Khanna A.S. (Ed.) High-Performance Organic Coatings: Selection, application and evaluation
Подождите немного. Документ загружается.
216 High-performance organic coatings
corrosion protection even after 60 days of immersion in 3.5% NaCl (Fig.
11.1).
One of the interesting properties of the coatings based on novel surface
treatments is that the degree of protection increases by increasing the
immersion time up to 60 days. Conversely, the corrosion resistance of
epoxy-treated specimens showed a sudden decrease when increasing the
immersion time in NaCl, as shown by the resistance curves of the Bode
plots (Fig. 11.2).
A general equivalent electrical circuit model (Randles Circuit) (Fig. 11.3)
was used to analyse the ac impedance spectra obtained from the novel
surface treatments after two months of immersion in NaCl. Table 11.1 sum-
1
the coating capacitance, R
2
the coating resistance and Q the constant phase
element associated to a capacitance because the CPE power n is equal to
0.9 (see Table 11.1). The n values of the CPEs are in the interval 0 n
1. The same equivalent circuit was used to analyse the unscratched speci-
inspection and microscopic examinations, the novel surface treatments +
been created in the coating, part of the substrate is directly exposed to the
corrosive environment. Consequently, an aluminium oxide layer enriched
Z (ohm)
1.0
M0.5 M0
0
0.5
M
1.0
M
1.5
M
2.0
M
3.0
M
2.5
M
Z ″ (ohm)
Epoxy
Molybdate 1
Molybdate 2
Ceria
Vanadia
11.1 Comparison between the novel pre-treatments and epoxy-treated
under scratched conditions.
marizes the fitting results. In this circuit, R is the electrolyte resistance, C
corrosion resistance in 3.5% NaCl [4, 6]. Based on the fitting results, visual
against filiform corrosion. In this case, when a small defect or a scratch has
with the same clear fluoropolymer top coat after 60 days in 3.5% NaCl
mens of epoxy primer + clear fluoropolymer top coat that showed excellent
clear fluoropolymer top coat specimens showed promising protection
© 2008, Woodhead Publishing Limited
Fluoropolymer coatings for corrosion protection 217
EpoxyEpoxyEpoxy
60 days
15 days
30 days
1 day
2.0
Log frequency (Hz)
Log Z
2.51.0 1.50.5
2.5
3.0
4.0
3.5
5.0
4.5
6.0
5.5
03.04.0
0
10
20
30
40
50
60
70
80
90
3.5
Theta
2.0
Log frequency (Hz)
2.51.0 1.50.5
2.5
3.0
4.0
3.5
5.0
4.5
6.0
6.5
5.5
03.04.0
0
10
20
30
40
50
60
70
80
90
3.5
Theta
VanadiaVanadia
60 days
15 days
30 days
1 day
Log Z
11.2 Comparison between Bode plots of epoxy-treated and novel
NaCl.
with vanadium, cerium or molybdenum will be formed [5, 7–10] to seal
the damage site in the coatings and, hence, improve the resistance against
coatings.
coating capacitances of the novel treatments decrease by increasing the
immersion time in NaCl. The decrease in capacitance indicates a decrease
sion. This can be explained by the continued formation of a vanadium,
against the attack of chloride ions.
