12 Organosilicon plasma polymer coatings containing Ce-based nanoparticles: characterisation of anti-corrosion properties Doriane Del Frari, Jérôme Bour, Julien Bardon, Olivier Buchheit, Claire Arnoult and David Ruch Centre de Recherche Public Henri Tudor (CRPHT), Laboratoire de Technologies Industrielles, Rue de Luxembourg 66, L-4002 Esch-sur-Alzette, Luxembourg, Belgium
[email protected]
12.1
Introduction
For many decades, chromate compounds have been successfully used as anticorrosive inhibitors in the surface treatment of aluminium and others alloys. The use of chromates is however restricted worldwide, as they are considered highly toxic and carcinogenic [1]. This has stimulated research aimed at the development of effective and environmentally acceptable alternatives to chromates. To obtain the beneficial properties of chromatation, two approaches can be utilised: passive [2] and active [3,4] corrosion protection. Passive protection is normally provided by a barrier film that prevents contact between the corrosive species and the metal surface and therefore hinders a corrosion process. However, when a defect is formed in the barrier layer, the coating cannot stop corrosion in that place. The second approach is active corrosion protection, which employs inhibitive species that can decrease corrosion activity. An important point is that both strategies must be used together to protect the metallic substrate adequately. Many alternative processes have been examined: among the possible candidates for the environmentally friendly protection of alloys are the organosilicon-based treatments [5], particularly for the protection of galvanised steel [5,6]. Dry processes such as plasma discharge deposition, could be preferred because they are solvent-free and, therefore, more environmentally friendly. Organosilicon layers such as plasma polymerised hexamethyldisiloxane (ppHMDSO) coatings can be deposited on metal parts (e.g. galvanised steel) by Dielectric Barrier Discharge (DBD) atmospheric plasma processes [7]. According to the literature, polymerised organosilicon-based coatings are known to be effective treatments to achieve physical barriers layers [8]. The major drawback of the siloxane coatings is their inert character regarding the corrosion processes. By themselves, ppHMDSO coatings do not provide any ‘active’ protection when barrier coatings fail and aggressive species reach the metallic surface and initiate corrosion activity. Recent research efforts have been focused on the modification of the bulk properties of organosilicon coatings by adding ‘active’ anti-corrosion species to improve corrosion resistance further, and/or to introduce self-healing ability in the coating [9–13]. It has been reported that the specific incorporation of a small amount (1–5%) 220
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of nanoparticles in organic polymers leads to improvements in the barrier properties of organic coatings. Van Ooij and colleagues [14] have shown that nanoparticle-filled silane films could be used to replace the toxic chromate-based treatment for aluminium alloys. Silane films can be thickened and strengthened by incorporating silica nanoparticles into the film. The authors suggest that the presence of silica suppresses the cathodic reaction (oxygen reduction). The reaction between hydroxyl ions and silica leads to the formation of SiO3– ions, which in a second step, react with Al3+ ions to form a passive silica film. Some studies have examined the possibility of combining plasma polymerisation techniques and nanocomposite elaboration methods. Nastase and colleagues [15–17] worked on nanocomposites with nanoparticles and nanotube fillers incorporated into a polymer matrix by plasma polymerisation. This process allows direct and easy deposition of the nanocomposite coating on the target surface. They demonstrated that this technique could be competitive with chemical synthesis and sol–gel processes. The development of anticorrosion coatings with properties similar to those of hexavalent chromium coatings which provide both self-healing and physical barrier effects is challenging. It is known that cerium-based coatings can also provide corrosion protection of metals thanks to their active properties. Indeed, cerium oxides and cerium hydroxides are reported to be cathodic inhibitors and have also been proposed as effective species for the protection of metals from corrosion: cerium possesses similar behaviour to that of chromium [18–20]. Hamdy [21] studied the effect of cerium treatment on corrosion behaviour. A silicate/cerate composite treatment has been tested and was found to improve the corrosion resistance due to the formation of an oxide-thickening layer that acts as a barrier to oxygen diffusion to the metal surface. Cerium is incorporated into the pores of the oxide film and is concentrated at the metal/film interface, leading to improved corrosion resistance. Moreover, it seems that a pitting auto-repair process takes place when high amounts of cerium and silicon are present. Additional improvement in corrosion protection was also observed in the case of silane pretreatments when cerium nitrate was introduced into the coating [15]. Therefore, innovative layers composed of siloxane/cerium and deposited by atmospheric pressure plasma have been tested. In this study, HMDSO was atomised and introduced as a precursor in an atmospheric pressure DBD plasma. The hybrid coating was obtained by mixing liquid precursor and nanoparticles (HMDSO and nanoAlCeO3) before atomisation. The properties of these different coatings were studied by scanning electron microscopy (SEM) and interferometry measurements. Their corrosion resistance has been determined electrochemically and their selfhealing properties have been demonstrated by a combination of electrochemical impedance spectroscopy (EIS) and a nanoscratch method. 12.2 12.2.1
Experimental Substrates and solutions
The samples of hot dip galvanised cold rolled steel (ALUZINC) to be treated were provided by Arcelor Mittal Dudelange (Luxembourg) and the galvanisation coating contained 55 wt% Al, 43.3 wt% Zn and 1.6 wt% Si. They were degreased using ethanol. The chromate coating on the reference sample contained about 20 mg/m2 of hexavalent chromium.
