Envi Paints

8/6/2019 Envi Paints

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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

)


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)



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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


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


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


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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