Volume 2 Paper 3
Iron Corrosion Protection by Plasma-Polymerised Coatings
C. Vautrin-Ul, C. Boisse-Laporte, A. Chausse, P. Leprince and R. Messina
Because you are not logged-in to the journal, it is now our policy to display a 'text-only' version of the paper. This version is obtained by extracting the text from the PDF or HTML file, and it is not guaranteed that the text will be a true image of the text of the paper. The text-only version is intended to act as a reference for search engines when they index the site, and it is not designed to be read by humans!
If you wish to view the human-readable version of the paper, then please Register (if you have not already done so) and Login. Registration is completely free.
JCSE Volume 2 Paper 3
Submitted 13th September 1999
Iron Corrosion Protection by Plasma-Polymerised Coatings
C. Vautrin-Ul1*, C. Boisse-Laporte2, A. Chausse1,
P. Leprince2, R. Messina1
1 Laboratoire Analyse et Environnement, UMR 85 87, C.N.R.S., 2 Rue H.
Dunant, 94 320 THIAIS (France )
email : mailto2('Christine.Vautrin-Ul','chimie.univ-evry.fr')
2 Laboratoire de Physique des Gaz et des Plasmas, UMR 85 78 Bat. 210,
Universit� Paris Sud 91 405 ORSAY Cedex (France)
Plasma-polymerized coatings deposited on iron were synthesized in microwave
plasma reactor. We have studied the influence of the HMDSO/O2 ratio
mixture on the film structure by Infrared Spectroscopy. The variation of the
HMDSO content in the feed gas provides a large wide of materials, from silica
(HMDSO content : 20%) to polymer (HMDSO content > 80%). The corrosion
protection properties of the coatings were estimated by Electrochemical
Impedance Spectroscopy. Coating parameters were obtained using a circuit
analog model. An efficient corrosion protection of iron has been obtained.
However, the protection depends on the structure of the coating.
§2 Keywords protective coating , HMDSO precursor , microwave plasma ,
electrochemical impedance spectroscopy.
In the last few years, the corrosion protective properties of plasma-polymerised
coatings based on organosilicone precursor have been studied [1-6]. These
films show interesting properties: they are highly crosslinked, insoluble,
pinhole-free and very adherent. Particularly, the organosilicone plasma-polymerised
seems to be a good candidate to form priming layer for subsequent coating [6,
7-9]. In some of our previous works [10-11], we studied the effects of
hexamethyldisiloxane (HMDSO)/O2 mixture on the excited species in
the plasma and on the structure of the coatings. The plasma polymerisation
occured in a microwave reactor which are characterized by high rates of
charged and neutral species (atoms, radicals ...) and consequently provided
short time of deposition. The aim of this paper is to study the influence of
HMDSO/O2 mixture on the corrosion protection properties. Plasma-polymerised
films were deposited on iron samples under different HMDSO/O2
ratios mixture and provide to a large wide of materials, from silica to
polymers. The coatings characterization was investigated by IR spectroscopy.
By using electrochemical impedance spectroscopy (EIS) measurements and
appropriate model (equivalent circuit), it is possible to obtain parameters
which can be correlated to the protective properties of several plasma-polymerised
§4 Materials and experimental procedure
The substrat materials, iron disks (purity 99,5%) 1,5 cm in diameter and 1 mm
in thickness, was supplied by Goodfellow. Hexamethyldisiloxane (purity 98%)
was purchased from Sigma-Aldrich. Plasma polymerisation was carried out in a
large microwave plasma reactor sustained by a surface wave ; this apparatus is
described in detail elsewhere .The microwave is excited at a 2.45 GHz
frequency and put through a quartz tube thanks to an excitator built on the
surfaguide principle. The microwave (MW) power delivered is 600 W.
Furthermore, the substrate holder was biased by means of a 13.56 MHz
radio-frequency (RF) generator at a fixed power 50W. The feed gases, oxygen
and argon, are injected at the top of the tube whereas the HMDSO is injected
in the flowing afterglow via a ring. The total pressure varied from 50 mTorr
to 80 mTorr depending on the HMDSO to O2 ratios. The flow rates
were fixed at 76 sccm for (HMDSO + O2) and at 5 sccm for the argon.