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SEM analysis of the AlCeO3 nanoparticles (Aldrich, 99%) showed them to have diameters varying from a few nanometres ( 98% grade) solution, purchased from Aldrich. The concentration of nanoparticles was calculated in weight percent. The solutions were homogenised by ultrasonic stirring at room temperature for 30 min. 12.2.2
Plasma deposition process
Coating experiments were performed in a semi-dynamic dielectric barrier discharge (DBD) reactor. The DBD discharge was switched on between an earthed bottom aluminium plate and two high voltage aluminium top plates protected by a 3.25 mm thick glass plate, as shown in Fig. 12.1. The gap between the earthed electrode and the dielectric plate was set to 2 mm. The liquid precursor solution was atomised at a constant pressure of 2×105 Pa and injected into the carrier gas flow before entering the plasma zone. The carrier gas composition, which was set by mass flow controllers, was a N2/O2 (97:3, vol.%) mixture with a total gas flow of 20 standard litres per minute. The gas mixture containing the precursor aerosol was injected into the plasma through a slit between the two top electrodes. The experiments were carried out at atmospheric pressure and ambient temperature. During the deposition experiments, the top electrode block moved back and forth over the sample at a constant speed (4 m/min) with a 380 mm displacement range. A treatment with 12 passes took 68.4 s. It has been demonstrated that the coating properties can be improved when an additional plasma post-treatment is performed on pre-existing plasma polymerised coatings [22]. This post-treatment, which has an effect on both the chemical composition and structure of the ppHMDSO layers,
12.1 Diagram showing the experimental atmospheric DBD plasma deposition reactor
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improves anti-corrosion passive properties. Consequently, the best deposition procedure consists of a coating step (sample exposed to plasma + precursor injection) and a post-treatment step (plasma only). The post-treatment was also 68.4 s duration. 12.2.3
Characterisation techniques
Scanning electron microscopy SEM analyses were carried out on a QUANTA 200 FEG from FEI equipped with an X GENESIS XM 4i EDS spectrometer supplied by EDAX. This microscope is a Variable Pressure SEM (VP-SEM) that enables the observation of non-conducting samples directly without additional coating [23,24]. Analyses were performed with specimen chamber pressures ranging from high vacuum (10–3 Pa) to 120 Pa depending on the charging effect. AlCeO3 powder specimens were prepared by sputtering powder onto a double-sided adhesive carbon film bonded onto an aluminium stub. Morphological investigations were performed in secondary electron mode. In order to provide better chemical contrast, the coating morphology and filler dispersion were investigated on coatings deposited onto a flat silicon substrate. Observations were performed using the backscattered electron imaging mode (BSE) and by X-ray elemental mapping (EDS). BSE analyses were carried out with a 30 kV electron probe. X-ray mapping microanalyses were carried out with a 5 kV electron probe for a live time of 900 ms per dot. Samples used to obtain general surface views were mounted directly on double-sided adhesive carbon film bonded onto an aluminium stub. To allow the observation of cross-sections, samples were broken in liquid nitrogen and held vertically in a gripping stub. Nanoscratch experiments Scratching was performed on galvanised steel samples coated with anticorrosion layers such as ppHMDSO or chromated layers. This simulates severe damage caused to these layers. Scratching experiments were carried out by a nanoscratch NST from CSM Instrument Company. A sphero-conical tip of diamond-like carbon-coated steel penetrated into the sample. All experiments, regardless of their nature (constant or increasing normal load with a constant displacement velocity of the indenter), included three successive steps performed at the same location: -
First, the profile of the original surface was determined by a scratch test at 1 mN constant normal load (prescan) The scratch test was then performed Then the residual profile of the scratch was determined by a 1 mN constant normal load scratch (postscan).
