Hydrophobic and superhydrophobic coatings for corrosion protection of steel LINA EJENSTAM Doctoral Thesis, 2015 KTH Royal Institute of Technology School of Chemical Science and Engineering Department of Chemistry Division of Surface and Corrosion Science 100 44, Stockholm, Sweden
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Hydrophobic and superhydrophobic coatings for corrosion protection of steel
LINA EJENSTAM
Doctoral Thesis, 2015
KTH Royal Institute of Technology
School of Chemical Science and Engineering
Department of Chemistry
Division of Surface and Corrosion Science
100 44, Stockholm, Sweden
TRITA-CHE Report 2015:54
ISSN 1654-1081
ISBN 978-91-7595-703-6
SP Chemistry, Materials and Surfaces Publication nr: A-3583
The work presented in this thesis was performed as a joint project
between the Division of Surface and Corrosion Science at KTH, and SP
Chemistry, Materials and Surfaces. My PhD project was designed to
further promote the collaboration between KTH and SP, with me
performing my work at both sites, and was started as a part of the SSF
(Stiftelsen för strategisk forskning) funded program: Microstructure,
Corrosion and Friction Control. Additional funding was provided by
RISE Research Institutes of Sweden. Valuable supervision has been given
by professor Per Claesson and professor Jinshan Pan at KTH, and by
adjunct professor Agne Swerin at SP, throughout the project. This joint
project has been highly educational to me and I am grateful I got the
chance to work with people, and procedures, both in academia and at a
research institute.
Since the topic of corrosion protection is interesting to a broad
audience, both in academia and in industry, the technical level of this
thesis has been adapted to be, hopefully, attractive to anybody with
interest in the field, without requiring specific background in either
wetting or electrochemistry.
Stockholm, October 2015
Lina Ejenstam
viii
ix
List of papers
This doctoral thesis is based on the following papers, which are referred
to in the text by their Roman numerals.
I. The effect of superhydrophobic wetting state on
corrosion protection – The AKD example
Lina Ejenstam, Louise Ovaskainen, Irene Rodriguez-Meizoso,
Lars Wågberg, Jinshan Pan, Agne Swerin and Per M. Claesson.
Journal of Colloid and Interface Science, 2013, 412, 56-64
doi: 10.1016/j.jcis.2012.09.006
II. Towards superhydrophobic polydimethylsiloxane-silica
particle coatings
Lina Ejenstam, Agne Swerin and Per M. Claesson.
Preprint, accepted for publication in Journal of Dispersion
Science and Technology, 2015
doi: 10.1080/01932691.2015.1101610
III. Corrosion protection by hydrophobic silica particle-
polydimethylsiloxane composite coatings
Lina Ejenstam, Jinshan Pan, Agne Swerin and Per M. Claesson.
Corrosion Science, 2015, 99, 89-97
doi: 10.1016/j.corsci.2015.06.018
IV. Long-term corrosion protection by a thin nano-
composite coating
Lina Ejenstam, Mikko Tuominen, Janne Haapanen, Jyrki M.
Mäkelä, Jinshan Pan, Agne Swerin and Per M. Claesson.
Applied Surface Science, In press, available online, 2015
doi: 10.1016/j.apsusc.2015.09.238
V. Comparison between AFM-based methods for assessing
local surface mechanical properties of PDMS-silica
composite layers
Hui Huang, Lina Ejenstam, Jinshan Pan, Matthew Fielden,
David Haviland and Per M. Claesson
Manuscript
x
The author’s contribution to the included papers:
Paper I: Major part of experimental work, major part of
manuscript preparation. The surface coatings were
prepared by Dr. Louise Ovaskainen, at the division of
Fibre and Polymer technology, KTH.
Paper II: All experimental work, major part of manuscript
preparation.
Paper III: All experimental work, major part of manuscript
preparation.
Paper IV: Major part of experimental work, major part of
manuscript preparation. The polyester base coat was
prepared at Akzo Nobel in Malmö in cooperation with
Dr. Majid Sababi, and the TiO2 coating was prepared by
Dr. Mikko Tuominen at Tampere University of
Technology, Finland.
