EIS)
was selected based on the already obtained results for
metal and metal-coated corrosion evaluation [29-32].
The main purpose of the present work was to find an
environmentally friendly conversion treatment able to
replace satisfactorily those passivating ones based on
Cr6+. Electrogalvanised steel covered with Cr6+-free pas-
sivating layers were investigated using AC and DC elec-
trochemical techniques. Morphological studies of the
coatings surface were also performed.
2. Experimental Details
2.1. Samples Preparation
Electrogalvanised steel samples (7.5 × 10 × 0.1 cm) were
industrially produced and covered with the following
conversion treatments: 1) blue-colored Cr3+ based pas-
sivation; 2) yellow-colored Cr3+ based passivation; 3)
green-colored Cr3+ based passivation. For each conver-
sion layer, an individual commercial conversion bath was
formulated and the coating was produced according to
the respective supplier recommendations.
2.2. Thickness Measurements
The coatings thickness was measured using the Helmut
Fischer equipment DUALSCOPE MP4.
2.3. Morphology
The coatings morphology was determined from scanning
electron microscopy (SEM) analyses using a LEICA
S440 microscope.
2.4. Electrochemical Behavior
The electrochemical cell consisted of a classic three-elec-
trode arrangement, where the counter electrode was a
platinum sheet, the reference one a saturated calomel
electrode (SCE) and the working electrode each coated
steel sample with a defined area of 7 cm2. All measure-
ments were performed at a constant room temperature
(22 3) in 0.6 N NaCl solution.
Potentiodynamic polarization experiments were car-
ried out using a Solartron 1280 electrochemical system at
a swept rate of 1 mV.s-1, over the range ±0.300 V(SCE)
from the open-circuit potential OCP). Before each swept,
the electrode in contact with the electrolyte was stabi-
lized for several minutes. The corrosion current density (j)
and corrosion potential (Ecorr) were obtained from a Tafel
slope by extrapolation of the linear portion of anodic and
cathodic branches.
Impedance spectra in the frequency range 2.10-2 Hz < f
< 4.104 Hz were performed in the potentiostatic mode at
the OCP, and as a function of the exposure time in the
electrolyte solution, using a Solartron 1260 Frequency
Response Analyzer (FRA) coupled to a Solartron 1286
electrochemical interface (EI). The amplitude of the ap-
plied AC voltage was 3 mV peak to peak. Each sample’s
surface evolution was analyzed until white corrosion
products could be seen by the naked eye. The experi-
mental spectra were interpreted on the basis of equivalent
electrical circuits’ models using the ZView fitting soft-
ware by Scribner Associates. All impedance measure-
ments were carried out by triplicate in a Faraday cage in
order to minimize external interference on the system
3. Results and Discussion
The overall coating thickness and description of the sam-
ples investigated in this work are reported in Table 1. In
it can be seen that these showed similar and uniform
thickness; besides, they also exhibited a bright appear-
ance throughout their extension. Unfortunately, informa-
tion related with the passive layer thickness was not pos-
sible to be obtained.
3.1. Morphology
The consideration of the coating morphology after the
coating/drying process is very important since the pres-
ence of flaws such as pores and/or other defects could be
areas were localized corrosion of the treated zinc surface
starts from its exposure to a given environment [33].
Therefore, after applying the conversion treatment, the
coatings surface morphology was observed up to 1,000X
Table 1. Characteristics of the samples.
(Zn + conversion
blue-colored Cr3+
passivation UniFix Zn-3-50
yellow-colored Cr3+
passivation UniYellow 3
C green-colored Cr3+
passivation SurTec S680® 10.4
Anti-Corrosion Performance of Cr+6-Free Passivating Layers Applied on Electrogalvanized
Copyright © 2010 SciRes. MSA
by SEM (Figure 1). All the samples presented surface
roughness. Besides, A samples, subjected to blue-colored
Cr3+-based passivation, exhibited surface fissures (indi-
cated by the red arrows) which reduce its protective
properties (Figure 1(a)), while the B samples, subjected
to yellow-colored Cr3+-based passivation, exhibited ho-
mogenous structure with nodular growth (Figure 1(b)),
Figure 1. Microstructure of the tested coatings. (a) sample
A; (b) sample B; (c) sample C.
and C samples, subjected to green-colored Cr3+ passiva-
tion, exhibited a gel-like structure (Figure 1(c)). The cha-
racteristic cracks of chromate coatings were not present,
perhaps due to its thin thickness [34].
3.2. Polarization Curves
Potentiodynamic polarization curves were performed at a
swept rate of a 1 mV.s-1 in the range ±0.300 V (SCE)
with respect to the OCP. This procedure has been re-
peated for all the investigated samples. Figure 2 shows
typical potentiodynamic polarization curves for passi-
vated electrogalvanised steel in chloride solution.
Corrosion potential, Ecorr, and corrosion current den-
sity, jcorr, values obtained from Figure 2 were reported in
Table 2. As it can be seen, the corrosion potential (Ecorr)
of A samples was more negative, i.e. less noble, and this
means that from the thermodynamic point of view these
samples type are more susceptible to be corroded. With
regard to B and C samples, both presented similar and
more positive corrosion potential values than A samples,
indicating that a corrosion resistance improvement took
place, probably due to the homogenous morphology of
the covering layer showed in Figures 1(b) and 1(c) pro-
vided a better barrier resistance.
