Surface and Coatings Technology 174–175(2003) 107–111

0257-8972/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0257-8972Ž03.00422-5

Application of plasma deposited organosilicon thin films for the corrosionprotection of metals

F. Fracassi *, R. d’Agostino , F. Palumbo , E. Angelini , S. Grassini , F. Rosalbinoa, a a b b b

Dipartimento di Chimica Universita degli Studi di Bari, IMIP CNR via Orabona, 4 70126 Bari, Italya `Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Italyb

Abstract

In this contribution it will be shown that plasma enhanced chemical vapor deposition(PECVD) of silicon containing organiccompounds is a promising approach for the corrosion protection of metals(steel and magnesium alloys). When the depositionprocess is preceded by a suitable plasma treatment, which depends on the particular metal under study, a marked increase of theprotective properties measured with electrochemical impedance spectroscopy(EIS) is detected. The highest impedance modulusobtained for Mg is 450 KVØcm , 8000 times higher than for the bare metal. Highly protective coatings are obtained for inorganic2

films, free of sylanols. A marked decrease of the impedance modulus of coated substrates has been registered after immersion inelectrolyte solution due to the presence of pinholes which represents defect points where localized corrosion starts.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Corrosion; PECVD; SiOx

1. Introduction

The protection of metals from corrosion is a veryimportant problem from the economical point of viewand for its implication on the environment. Corrosionprevents the wide utilization of some interesting mate-rials such as magnesium alloys in strategic applications,such as transport and aerospace and therefore manystudies have been carried out in order to developsuccessful corrosion prevention strategies. Magnesiumalloys are usually protected by chromate conversioncoatings, anodizing andyor polymer coatings. Unfortu-nately, despite of their simplicity, these approaches makeuse of toxic reactants and produce toxic wastes andenvironmental pollutants(e.g. hexavalent chromium ionis highly toxic and carcinogenic) and therefore alterna-tive technologies should be studied and developed.Similar problems are encountered also for the protectionof steel manufactures. In this scenario, the plasmaenhanced chemical vapor deposition(PECVD) fromorganosilicon precursors appears to be an attractiveapproach to increase the corrosion resistance of metallic

*Corresponding author. Tel.:q39-080-544-2009; fax:q39-080-544-3405.

E-mail address: fracassi@chimica.uniba.it(F. Fracassi).

materials, since it is a clean environmentally friendlytechnique, which does not produce wastes or appreciableatmospheric emissions.Among the main technological advantages of PECVD

it is important to mention the possibility of carrying outsubstrate pre-treatments, just before the deposition pro-cess in the same reactor, in order improve the coating-substrate performances.In this work the evaluation of the corrosion resistance

has been performed by means of electrochemical impe-dance spectroscopy(EIS), a rapid indirect diagnostictechnique which allows a quantitative evaluation of theinhibiting effect of the applied coating and to obtaininteresting information on the failure mechanism. Thedetailed description of EIS can be found elsewherew1x,the measurements are performed by applying an alter-nating potential to the coated substrate immersed in anelectrolytic solution(Na SO or NaCl). By changing2 4

the frequency of the alternating potential(v), it ispossible to measure the current intensity as a functionof v and therefore to calculate the impedance(Z) atvarious frequency values. A convenient way to representthe experimental data is the Nyquist plot, i.e.—ImZ vs.ReZ (ImZ and ReZ are the imaginary and the real partof Z, respectively). A suitable equivalent electricalcircuit representing the system under studyw2x allows

108 F. Fracassi et al. / Surface and Coatings Technology 174 –175 (2003) 107–111

Fig. 1. Nyquist plot of a Mg alloy sample coated with SiO filmx

deposited after a hydrogen plasma pretreatment, in plasma fed with a3:50 TEOSyO mixture after 1 h of immersion in 0.1 M NaCl solution.2

Fig. 2. Electrical circuit utilized to fit the EIS data obtained withSiO coated Mg substrates immersed in aerated 0.1 M NaCl solution.x

to fit the experimental points of Nyquist plot andtherefore to obtain the values of the circuital elementscorresponding to the system and in particular the valuesof the charge transfer resistance,R . According to thect

scientific literaturew3x the inhibition capability of theSiO coating has been correlated toR which is relatedx ct

to the corrosion process occurring at the metalycoatinginterface.

