envi paints
TRANSCRIPT
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Evaluation of environmentally friendly paints over weathering
galvanised steel: the influence of the surface roughness
Collazo, A.(1)
; Prez, C.(1)
; Izquierdo, M.(2)
; Merino, P.(1)
(1) Materials Engineering, Applied Mechanics and Construction Department.
(2) Chemical Engineering Department.
E.T.S.E.I.M., University of Vigo. Lagoas-Marcosende, 9. 36280 Vigo. Spain
Abstract
A comparative study between a high solid paint (P1) and a traditional coat (P2) was
made using EIS technique, both paints were applied over weathering galvanised
steel. The results indicated that the high solid paint had the better behaviour and so, itrepresents a good environmentally friendly alternative. Previously, the optimum
weathering degree was assessed based on the impedance modulus evolution and the
surface roughness profiles. In the last part of this work, the macroscopic behaviour of
a new paint system, based on P1 priming coat and a water-borne resin as a topcoat,
was evaluated by submitting to different accelerated tests. The influence of
weathering galvanised degree was analysed, as well. Regardless of the test, the
increase of such parameter was corresponded to an behaviour improvement.
Keywords
Environmentally friendly paint, weathering degree, surface profile, modes of failure.
Introduction
Painting galvanised steel is a widely used method of corrosion control in atmospheric
conditions. Transmission towers are a field of application particularly indicated for
such duplex coatings in order to obtain maximum durability. Due to the exposed
situation, their frequent reconditioning can be difficult [1]. Nevertheless, some
failures, mainly loss of adhesion, are common. In that sense, the selection of
appropriate paint system and zinc surface stage are the key to guarantee a long
service life [2].
To get a good adherence between zinc surface and paint is a critical point in the
duplex system behaviour, in that sense, the smooth surface of fresh galvanised steel
can lead to poor zinc-paint adhesion, as Figure 1 depicts. It is widely accepted that
certain weathering degree of zinc surface improves the paint adhesion, due to the
associated roughness increasing provides more anchorage points [1]. The way to get
such weathering surface is a topic submitted to strong controversy, the type of
atmosphere seems to be determinant because of the generated zinc corrosion
products must be protect the surface. So, non-contaminant environments are the most
adequate [3,4]. In such way, it is generally assumed that a weathering time between
one year to 20 months in a rural atmosphere leads a good zinc surface paintability [1-
2345].
Corresponding author. e-mail: [email protected] Phone: +34 986 812603 Fax: +34 986 812201
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Fig. 1. Photograph showing the poor adherence of duplex system over fresh
galvanised steel.
The long time required for getting the good surface conditions has forced to develop
different accelerated tests, the most interesting are those based on wet-dry cycles
and/or darkness-UV radiation, because their conditions are closer to the atmospheric
ones [6 - 9].
Respect to the paint selection, total compatibility between both coats is required. In
that sense, certain types of resins can react with zinc surface leading to paint
delamination [2, 8]. Fortunately, many others are recommended to be applied over
galvanised surfaces. Traditionally, paints based on chlorinated rubber or epoxy resins
have mainly been used to be coat galvanised transmission towers [5], such productsimply high Volatile Organic Compounds (VOCs) emissions. Nevertheless, current
environmental legislations aimed at drastically reduction of VOCs emissions. This
fact has forced the paint industry to develop new alternative formulations [10]. Two
families which satisfies this constraint are high-solid systems with low VOCs and
water-borne paints [11 -13].
Following this line, this work, which is part of a broader project supported by the
main Power Companies in Spain, deals with the evaluation of environmentally
friendly paint systems to be applied over galvanised transmission towers as an
alternative to the conventional ones. Previously, the galvanised surface was studiedto find out the possible correlation between weathering degree and zinc surface stage
to be painted. Electrochemical Impedance Spectroscopy (EIS) has demonstrated to
be a useful tool to study galvanised steel [14,15] and the protection mechanism of
the paint in the duplex system [16-20]. Besides, the macroscopic behaviour of an
alternative paint system was evaluated using different accelerated tests. Such
procedure makes possible to analyse the characteristic modes of failure depending on
the conditions [8,21].
