Coatings produced by electrophoretic deposition from nano-particulate silica sol gel suspensions

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Surface and Coatings Technology 18 (004) 199 03 Coatings produced by electrophoretic deposition from nano-particulate silica sol gel suspensions Y. Castro, B. Ferrari*, R. Moreno, A. Duran Instituto de Ceramica y Vidrio (CSIC), Campus de Cantoblanco, Camino de Valdelatas syn, Madrid 8049, Spain Received 3 April 003; received in revised form 31 July 003; accepted 31 July 003 Abstract Sol gel technology allows the production of hybrid coatings on metals using low temperatures for densification, but has an important limitation related with the maximum coating thickness attainable, typically lower than mm. The incorporation of nano-particles to the sol can make it possible to increase the coating thickness, without increasing the sintering temperature. This work deals with the preparation of thick sol gel coating by electrophoretic deposition (EPD). Commercial SiO nanoparticles are suspended in an acid-catalysed SiO sol, whose stability is largely increased by adding tetramethylammonium hydroxide (TMAH) up to ph 6. Coatings are formed by dipping and by EPD on AISI 304 stainless steel substrates. The protective behaviour against corrosion in aggressive media (i.e. marine water) of the produced films is studied. EPD leads to coatings as thick as 5 mm with good corrosion resistance, while the suspension stability is increased by 15 times compared with that of the starting acid suspension. 003 Elsevier Science B.V. All rights reserved. Keywords: Sol gelyslurry; Nanoparticles; Electrophoretic deposition; Immersion test 1. Introduction The use of some steels is limited in aggressive media such as corrosive gases, seawater, etc., in which the steel undergoes degradation and corrosion, affecting its properties. Different routes can be used to increase the corrosion resistance, the sol gel process being one w1x. Corrosion studies with inorganic sol gel coatings have demonstrated that they provide good protection against oxidation and nitridation by anhydrous ammonia wx, but when they are used in electrolytic media, corrosion takes place due to the presence of micro-cracks and porosity. Hybrid coatings are able to stop the electrolytic corrosion, since the presence of organic groups increases the ductility and the thickness of the coating w3,4x. The critical thickness of the films is normally around ; mm, restricting the protective effect in certain cases. One way to obtain thicker coatings is to prepare sols by adding commercial colloidal suspensions to the precursor sols w5,6x. Another route consists of the direct *Corresponding author. Tel.: q34-917-355-870; fax: q34-917- 355-843. E-mail address: bferrari@icv.csic.es (B. Ferrari). addition of particles to the sol w7x. In this case, additional aspects, such as the type of additives necessary to stabilise the suspension, have to be considered. The resulting coatings present lower shrinkage rate, which reduces the final internal stresses. An attractive method for increasing the thickness of the sol gel coatings is electrophoretic deposition (EPD). This technique has been widely applied in ceramics for conventional suspensions, but only a few studies dealing with the application of EPD to sol gel suspensions have been reported w8,9x. The aim of the present work has been to prepare hybrid organic inorganic suspensions via acid catalysis to study their stability and to optimise the parameters involved in the preparation of thick coatings by EPD. The protective behaviour of the coatings against corrosion is studied through electrochemical tests.. Experimental Suspensions were prepared by mixing tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) with a colloidal suspension of silica (Levasil 00A, ph 9, 057-897/04/$- see front matter 003 Elsevier Science B.V. All rights reserved. doi:10.1016/j.surfcoat.003.07.001

00 Y. Castro et al. / Surface and Coatings Technology 18 (004) 199 03 Table 1 Properties of sol gel suspensions prepared to different concentrations, and thickness of coatings produced by dipping at 35 cmymin Concentration ph Viscosity Time of stability Conductivity Dip-coating (gyl) (MPa s) (h) (msycm) thickness 340 (109) ;4.0 300 (96) ;3.8 564 3.5 55 (8) ;3. 50 598.5 6 ;3. 840 6.5 1 (71) ;. 100 590 1.5 particle size 0 nm) in acid catalysis conditions by adding HNO3 up to ph. The total silica concentration was 340 gyl, with 109 gyl of colloidal particles, which is labelled as SiLev340(109). Improved adherence of the coatings was reached by diluting with ethanol to concentrations of 300(96) gyl, 55(8) gyl and 1(71) gyl, respectively. The ph of the 55(8) gyl suspension was modified by adding tetramethylammonium hydroxide (TMAH) up to ph 6. A reference hybrid sol without particles was also prepared by mixing the same precursors via acid catalysis with HNO3 up to ph. The rheological characterisation of the suspensions was done by measuring the evolution of viscosity with concentration, ph and time, using a rotational rheometer (Haake, RS550, Germany) at 5 8C. Dipping tests were performed at withdrawal rates ranging from 9 to 65 cmymin on glass substrates and stainless steel AISI 304 in order