2. Experiments

Protective SiO thin films have been deposited onx

mild Q panel steel(Fe 99%, Mn-1%) and WE 43(Mg94%, Y 3%, Nd 3%) Mg alloy substrates, positioned onthe ground electrode of a home-made parallel platereactor(13.56 MHz), at the floating plasma temperature(T-60 8C) and 100 mTorr of pressure. The feed gascontained the organosilicon monomer in the mixturewith oxygen and argon. The following organosiliconmonomers have been tested up to now: hexamethyldi-siloxane(HMDSO) for steel and Mg, tetraethoxysilane(TEOS) and tetramethoxysilane(TMOS) for Mg. Pre-treatments have been performed in the same reactor,immediately before the application of the SiO coating,x

in plasma fed with oxygen, hydrogen or CF –O(20%).4 2

Electrochemical impedance measurements have beenperformed in aerated 0.1 M Na SO solution for steel2 4

and in aerated 0.1 M NaCl solution for Mg, at 25"18C, using a frequency response analyzer EIS300(GamryInstruments). Impedance spectra were recorded at theopen circuit potential with a three-electrode electrochem-ical configuration(lead–lead sulfate electrode as refer-ence and titanium counter electrode).

3. Results

A typical Nyquist plot for a Mg alloy coated with a1300 nm thick organosilicon film after 1 h of exposureto a 0.1 M NaCl solution is reported in Fig. 1. Theequivalent circuit model reported in Fig. 2 was consid-ered for an accurate analysis of the impedance diagram.In this modelR is the solution resistance,R the charges ct

transfer resistance(electron transfer) of the faradicprocess on the metal surface, in parallel with a constant-phase element(CPE), C is the coating capacitance, inc

parallel with the coating resistanceR , C (CPE) andc diff

R are related to a possible diffusion process ofdiff

reagents from the bulk solution toward the samplesurface. The impedance of a CPE is represented by theequationZsCØ1y(i v) .a

Notwithstanding the low thickness of the depositedcoating (lower than 1300 nm), a marked inhibitingaction toward the corrosion process occurring at themetal surface is observed with EIS, when HMDSO orTEOS are utilized as film precursors, provided a suitablepre-treatment is performed and the deposition conditionsare optimized. The highest charge transfer resistanceobtained for steel and Mg substrates are summarized inTable 1. A consistent improvement with respect to thebare steel is detected(from 250 to 2 200 000V cm )2

when an oxygen plasma pre-treatment is performed,while for Mg alloy, the best results are obtained with aH plasma pretreatment(from 60 to 480 000V cm )22

w4x. These values are indicative of a marked slowingdown of the corrosion process occurring at the metalsurface. When TMOS is utilized as a film precursor, the

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Table 1Comparison between the best charge transfer resistance obtained withEIS on steel and Mg alloy as a function of the pretreatment

Untreated Coated(1300 nm) Pretreatedand coated

Steel 250V 1275V 2.2 MV (O )2Mg 60 V 1150V 125 kV (CF –O )4 2

480 KV (H )2

EIS measurements have been performed in 0.1 M Na SO for steel2 4

and 0.1 M Nacl for Mg(1 h immersion).

Fig. 4. O 1s region of Mg substrates after 5 min of Arq sputtering(3 KeV). (A) as received;(B) after hydrogen plasma treatment.

Fig. 3. Mg surface depth profile(Auger KLL transition). Without (a)and with(b) hydrogen plasma treatment.

Fig. 5. FT-IR of a SiO thin film(TEOSyO yArs3y30y36, 100 mTorr,x 2

100 W) immersed in 0.1 M NaCl :(A) as deposited;(B) after 24 h;(C) after 720 h.

best charge transfer resistance obtained is orders ofmagnitude lower than with HMDSO and TEOS.The results on steel have been explained by means of