Paper V: Part of initiating measurements, part of manuscript
preparation. All measurements shown in the manuscript
were performed by Hui Huang at the division of Surface
and Corrosion Science, KTH, in cooperation with the
section of Nanostructure Physics, KTH. Hui Huang also
prepared the major part of the manuscript.
xi
Summary of papers
Paper I:
The effect on corrosion protective properties of coatings representing
different wetting states was evaluated using a model system consisting of
alkyl ketene dimer (AKD) wax coatings exhibiting different hierarchical
structures. All coatings had the same surface chemistry and the effect
seen could therefore be attributed only to the difference in surface
structure. A remarkable increase in the measured electrochemical
impedance was found for the coating wetting in the superhydrophobic
Lotus state, originating from the air layer present at that surface, which
slows down transport of corrosive ions from solution to substrate, to the
point where the electrical circuit is broken and no corrosion can proceed.
Paper II:
The development of a one-pot superhydrophobic coating formulation
was addressed through systematic addition of hydrophobic silica
nanoparticles (16, 60-70, 250 and 500 nm) to a polydimethylsiloxane
(PDMS) matrix. Superhydrophobicity in the Lotus state was only reached
when particles were added at such high amount (40 wt%) it caused the
coating to crack, while the superhydrophobic rose state was achieved for
non-cracked coatings, which lead to further understanding of how the
shapes of the protrusions affect the resulting wetting state.
Paper III:
The corrosion protective properties of the most promising coating
system developed in Paper II was evaluated by coating carbon steel
substrates with formulations consisting of PDMS containing 0, 10, 20 or
40 wt% hydrophobic 16 nm particles. Due to crack formation allowing
easy passage of electrolyte through the coating, the 40 wt% coating did
not provide any corrosion protection to the underlying substrate. The 20
wt% coating performed very well, although it did not reach
superhydrophobicity but displayed hydrophobic wetting behaviour. The
protective properties were assigned to the synergistic effect of the
hydrophobicity of the matrix and particles, as well as the elongated
diffusion path arising from particle addition. Beyond this, another very
interesting effect was seen. The coatings with 0 and 10 wt% particles
exhibited a surprising increase in measured impedance during the first 24
xii
hours. This effect is so large that the most likely reason is passivation of
the underlying metal, promoted by the coating properties.
Paper IV:
To further understand the corrosion protective properties seen in
paper III, a coating system consisting of a polyester acrylate (PEA) base
coat, covered by a layer of TiO2 nanoparticles (< 100 nm) which in turn
was covered by a thin hexamethyl disiloxane (HMDSO) coating was
developed. The systematic evaluation of the protective properties of the
layers one by one, two by two, and in the three layer combination showed
that to reach a long-term stable, reproducible, protective coating a
combination of properties from all three layers is needed.
Paper V:
A comparison of coating properties on the nanoscale level using three
different operating modes of atomic force microscopy (AFM) was
conducted using the polydimethylsiloxane coating (Papers II and III)
without and with the addition of 20 wt% hydrophobic silica nanoparticles
(16 nm). The surfaces were found to be quite heterogeneous on the
nanoscale level, and different regions of softer and stiffer polymer was
encountered and attributed to variations in crosslinking degree. The 16
nm hydrophobic silica particles were found to aggregate and form
structures in the size 20 - 110 nm. In addition to the particles, spherical
shapes were also seen on the surface which were recognized as air
bubbles trapped in the matrix during curing.