Four different percentage in the HMDSO/O2 mixture have been studied
: 20/80, 50/50, 80/20 and 100/0. Before coating deposition, samples were
treated in the microwave reactor without RF bias by oxygen plasma (150 sccm)
during 10 min. The duration of coating deposition were varied from 6 min to 45
min. After coating, microwave and RF powers were turned off and samples were
left in the reactor during 15 min in an argon flow (900 sccm) for cooling. The
film structure was analysed by means of reflection absorption IR (RAIR)
measurements performed on a Br�cker IFS28 spectrometer. The spectra of the
plasma-polymerized coatings were typically obtained by averaging 20 scans and
then substracting the spectrum obtained from the substrate before deposition
of the film. The film thickness was measured with a profilometer (Dektak 3030,
Sloan). The samples were weighted before and after coating with a micro-scale
balance (Sartorius MC210). The film weight was deduced by difference. The
corrosion protection of the film was analysed by means of electrochemical
impedance spectroscopy (EIS) measurement. EIS data were obtained using a
Schlumberger solartron 1255 frequency response analyser coupled to an EGG PAR
273 potenstiostat/galvanostat. Impedance data were collected at frequencies
ranging from 10 mHz to 100 kHz. Typically measurements were made at 5
points/decade, 10 mV a.c. amplitude. An electrochemical cell with three
electrodes was used. Coated iron sample was working electrode (area 0,8 cm2)
and a platinum grille served as counter electrode. A saturated calomel
reference electrode was employed. Experiments were conducted at room
temperature with NaCl 0,1 mol.L-1 electrolyte under Ar atmosphere.
The experimental impedance spectra were interpretated on the basis of
equivalent electrical circuits using a fitting software (EGG).
§5 Results and discussion
Deposition rate and density
Table 1 shows weight, thickness and density for plasma-polymerised coating
from different HMDSO/O2 mixtures and at several time deposition.
One may notice the short deposition time values due to the used of a microwave
plasma reactor. The density values are obtained with precision near to 20%. At
low HMDSO percentage, the highest density is obtained : it is close to the one
of thermal silica (2,22 g.cm-3). For higher HMDSO content in the
feed, the densities seem to be constant with a value around 1,5 g.cm-3.
§6 Table 1 : Weight, thickness and density of several plasma-polymerised
coating obtained from different HMDSO/O2 ratio mixtures, deposited
on iron samples.
Deposition time (min)
Coating weight (mg)
§7 Coating characterization by IR spectroscopy
The structure and bondings in the deposited films have been studied by
means of IR spectroscopy (Figure 1). According to the litterature [11-13],
the absortion bands can be assigned as follow : 790-800 cm-1 (Si-C
stretching and CH3 rocking in Si-(CH3)2), 835
cm-1 (Si-C stretching and CH3 rocking in Si-(CH3)3),
880 cm-1 (Si-(CH3)2 rocking and bending),
1000-1100 cm-1 (Si-O asymmetric stretching), 1260 cm-1 (CH3
symmetric bending in Si-(CH3)X), a weak band at
1350 cm-1 (-CH2- scissoring and wagging vibrations in
disilymethylene Si-CH2-Si), 1408 cm-1 ( CH3 symmetric
bending in Si-(CH3)X), 1458 cm-1
((CH)X asymmetric bending), 1710 cm-1 (C=O groups),
2900-2960 cm-1 (CHX symmetric and asymmetric
stretching), 2100-2250 cm-1 (Si-H stretching vibration). With
increasing O2 in the HMDSO/O2 mixture, the relative
intensities of the absortion band at 1260 cm-1 (characteristic of
Si-(CH3)X) is seen to decrease and the one of the band
at 1000-1100 cm-1 (Si-O) increase. We consider that this is because
of a decrease in the relative amount of organic content and to the high degree
of crosslinking in the coating. This view is supported by measurement of the
film density. The highest density is obtained at high O2 content in
the feed. One may conclude that plasma-polymerised coatings obtained from
HMDSO/O2 could be considered as polymer like or silica like as a
function of O2 content in the feed.
§8 Figure 1 : Infrared spectra of plasma polymerized coating deposited from
various HMDSO/O2 ratios : (a) HMDSO/O2 100/0 ; (b) HMDSO/O2
80/20 ; (c) HMDSO/O2 50/50 ; (d) HMDSO/O2 20/80.