In order to make easier the comprehension of this work, it had been divided into
three parts, with different aims. Thus, the first part deals with the correlation between
weathering degree and roughness profile in order to find out a parameter to assess thezinc surface stage. Once the optimum weathering galvanised has been chosen, in the
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second part a comparative study between one traditional priming coat and a high-
solid paint was made using EIS technique. Finally, in the third part, a new paint
system based on the better priming coat was submitted to different accelerated test to
assess its macroscopic behaviour. Each part has different experimental set-ups and
the results are dissimilar too, for that reason, those will be exposed separately.
FIRST PART: STUDY OF WEATHERING GALVANISED STEEL (WGS)
Experimental Procedure
Carbon steel plates, 220x120x2mm dimensions, were hot-dip galvanised in a molten
commercial zinc bath. The average thickness of galvanised layer was 70 m.
Weathering galvanised steel (WGS) samples were obtained using two different ways.
Accelerated agedcarried out by the introduction of fresh galvanised steel samples in
a Weathering Cyclic Chamber (WCC). This is based on alternating periods of UV-IRradiation, supplied by three lamps with the light spectrum close to the sunlight
radiation, which allows the comparison between artificial and natural weathering.
Each exposure cycle, with two hours duration, consisted on light plus a spray with
fresh water (pH = 6-6.5) and darkness plus a spray with the same fresh water. The
relative humidity obtained inside the chamber was high, at least 90 %.
The natural aged of galvanised steel samples were made by exposition to natural
environment for a long time (20 months), in rural atmosphere with C2 corrosivity
category according to ISO 9223 [22]. The reason to choose such low aggressive
environment was to obtain non-contaminant corrosion products on the zinc surface.
To characterise the weathering galvanised degree, the samples were periodically
removed from the chamber to do the impedance measurements and the roughness
profile characterisation, after that, they were reintroduced until the next
measurement. The same procedure was carried out with samples exposed to natural
environment, just at the end of the exposition.
The impedance measurements were perform with an impedance analyser Autolab at
open circuit potential, the frequency range scanned was from 105
Hz down to 0.1 Hzand the signal amplitude was 20mV. The electrochemical cell consisted in a classical
three electrode arrangement: the counter electrode was a graphite sheet, a saturated
calomel electrode (SCE) was used as reference one and the WGS, defining an area of
13 cm2, was the working electrode. The employed electrolyte was Na2SO4 1N.
The roughness profile was evaluated by Ra ( m) parameter, which is defined as the
arithmetic mean of the absolute values of the profile deviations, |y| [23].
Ra = l
0
dx|)x(y|L
1
=
n
1i
i |y|n
1
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Figure 2 illustrates the physical meaning of Ra and the related parameters. L
represents the total path length (5 mm) and m is the mean line obtained by least
square fitting of the profile.
Fig. 2. Schematic representation of Ra meanning and the related parameters, m is the
mean line, L represents the total path length and |yi| the absolute values of the profile
deviations.
A Surtronic 10 equipment with 0.1 m accuracy was used to perform the
measurements. The path length was 5 mm and 20 measurements were made in each
sample.
Results and DiscussionFigure 3 depicts the Nyquist plots of galvanised steel at different exposure times in
WCC. It is characteristic the impedance increasing with the weathering degree.
Surface P rofile
Ox
y
L = 5 mm
m (mean line)
Ra
O
|yi|y
x
L = 5 mm
m (mean line)
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0 1000 2000 3000 4000 5000 6000
0
1000
2000
3000
4000
5000
6000
100mHz25 mHz
10 Hz
100 mHz
-ZImg
(ohm.cm
2)
ZReal
(ohm.cm2)
beginning
after 8 days in WCCafter 14 days in WCC
0 400 800 1200 16000
400
800
1200
1600
Fig. 3. Nyquist plots of galvanised steel at different weathering degree. The 10 Hz
frequency is indicated by a solid symbol.