to evaluate the critical thickness. EPD tests were carried out at current densities from 0.1 to 3.8 maycm, and deposition times up to 60 min by using a potentiostateygalvanostate (AMEL 551, UK). Galvanostatic conditions were used to prepare the EPD coatings in order to maintain a constant electric field. Polished stainless steel AISI 304 was used as working electrode, and graphite as counter-electrode. All the coatings were sintered at 500 8C for 30 min with cooling and heating rates of 8 8Cymin. The thickness of the coatings obtained on glass substrates was determined by profilometry (Talystep, Taylor Hobson, UK), and that of coatings obtained onto stainless steel was measured by gravimetry and scanning electronic microscopy (SEM). SEM was also used to characterise the coatings (adherence, cracking, etc.). The protective behaviour against corrosion was studied in 0.6 N NaCl, by means of polarisation curves and polarisation resistance, using a potentiostate AMEL 56. Polarisation curves were determined applying a sweep rate of 166 mvyseg at 5 8C. Polarisation resistance was measured by changing the potential by 30 mv around the open circuit potential w10x. The experimental set-up for the electrochemical tests involved three electrodes: a reference of saturated calomel electrode (SCE), a platinum counterelectrode and the working electrode, which was the studied specimen. An area of 1 cm was masked with resistant adhesive tape for the test. Tests were performed without stirring and purging the cell. 3. Results and discussion 3.1. Rheology and stability of the suspensions The rheological behaviour of the SiLev suspensions with different concentrations and phs was studied by measuring the viscosity curves at 5 8C. Table 1 shows the viscosity values of the suspensions obtained by fitting the flow curves to the Newtonian model. The maximum time of the stability, the conductivity and the thickness of the dip-coatings obtained at a withdrawal rate of 35 cmymin are also shown. The viscosity of the concentrated suspension is 4 MPa s and decreases with dilution. The 55(8) and 1(71) gyl suspensions were selected to evaluate the time stability by periodically measuring their rheological behaviour. The 55(8) gyl suspension maintained stable for ;50 h. When diluting to 1(71) gyl the stability is preserved for ;100 h. The effect of ph was studied for the 55(8) gyl suspension, using strong base (TMAH) to shift the ph up to 6. The addition of TMAH does not modify the viscosity of the fresh suspension, but significantly improves its stability. The ph 6 suspension maintains stable for 840 h, more than one order of magnitude higher than the starting suspension. TMAH was added to move far away from the isoelectric point of the SiO suspension, modifying the surface charge of the particles. TMAH in solution dissociates and the hydroxyl ions neutralize protons, and the suspension conductivity decreases (Table 1). Colloidal studies show that SiO suspensions at ph 6 should have particles negatively charged. However, the surface charge maintains positive at ph 6 after TMAH addition. A possible explanation is that TMAH promotes the specific adsorption of tetramethyl ammonium ions q w(ch 3) 4N x on the particles surface, shifting up the isoelectric point w11x. 3.. Characterisation of the dip-coatings Coatings produced on steel using the concentrated suspension released during drying, indicated a poor

Y. Castro et al. / Surface and Coatings Technology 18 (004) 199 03 01 Fig. 1. Weight per unit area vs. current density for 5 min deposition time of the 55(8) gyl suspension at ph and 6, and 1(71) gyl suspension at ph. adherence, likely a consequence of the high concentration of water used in the synthesis. Thus, the suspension was diluted with ethanol to increase wettability and adherence. The thickness of the coatings obtained at 35 cmymin from the prepared suspensions is shown in Table 1. All the coatings produced at larger withdrawal rates than 35 cmymin present heterogeneities and defects. A maximum defect-free thickness of ;3.5 mm was obtained for the 300(96) gyl suspension. This thickness duplicates that obtained from the particle-free hybrid silica sols ( mm) w1x. However, the maximum attainable thickness decreases with dilution, while stability increases. Related to the addition of TMAH, it is worthy to note that coatings obtained by dipping from suspensions prepared at ph 6 had a similar thickness than those obtained from ph suspension (Table 1). Hence, the addition of TMAH is useful since it allows to maintain the homogeneity and thickness of the coatings, and largely increases the stability. and the decrease of the suspension conductivity, promoted by the addition of TMAH. A sharp deposit growth is also observed for current densities higher than 0.5 maycm. In all cases, deposition was limited by water electrolysis. EPD test performed at potentials higher than 1 V led to water decomposition. When H is adsorbed onto the surface of the cathode (stainless steel substrates), a heterogeneous deposition takes place. Consequently, it could be expected that homogeneous coatings were successfully obtained from the 55(8) gyl sols by applying lower current densities. Thus, the deposition kinetics of ph 6 and ph