Volume 2 Paper 3

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 :

2 Laboratoire de Physique des Gaz et des Plasmas, UMR 85 78 Bat. 210, Université Paris Sud 91 405 ORSAY Cedex (France)

Abstract

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.

Keywords protective coating , HMDSO precursor , microwave plasma , electrochemical impedance spectroscopy.

Introduction

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 coatings.

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 [11].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 Brücker 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).

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.

Table 1 : Weight, thickness and density of several plasma-polymerised coating obtained from different HMDSO/O2 ratio mixtures, deposited on iron samples.

Sample name A B C D E F G
HMDSO/O2 (%) 100/0 80/20 50/50 20/80 80/20 80/20 80/20
Deposition time (min) 12 12 12 12 6 30 45
Coating weight (mg) 0,3 0.3 0.25 0,2 0,2 0,6 1,1
Thickness (mm) 1,0 1,2 0,9 0,5 0,5 3,1 4,1
Density (g.cm-3) 1,5 1,4 1,4 2,2 2,0 1,3 1,4

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. 

 

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.

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 NaCl.

Figure 2 : The equivalent circuit used to model coated electrode  

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 [10]. 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 [14]. The volume fraction V of electrolyte absorbed by the coating can be generally determined from the experimental values of Cc [18] 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[17]. 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 [14]. 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.

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

Exposure time (days) 0 0.5 1 1.5 2 3 4 7 8
Ad (cm2) 10-4 0.003 0.003 0.007 0.006 0.010 0.017 0.016 0.015
  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.

Conclusions

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 protective properties.

 

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

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