The penetration depth of the tip during the test was determined by comparing the original profile (prescan) with the vertical position of the indenter during the scratch test. Scratches of 3 μm depth were produced so that anticorrosion coatings of thicknesses much lower than 3 μm would fail. The radius of the spherical tip was 2 μm and the included angle of the conical section was 90°. A scratch with a constant normal load (50 mN) was made on samples at a scratching speed of 1 mm/min. The scratched
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length was 0.5 mm and the appearance of the residual scratch track was observed with an optical microscope after the scratch test. Interferometry experiments Three-dimensional profilometry pictures of hybrid coatings were obtained with a Wyko NT3300 white light interferometer (Veeco Company). Further topography data treatment was performed using Mountains Map software (Digital Surf Company). The coating thickness was determined by measuring the step height between the substrate surface and the deposited layer. Here the substrate under test was a glass slide, which had a very flat surface topography. Topographical grain analyses, which characterise the peak structure of ppHMDSO layers containing AlCeO3 particles, were performed using the following parameters: 0.1 μm lateral resolution (theoretical resolution), vertical shifting interferometry (VSI) mode, VSI filter, measurement size 150 μm by 150 μm, 1 measurement per sample. Data treatment consisted of filling-in non-measured points and applying a 12-order polynomial to remove the form. This high-order correction was chosen because the following steps involve working on the upper part of the AbbottFirestone curve1 to isolate the peaks clearly. It is thus of prime importance that the surrounding surface is as flat as possible. After form correction, a slight truncation of the surface was performed to remove aberrant data points (in both peak and valley areas). Peaks were then isolated using the Sr1 parameter (the 3D equivalent of Mr1 – ISO 13565-2:1996) of the Abbott-Firestone curve. This parameter, called the ‘upper material ratio’, gives the bearing ratio linked to the individual peaks. A second truncation of surface heights, from the highest peak down to the height related to Sr1, was then undertaken (Fig. 12.2). Once highlighted, peaks were counted, and their mean height, mean area and height to area ratio (i.e. peak transverse area at 15% bearing ratio) were computed. Electrochemical measurements Electrochemical experiments on galvanised steel samples were performed using a PARSTAT 2273 potentiostat/galvanostat (Princeton Applied Research) and a GAMRY 600 with a PCI4 Controller Board. Tests were carried out in 35 g/L NaCl non-deaerated aqueous solution, using a three-electrode electrochemical cell, consisting of the working electrode (1 cm2 exposed area), a saturated calomel electrode (SCE) as the reference electrode and platinum grid as the counter electrode. The measurements were performed at room temperature, during immersion of the sample in solution. The polarisation curve was measured by scanning the potential from +20 mV/SCE down to –600 mV/SCE versus the rest potential, which was the stabilised open circuit potential. This curve allowed measurement of the corrosion current, which is directly
1
The Abbott-Firestone or bearing ratio curve is the integral of the amplitude distribution function (ADF). This integral is performed from the highest peak downward. Thus, each point on this plot has the physical significance of the fraction of the projected surface above a given height, or the surface bearing ratio Ar (expressed in percent) at this height.