§9 The RAIR spectra of coatings, obtained from several HMDSO/O2
mixture after exposure of the iron coated samples to NaCl for eight days,
didn't show any change in the films. This fact demonstrates the chemical
stability of these plasma-polymerised coatings. Corrosion protection
properties by EIS The frequency dependence of the electrochemical
impedance of a polymer coated iron sample usually can be modeled by an
equivalent circuit shown in Figure 2 [14 - 17]. This model will be tested for
the thin HMDSO plasma-polymerised coating. The non-conducting organic coating
appears as a capacitor Cc called coating capacitance. Ionically
conducting paths either due to the presence of defects or pores, or due to the
slight degree of solvatation of water and ions by the coating, produce finite
resistance Rpor (pore resistance), that short the coating. In
series with this resistive element is a parallel resistance Rpol (polarisation
resistance) and the double layer capacitance Cdl, representing the
corrosion process at the electrolyte-saturated coating/metal interface.
Finally, in series with the entire network representing the coated surface is
Rsol : the electrolyte resistance due to ohmic drop within the
electrolyte which is negligible in the case of coatings immersed in 0,1 mol.L-1
§10 Figure 2 : The equivalent circuit used to model coated
§11 The initial coating capacitance Cc is the lowest for silica-like
coating (D) which has the thickest coating layer (Figure 3(a)). Coatings A, B
and C have the same thickness but sample A has a lower Cc value
This fact could be correlated to the excited species present in the plasma
during deposition . The low atomic oxygen concentration in the plasma
provide to a coating less crosslinked. And hence this coating possesses the
lower barrier properties. The increase of Cc in the first day of
exposure is considered due to water uptake by the coating. In fact, the
coating capacitance increases with increasing water uptake . The volume
fraction V of electrolyte absorbed by the coating can be generally determined
from the experimental values of Cc  as follow: v = log(Cc(t)/Cc(0))/log80
(1) where Cc(t) is the coating capacitance at time t and Cc(0)
is the initial coating capacitance. The application of equation (1) allow to
high volumique fraction of water penetrated close to 1. These values have not
been confirmed by weight mesearements. In fact, the thickness of the plasma
polymerised coating is very low comparatively to the classical organic
protective coating and consequently, Rpor is so low and Cc
cannot be determined with sufficient accuracy to determine water uptake.
Figure 3(b) shows that the pore resistance Rpor decreased the most
for coating A. At the end of exposure, Rpor was the lowest for
sample A but the Rpor values of B, C and D coatings were similar.
The increase of Cdl (Figure 3(c)) can be considered as evidence
that the area at which delamination was increasing. This increase occured
first and was the largest for sample A. The initial Cdl value of
the silica-like coating (D) is the lowest but it doesn't vary very much with
time exposure. The decrease of Rpol (Figure 3(d)), which suggests
an increase of the delaminated area at the metal/coating interface is the
largest for sample A. This analysis of EIS data for this four coatings systems
shows qualitatively that the polymer-like coating A suffer the higher
degradation and corrosion at the metal/coating interface during exposure to
NaCl for eight days while B, C and D samples seem to have similar protective
properties and show good performance relatively to the low thicknesses of all
the coatings. Several parameters have been related to the delaminated area
. The polarization resistance has been used to estimate the delaminated
area because Rpol is obtained with most accuracy. Assuming that Ad
is equal to the corroding area, one obtains the following relation ship : Ad
= R0pol/Rpol (2) The specific polarization
resistance, R0pol is associated with the charge transfer
behavior of the metal substrate and can be estimated using the linear
polarization of an uncoated sample. The value of R0pol
is assumed to be constant. Ad has been calculated for the plasma-polymerised
coating obtained from the HMDSO/O2 80/20 mixture. The results are
reported in table 2 and is coherent with the corroding area observed. The
delaminated area seems to stabilize around 0.015 cm2 after 4 days
of immersion in NaCl.
§12 Table 2 : Delaminated areas calculated from Equation (2) for the plasma-polymerised
coating obtained from HMDSO/O2 80/20 mixture as a function of
Exposure time (days)
comment(13) To improve the barrier properties of the plasma-polymerised coatings,
we have increased the thickness to 0,5 mm to 4 mm
(samples B, E, F and G). This serie of coating were synthezised from a HMDSO/O2
80/20 mixture. The EIS results for the 0,5 mm thick
coating were bad probably due to pinholes. In the case of 3 mm
and 4 mm thick coatings, the initial value of Cc,
Rpor, Rpol and Cdl were in the same magnitude
order than the results of sample B (1mm). It is
well known that the increasing of thickness of plasma-polymerised coating
provide to the increasing of the strains and could favorise microcracks
formation. This fact could explain the best properties obtained only for 1 mm
thick coating when the exposure time increase.