filiform corrosion. Such processes are usually referred to as ‘self-healing’
According to the fitting data (Table 11.1) and Bode plots in Fig. 11.2, the
in water uptake and, hence, an increase in the resistance to filiform corro-
cerium or molybdenum enriched Al oxide film, which protects the metal
surface treatment coated clear fluoropolymer over two months in
© 2008, Woodhead Publishing Limited
218 High-performance organic coatings
60 days
60 days
Molybdate 1
Molybdate 2
30 days
30 days
15 days
15 days
1 days
2.0
Log frequency (Hz)
Log Z
2.51.0 1.50.5
2.5
3.0
4.0
3.5
5.0
4.5
6.0
5.5
6.5
0 3.0 4.0
0
10
20
30
40
50
60
70
80
90
3.5
Theta
Log Z
2.0
Log frequency (Hz)
2.51.0 1.50.5
2.5
3.0
4.0
3.5
5.0
4.5
6.0
5.5
6.5
0 3.0 4.0
0
10
20
30
40
50
60
70
80
90
3.5
Theta
Ceria
60 Days
15 Days
30 Days
1 Day
Ceria
60 days
15 days
30 days
1 day
2.0
Log frequency (Hz)
Log Z
2.51.0 1.50.5
2.5
3.0
4.0
3.5
5.0
4.5
6.0
6.5
5.5
0 3.0 4.0
0
10
20
30
40
50
60
70
80
90
3.5
Theta
11.2 Continued
© 2008, Woodhead Publishing Limited
Fluoropolymer coatings for corrosion protection 219
under scratched conditions
Pretreatment + clear
FLBZ 1074 top coat
Immersion
time (days)
R
1
(kΩ)
C
(nF)
R
2
(MΩ)
Qn
Vanadia 30
60
0.50
0.50
47
39
35
40
0.18e-08
0.18e-08
0.9
0.9
Molybdate 1 30
60
0.50
0.99
62
50
25
34
0.29e-08
0.28e-08
0.9
0.9
Molybdate 2 30
60
0.50
0.99
90
59
10
14
0.40e-08
0.67e-08
0.9
0.9
Ceria 30
60
0.50
0.99
60
57
1.9
7
0.56e-08
0.70e-08
0.9
0.9
mens, the equivalent electrical circuit described in Fig. 11.3 that was
equivalent electrical circuit (shown in Fig. 11.4) was proposed to model the
EIS spectra after scratching, considering the new corrosion process due to
1 1
C
R
1
R
2
Q
11.3 The equivalent electrical circuit for the novel surface treatment-
Circuit).
C
1
R
1
C
2
R
2
R
3
11.4 The equivalent electrical circuit for scratched epoxy-based
On the other hand, as a result of the filiform corrosion process that took
suitable to fit the EIS spectra of the same coatings under uniform coatings
[4, 6] became unsuitable to fit the impedance curves after scratching. A new
filiform corrosion. In this circuit, R is the electrolyte resistance, C the
Table 11.1 Data fitting of the novel treatments + clear fluoropolymer top coat
based fluoropolymer coatings under scratched conditions (Randles
fluoropolymer coatings.
place in the scratched epoxy primer + clear fluoropolymer top coat speci-
© 2008, Woodhead Publishing Limited
220 High-performance organic coatings
coating capacitance, R
2
the coating resistance, C
2
the capacitance of the
double layer at the base of the scratch, and R
3
the charge transfer resistance
of the corrosion reaction at the base of the scratched part in the coat. The
corrosion resistance decreased by increasing the immersion time in NaCl,
indicating the weak performance of the epoxy primer to protect the metal
breakdown occurred after 15 days only, whereas such breakdown did not
occur till 60 days for the novel surface treatment specimens as shown
from the relation between the resistance and frequency in the Bode plot
(Fig. 11.2).
The corrosion resistance of epoxy treated samples was increased by pigment
addition. Conversely, the novel treatments showed a dramatic decrease in
the corrosion resistance. Pigment addition affects negatively the adhesion
performance for epoxy treated specimens and the novel treatments as well.
in NaCl.We suggest that the presence of pigments in the epoxy layer
increases the capacitance of the resin because Fe
3+
can capture water and
be hydrated to iron hydroxide. The hydrated oxides partially block the
pores of the resin, and hence the corrosion resistance of epoxy primer pig-
Moreover, the presence of iron oxide and titanium oxide as pigment
decreases the porosity of epoxy and hence improves the corrosion
resistance [20, 21].
ments (Fig. 11.5) in which the surface resistance of the epoxy primer pig-
5 2
coat (2 × 10
5
Ω cm
2
). This result indicates that porosity plays a vital role in
the mechanism of protection [20].
