Oil removal from iron surfaces by atmospheric-pressure barrier discharges

Surface and Coatings Technology 115 (1999) 66–69

Oil removal from iron surfaces by atmospheric-pressure barrier discharges G. Baravian a, *, D. Chaleix b, P. Choquet b, P.L. Nauche b, V. Puech a, M. Rozoy a a LPGP, Associe´ au CNRS, Universite´ de Paris-Sud, 91405 F-Orsay, France b IRSID, BP 30320, Maizie`re-le`s-Metz, France Received 03 December 1998; accepted 12 March 1999

Abstract An atmospheric-pressure dielectric barrier discharge plasma is used for oil removal from iron surfaces. The first results show that oiled iron surfaces cleaned in liquid solvent or by an atmospheric-pressure dielectric barrier discharge plasma in oxygen provide the same X-ray photoelectron spectra to high sensitivity, indicating an efficient oil-removal effect of the plasma. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Dielectric barrier discharge; High-pressure discharge plasmas; Metal surface cleaning; Plasma–surface interactions

1. Introduction Recent international rules concerning environmental pollution have led industry to search for and develop dry ways for cleaning processes of metals. It was shown that treatment processes using low-pressure, non-thermal-equilibrium reactive plasmas, in which the reactive species are produced by energetic electrons, are efficient possible solutions to this problem [1–5]. Unfortunately, the use of these low-pressure plasma-based treatments adds a complication related to the necessity of large vacuum equipment to allow the on-line cleaning of continuously moving metal sheets. Therefore, the development of techniques relying on the use of plasmas produced at atmospheric pressure will be an improvement from an economics point of view. While some efficient excitation schemes at atmospheric pressures have been developed in the frame of laser technology to produce uniform plasmas [6–9], these techniques are somewhat complicated to implement and by far more costly than the so-called dielectric barrier discharges (DBDs). At atmospheric pressure the DBD plasmas, which comprise of many individual filamentary micro-discharges statistically distributed over the electrode surface, have long been used for ozone

generation, although an understanding of the physical processes explaining these discharges is more recent [9– 11]. These DBD plasmas are attracting renewed interest owing to their numerous possible industrial applications in processes for the treatment of surfaces and polluted atmospheres [10–17] and also in metals cleaning [18,19]. For this last application, these discharges have the advantages of the dry processes without the expansive constraints of the vacuum installations. In this paper, we present the reactor used and first results obtained from experiments on cleaning of oiled iron surfaces by atmospheric-pressure DBD plasmas in oxygen. The discharge is supplied by a high-voltage pulser. When the metal surface is at a negative potential, the process combines the chemical action of reactive species with the bombardment of energetic ions. Ex situ X-ray photoelectron spectroscopy ( XPS) was used to test the efficiency of the discharge for oil removal. The XPS spectra of iron samples obtained after plasma cleaning are compared with those obtained when the surfaces are cleaned with liquid solvents (acetone and ethanol ), as is usual in the laboratory. The comparison shows that almost the same quality of cleaning can be achieved by these two cleaning processes.

2. Experimental details * Corresponding author. Tel: +33-169-158182; fax: +33-169-157844. E-mail address: [email protected] (G. Baravian)

The DBD experimental reactor was a parallelepipedshaped chamber (25 cm×15 cm×10 cm ), made from