Two time constants are clearly distinguished. Our interest is focus on the high
frequency time constant, which is related to the double layer capacitance, Cdl, and
charge transfer resistance, Rct, of the zinc corrosion process, a detailed study of its
evolution was already made in a previous work [24]. The impedance increasing has
been explained in terms of the corrosion products nature generated on the zinc
surface. By XDR technique, hydrozincite, Zn5(CO3)2(OH)6, was identified as the
main compound. Related to the low frequency time constant, several researchers
pointed that it is associated with oxygen diffusion and/or ZnI-Zn
IIequilibrium
developing at the zinc surface [25].
In order to find out the correlation between impedance value and roughness profile at
different weathering degrees, one frequency was chosen to obtain its impedancemodulus value. Such selection is a crucial point due to the study of its evolution
should give information about the state of the zinc surface depending on the
weathering degree. Considering the high frequency loop, i.e., the zinc corrosion
process, the better choice could be its characteristic frequency, which is directly
related to Rct, such election implies the impedance diagrams fitting. As it was
mentioned above, this work is framed in a project to reconditioning transmission
towers, so, the procedure should be simple and short duration in order to its potential
applicability on field. Taking account this point of view, fitting the impedance data
can be an inconvenient. Based on the obtained Nyquist plots, a 10 Hz frequency was
chosen, which is placed as solid symbol in figure 3. As it can be seen, this is located
in the high frequency loop and it corresponds with a resistive behaviour,approximately. So, the impedance modulus at 10 Hz can be considered as a good
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parameter to correlate with surface roughness and, at the end, with the zinc surface
state.
Figure 4 shows the impedance modulus evolution with the exposure time in WCC,
the solid symbols diagram corresponds to the sample exposed during 575 days (20
months) in a rural environment. The 10 Hz frequency falls in the first part of thediagram related to the high frequency time constant.
10-1
100
101
102
103
104
105
101
102
103
104
105
|Z|(ohm.cm
2)
Frequency (Hz)
beginning
8 days
14 days
21 days
36 days
66 days
91 days
575d atmosphere
Fig. 4. Impedance modulus plots of fresh galvanised steel and at different exposure
times in WCC. The diagram corresponded of samples exposed in a rural atmosphere
during 575 days is included, as well.
Figure 5 shows the evolution of the impedance modulus at 10 Hz and the Ra
parameter as indicative of the surface roughness.
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0 50 100 450 500 550 600
100
1k
10k
Weathering time (days)
|Z|1
0Hz
(ohmcm2
)
|Z|10Hz
0.1
1
10
Ra(microms)
Atmospheric Exposition
(Rural Environment, C2)
Ra
Accelerated
Aged (WCC)
Fig. 5. Ra values and impedance modulus evolution with weathering time.
It is remarkable the same tendency observed in both parameters, those increase with
weathering time. Such result can be surprised, because as the roughness is higher the
active surface becomes greater and the total impedance should be lower. The
explanation of that opposite evolution can be done in terms of the zinc corrosion
products generated on the surface. As it was stated in previous works [ 24, 26], at
open-air conditions the main corrosion product is hydrozincite, which covers the zinc
surface and passivates it. So, the impedance increases when the corrosion products is
getting larger and the roughness in higher, as well. When such film is broken the
impedance decreases up to the cracks blockage occurs, this phenomenon explains the
lower values observed at several exposure times.
To decide the optimum weathering galvanised degree is a critical point. The analysis
of the roughness profile evolution with weathering time (figure 5) suggests two
different slopes. Initially, the roughness increasing is higher and after several days in
WCC, this growth becomes smaller. The same tendency can be observed in the
|Z|10Hz evolution, although more fluctuations are appreciated, which could beexplained taking account the dynamic nature of passivation process. The transition
between both stages can be located about 36 weathering days, which represents a
good compromise between good surface properties and time consuming to get them.