xiii
Nomenclature
Scientific abbreviations
ACA Advancing contact angle
AFM Atomic force microscopy
AKD Alkyl ketene dimer
CA Contact angle
CAH Contact angle hysteresis
CE Counter electrode
CPE Constant phase element
DMT Derjaguin-Muller-Toporov
EIS Electrochemical impedance spectroscopy
HMDSO Hexamethyl disiloxane
LFS Liquid flame spray
OCP Open circuit potential
PDMS Polydimethylsiloxane
PEA Polyester acrylate
RCA Receding contact angle
RE Reference electrode
RESS Rapid expansion of supercritical solutions
WE Working electrode
Symbols
C capacitance (F)
θ contact angle (°)
γ surface energy (J/m2)
r roughness ratio
λ wavelength (nm)
xiv
xv
Table of contents
Abstract ……………………………………….……….………..…….iii
Sammanfattning………………………………………….…………...v
Preface…………………………………………………….…………..vii
List of papers…………………………………………………………ix
Summary of papers………………………………………………….xi
Nomenclature……………………………………….……………….xiii
Table of contents……………………………………………………xv
1. Introduction ............................................................................. 1 1.1. Aim of this work .................................................................................... 2
4. Results and discussion......................................................... 25 4.1. The AKD system ................................................................................. 25 4.2. The PDMS-hydrophobic silica nanoparticle system ....................... 27
xvi
4.3. The three layered composite system ................................................ 29 4.4. Unifying concept 1: Superhydrophobicity........................................ 30 4.4.1. Shape, size and distribution of protrusions .............................................. 30 4.4.2. Air film stability under water .................................................................... 34
4.5. Unifying concept 2: Corrosion protection ........................................ 36 4.5.1. Effect of wetting state on corrosion protection ......................................... 36 4.5.2. Barrier effect from particles ..................................................................... 39 4.5.3. Possible passivation of underlying substrate ........................................... 41
4.6. Commercial PDMS .............................................................................. 43 4.7. Mechanical properties of PDMS coating........................................... 45 4.7.1. Nanomechanical properties of PDMS coating ......................................... 45
The most likely explanation to this large increase in impedance is that
the coating can promote passivation of the underlying metal surface. Iron
can form conductive oxides that are stable at high pH, and the major
reaction product of the cathodic reaction is OH- through the reduction of
O2 [5]. It has also been suggested that accumulation of OH- under surface
coatings can take place since organic coatings often exhibit selective
permeability [78].
42 | RESULTS AND DISCUSSION
Iron oxides are not stable in an environment containing Cl- ions,
making the electrolyte containing 3 wt% NaCl used here, a very corrosive
environment. However, because of selective permeability the diffusion of
Cl- through a polymer coating is much slower than that of water [78], and
by combining the slow diffusion of Cl- and local increase in pH,
passivation of the substrate is possible. This may explain the large
increase in measured impedance. However, at some point, the Cl- ions
will penetrate through the coating and cause the passive oxide layer to
break.
Another factor indicating passivation is the color of the corrosion
products. For example, in Paper IV, for the PEA-TiO2-HMDSO coating
where the corrosion was seen to stop (Figure 20) the corrosion products
were black or dark grey, which is characteristic of the more stable iron
oxide Fe3O4, while corrosion causing failure of the coatings has been
noted to be red, characteristic of the porous Fe2O3 iron oxide.
Furthermore, there have been reports showing how stable Si-Fe oxides
can form and enhance the corrosion protection achieved when silica
nanoparticles are present in the coating under a mildly alkaline
environment. These Si containing oxides also resist Cl- ion penetration
allowing a Cl- free environment to be created near the metal/coating
interface which can stabilize other iron oxides [97, 99, 101, 102]. This
effect could help explain the surprisingly good corrosion protection seen
for the PDMS system with added hydrophobic silica particles, studied in
Paper III.
RESULTS AND DISCUSSION | 43
4.7. Commercial PDMS
PDMS coatings without added nano-particles were also prepared from
a commercially available PDMS formulation (results not included in
papers) for comparison. A condensation cure PDMS was chosen to match
the curing mechanism of the model PDMS described in Papers II and III.
The formulation was also coated, and subsequently evaluated, in the same
way as the model one to facilitate comparison. The water contact angles
and coating thicknesses for the two PDMS coatings are shown in Table 5.