suspensions was studied at current densities of 0.35 maycm and 1.5 maycm, respectively, the maximum current densities to avoid H adsorption. The deposited mass (open symbols) and the coating thickness (full symbols) evolution related to the deposition time, for both ph conditions, is plotted in Fig.. The potential maintains constant during EPD. The deposition rates were estimated dividing the slope of both straight lines by the potential measured during the test. The modification of ph increases the deposition rate by two orders y5 y3 of magnitude, from 6.1=10 to 8.1=10 mgycm s V. The improvement of the particles dispersion and a more accurate control of the EPD process allows to obtain homogeneous and crack-free coatings at ph 6, using low current densities, and deposition times lower than 5 min. Coatings thicker than 5 mm peel out, so the deposit forming is limited by the drying process. However, this is roughly twice the maximum thickness of the 55 gyl sol at ph, (.8 mm). Fig. 3 shows a SEM microphotograph of the cross section of a glass-like silica coating of ;4 mm, obtained from a ph 6 suspension by EPD at 0.8 maycm for 3 3.3. Characterisation of EPD coatings EPD tests were performed applying current densities ranging from 0.4 to 3.6 maycm for a constant deposition time of 5 min. Fig. 1 shows the evolution of weight per unit area with the current density for sols 55(8) gyl and 1(71) gyl at ph, and 55(8) gyl atph6. Deposit growth is negligible for the 1(71) gyl suspension. For the 55(8) gyl suspension at ph, deposition slowly increases for small current densities, but there is an important increase for current densities higher than maycm. However, the deposition of the ph 6 (55(8) gyl) suspension is much faster, due to the increased electrophoretic mobility of the particles Fig.. EPD kinetics at current densities of 0.35 and 1.5 maycm for 55(8) gyl suspension at ph 6 and. Open symbols correspond to the weight per unit area evolution, and full symbols correspond to the thickness evolution.

0 Y. Castro et al. / Surface and Coatings Technology 18 (004) 199 03 Fig. 3. SEM microphotograph of a coating obtained by applying 0.8 maycm for 3 min with the 55(8) gyl suspension at ph 6, and sintered at 500 8C for 30 min. min. The film is uniform and appears strongly adhered to the stainless steel substrate. 3.4. Corrosion behaviour of coatings The protective behaviour of the coatings was studied through potentiodynamic tests. Fig. 4 shows the polarisation curves of the uncoated steel, a 1.8 mm coating obtained by dipping with a 55 (8) gyl suspension at ph, and a 3.3 mm coating produced by EPD, applying 0.35 maycm for 1.5 min, with a 55 (8) gyl suspension at ph 6. The uncoated 304 steel has a passive range of 0.69 V and shows a corrosion current density y7 of 1.8=10 Aycm. The corrosion resistance of the dip-coated steel is strongly improved when coatings are higher than 1.8 mm, showing a corrosion current density four orders of magnitude lower than uncoated steel, and a passive range of 0.88 V. Similar tests were performed with coatings obtained by dipping using the suspension with ph 6, which gave worse results. Possibly, the interaction of the structural network of the sol and the quaternary ammonium ions adsorbed on the particles surface produce defects in the coatings that promote an earlier contact between the substrate and the electrolytic medium during the electrochemical tests. In the case of EPD-coatings obtained from ph 6 suspension, the corrosion resistance is similar to that obtained for coatings produced by dipping from ph suspensions. The protective behaviour is improved when the coating is thicker than 3 mm. The current density y7 was reduced from 10 Aycm for the uncoated 304 y11 steel to 10 Aycm for the 3 mm EPD-coating, showing a passive range of 0.93 V. The variation of polarisation resistance (R p) with immersion time into the electrolyte (marine water, i.e. Fig. 4. Polarisation curves of uncoated AISI 304 and coatings with a thickness of 1.8 and 3.3 mm obtained by dipping and EPD using the 55(8) gyl suspension at ph and 6. 0.6 N NaCl) was studied for 1.8 mm and 3.3 mm coatings obtained from suspensions with ph and ph 6 by dipping and EPD, respectively (Fig. 5). The behaviour of the uncoated steel is also shown for comparison purposes. No experimental data of Rp could be registered for immersion times below 100 h, the values being higher than the detection limit of the equipment. After 100 h, the polarisation resistance starts to decrease and reaches the Rp of the uncoated steel after 550 h and 800 h for EPD-coatings and dip-coatings, respectively. The coatings produced by either EPD or dipping act as an effective barrier against corrosion in sea water for approximately 1000 h. It must be remarked that the EPD coatings produced by shifting the ph of the suspension up to a value of 6 maintain the good properties reached by dipping in ph suspensions, but the stability of the suspension is enhanced by 15 times, thus allowing a better constancy Fig. 5. Polarisation resistance with the immersion time in 0.6 N NaCl for uncoated steel, a 1.8 mm coating obtained by dipping with the 55(8) gyl suspension at ph, and a 3.3 mm coating obtained by EPD with the 55(8) gyl suspension at ph 6.