On the other hand, the corrosion current density (jcorr)
of C samples is one order of magnitude less than the
corresponding to the other two sample types tested, i.e.,
Figure 2. Polarization curves of the samples tested in 0.6 N
NaCl solution, v = 1 mV/s.
Table 2. Ecorr and jcorr values of Zn coatings after applying
the conversion treatment.
Conversion TreatmentEcorr V(SCE) jcorr A/cm2
A 1.10 0.2
B 1.04 0.4
C 1.04 0.02
Anti-Corrosion Performance of Cr+6-Free Passivating Layers Applied on Electrogalvanized
Copyright © 2010 SciRes. MSA
its corrosion rate is lower.
3.3. Electrochemical Impedance Spectroscopy
EIS measurements carried out in the 0.6 N NaCl solution
were discontinued upon the white corrosion products on
the surface could be seen by naked eye.
Figure 3 shows a Nyquist representation of the time
dependent electrochemical impedance, while Figure 4
illustrates the electrical equivalent circuits able of simu-
lating the physicochemical processes taking place at the
coated steel surface. It is important to emphasize that
experimental impedance data obtained for A and B sam-
ples were analyzed on the basis of the electric equivalent
circuit depicted in Figure 4(a)., while for the C samples
was used the shown in Figure 4(b) [35]. In these figures,
Rsol represents the electrolyte resistance between the ref-
erence and working (coated steel) electrodes; the first
time constant (R1Q1) - where R1 and (Q1 C1) are re-
spectively the resistance to the ionic flux and the dielec-
tric capacitance of the conversion layer - appears at the
higher frequencies. Once the permeating and corro-
sion-inducing chemicals (water, oxygen and ionic species)
reach electrochemically active areas of the substrate,
particularly at the bottom of the coating defects, the me-
tallic corrosion becomes measurable so that its associated
parameters, the charge transfer resistance, R2, and the
electrochemical double layer capacitance, (Q2 C2), can
be estimated [3]. Sometimes, the Q2 parameter can be
associated to a diffusional process, which not only could
be the rate-determining step (rds) of the corrosion reac-
tion but also mask part of - or completely its time con-
stant. It is important to remark that R2 and C2 values
vary directly (R2) and inversely (C2) with the size of the
electrochemically active metallic surface.
Distortions observed in these resistive-capacitive con-
tributions indicate a deviation from the theoretical mod-
els in terms of a time constants distribution due to either
lateral penetration of the electrolyte at the metal/coating
interface (usually started at the base of intrinsic or artifi-
cial coating defects), underlying metallic surface hetero-
geneity (topological, chemical composition, surface en-
ergy) and/or diffusional processes that could take place
along the test. Since all these factors cause the imped-
ance/frequency relationship to be non-linear, they are
taken into consideration by replacing one or more ca-
pacitive components (Ci) of the equivalent circuit trans-
fer function by the corresponding constant phase element
Qi (CPE), for which the impedance may be expressed as
Figure 3. Evolution of the A and C samples impedance
(Nyquist representation). (a) sample A; (b) sample C.
Figure 4. Equivalent circuit models used for fitting the im-
pedance data.
Z() impedance of the CPE (Z = Z´ + jZ´´)()
j imaginary number (j2 = 1)
angular frequency (rad)
Anti-Corrosion Performance of Cr+6-Free Passivating Layers Applied on Electrogalvanized
Copyright © 2010 SciRes. MSA
n CPE power: (n = /
constant phase angle of the CPE (rad)
Y0 part of the CPE independent of the frequency
Difficulties in providing an accurate physical descrip-
tion of the occurred processes are sometimes found. In
such cases, a standard deviation value (2 < 10-4) be-
tween experimental and fitted impedance data may be
used as final criterion to define the most probable circuit.
The comparison between simulated and experimental
data at different exposure times are omitted for simplicity,
however, in all cases, the experimental data were in good
agreement with the model predictions.
The more interesting data to discuss are the exposure
time dependent resistance R1 of the passivation treatment
(giving information on the barrier properties of the con-
version layer) coupled in parallel with its Q1 (related to
the coating capacitance) and the charge transfer resis-
tance R2 (giving information on the kinetic of the corro-
sive process). These values, estimated from the fitting
analysis of the impedance spectra, are reported in Fig-
ures 5 to 7, respectively.
Figure 5 shows the trend of the parameter R1, which
was associated to the evolution of the coating barrier
properties and consequently with its degradation during
exposure time in the aggressive aqueous solution. At zero
time, the same and low R1 values for A and B samples
suggest poorer barrier properties when compared with
the afforded by C samples. Then, it is observed a slight
increase of the R1 values until one hour of immersion for
C samples and three hours for A and B samples. This
was attributed to the blockage of the intrinsic and struc-
tural conversion layer defects with the soluble metallic
Figure 5. Values of R1 as a function of immersion time in
0.6 N NaCl solution obtained from impedance data fitting
for A, B and C samples.