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12.2 A ppHMDSO coating containing 4% nanoparticles – peak heights range from white down to black. Here the Sr1 parameter is 20.1%, or 1.34 μm under the highest peak. Peak discrimination is correct
proportional to the corrosion rate, in the case of general corrosion. The EIS measurements were performed at the open circuit potential. The measurement frequency ranged from 100 kHz down to 10 mHz, with a 10 mV amplitude sinusoidal voltage. 12.3 12.3.1
Results and discussion Benchmark without AlCeO3
Anti-corrosion properties were measured using electrochemical methods: cathodic polarisation Tafel plots, which are not presented here, and Bode plots for plasma polymerised hexamethyldisiloxane ( ppHMDSO) samples (Fig. 12.3). The Tafel curves chosen correspond to the best result obtained for a series of three measurements on each sample. The open circuit potential after 3 h of immersion was taken as the reference potential (Ecorr) for each curve; hence it was possible to compare the cathodic current between different curves and therefore the anti-corrosion properties of different coatings. The corrosion current, in agreement with Tafel theory, was calculated by extrapolating the oxygen reduction plateau at the corrosion potential [25,26]. In comparison with the untreated substrate, a significant decrease in the corrosion current was observed for ppHMDSO-coated samples. Indeed, the average value corresponding to these siloxane films was about 1.2×10–7 A/cm2 whereas the aluzinc sample gave a cathodic current of 3.5×10–7 A/cm2. As a reference, the curve for the chromated coating gave a value of 5.0×10–8 A/cm2. The plasma polymerised coatings improved the corrosion resistance by a factor of three compared to the untreated sample, and the current value was close to the value for chromated coatings. The corrosion resistance of these coatings is due to the development of a dense –Si–O–Si– network, which
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12.3 EIS Bode plots of galvanised steel substrates, chromated and ppHMDSO layers recorded after 50 h of immersion
hinders the penetration of aggressive species towards the metallic substrate. The corrosion protection effectiveness of siloxane coatings is strongly dependent on their barrier properties. Improvement of the barrier properties of siloxane coatings is a prominent objective towards the development of corrosion protection layers. The impedance spectra can be investigated to allow modelling of the physicochemical processes that occur in the coating–substrate system during corrosion tests. Figure 12.3 shows the evolution of the impedance spectrum of ppHMDSO coating with, for reference purposes, spectra from untreated and chromated samples. Experiments are commenced after 50 h of immersion in NaCl solution. Concerning the Z modulus curves, at high frequencies, chromated and ppHMDSO coatings display the same characteristics. At low frequencies, slightly better corrosion resistance is observed for the chromated film (300 kΩ) than the polymer one (100 kΩ). On the other hand, the resistance of the untreated substrate was lower by one order of magnitude. Therefore the results obtained from Tafel plots and impedance spectra are consistent. 12.3.2
Addition of AlCeO3 nanoparticles
Morphological investigation Elemental cartography and topography Figure 12.4a shows a general view at low magnification of the surface of a ppHMDSO coating containing 3% AlCeO3 nanoparticles with a thickness of 300 nm. The coating surface is neither smooth nor flat. Numerous irregularities can be seen,
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especially protuberances that appear white in the micrograph. These protuberances were present on the whole surface and were uniformly distributed in the matrix. Their diameter ranged from a few hundred nanometres to a few micrometres. Aluminium elemental mapping (Fig. 12.4b) showed clearly that these protuberances correspond to Ce-based particles, which were the only constituents containing aluminium. Since the dimensions of these structures were of micrometre magnitude, the AlCeO3 particles must have formed agglomerates that were distributed homogeneously in the matrix. The elemental mapping for silicon (Fig. 12.4c) confirmed this observation. The silicon signal was almost undetectable at positions corresponding to nanoparticle agglomerates. It came from the substrate and the ppHMDSO coating. Elemental mapping for oxygen (Fig. 12.4d) was perfectly consistent with these observations. The most intense signals correspond to the particles, the signals of medium intensity to the extra thickness around the particles, and the weakest signals to the matrix background. It can be noted that the darkest areas of oxygen mapping correspond to the masking effect of the largest agglomerates. To confirm these observations, a thorough study was undertaken on both the sample surface and cross-sections (Fig. 12.5). Particles are gathered into agglomerates whose diameter ranges from 100 nm to several micrometres. It was noticed that similar agglomerates were present in the initial powder. Figure 12.5b, obtained on a cross-section with a tilt of 5°, confirmed that particles were embedded in the polymer matrix. Agglomerates whose diameter was greater than 200 nm (the thickness of the coating) were thicker than the surrounding film. Indeed, an excess thickness of 150 nm relative to the agglomerate is visible in this image. Figure 12.5b1 represents agglomerates in the BSE imaging mode, where nanoparticles appear white and are embedded in the matrix.