On the other hand, an attempt to extrapolate the accelerated aged to the natural
weathering was made, for that, samples were submitted to rural environment during
20 months. Such period was chosen based on field studies carried out over
galvanised steel exposed at different atmospheres [1- 4]. As it was pointed in the
introduction section, the best surface properties can be reach after 1 year to 20
months exposition in non aggressive environments. The |Z|10Hz and the surface
profile after 20 months in a rural atmosphere is close to those parameter values after36 days in WCC. Spite of that, we consider that the direct extrapolation of
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accelerated results to natural environment can be risky and it should be necessary to
do more property studies.
Based on these considerations and assuming the above reticence, the weathering
degree obtained after 36 days in WCC has been chosen to apply the different type of
paints studied in the second part of this work.
SECOND PART: COMPARATIVE STUDY BETWEEN TWO PRIMING
COATS APPLIED OVER WEATHERED GALVANISED STEEL USING EIS
Experimental Procedure
Weathering galvanised steel samples were painted with one of the two studied
priming coats. Their main features are displayed in table 1.
Tab. 1. Main features of the employed priming coats.
The same equipment and the electrochemical cell than those used in the first part
were employed to perform the impedance measurements, the frequency range was
from 105
Hz down to 0.01 Hz, the working electrode was coated weathering
galvanised steel (CWGS) and the measurements were made in immersion conditions
with Na2SO4 as the electrolyte.
The impedance data analysis was based on the Nelder and Mead algorithm [27],
which employs the simplex method to minimise the 2 function, given by equation(1):
+
=
=
N
1i
2
ei
''ci
''ei
2
ei
'ci
'ei2
|Z|01.0
ZZ
|Z|01.0
ZZ(1)
where N is the total number of scanned frequencies, Z 'ei and Z''
ei the real and
imaginary parts of the experimental impedance Zei, |Zei| the experimental
impedance modulus at frequency i, and Z 'ci and Z''
ci the corresponding real and
imaginary parts of the calculated impedance at frequency i. More details were explained
in previous works [18,28].
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Results and Discussion
Figures 6 and 7 depict the duplex systems evolution. The impedance decreasing with
immersion time, mainly for polyamide epoxy paint. After long immersion periods,
up to three time constants can be appreciated in the impedance plots.
10-2
10-1
100
101
102
103
104
105
103
105
107
109
1011
|Z| beginning
|Z| 190 hours|Z| 260 hours
|Z
|(ohm.cm
2)
Frequency (Hz)
0
20
40
60
80
100
Polyamine Epoxy Paint (P1)
Phase beginningPhase 190 hoursPhase 260 hours
-Phase
angle(degrees)
Fig. 6. Bode plots of polyamine epoxy paint (P1) at different immersion times.
10-2
10-1
100
101
102
103
104
105
104
105
106
107
108
109
1010
Polyamide Epoxy Paint (P2)
Modulus
beginning
175 hours
260 hours|Z
|(ohm.cm
2)
Frequency (Hz)
0
20
40
60
80
100
Phase
beginning175 hours
260 hours
-Phaseangle(degrees)
Fig. 7. Bode plots of polyamide epoxy paint (P2) at different immersion times.
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The equivalent circuit used to model such behaviour is showed in figure 8 and the
associated impedance is given by equations (2) to (4). Such expressions are used in
equation (1) to fit the experimental data.
p2
pp
p
e
R/Z1
1)CRj(
RR)(Z
p
++
+=
(2)
being
cp3
cpcp
cp
2
R/Z1
1)CRj(
R)(Z
cp
++
=
(3)
and1)CRj(
R)(Z
dl
dlct
ct
3 += (4)
Ccp/cp
Cdl/dl
Rcp
Rct
Re
Rp
p p
Fig. 8. Electrical equivalent circuit employed to model the behaviour of the duplex
system.
Re accounts for the electrolyte resistance, Rp represents the paint resistance and Cpits dielectric capacitance. The Rcp.Ccp time constant is associated to the corrosion
products layer. The parameters Cdl and Rct account respectively for the double layer
capacitance and the charge transfer resistance corresponding to the zinc corrosion
process. The i parameters account for the Cole-Cole dispersion of the RiCi timeconstants.