Figure 6. Values of Q1 and its exponent n1 as a function of
immersion time in 0.6 N NaCl solution obtained from
impedance data fitting for A, B and C samples.
corrosion products formed due to the fast permeation of
the corrosion inducing chemicals through the thin con-
version layer. After that, these values started to decrease
probably because of the interfacial corrosion reactions
caused an increasing number and/or area of the coating
defects and, consequently, of the exposed zinc area at the
conversion layer/Zn interface. It is interesting to note that
as the R1 values decrease, the correspondingto Q1 in-
crease (Figure 6(a)); such a behavior indicates that the
involved relaxation process takes place at the same area
[38]. At the end of the immersion test, an oscillating be-
havior with values less than the initial ones could be ob-
Figures 6(a) and 6(b) show values of Q1 and its ex-
ponent n1 (see equation expressing the CPE definition)
as a function of the immersion time in the 0.6 N NaCl
Anti-Corrosion Performance of Cr+6-Free Passivating Layers Applied on Electrogalvanized
Copyright © 2010 SciRes. MSA
Figure 7. R2 as a function of immersion time in 0.6 N NaCl
solution obtained from impedance data fitting for A and B
solution. The initial conversion layer capacitance was the
lowest for C followed by B samples, result attributed to
the more uniform and compact morphology of C samples
(Figures 1(b) and 1(c)). During the first hours of immer-
sion, the Q1 values of C and B samples increased, being
much more evident (two orders of magnitude) the corre-
sponding to C samples. These changes, coupled to the n1
values evolution, can be explained assuming that despite
the blockage of the fissures and pores of the conversion
layer with the corrosion products, these last provides a
poor dielectric behavior and, therefore, are unable to in-
hibit the corrosion process. For the first hours, A samples
exhibited a trend to decrease the Q1 values followed by
an increase of approximately one order of magnitude,
which is indicative of conversion layer degradation [39].
The decreasing n1 values shown in Figure 6(b) for B
and C samples may be interpreted as a trend to change
from capacitive to diffusional behavior, or a mix of both.
On the other hand, the A samples showed an opposite
response [40]. After several hours of exposure, all these
changes followed the observed for the Q1 values.
The analysis of R2 (charge transfer resistance) as a
function of immersion time is a useful tool for the corro-
sion rate evaluation since it gives information about the
kinetic of the corrosive process. In such sense, Figure 7
shows that according to the equivalent circuit utilized for
fitting the impedance data (Figure 4(b)), C samples did
not present the time constant (R2Q2) corresponding to
the faradaic process. It means that this type of conversion
layer provided barrier properties (Figure 5) high enough
as to inhibit the corrosion process throughout the test. On
the other hand, the fact that the R2 values were greater
for B samples than for A samples means that those showed
lower corrosion rates. This was attributed to the fact that
the zinc corrosion products gathered in the conversion
layer defects acted as a better partial barrier, but also that
such an effect disappeared as the time of exposure elaps-
This analysis of EIS data for the three Cr3+-based con-
version treatments showed that a deficient deposition of
the conversion layer produces coatings with lower barrier
properties and, therefore, lower corrosion protection (as
particularly found in the case of A samples).
Summarizing, green-colored Cr3+ passivation exhibited
higher protective capacity than yellow-colored Cr3+ and
blue-colored Cr3+ passivation layers, which is clearly
noted in the polarization curves data (Figure 2).
4. Conclusions
From the results generated during this investigation for
three alternative conversion treatments applied on elec-
trogalvanised steel, the following conclusions can be
made with regard to their corrosion performance in con-
tact with a chloride solution at room temperature:
the more uniform coating presented lower corro-
sion rate;
the electrochemical techniques demonstrated to be
a very useful tool to characterize the corrosion
protection provided by different conversion treat-
the EIS data analyses based on equivalent circuit
models showed that green-colored Cr3+ conversion
treatment (C samples) presented the highest corro-
sion protection followed by the yellow-colored
Cr3+ conversion treatment (B samples) and blue-
colored Cr3+ conversion treatment (A samples), re-
spectively. This behavior was in agreement with
the results obtained of the polarization curves;
the conversion treatments investigated shown in-
teresting results but other experiments need to be
performed in order to evaluate alternatives to the
traditional and highly effective, but toxic and pol-
lutant, Cr6+ based conversion treatment.
In the near future it is likely that stringent legislation
will require the total removing of hexavalent chromium
as anticorrosive treatment. Consequently, more studies
are needed concerning the corrosion protection, ecologi-
cal and toxic effects afforded by new alternative treat-
5. Acknowledgements
The authors acknowledge CNPq/PROSUL (Process 490
116/2006-0) of Brazil, CAPES/MINCyT (Process 158/09
of Brazil and BR/08/04 of Argentina), and Comisión de
Investigaciones Científicas de la Provincia de Buenos
Aires (CIC) and Consejo Nacional de Investigaciones
Científicas y Técnicas (CONICET) of Argentina by their
Anti-Corrosion Performance of Cr+6-Free Passivating Layers Applied on Electrogalvanized
Copyright © 2010 SciRes. MSA
financial support to this research.
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