0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 16 2 - 0

G. Baravian et al. / Surface and Coatings Technology 115 (1999) 66–69

Plexiglas, containing two plane-parallel metal electrodes. The upper electrode was covered by a dielectric plate and was connected to a pulsed high-voltage power supply; the lower one, electrically grounded, was the oiled iron surface. In the present experiment, we used oiled plates directly as-manufactured. The plates were sprayed with oil in such a way that the thickness of the protective film is unknown and largely non-uniform. The inter-electrode gap was swept by an oxygen gas flow at atmospheric pressure. The gas flow allows renewal of the reactive gas, cooling, and avoids redeposition onto the metal sheet of the removed or transformed oil components. A schematic drawing of the experimental apparatus is given in Fig. 1. We used a high-voltage pulse generator providing voltage pulses up to 40 kV, with a rise time of about 200 ns and an adjustable repetition rate up to 1000 Hz. The actual voltage waveform is related to the nature of the load on which the generator is coupled. The DBD mainly constitutes a capacitive load whose capacitance value depends on the discharge surface and on the value of the dielectric constant, e, of the dielectric material. In the present experiment, the DBD capacitance was always smaller than the internal capacitance value of the generator. In this particular case, a resistance of 1 kV put in parallel with the discharge allowed us to obtain, for each cycle, two opposite polarity voltage pulses, necessary to the working of the DBD. The voltage was measured with a resistive Tektronix P6015 probe with bandwidth of 75 MHz, while the discharge current was measured by using more than one GHz-bandwidth, 0.05 V resistive shunt put in the circuit of the grounded electrode. The oscillograms given in Fig. 2 represent the temporal variations of the voltage and current for the total duration of one cycle [Fig. 2(a)] and, on a shorter time scale [Fig. 2(b)], a detailed view of the electric behaviour of the discharge. Negative and positive current pulses are separated by about 6 ms, the negative one being the highest. It can be noted that the time duration of the current pulses (~10 ns) is shorter than that of the voltage pulses (~6 ms). This results from a general property of DBD for which, for a given configuration and for a single pulse, the current pulse duration is limited by the time necessary for the charges accumu-

Fig. 1. Experimental set-up.

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Fig. 2. Typical voltage ( lower trace, 5 kV per division) and current (upper trace, 20 A per division) oscillograms obtained for an electrode surface equal to 50 cm2, a gap of 3.4 mm and a ceramic plate thickness of 3 mm. Time scale: (a) 2 ms per division; (b) 50 ns per division.

lated on the dielectric surface to produce a space–charge field which annuls the external potential applied to the electrodes. This current limitation induces a limitation on the energy that can be transferred to the gas. The potential polarity reversal removes the charge deposited on the dielectric barrier and allows the discharge to start again at the next cycle. Owing to the very short rise time of the pulse provided by our high-voltage generator supply, we would like to emphasise that at each cycle only one current pulse is obtained as shown in Fig. 2(b); this is in contrast to the classical DBD, usually powered by a low-frequency (<50 kHz ) alternating sinusoidal power supply, which, at each voltage reversal, is characterised by multiple current pulses [16 ]. As a result, in our case, all the micro-discharges are produced simultaneously by a unique current pulse which appears, from one shot to the next, at the same voltage threshold. This working mode presents the advantage of generating reproducible plasmas having well-defined characteristics. The spatial structure has been studied with a chargecoupled device (CCD) gated camera. As shown in Fig. 3, a multi-filamentary discharge structure is obtained for

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G. Baravian et al. / Surface and Coatings Technology 115 (1999) 66–69

Fig. 3. Typical cameragram obtained for a gate of 80 ms. The dielectric is at the top of the image. The discharge parameters are: air gap of 2.85 mm, dielectric thickness of 3 mm, rate frequency of 10 Hz. One pixel corresponds to about 0.03 mm of spatial resolution.

each single current pulse [Fig. 2(b)]. The influence of various discharge parameters (gap, dielectric thickness, e, repetition rate) on the spatial homogeneity of the plasma was investigated. The surface density of these filaments increases as the repetition rate is increased and as the gap is decreased. For the two dielectric materials used, with e of 6 and 7.6, and for plate thickness between 1 and 5 mm, there is no sensitive change of the surface density of the filaments. In the cleaning experiments by DBD plasma, a thin spray of protective oil covered the sheet samples. Dielectric plates of 3 to 6 mm thickness, with a dielectric constant of 7.6, were used as an isolating dielectric barrier, and the inter-electrode gap was varied from 1 to 3.3 mm. The upper isolated electrode was a rectangular adhesive foil of cooper, pasted onto the dielectric plate, leading to the same treated surface area on the opposite lower electrode. These characteristics fixed the electric capacitance value of the reactor to between 10 and 30 pF, and determined the energy that can be transferred from the high-voltage source to the gas. Surface diagnosis by XPS was performed by using a Physical Electronics Industries W5500 device. The spectra were recorded over an energy range of 0 to 1100 eV. Sample surfaces of 0.6 mm×2 mm area were analysed under a depth of 10 nm. The sensitivity was about 1000 ppm. The samples were introduced into the XPS analyser chamber immediately after treatment in order to limit external contamination.