Figures 9 (a) and (b) depict the evolution of the high frequency time constant, Cp.Rp
which is associated with the paint dielectric properties.
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0 50 100 150 200 250 300
40p
60p
80p
100p
Cp
Polyamine epoxy paint (P1)
Cp
Polyamide epoxy paint (P2)
Cp(F.cm
-2)
Immersion time (hours)
0 50 100 150 200 250 300
10M
100M
1GR
pPolyamine epoxy paint (P1)
Rp
Polyamide epoxy paint (P2)
Rp
(ohm.cm
2
)
Immersion time (hours)
Fig. 9. Evolution of (a) the dielectric capacitance, Cp and (b) the pore resistance, Rp
for both paints. The solid line indicates the second and third time constants
appearance in polyamide epoxy paint(P2) while the dot line denotes the third time
constant in polyamine epoxy paint (P1).
The evolution is the expected one, the paint capacitance undergoes an increase with
the immersion time, which reflects the water uptake in the film. Such process seems
slightly different depending on the paint. In polyamine epoxy paint (P1) can beinitially appreciated a clear increasing following by an stabilization period with
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certain fluctuations at longer immersion times. In the case of polyamide epoxy paint
(P2) the capacitance evolution is characterised by a continuous increase with no
well-stated periods. The evaluation of the water uptake amount is not easy because
there is no clearly stated a saturation stage, mainly in P2 paint. For P1 paint it could
be estimated about 5%.
Respect to the paint resistance evolution, it is remarkable its high values, not only at
the beginning, in the order of G.cm2, but at he end of the immersion time withvalues higher than 10 M.cm2. Spite of that, it noticeable the stronger decreaseobserved in P2 paint, which indicates its poorest barrier properties, although the
initial values were higher.
At the beginning, the paint is intact and just the time constant related to the dielectric
properties of the paint is observed with a characteristic frequency about 1Hz.
Nevertheless, after a few immersion hours the paint resistance decreases markedly
and the associated time constant is shifted towards higher frequencies, from 10Hz up
to 100Hz depending on the immersion period. As a consequence, the time constantrelated to the zinc corrosion layer, Ccp.Rcp, which is located about 0.1-10 Hz range,
can be noticed. Figures 10 (a) and (b) depict the evolution of the related parameters.
This is quite different depending on the type of paint.
0 50 100 150 200 250 300
10p
100p
1n
10n
100n
1C
dlP2 paint
Cdl
P1 paint
Ccp
P2 paint
Ccp
P1 paint
Capacitance
(F.cm-
2)
Immersion time (hours)
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0 50 100 150 200 250 300
1M
10M
100M
1G
10G
100G
Rct
P2 paint
Rct
P1 paint
Rcp
P1 paint
Rcp
P2 paint
Immersion time (hours)
Resistance(ohm
.cm
2)
Fig. 10. Evolution of: (a) the double layer capacitance, Cdl (solid symbols), and
corrosion products capacitance, Ccp. (b) The associated resistances, Rct (solid
symbols) and Rcp with immersion time for both types of paints
The initial Ccp capacitance values are similar, but at longer immersion times, a
continuous increasing is observed for polyamide epoxy (P2) paint. Parallel evolution
is observed for the associated resistance, Rcp, which undergoes a monotonically
decreasing.
The Ccp.Rcp evolution for polyamine epoxy (P1) paint is characterised by a longperiod with no significant variations, mainly in the capacitance values, following by
an strong decreasing at longer immersion periods. This change occurs when the third
time constant is appreciated. Figures 11 and 12 depict the measured and fitted values
corresponded to the impedance diagrams obtained at this transition. After that, both
parameters increase with immersion time.