12.4 SEM micrograph (a) and mapping scan of Al (b), Si (c) and O (d) of a ppHMDSO coating containing 3% AlCeO3 nanoparticles
12.5 Backscattering SEM micrograph of a ppHMDSO coating containing 3% AlCeO3 nanoparticles: (a) surface; (b) cross-section; (b1) BSE; (b2) SE
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Influence of nanoparticle concentration The influence of particle concentration on the coating structure and morphology was studied by correlation of SEM results and interferometer analyses. Samples with nanoparticle concentrations of 1% to 4% were analysed. Figure 12.6 shows the microscopy results. The SEM micrographs correspond to an area of 250 × 250 μm. Particle agglomerates appear in white because they are composed of heavier elements than the plasma polymer. These micrographs enable evaluation of the structure and distribution of the smallest agglomerates as a function of the concentration of particles. The number of observable agglomerates increases with the concentration of particles. These hybrid coatings were also studied by interferometry: mean values are reported in Table 12.1. Figure 12.6(f to j) shows the full three-dimensional measurement before peak isolation. Peaks become more visible as the concentration of nanoparticles is increased, i.e. they become more numerous and higher (especially for a concentration of 4%). Since the reference state (no particles) is free of peaks (Fig. 12.6f), these peaks can be linked directly to the presence of cerium particles. The numerical values from Table 12.1 confirm this visual trend. When the three other samples were studied further, calculations showed that the peaks became more numerous with increasing nanoparticle concentration, without significant change in both mean height or area value, but at higher concentration levels, they became significantly higher and thinner (see height over surface ratio). These observations are in good agreement with SEM measurements (same order of magnitude of peaks in both techniques, see Fig. 12.5). Further topographical analyses of peaks will be carried out. Electrochemical measurements Influence of nanoparticles Plasma polymerisation of coatings incorporating AlCeO3 nanoparticle concentrations in the range 1 to 4% was performed. Tafel plots corresponding to these films are presented in Fig. 12.7a in comparison with chromated coatings. The curves are characterised by a current plateau relative to the limiting current of dissolved oxygen; depending on the conditions, this cathodic process can be explained by two different mechanisms, as described by equations 12.1 and 12.2, respectively: O2 + 2H2O + 2e– Æ H2O2 + 2OH–
[12.1]
O2 + 2H2O + 4e Æ 4OH
[12.2]
–
–
This phenomenon is usually observed for experiments carried out on bare metals immersed in corrosive solutions, showing that the corrosion mechanism is under diffusion control [26,27]. In the –0.1 to 0 V range, corresponding to dioxygen reduction, corrosion currents relative to filled samples decrease slowly and range between 9.0×10–8 and 1.2×10–7 A/cm2. The presence of nanoparticles improves corrosion resistance when the concentration of particles is greater than 2%. The main anodic reaction during the corrosion of untreated samples is oxidation of aluminium and zinc. At cathodic sites, due to the local pH increase (equations 12.1 and 12.2), the deposition of cerium(III)
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12.6 SEM micrographs and 3D interferometry pictures of ppHMDSO containing AlCeO3 at concentrations of 0% (a, f), 1% (b, g), 2% (c, h), 3% (d, i) and 4% (e, j)
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Table 12.1 Influence of nanoparticle concentration on article characteristics
Number of peaks Mean height Mean area Height/area ratio
[nm] [μm2] [nm/μm2]
1% of particles
2% of particles
3% of particles
4% of particles
232 78 6.4 12.0
294 40 4.2 9.6
293 74 4.8 15.4
319 198 12.0 16.5
hydroxide may occur, blocking the cathodic reaction and hindering the whole corrosion process. EIS spectra measured on these coatings are presented in Fig. 12.7b. These impedance values confirm that the barrier properties, visible at low frequencies, are improved by AlCeO3 incorporation in comparison with the reference sample. The best corrosion resistance is achieved when the concentration of particles is 3%: beyond this value, it seems that further incorporation does not improve corrosion resistance. The incorporation of this kind of particles enhanced the ppHMDSO barrier properties compared to those of pure ppHMDSO films. The corrosion resistance results are consistent between the Tafel plots and Bode plots. In both cases, the best corrosion resistance is obtained with 3% incorporation. Immersion time During immersion, defects appear in the protective coating due to water uptake and they become preferential sites for corrosion. In order to see the evolution of these defects with the different kinds of coating, the films were immersed in the aggressive solution for a long time. Figure 12.8 shows the evolution of the Z modulus for pure and AlCeO3-filled ppHMDSO films. The coatings were immersed in NaCl solutions for 5 days. After this period, the good barrier properties of ppHMDSO films decrease. Similarly, the global impedance values decrease by 80 kΩ to 40 kΩ. On the other hand, curves corresponding to ppHMDSO containing AlCeO3 nanoparticles are more or less superimposed, whatever the immersion time. Passive properties (barrier effect) and global values of Z are similar. The addition of corrosion inhibitor to the siloxane matrix can confer additional active corrosion protection. Cerium salts can provide self-healing ability to supplement the good barrier properties of the films. Corrosion inhibition at localised defects In order to investigate the self-healing ability of different siloxane coatings, artificial defects were made in the surface of different films. When the barrier coating is damaged, corrosive species can reach the metal substrate, leading to corrosion activity. Thus, the presence of an active inhibiting component in the protective coating helps to lower corrosion activity. The main goal of cerium additions to the siloxane coating is to impart active corrosion protection properties to the ppHMDSO barrier layer. Active corrosion protection means that the sample can hinder corrosion activity due to damage of the barrier layer at the defects. Therefore, artificial defects were created in the siloxane coatings to determine how hybrid films behave when the barrier is damaged.