300 cm2, the total energy and charge transferred to the plasma vary linearly with the surface in such a way that the surface energy density (J cm−2) and charge (C cm−2) appear as good parameters for characterisation of the DBD plasma. Fig. 4 presents measurements of surface energy density transferred, at each pulse, to the gas for various values of the inter-electrode gap and dielectric thickness. This study shows that for the dielectric used, the gap value has a relatively small effect on the energy transferred per pulse to the plasma, while this energy decreases strongly with increasing thickness. This behaviour exhibits one of the aspects of the DBD plasmas which, for large surface treatments, must use relatively high-thickness dielectric plates for reasons of mechanical rigidity. This thickness limits the energy that can be transferred per pulse. Treatment times acceptable for moving iron sheets must take into account this characteristic by choosing an adapted pulse-repetition frequency. For example, under his working conditions, Pochner [18] indicates that an energy of 1 J cm−2 was necessary to achieve cleaning by DBD. The curve of Fig. 4 shows that this energy is obtained after some thousand pulses. Under our operating conditions, the efficiency of oil removal was tested by the X-ray photoelectron spectroscopy technique. Fig. 5(a) shows the XPS spectrum obtained (with intensities normalised to the maximum peak intensity) for oiled iron sheets before cleaning treatment. The carbon line, characterizing one of the major oil components, dominates the spectrum and iron lines do not appear. In Fig. 5(b) we can see the spectrum obtained after ultrasonic cleaning of the surface in acetone followed by rinsing with ethanol. The carbon line is weaker and the spectrum is dominated by the O 1s line of oxygen; the iron line and those of other heavy atoms — resulting from the composition of the protective oil used — appeared with relatively strong intensities. This spectrum was taken as the reference spectrum of cleaning quality. Fig. 5(c) gives the

3. Results and concluding remarks It was of importance to identify the main parameters acting on the transfer of electrical energy to the gas. Extensive measurements versus the electrode surface have revealed that, in our device and for surfaces up to

Fig. 4. Energy deposited in the gas versus the ceramic plate thickness for various gap values.

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flushed through the reactor with a DBD plasma running for 10 min at 600 Hz. In these experiments the thickness of the oil film removed was unknown; work is in progress to determine the actual mass of oil removed versus the input energy density in order to optimise the treatment efficiency.

Acknowledgements The authors wish to thank Dr Bernard Lacour from CILAS Company for providing the pulsed power generator and S. Steer for XPS analysis.

References

Fig. 5. XPS analysis of: (a) oiled iron sheet; (b) sheet after cleaning with acetone and then ethanol; (c) sheet after DBD plasma cleaning.

spectrum obtained after a typical cleaning by the DBD plasma in pure oxygen at atmospheric pressure. In these experiments, the gas flow was greater than 1000 l min−1, the inter-electrode gap was 3 mm and the thickness of the dielectric (e=7.6) was 4 mm. The pulsed generator worked at a 600 Hz repetition frequency with a maximum voltage of 40 kV and a current pulse duration of 10 ns by period. The total treatment time was 10 min. Comparison between the XPS spectra of surfaces cleaned by the chemical process and surfaces cleaned by the DBD plasma show certain similarities. Particularly in Fig. 5(c) we observe that, although the carbon line dominates the spectrum, lines from oxygen, iron and other component atoms of the oil appear with intensities comparable to the reference spectrum. The detection of the iron lines is a clear indication that the residual oil thickness is of the order of one atomic layer, which is a cleaning quality acceptable for application to a large number of coatings. In summary, in this work we present the characteristics of a laboratory DBD plasma reactor, working at atmospheric pressure, which was used for the cleaning of oil-protected iron plates. XPS analysis of the chemical composition of the surface of DBD-plasma-cleaned plates reveals a cleaning quality as good as that obtained by reference laboratory wet techniques (ultrasonic cleaning in acetone, followed by rinsing in ethanol and evaporation) when an atmospheric-pressure gas flow of pure oxygen at a rate greater than 1000 l min−1 was

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