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0 2G 4G 6G 8G 10G 12G0
2G
4G
6G
8G
10G
12G
175 immersion hours
0.1 Hz
Enlarged
area
Experimental
Fitting
-ZImg
(ohm.cm
2)
ZReal
(ohm cm2)
0 500M 1G0
500M
1G
1 Hz
10-2
10-1
100
101
102
103
104
105
106
107
108
109
1010
1011
|Z|(ohm.cm
2)
Frequency/ Hz
0
30
60
90
175 immersion hours
ExperimentalFitting
-Phaseangle(degrees)
Fig. 11. Measured (O) and fitted (X) Nyquist (a) and Bode (b) plots for P1 paint after
175 immersion hours. The best fitting parameters are: Rp = 78 M.cm2,Cp = 89pF.cm
-2, p= 934m, fp = 23Hz. Rcp = 15G.cm2, Ccp = 180pF.cm-2,
p = 650m, fcp = 60mHz.
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0.0 400.0M 800.0M 1.2G 1.6G 2.0G0.0
400.0M
800.0M
1.2G
1.6G
2.0G
186 immersion hours
10 Hz
Enlarged
area
ExperimentalFitting
0.1 Hz
10 Hz
-ZImg
(ohm.cm
2)
ZReal
(ohm cm2)
0 80M 160M0
80M
160M
10-2
10-1
100
101
102
103
10410
4
105
106
107
108
109
1010
186 imersion hours
Experimental
Fitting
|Z|(ohm.cm
2)
Frequency (Hz)
0
30
60
90
-Phaseangle(degree
s)
Fig. 12. Measured (O) and fitted (X) Nyquist (a) and Bode (b) plots for P1 paint after
186 immersion hours. The best fitting parameters are: Rp = 52M.cm2,Cp = 85pF.cm
-2, p = 936m, fp = 36Hz. Rcp = 937M.cm2, Ccp = 31pF.cm-2,
cp=688m, fcp = 5Hz. Rct = 2G.cm2,Cdl = 13nF.cm-2, dl=525m, fdl=58mHz
At lower frequencies, in the order of mHz, a third time constant, related to the
corrosion process is observed. Its evolution is markedly different depending on thepaint, as already happened with the second time constant. In the case of P2 almost
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The evolution of this third time constant is in the same way that the explained for
RcpCcp, that is to say, a regular increase of the Cdl and the corresponded Rct
decreasing. For P1 paint, it is no possible to distinguish this third time constant until
longer immersion periods, as it was mentioned before, when Rcp and Ccp fall. In such
situation, the frequency is shifted towards higher values (see the characteristic
frequencies in figures 11 and 12). From that immersion time (about 186 hours), bothtime constants have the same tendency, characterised by a regular increasing of
capacitances and resistances, simultaneously.
As it was stated, the evolution of these two time constants is quite different
depending on the paint. Such behaviour can be explained in terms of their protective
features.
The comparative study of the dielectric properties (CpRp time constant) reveals that
the polyamine epoxy (P1) paint has better barrier features, so, it can protect better the
zinc corrosion products film. The worst barrier properties of polyamide epoxy (P2)
paint can explain that the three time constants are observed at early stages. As thecorrosion progresses, the Cdl is bigger and Rct smaller, as a consequence, the active
surface becomes larger. Even so, the Cdl values are quite small, assuming that Co
dl for
bare zinc is about 30 m.cm-2
[29], the active surface is less than 0.4%. Respect to
the time constant associated with these corrosion products, the capacitance increasing
and the resistance decreasing is in accordance to the larger amount of them.
The better dielectric properties of P1 paint justifies that only two time constants are
observed during long immersion time. Besides, the one associated with the corrosion
product film does not change significantly during that period, as corresponds to a
good barrier layer. When the third time constant appears a drastic change occurs, thatcan be explain if a breaking on this film takes place, this assumption is supported by
the marked decrease in the Rcp values. The explanation of the gap observed in the Ccp
parameter has more difficult explanation. If we consider that the initial capacitance
values are, actually, the sum of Ccp + Cdl ones (they are parallel combination in the
proposed circuit), this gap reveals that the real values of the film are lower than
those observed initially. Assuming that, the corrosion process already happens during
this first period, when just two time constants are observed, but its extension is
negligible, the active surface is in the order of 0,003%. When the breaking film
occurs, the Cdl values increase significantly, about two orders of magnitude, in the
10nF.cm-2
range.