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12.7 Tafel plots (a) and impedance plots (b) of ppHMDSO layers containing AlCeO3 nanoparticles
Figure 12.9 presents impedance spectra obtained on chromated (a), ppHMDSO (b) and ppHMDSO containing 3% AlCeO3 (c) samples, in which an artificial defect was made by the introduction of a nanoscratch after 1 day of immersion. The samples were first immersed in NaCl solution for 24 h. After this period, a defect was created on the surface and new EIS spectra were measured. According to the literature, two kinds of behaviour can be expected. In the first [28], a decrease in impedance values after the creation of the defect is observed, but after more than 24 h of immersion,
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12.8 Evolution with immersion time of impedance plots of ppHMDSO layers with and without AlCeO3 nanoparticles; 1d, 1-day immersion in NaCl solution; 5d, 5-day immersion in NaCl solution
this impedance remains approximately constant: the presence of active nanoparticles blocks further evolution of corrosion. In the second behaviour, a decrease in the Z value occurs after the scratch, followed by a slight increase in impedance at low frequencies, revealing that corrosion has slowed down [29]. Chromated coatings are well known for their self-healing properties, and the impedance spectra (Fig. 12.9a) confirm this behaviour: whatever the time of immersion or presence of a scratch, the curves can be superimposed. A slow decrease in the global resistance is observed after 3 h, then stabilisation occurs: chromate ions play their inhibiting effect and stop corrosion activity. With the pure ppHMDSO layer (Fig. 12.9b), the global corrosion resistance decreases slightly 3 h after the creation of the defect, and then more strongly after 24 h. Corrosion activity increases when the metal is in contact with an aggressive solution, which is consistent with the evolution of the curve. The siloxane-based polymer does not provide any active protection. On the other hand, when AlCeO3 particles are incorporated, a decrease in the global corrosion resistance is observed immediately after the scratch (Fig. 12.9c). However, after longer immersion, the resistance increases strongly and tends to the initial value. The anticorrosive effect of the cerium ions entrapped into the siloxane coating is due to an inhibiting effect and a self-healing mechanism, probably with cerium hydroxide precipitation. Indeed, the inhibitive action of Ce3+ is based on the formation of highly insoluble hydroxides, which can be formed in the place where the cathodic reaction occurs according to the following equation: Ce3+ + 3OH– Æ Ce(OH)3
[12.3]
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12.9 Impedance spectra of chromated (a), ppHMDSO (b) and ppHMDSO layer containing 3% AlCeO3 (c) – one scratch
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Indeed, when nanoparticles are embedded in the siloxane coating, the following mechanism can be assumed: the silica network progressively breaks down, which releases the nanoparticles that precipitate on the electrode surface. Hence, they form complexes with charged species, which reinforces their protective role. The formation of these stable corrosion products decreases the active corrosion area, hence slowing down the corrosion activity. 12.3.3
Optimisation of active properties
In order to improve our understanding of the active properties of coatings containing Ce-based nanoparticles, two kinds of experiments were undertaken. On the one hand, more scratches were made on the best coating, i.e. ppHMDSO containing AlCeO3, and on the other hand, an attempt was made to improve the dispersion of the nanoparticles. The EIS spectra resulting from these tests were studied. Increase in localised defects Figure 12.10 shows results obtained for samples scratched five times: this corresponds to five times more scratches than in the previous experiments (Fig. 12.9). A slight decrease in the global corrosion resistance was observed 1 day after immersion and 3 h after creation of the defects. However, after immersion for 24 h following the introduction of the scratches, this decrease stopped and the curves became superimposed. The evolution of the global impedance (in percent of the arbitrary best Z value obtained after 3 h of immersion) was calculated and results are presented in Table 12.2.