Epoxy + clear FLBZ
topcoat
Immersion
time (days)
R
1
(kΩ)
C
1
(nF)
R
2
(MΩ)
R
3
(MΩ)
C
2
(μF)
Epoxy 30
60
0.99
0.99
85
80
0.10
0.12
0.60
0.65
0.50
0.60
Table 11.2 Data fitting of scratched epoxy pretreated FLBZ 1074 top coat
data fitting is shown in Table 11.2. It can be seen from Fig. 11.2 that the
from corrosion (mainly filiform corrosion). Moreover, the passive film
Severe filiform corrosion was observed after less than 30 days of immersion
The previous explanations were confirmed by the impedance measure-
11.4.2 Pigmented fluoropolymer top coat
mented fluoropolymer top coat specimens showed some improvement in
comparison with the epoxy primer clear fluoropolymer top coat ones.
mented fluoropolymer top coat specimens (12 × 10 Ω cm ) was about six
times higher than the resistances of epoxy primer clear fluoropolymer top
© 2008, Woodhead Publishing Limited
Fluoropolymer coatings for corrosion protection 221
On the other hand, the addition of iron oxide and titanium oxide as
resistance of the novel surface treatments in NaCl. According to the Nyquist
plots (Fig. 11.5), visual inspection and microscopic examinations [20], the
surface resistance decreased dramatically from about 2.8 × 10
5
Ω cm
2
to
0.1 × 10
6
Ω cm
2
.
The dramatic decrease in the corrosion protection and the adhesion
performances of the novel surface treatments can be attributed to the com-
petitive action due to formation of two different oxides, one from the treat-
ment layer (vanadium, molybdenum or cerium oxide) and the other from
the pigment (namely iron or titanium oxide) when the aggressive solution
ions diffuse through the top coat. Formation of two different oxides makes
11.5 Salt spray chamber and adhesion
performance tests
less than 40 h of exposure to salt spray. Conversely, the novel surface treat-
rosion even after 2000 h of exposure in the salt spray chamber.
The presence of pigments improves the durability of the epoxy-treated
specimens up to 170 h. However, the novel surface treatments + pigmented
Epoxy + Pigmented
3.0
M
2.5
M
2.0
M
1.5
M
1.0
M
0.5
M
0
Z′′ (ohm)
Z (ohm)
0 0.5
M 1.0
M 1.5
M 2.0
M
11.5 Comparison between the corrosion resistance of clear and
novel treatments.
the surface film inhomogeneous and hence water can easily diffuse through
the pores among the oxide particles, causing filiform corrosion.
Vanadia + Clear fluoropolymer
fluoropolymer
Epoxy + Pigmented fluoropolymer
Vanadia + Pigmented fluoropolymer
pigmented fluoropolymer for the epoxy-treated specimens and the
pigment to the fluoropolymer top coat affects negatively the corrosion
Epoxy-based + clear fluoropolymer coatings showed filiform corrosion after
ments + clear fluoropolymer top coat did not show any sign of filiform cor-
fluoropolymer top coat specimens were affected negatively by adding the
© 2008, Woodhead Publishing Limited
222 High-performance organic coatings
than 30 h (Table 11.3).
The presence of a vanadia, ceria or molybdate component enables the
to provide impermeability for the coating. Moreover, the presence of one
of these components promotes the uniformity of the dry mix, to provide
the opportunity for the application of a uniform composition to the
substrate.
ings, have improved both wet adhesion and low water/oxygen permeability
and have maintained these performance characteristics over protracted
periods of use.
11.5.1 Adhesion performance tests
Table 11.4 compares the adhesion performance of the novel treatment and
top coat. It seems that the presence of pigments affects negatively the
spray chamber test using ASTM B117
Surface treatment
top coat (h) top coat (h)
Epoxy 40 170
Vanadia >2000 30
Molybdate 1 >2000 27
Molybdate 2 >2000 24
Ceria >2000 26
coatings using ASTM D 3359-87 cross-cut method
Surface treatment
top coat (h) top coat (h)
Epoxy 100 75–85
Vanadia 75–85 50–60
Molybdate 1 75–85 55–60
Molybdate 2 75–85 50–55
Ceria 75–85 50–60
pigments and showed a dramatic increase in filiform corrosion after less
Table 11.3 Durability of clear and pigmented fluoropolymer coatings to salt
Clear fluoropolymer Pigmented fluoropolymer
Table 11.4 Adhesion performance of clear and pigmented fluoropolymer
Clear fluoropolymer Pigmented fluoropolymer
fluoropolymer component to be uniformly applied to the substrate, so as
The novel treatments, when applied with clear fluoropolymer top coat-
epoxy-treated specimens after applying clear and pigmented fluoropolymer
© 2008, Woodhead Publishing Limited
Fluoropolymer coatings for corrosion protection 223
adhesion performance of both the epoxy-treated and the novel surface
treatments.