Static and advancing contact angles match very well, while a discrepancy
can be seen for the receding contact angle. Since an increase in receding
contact angle was noted for the experimental PDMS coating in Paper II
during the first week of curing, the difference here indicates that the
coating used in Papers II and III presents a less homogenous surface
layer. When measuring coating thickness for the commercial PDMS a
difference was noted between different samples, and in Table 5, the
thicknesses for two samples are shown. This difference is most likely due
to the short tack-free time of the formulation. When preparing the last
coatings the formulation had already begun to cure and was therefore
more difficult to spin coat onto the samples.
Table 5. Water contact angles, and coating thicknesses, for the commercial and model PDMS
coatings.
* the two thicknesses for commercial PDMS are measured on two samples made from the
same batch, one made early during the coating process and one late, showing an increase
in thickness due to the short tack free time of the formulation causing the viscosity to
increase during the coating process.
The commercial PDMS coatings performed well during
electrochemical testing, showing results early after exposure that indicate
the presence of air at or in the surface and with a very good long term
stability of more than 100 days. The electrochemical data collected for the
commercial PDMS are shown in Figure 22. As for the model PDMS three
replicates were tested also for the commercial one and the best obtained
Sample SCA (°) ACA (°) RCA (°) Thickness (µm)
PDMS
commercial
120 ± 1 120 ± 0 103 ± 3 16 ± 4,
13 ± 2*
PDMS model 121 ± 1 120 ± 1 69 ± 1 13 ± 3
44 | RESULTS AND DISCUSSION
results are shown here. The OCP exhibit some turbulent behavior in the
beginning, and in correlation, the EIS at the lower frequency limit (0.01
Hz) is very high, on the border of what can be measured with the
instrument, and also somewhat scattered. Papers I and III explain this by
the presence of air at or in the coating obstructing the flow of the
electrical circuit. Both the indication of air present at the beginning of
exposure, as well as the high stable impedance modulus (at the lower
frequency limit), are different compared to the results obtained in Paper
III where the experimental PDMS coating exhibit a impedance of just
above 108 Ωcm2 at the beginning that then continuously deteriorate until
the coating fails after approximately 20 days. The mean time to failure for
the model coating was 14 ± 7 days. For the commercial PDMS one
replicate failed after 32 days, while the other two stayed stable for the
whole exposure time of 100 days.
Figure 22. Electrochemical data collected for the commercial PDMS coating during
exposure to 3 wt% NaCl water solution, in a) OCP vs. time and in b) impedance modulus
vs. frequency at different times.
The receding contact angle is the only parameter evaluated here which
is significantly different between the model and commercial PDMS
samples, but another important difference is that the commercial coating
contains adhesion promoters. Thus the model coating will have a greater
tendency to locally delaminate than the commercial PDMS samples,
which at least partly explains why the commercial coating performs better
in the corrosion protection test. However, since the content of the
RESULTS AND DISCUSSION | 45
commercial formulation is not fully known and may, most likely, contain
additives which enhance its performance, not too much weight should be
put on the differences seen here at this stage.
4.8. Mechanical properties of PDMS coating
Evaluation and improvement of the coatings regarding wear and
friction control clearly needs further development. Macroscopic wear test
of the model PDMS based coatings was performed using a minitraction
machine, and the results showed that the coating adhesion to the
underlying substrate is an issue that needs to be further addressed.
Furthermore, evaluation of the nanomechanical properties of the coating
is ongoing and some results are presented in this section.
4.8.1. Nanomechanical properties of PDMS coating
The model PDMS coating without, and with, the addition of 20 wt%
hydrophobic nanoparticles (16 nm) was evaluated using three different
AFM modes; tapping mode, PeakForce QNM and Intermodulation AFM
(ImAFM). The results show that both PeakForce QNM and ImAFM can
be used to investigate the nanomechanical surface properties of the
coatings.
Using the ImAFM mode, effective elastic modulus and adhesion were
extracted for the PDMS without added particles using the DMT model,
and the results are shown in Figure 23a. Red areas in the map showing
effective elastic modulus (left hand side) represent areas that are less
deformable. One possible explanation to why these patches occur is that
the PDMS coating has not cured in a homogenous way but instead
contains areas exhibiting higher crosslinking density, making those areas
less deformable.