After that gap, the evolution of both time constants, mainly the increase of Rcp and
Rct, suggests the blockage of those breaks with new zinc corrosion products which
offer a good barrier features. Thus, at longer immersion times the impedance
becomes higher, as can be appreciated in figure 6.
Based in all these considerations, it can be concluded that the better behaviour
corresponds to polyamine epoxy paint, denoted as P1.
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Tab. 2. Relation between the ASTM standard references and the classification
established in this work.
Results and Discussion
The main mode of failure was different depending on the accelerated test. Figures 14
(a) and (b) depict the behaviour of DSW1 and DSW2 samples in Salt Spray Fog
Chamber.
0 500 1000 1500 2000 2500 3000 35000
40
80
120
SSFC
CWGS. Weathering degree: 14 days in WCC (W1)
Performance
Exposure Time (hours)
Blistering
Failure at scribe
Adherence
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0 500 1000 1500 2000 2500 3000 35000
40
80
120
SSFC
CWGS. Weathering degree: 36 days in WCC (W2)
Performance
Exposure Time (hours)
Blistering
Failure at scribe
Adherence
Fig. 14. Macroscopic behaviour of duplex system in SSFC. (a) For samples with
weathering galvanised degree obtained after 14 days in WCC and (b) after 36 days.
As the plots reveal, the behaviour is markedly different depending on the weathering
degree, in that sense, a significant improvement can be observed in all parameters for
DSW2 samples, it is noticeable the good adherence even at long exposure times. As
it could be expected, the main mode of failure is the cathodic blistering,
characteristic of that kind of environments. Figures 15 (a) and (b) display the generalaspect of both types of samples after SSFC exposition.
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Figure 16 depicts the behaviour of the DWS1 and DSW2 specimens submitted to
accelerated test in a weathering cyclic chamber. This was big different than the
observed in salt spray fog chamber.
0 500 1000 1500 2000 2500 3000 35000
40
80
120
WCC
CWGS. Weathering degrees: 14 (W1) and 36 (W2) days in WCC
Performance
Exposure Time (hours)
Blistering
Failure at scribeAdherence (W1)
Adherence (W2)
Fig. 16. Evolution of the duplex system behaviour with exposure time in the WCC.
Two different weathering galvanised degrees (W1 and W2) are studied
The only mode of failure that can be appreciated after long exposure time is the
adhesion loss, which reaches important values, about 35% of removed area for
samples with the lower weathering degree (DSW1). In the same way that it was
observed in SSFC test, the aged increasing leads to an improvement of the
macroscopic behaviour. Figures 17 (a) and (b) confirm that assumption.
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Fig. 17. Appearance shown by (a) DSW1 samples and (b) DSW2 samples after 3500
hours in weathering cyclic chamber.
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Conclusions
In the study of weathering galvanised steel, impedance modulus at 10 Hz andsurface profile parameters were used to follow the zinc surface evolution. Both
parameters increased with weathering time.
The weathering galvanised degree obtained after 36 days in WCC was chosen asan optimum compromise between good surface properties and time consuming.
Respect to the comparative study between two priming coats, the duplex systembehaviour had been modelled by three RC parallel combinations. RpCp related to
the dielectric properties of the paints, RcpCcp associated with the properties of the
zinc corrosion film and RctCdl made reference to the corrosion process.
Different RcpCcp and RctCdl evolution were observed depending on the paint. Thebetter behaviour of polyamine epoxy one (P1) had been explained by its better
barrier properties.
The CWGS samples submitted to different accelerated tests revealed animprovement with the weathering degree for all the evaluated parameters.
The characteristic mode of failure in SSFC was cathodic blistering. Whereas theadhesion loss was the only defect observed in WCC.
ACKNOWLEDGMENTS
The authors acknowledge the Main Power Companies in Spain, Iberdrola, Redesa
and Fenosa, for the financial support.
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