11.6 Conclusions
1.
treatments using vanadia, ceria and molybdate salt solutions were pro-
ropolymer coating systems.
2. The performance of the new coating systems was compared with that
from the corrosion protection, adhesion and durability points of view.
3. The novel surface treatments showed outstanding corrosion resistance
even after two months under scratched coating conditions. The durabil-
ity of such new coatings using a salt spray chamber test was very promis-
specimens after only 40 h of exposure.
4. The novel surface treatments are characterized by their safety to the
environment and human body, very low cost, ease of application and
very fast curing (not exceeding 5 min).
5. The mechanism of protection using the novel treatments depends on
the formation of a highly protective aluminium oxide layer enriched
with vanadium, molybdenum or cerium, which has a dual effect of
improving the corrosion resistance and of maintaining the adhesion
performance within the acceptable range.
6. Pigment addition affects negatively the adhesion performance for both
epoxy-treated specimens and the novel treatments.
11.7 References
1. Dolbier W (2005), ‘Fluorine chemistry at the millennium’, J. Fluorine Chemi.,
126, 157–163.
2. Munekata S (1988), ‘Fluoropolymers as coatings materials’, Prog. Org. Coat.,
16, 113–134.
US Patent 5,972,494.
4. Hamdy A S, Beccaria A M and Temtchenko T (2002), ‘Corrosion protection
155, 176–183.
5. Hamdy A S, Beccaria A M and Temtchenko T (2002), ‘Improving the corrosion
protection of AA6061 T6–10% Al2O3 using new surface pre-treatments prior
6. Hamdy A S, Beccaria A M and Temtchenko T (2001), ‘New high performance
surface pre-treatments for aluminum composites’, 20th Annual Conference
ing (>2000 h), while filiform corrosion was observed for the epoxy-treated
Newly developed fluoropolymer coating systems based on novel surface
posed as alternatives to the commercially available epoxy-based fluo-
of clear and pigmented epoxy-based fluoropolymer coating systems
3. Janssens R (1999), ‘Filled fluoropolymer composition for corrosion resistance’,
of AA6061 T6 by fluoropolymer coatings in NaCl solution’, Surf. Coat. Techn.,
to fluoropolymer coatings’, Surf. Coat. Techn., 155, 184–189.
© 2008, Woodhead Publishing Limited
224 High-performance organic coatings
‘Corrosion Problems in Industry’, 6–8 November 2001, Egyptian Corrosion
Society, Red Sea, Egypt.
7. Hamdy A S (2004), ‘A novel coating system for the corrosion protection of
aluminum alloys and composites in marine environments’, presented at the IIW
International Congress on Welding and Applied Processes, 29 November–1
December 2004, CMRDI, Cairo, Egypt.
8. Hamdy A S (2005), ‘Novel eco-friendly surface coatings for automotive and
aerospace materials’, presented at the Arabian–European Symposium on
Environmental Protection, 27–28 September 2005, Tunisia.
9. Hamdy A S, ‘Novel coating systems for corrosion protection of aluminum alloys
and composites in marine environment’, presented at Symposium Materials
Processing Challenges for the Aerospace Industry, Materials Science &
Technology Conference and Exhibition (MS&T’06), 15–19 October, 2006,
Cinergy Center, Cincinnati, OH, pp. 667–674.
10. Hamdy A S and Butt D (2007), ‘Eco-friendly conversion coatings for automo-
tive and aerospace materials’, invited presentation at the European Coatings
Conference ‘New Concepts for Anti-Corrosive Coatings’, 14–15 June 2007,
Berlin, Germany.
alloys’, Mater. Sci. Forum, 95, 217–221.
12. Greene H J and Mansfeld F (1997), ‘Corrosion protection of aluminum metal–
matrix composites’, Corros. NACE, 53, 920–924.