The 16 nm hydrophobic silica particles were clearly visible in contrast
to the surrounding soft matrix using all three AFM modes when
evaluating the PDMS with 20 wt% particles. In topography
measurements of this sample it can be clearly seen that the particles
aggregate to form large secondary, and tertiary, structures as proposed
earlier by others [103]. The dimension of the particle aggregates observed
was estimated to measure between 20 and 110 nm.
46 | RESULTS AND DISCUSSION
Furthermore, a discussion has been raised regarding the rounded
shapes also seen in the lower right corner of Figure 23c. These rounded
shapes were seen also when using the PeakForce QNM mode and the
mechanical properties extracted for these rounded shapes are different
compared to those exhibited by particle aggregates with smaller
deformation depth, lower adhesion and lower energy dissipation. Due to
this, these rounded shapes were assigned to nanobubbles of air trapped in
the polymer matrix due to the increase in viscosity caused by the addition
of particles. This is an interesting, yet speculative, notion that could help
understand the excellent corrosion protective properties of these
coatings.
Figure 23. ImAFM mode scans of the PDMS without and with 20 wt% silica, the DMT model
was used for evaluation. In (a) effective elastic modulus and in (b) adhesion for the PDMS
coating without particles, in (c) effective elastic modulus and (d) adhesion for the PDMS
coating with addition of 20 wt% silica nanoparticles. All images are 1 x 1 µm.
CONCLUSION | 47
5. Conclusions
Three different coating systems, with focus on hydrophobicity and
superhydrophobicity, have been used to investigate which properties a
top coat layer in a corrosion protective coating system would benefit
from.
Through the use of AKD wax coatings, exhibiting the same surface
chemistry but different wetting states due to different surface structures,
it was possible to relate corrosion protective properties to the wetting
state. The superhydrophobic Lotus state can provide corrosion protection
to underlying substrates through the insulating air layer that forms on top
of such surface when it is immersed in the electrolyte, the air layer
obstructs the transport of corrosive ions to the surface to the point where
the electrical circuit is broken and corrosion cannot occur. This effect was
found to be efficient as corrosion protection during short exposure times,
in 3 wt% NaCl water solution, of less than ten days.
The aim of developing an easy-to-use, one-pot, superhydrophobic
coating was undertaken using nanoparticle containing PDMS coatings.
The conclusion is that high enough particle load to achieve
superhydrophobicity is hard to reach without causing the coating to
crack. It was also seen how the shape of the protrusions is an important
factor deciding which state of superhydrophobicity is formed; the Lotus
state is promoted by sharp, needle-like, structures, while rounded shapes
will promote the rose state.
Although not superhydrophobic, the hydrophobic PDMS coatings
containing silica nanoparticles were evaluated with respect to corrosion
protective properties and were found to perform better than expected for
coatings this thin (10 - 20 µm). The corrosion protective properties were
found to increase with increase in amount of added particles up to 20
wt%, where the coating provided corrosion protection to the underlying
substrate for almost 80 days when immersed in 3 wt% NaCl. The
properties of this coating system were concluded to be a synergistic effect
from the hydrophobic matrix and the addition of hydrophobic
nanoparticles.
48 | CONCLUSION
To further understand the corrosion protective mechanism shown by
the PDMS coating, a three layer composite coating system was developed.
Investigations of this coating system lead to the conclusion that several
factors are needed in combination to achieve the enhancement in
corrosion protection seen; good coating adhesion, blocking of pores and
an elongated diffusion path from addition of nanoparticles, and
hydrophobicity to further slow down the transport of water containing
corrosive ions. It was also noted that the hydrophobic top coat was
needed to exploit a positive effect from particle addition, if no
hydrophobic coating was applied; the hydrophilic particles instead
quicken the transport of water through the coating.
An additional conclusion, reached through comparison of the different
coating systems evaluated, is that if the insulating property of a
superhydrophobic surface in the Lotus state is not reached, the exact
wetting state is of less importance, as long as the surface is hydrophobic.