XPS depth profile(Ar ion sputtering) w5x. In agree-q

ment with published data, oxygen plasma removes thesurface contamination of adventitious carbon, increasesthe surface concentration of oxidized Fe and decreases3q

the density of reduced species(e.g. Fe ), i.e. the2q

electron donor species. As a consequence of this modi-fication, the reduction of atmospheric oxygen on thesteel surface, which is responsible for the corrosiondiffusion, slow down w5x. In the case of magnesiumsubstrates, the effect of the best pretreatment(H plas-2

ma) is obviously different since it reduces the extent ofsurface oxidation. In fact XPS depth profile shows thatwhen the Mg surface is treated in H plasma, a marked2

increase of the unoxidized Mg component is revealedunderneath the uppermost layers(Fig. 3), which arereasonably oxidized during the short time elapsedbetween the H plasma treatment and XPS analysis. The2

comparison of the O 1s XPS region(after 5 min ofAr sputter cleaning) shows that when the magnesiumq

surface is treated in H plasma a preferential removal of2

OH groups occurs, in fact in Fig. 4 it is shown that thecontribution of hydroxyl groups at 533.2 eV becomeslower than that of the oxide one(531.0 eV) after H2plasma treatmentw6x. A contribution of preferentialsputtering and surface rearrangement due to the sputtercleaning procedure, however, cannot be ruled out. Theimprovement of corrosion resistance reported in Table 1

for the pretreatment in CF –O(20%) plasma has been4 2

ascribed to the formation of MgF ;w6x the presence of2

oxygen in the feed gas avoids the massive formation offluorocarbon polymer contamination. The reasons of thebeneficial effect of the pretreatment on the properties ofSiO coatings is not completely understood, at presentx

the following factors appear to play a key role: elimi-nation of surface contamination(i.e. hydrocarbon layer);modification of the chemical composition and conse-quent passivation of the metal surface at the metal-coating interface; improvement of the coating-metaladhesion and reduction of film defectiveness.

110 F. Fracassi et al. / Surface and Coatings Technology 174 –175 (2003) 107–111

Fig. 6. SEM photograph of a SiO coating;(a) as deposited;(b) after 24 h of immersion in NaCl 0.1 M.x

Fig. 7. Trend of the coating capacitance,C , as a function of exposurec

time in 0.1 M NaCl solution.

The exploitation of various experimental conditionsand film composition has shown that the best corrosionprotections are obtained for inorganic SiO -like coating,x

when any organic and Si–OH(silanol) groups aredetected by infrared spectroscopy. This is in agreementwith what was recently reported on the utilization ofSiO thin film as a barrier to oxygen and water vaporx

on PET for food packagingw7x. Inorganic films aremore compact, more cross-linked and therefore they area good barrier to the permeation of liquids and gases.Silanol groups decrease the crosslinking of the coatingand represent reactive points where the degradation ofthe film immersed in water or exposed to humiditystarts. This can be appreciated in Fig. 5, where it isshown that, for SiOH-containing coatings, the IR absorp-tion of silanol groups increases with the time of immer-sion in NaCl solution. If the SiO film does not containx

silanols, the FT-IR spectrum is not modified duringimmersion in the electrolytic solution. The best approachto obtain inorganic deposits from organosilicon precur-sors is to increase the oxygen content of the feed gas;oxygen atoms generated by plasma activation ‘burns’the organic groups and leaves an inorganic SiO -likex

film. Unfortunately this can result in a contemporaneousincrease of silanol concentrationw8x which reduces thechemical stability of the coating and therefore its cor-rosion resistance. This inconvenience can be solved byrising the input power, which decreases the SiOH con-centration as well as that of the residual organic groups.In order to study the stability of the film, EIS

measurements have been performed on substrates coatedunder the best conditions as a function of the time ofimmersion in the electrolyte solution. A decrease of thecorrosion protection with time has been registered. Infact the R of a steel sample immersed for 870 h inct

Na SO 0.1 M is reduced to 16 KV cm (from 2.2 MV22 4

cm ), that of a Mg after 192 h immersion in NaCl2

passes from 480 to 2 KV cm . Fig. 6 shows that this2

effect can be ascribed to a localized corrosion that startsat a few defect points(not present on the as depositedcoating) representing preferential electrolyte penetrationpath. The corrosion process remains localized at thesedefect points also after long immersion times, in factthe SiO coating is stable, its chemical composition isx

not affected by the interaction with the electrolyte sinceany variation is detected, even after long immersiontime by FT-IR. The chemical stability of deposited filmsis also confirmed by the trend of the coating capacitance,C (see Fig. 2), as a function of exposure time, reportedc

in Fig. 7. As a matter of fact, some authors relate thecoating degradation to a progressive increase ofCc

values, w9x while in our case an opposite trend isobserved.

Acknowledgments

This research has been partially supported by theItalian ‘Ministero dell’Universita e della Ricerca Scien-`tifica e Tecnologica’(DM n. 10 del 23 gennaio 2001).

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