13. Traverso P and Beccaria A M (1998), ‘Effect of magnesium ions on the protec-
tion power of corrosion products formed on the surface of AL 6061 T6 alloy–
10% Al
2
O
3
(v/v) composite in solutions containing chlorides’, Surf. Interface
Anal., 26, 524–230.
14. Hamdy A S, Beccaria A M and Spiniello R (2001), ‘The effect of cerium pre-
treatment on the corrosion behavior of aluminum composites’, Corrosion
Prevention & Control, 48(3), 101–108.
15. Hamdy A S and Beccaria A M (2001), ‘Corrosion protection performance of
thickened oxide conversion coatings containing vanadium ions formed on alu-
minium composites’, Corrosion Prevention & Control, 48(4), 143–149.
16. Hamdy A S and Beccaria A M (2002), ‘Corrosion protection of aluminum
metal–matrix composites by cerium conversion coatings’, Surf. Interface Anal.,
34, 171–175.
17. Hamdy A S and Beccaria A M (2005), ‘Effect of surface preparation prior to
cerium pretreatment on the corrosion protection performance of aluminum
composites’, J. Appl. Electrochem., 35, 473–478.
18. Hamdy A S, Beccaria A M and Traverso P (2005), ‘Corrosion protection of AA
6061 T6–10% Al
2
O
3
composites by molybdate conversion coatings’, J. Appl.
Electrochem., 35, 467–472.
19. Hamdy A S (2006), ‘Corrosion protection of aluminum composites by silicate/
cerate conversion coatings’, Surf. Coat. Techn., 200(12–13), 3786–3792.
20. Hamdy A S (2006), ‘Enhancing corrosion resistance of aluminum composites
218–224.
21. Hamdy A S and Temtchenko T (2005), ‘Corrosion protection performance of
at the European Corrosion Congress – EUROCORR 2005, Portugal.
11. Thompson G E (1996), ‘Corrosion and filming behaviour of aluminium and its
in 3.5% NaCl using pigmented epoxy fluoropolymer’, Prog. Org. Coat., 55(3),
pigmented epoxy fluoropolymer coatings on aluminum composites’, presented
© 2008, Woodhead Publishing Limited
225
12
Polysiloxane coatings for corrosion protection
K-H KAESLER,
Momentive Performance Materials Inc., Germany
12.1 Introduction: what is a polysiloxane?
Polysiloxane and silicone are synonymously used expressions. Although
silicone is the more commonly used term, we mainly use the term polysi-
loxane in this chapter, because it is clearer with respect to the chemistry
and avoids misunderstandings with the metal silicon, which is the chemical
core of any silicone or polysiloxane chemistry.
answer the question, ‘What is a polysiloxane?’ A polysiloxane is any one
of a class of synthetic materials characterized by silicon and oxygen linked
together in the siloxane bond, having organic groups attached to silicon
which have
a distinct bearing on the properties (see Fig. 12.1). Using
siloxy units with different numbers and types of organic substutients, one
can synthesize a broad variety of polymer types. These polysiloxane materi-
a water-soluble salt. The resin types are the ones used in the protective
Just as carbon atoms are the building blocks in conventional organic
chemistry, so are silicon atoms the basic blocks to which we add organic
side chains to build polysiloxane compounds having a wide variety of usages
fourth group in the Periodic Table of the elements, we can expect certain
To begin with, both have a valence of four, and both form analogous series
carbon and polysiloxane compounds. In contrast to those of carbon, the
hydrides of silicon are very vulnerable to thermal degradation and oxida-
tion. The very strong tendency of silicon compounds to combine with
oxygen is illustrated by the large quantities of silica, such as beach sand,
found in nature, and the equally great scarcity of unoxidized silicon. In
Before getting into the functions per formed by polysiloxanes, let us first
coatings field.
and properties. Since carbon and silicon are the first two members of the
similarities, but we should likewise be mindful of significant dissimilarities.
of hydrides. A significant difference is the relative reactivity of analogous
als may take the form of fluids or oils, elastomers, greases, resins or even
© 2008, Woodhead Publishing Limited