For example, the hydrophobic PDMS with 20 wt% hydrophobic
nanoparticles showed corrosion protective properties comparable to the
three layered composite coating wetting in the superhydrophobic rose
state. Therefore, when Lotus state superhydrophobicity is excluded, it was
concluded that other factors, such as elongation of diffusion path, and
blocking of pores by particles addition, were more important to the
resulting corrosion protection than the question of whether the coating
was hydrophobic or superhydrophobic.
FUTURE WORK | 49
6. Future work
There are several promising ways to continue research in this area.
One would be to start adapting either the PDMS or the PEA-TiO2-
HMDSO coating to commercial standards. To do this, coating adhesion to
the substrate, wear resistance and facilitation of the application process
have to be addressed. Another important factor of commercial corrosion
protection is the addition of active inhibitors; it would be interesting to
evaluate addition of such inhibitors to either of the coatings mentioned
above. However, care must be taken since most inhibitors are designed to
function in the presence of water, which makes them unsuitable to
combine with water repelling coatings.
When discussing commercial applications the concept of particle
volume concentration and also of critical particle volume concentration of
filler particles would be interesting to apply to facilitate understanding of
significant changes in properties in relation to filler concentration. This
calculation is, however, not entirely straight forward to perform for the
system investigated here since the fumed silica nanoparticles exhibit an
uneven surface, and it is not known whether the polymer can access all of
these structures, leading to uncertainty in the effective density of the
silica particles. One way of elucidating this is through oil absorption
measurements, and another one could be through further AFM studies of
the interphase between the polymer and the particle.
Another interesting path would be working on understanding the
corrosion protective mechanism of these coatings. One interesting
approach regarding the PDMS coating would be to look at enhancing the
wetting properties by patterning the surface, and then comparing the
corrosion protection with or without the addition of particles to evaluate
if wetting or elongated diffusion path is most important. It would also be
interesting to evaluate the diffusion of oxygen through the coatings. This
could be done using free films of the coatings, or by coating them onto
glass substrates of known porosity.
Furthermore, the addition of particles to the commercially available
PDMS formulation would be of great interest since the initial testing of
50 | FUTURE WORK
this PDMS gave promising results. This is however, not without
challenges since the formulation cures by humidity, making particle
addition in ambient conditions impossible. This could be overcome by
working in a controlled environment, such as N2 gas. Another alternative
would be to try yet another commercial PDMS formulation that does not
cure by humidity, but instead by, for example, heat.
ACKNOWLEDGEMENTS | 51
7. Acknowledgements
I would like to thank my supervisors, professor Per Claesson and
adjunct professor Agne Swerin, for giving me the opportunity to perform
this work, for countless interesting discussions, invaluable guidance and
keeping me on the right track (or at least one track). I would also like to
thank professor Jinshan Pan for sharing his expertise on electrochemistry
with us.
Thanks to all my co-authors, I enjoyed working with you and I am
proud of the publications we produced together. I am also grateful for the
funding provided by the SSF (stiftelsen för strategisk forskning) and RISE
Research Institutes of Sweden that made it possible for me to carry out
this work.
I have spent some great years at the division of Surface and Corrosion
Science, and at SP Chemistry, Materials and Surfaces, and a lot of the joy
during the work was due to truly wonderful colleges, both current and
past, thank you! Special thanks go to Petra, Josefina and Hanna who have
been there during both ups and downs to encourage me, both
educationally and privately.
Finally, I would like to thank my family. My parents who have always
believed in me and throughout the years always encouraged me to
“plugga så länge du bara orkar Lina!”, and my sister who has been there
to cheer me on whenever that has felt tough, and who always
congratulates me on every smallest progress.
Jesper, thank you for understanding how it feels to write a thesis, and
for taking care of our life while I did. Thank you for your endless support,
and for always being able to make me laugh, even at times when I do not
want to. We certainly are an effective team.
Henry, thank you for bringing perspective into all this, you will always
remain the most important thing in my life.
52 |
REFERENCES | 53
8. References
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