Y. Castro et al. / Surface and Coatings Technology 18 (004) 199 03 03 of properties and improved reliability, and opening the possibilities of industrial scaling. 4. Conclusions The dilution with ethanol of the as-prepared suspension improves the stability and adherence of the coatings to the stainless steel substrates. The addition of TMAH enhances the electrophoretic mobility of the particles, and does not change the sign of the surface charge, which suggests that TMAH specifically absorbs on the particles surface. TMAH decreases the suspension conductivity, which also increases the electrophoretic rate of the particles. Homogeneous and crack-free coatings are obtained by dipping for withdrawal rates F35 cmymin, with a maximum coating thickness of 3.5 mm obtained for the 300(96) gyl suspension. However, for this concentration the suspension maintains stable for storage times lower than 50 h. Homogeneous, crack-free and thicker coatings have been obtained by EPD from the 55(8) gyl suspension at ph 6. The attainable coating thickness is limited by the peel out of the coating during drying. Coatings with 5-mm thickness have been obtained applying 0.35 may cm for 4 min. Coatings G mm obtained by dipping from the 55(8) gyl suspension at ph behave as excellent barrier against corrosion. Coatings obtained by EPD from the 55(8) gyl suspension at ph 6 also show a good behaviour against corrosion, but thicker coatings (approx. 3 mm) are required to achieve the best results obtained from dipping. The addition of TMAH up to ph 6 increases the stability against time by more than 15 times, while maintaining the same viscosity and ensuring the same properties when coatings are produced by EPD. The possibility of storing the sol gel suspensions for 1 month at room conditions makes the EPD an attractive route for scaling-up to the industry. Acknowledgments This work has been supported by projects MAT000-095-C0-01 (CICYT, Spain) and EC BRITE Programme (BE97-5111) in collaboration with INM (Germany), Miele (Germany), Corus (The Netherlands), Ferro (France) and ABB (France). Dr Mennig and Dr Niegisch (INM) are gratefully acknowledged for sol processing. Dr Ferrari acknowledges CSIC and European Social Fund for their support. References w1x M. Guglielmi, J. Sol Gel Sci. Technol. 8 (1997) 443 449. wx O. de Sanctis, L. Gomez, N. Pellegri, A. Duran, Surf. Coat. Technol. 70 (1995) 51 55. w3x P. Galliano, J.J. Damborenea, M.J. Pascual, A. Duran, J. Sol Gel Sci. Technol. 13 (1998) 73 77. w4x M. Menning, C. Schelle, A. Duran, J.J. Damborenea, M. Guglielmi, G. Brusatin, J. Sol Gel Sci. Technol. 13 (1998) 717 7. w5x M. Menning, G. Jonschker, H. Schmidt, SPIE Sol Gel Opt. 1758 (II) (199) 15 134. w6x H. Schmidt, G. Jonschker, S. Goedike, M. Menning, J. Sol Gel Sci. Technol. 19 (000) 39 51. w7x J. Gallardo, P. Galliano, R. Moreno, A. Duran, J. Sol Gel Sci. Technol. 19 (000) 107 111. w8x M. Guglielmi, A. Liculli, S. Mazzarelli, Ceram. Acta and 3 (1994) 19 5. w9x Y. Castro, B. Ferrari, R. Moreno, A. Duran, Adv. Mater. 14 (00) 1505 1508. w10x ASTM G59-91, pp. 16 19. w11x W.R. Canon, R. Becker, K.R. Mikeska, Advances in Ceramics: Ceramic Substrates and Packages for Electronic Applications, 6, American Ceramic Society, Westerville, OH, 1989, pp. 55 541. w1x P. Innocenzi, M.O. Abdirashid, M. Guglielmi, J. Sol Gel Sci. Technol. 3 (1994) 47 55.