12.10 Impedance spectra of ppHMDSO layer containing 3% AlCeO3 – 5 scratches
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Table 12.2 Variation of impedance with time as a percentage of the maximum value (3h before scratching) Chromated
3 h before scratch 24 h before scratch 3 h after scratch 24 h after scratch
1 scratch
ppHMDSO +AlCeO3 3% 1 scratch
ppHMDSO ppHMDSO +AlCeO3 3% +AlCeO3 3% (EtOH) 5 scratches 1 scratch
Z max = 100 67% 59% 59%
Z max = 100 92% 36% 54%
Z max = 100 76% 64% 59%
Z max = 100 98% 84% 79%
Comparison shows that almost 60% of Zmax remains after scratching the chromated sample, and this value is constant after 24 h of immersion. For ppHMDSO containing AlCeO3, this percentage decreases strongly to 36% immediately after the introduction of one scratch then moves up to 54% thanks to its self-healing properties. On the other hand, the evolution of this kind of coating in relation to five scratches is different. The global impedance decreases to 64% after scratching and stabilises at around 60%. In spite of the greater number of defects on the surface, the corrosion does not increase as much as might be expected. It seems that a sufficiently large release of cerium ions occurred to slow down the corrosion reactions. In conclusion, more scratches result in a more pronounced self-healing reaction: there is not an increase in Z value as observed with one scratch but a tailing off of Z decrease. Improvement of nanoparticle dispersion Self-healing properties depend on cerium ions and their ability to be released into the polymer matrix. In order to optimise the dispersion of nanoparticles, a small amount of ethanol was added to the initial solution of HMDSO and AlCeO3 (3%) powder. Figure 12.11 presents SEM results for this surface, in comparison with those obtained without solvent. Some thick aggregates, which are present in Fig. 12.11a, disappear when the particles are dispersed in ethanol. Protuberances are less evident in this case and it seems that nanoparticles do not agglomerate. Concerning anti-corrosion properties, Tafel curves allow the calculation that the corrosion current for this kind of coating is slightly below that for classical Ce-based ppHMDSO films. Images of HMDSO treated samples after 25 days in a salt spray chamber are shown in Fig. 12.11(c,d) which compare the resistance of HMDSO plasma treatments with and without ethanol. An increase in corrosion protection is afforded by the presence of solvent, since pitting corrosion is reduced. This last result corroborates those obtained by electrochemistry. Active properties have also been studied using the same protocol as previously. The results are presented in Table 12.2. After 24 h of immersion in corrosive solution, 98% of the maximum impedance is retained, which is better than for the chromated sample or the (ppHMDSO+AlCeO3) coating. The same is true of values obtained after scratching, whatever the time of immersion. Indeed, the global impedance decreases extremely slowly and is close to 80% of maximum of Z after 1 day of immersion. AlCeO3 nanoparticles seem more dispersed into the polymer matrix and slow down corrosion activity thanks to good self-healing properties.
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12.11 SEM micrographs and salt spray test images of ppHMDSO containing AlCeO3 (3%) without (a, c) and with ethanol (b, d)
12.4
Conclusion
Plasma polymerised HMDSO coatings containing AlCeO3 nanoparticles and deposited on galvanised steel by atmospheric pressure dielectric barrier discharge have been studied. Morphological investigations have confirmed that nanoparticles are incorporated into the polymer matrix with an effective distribution, whatever the concentration. In addition, an excess thickness of these particles within the matrix (protuberances) is sometimes visible. This phenomenon has been confirmed by interferometry measurements, which show a surface morphology with a number of peaks that is proportional to the concentration of nanoparticles. Electrochemical experiments have confirmed the good barrier properties of hybrid siloxane coatings and also the active corrosion protection effect of cerium ions. Indeed, polymer coatings with cerium incorporation performed at least as well as the undoped siloxane layers: the best barrier properties were obtained with a 3% AlCeO3 concentration. Concerning the self-healing behaviour, ppHMDSO coatings incorporating cerium nanoparticles provide improved long-term corrosion protection and active properties.
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