In this work, plasma-polymerized coatings deposited on iron were synthesized
in microwave plasma reactor. We have studied the influence of the HMDSO/O2
ratio mixture on the film structure (RAIR spectroscopy) and on the protective
properties (electrochemical impedance spectroscopy). The structure of the
plasma-polymerised coatings has been found close to the thermal silica for low
HMDSO content in the feed (HMDSO/O2 : 20/80) and polymeric for
higher HMDSO/O2 ratios mixture. From the results obtained by EIS
measurements for different plasma-polymerised coatings, deposited on iron,
exposed in NaCl during eight days, the following conclusions were drawn : - An
efficient corrosion protection of iron has been obtained by these coatings
synthesized in the microwave plasma reactor. Nevertheless, the quality of the
protection depends on the HMDSO content in the feed gas. - The plasma-polymerised
coating from pur HMDSO had the lowest barrier and protective properties. - The
best protection is achieved for the plasma-polymerized coating at 80% HMDSO in
the mixture with a thickness of 1 mm. However the
results obtained for coating at 50% and 20% HMDSO lead nearly to similar
§15 Figure 3 : Time dependence of Cc (a), Rpor (b), Cdl (c) and Rpol (d) for
plasma-polymerised coating on iron substrates for various HMDSO/O2 ratios :
(circle) HMDSO/O2 100/0 ; (square) HMDSO/O2 80/20 ;
(cross) HMDSO/O2 50/50 ; (bold circle) HMDSO/O2 20/80
 H. P. Schreiber, M. R. Wertheimer and A. M. Wrobel, Thin Solid Films,
72 (1980) 487.
 K. D. Conners, W. J. van Ooij, S. J. Clarson and A. Sabata, J. Appl.
Polym. Sci. : Appl. Polym. Symp., 54 (1994) 167.
 E. Sacher, J. E. Klemberg-Sapieha, H. P. Schreiber and M. R. Wertheimer,
J. Appl. Polym. Sci. : Appl. Polym. Symp., 38 (1984) 163.
 W. J. van Ooij, A. Sabata and I. H. Loh, Pro. Eng. Symp., (1994)
 W. J. van Ooij and N. Tang, Polym. Mater. Sci. Eng., (1996) 155.
 J. S. Tonge, T. H. Lane, P. A. Giwa Agbomeirele and H.M. Klimisch, Electrochem.
Soc., INC, "Advances in Corrosion Protection by Organic Coatings",
13 (1995) 151
 W. J. van Ooij, A. Sabata, D. B. Zeik, C. E. Taylor, F. J. Boerio and
S. J. Clarson, Journal of Testing and Evaluation, JTEVA, 23 (1) (1995)
 T. F. Wang, T.J. Lin, D. J. Yang, J. A. Antonelli and H.K. Yasuda, Progress
in Organic Coatings, 28 (1996) 291.
 M. Statmann, R. Feser and A. Leng, Electrochimica Acta, 39 (8/9)
 C. Vautrin-Ul, C. Boisse-Laporte, N. Benissad, A. Chausse, P. Leprince
and R. Messina, to be published in Progress in Organic Coatings
 N. Bennissad, C. Boisse-Laporte, C. Vallee and A. Goullet, to be
published in Surf. Coat. Tech.(1999).
 A.M. Wrobel, M.R. Wertheimer, J. Dib and H.P. Schreiber, J.
Macromol. Sci. -Chem., A14 (3) (1980) 321.
 N. Inagaki and M. Taki, J. Appl. Polym. Sci., 27 (1982) 4337.
 F. Mansfeld, Journal of Applied Electrochemistry, 25 (1995)
 J. M. McIntyre and H.Q. Pham, Progress in Organic Coatings, 27
 P.L. Bonora, F. Deflorian and L. Fedrizzi, Electrochimica Acta,
41 (7/8) (1996) 1073.
 M. Kendig, S. Jeanjaquet, R. Brown and F. Thomas, J. Coat. Technol.,
68 (863) (1996) 39.
 D. M. Brasher and A.H. Kingsbury, J. Appl. Chem., 4 (1954) 62.