Preparation of sol-gel SiO2 coatings on steel and their corrosion resistance

Transcription

1 Preprtion of sol-gel SiO2 cotings on steel nd their corrosion resistnce Irin Stmolov 1, Stncho Yordnov 2*, Lyuen Lkov 2, Blskov 1, nd Bojidr Jivov 2 Ssho Vssilev 3, Vldimir 1 Institute of Generl nd Inorgnic Chemistry Bulgrin Acdemy of Sciences, Acd. G. Bonchev St., l.11, 1113 Sofi, Bulgri 2 Institute of Metl Science, Equipment nd Technologies with Hydro- nd Aerodynmics Centre Acd. A. Blevski, Bulgrin Acdemy of Sciences, Shipchenski Prohod Blvd. 67, 1574 Sofi, Bulgri 3 Institute of Electrochemistry nd Energy Systems, Bulgrin Acdemy of Sciences, Acd. G. Bonchev St., l. 10, 1113 Sofi, Bulgri Astrct. A protective SiO2 coting ws prepred y sol-gel method on stinless steel pltes, using precursor solutions hving 2 different concentrtions: low (0.5M) nd high (2M) concentrtion. The dip cotingdrying cycles hve een repeted three times, fter tht the cotings were thermlly treted t 300 o C or 500 o C in ir. Scnning electron microscopy (SEM), AFM nd X-ry diffrction nlyses hve een pplied. The corrosion resistnces of SiO2 coted steels were exmined in NCl medium for 346 hours. Mny microcrcks nd crters in the smples, prepred from solution of high concentrtion pper fter corrosion ttck. The cotings otined from solution of low concentrtions revels lmost unchnged surfce structure without visile crcks nd pits. The evluted corrosion rte of this coting is lower thn those of uncoted steel. 1 Introduction The corrosion control of metls is of economicl, technicl nd environmentl importnce. Corrosion processes re ccompnied y numer of rections tht chnge the composition nd properties of the metl surfce nd cn e reduced with the selection of pproprite mterils for prticulr ppliction, y using new lloys, corrosion inhiitors or protective films nd cotings deposited onto the metl surfce, especilly in ggressive environments. Oxide films nd cotings such s SiO 2, ZrO 2, Al 2O 3 nd TiO 2 cn e deposited on metls to improve their surfce nd nticorrosion properties [1-6]. There re different methods for the otining of thin films nd cotings on metl surfce, such s electrochemicl deposition [7], physicl vpor deposition (PVD) [8], chemicl vpor deposition (CVD) [9], plsm sprying [10] nd sol gel process [1, 5, 11-16]. The dvntges of sol gel technique re: low processing temperture, good homogeneity, use of compounds tht do not introduce impurities into the end product s initil sustnces. Cost of precursor solutions hs limited *Corresponding uthor: stncho14@v.g The Authors, pulished y EDP Sciences. This is n open ccess rticle distriuted under the terms of the Cretive Commons Attriution License 4.0 (

2 the use of sol-gel processing in mny pplictions [17]. The sol gel method is wetchemicl technique nd involving severl stges: hydrolysis nd polycondenstion, geltion, ging, drying, densifiction nd crystlliztion. The sol gel technology is sed on hydrolysis nd condenstion rections [18, 19]. Typiclly the ddition of ctlysts increses the hydrolysis rte. The type of ctlyst determines the ph of sols nd ffects the form of sol-gel mterils, otining the films, powders or monoliths. The ph vlue of the rection mixture, which is dependent on the used ctlyst, influences the hydrolysis nd condenstion rection nd ffects the form nd structure of the received mterils. One of the most technologiclly importnt spects of sol-gel processing is the ility to produce cotings with sintering tempertures lower thn 500 C. Depending on het tretment conditions, specilly the finl tretment temperture, the structure of sol-gel cotings my undergo chnges. A sol gel coting cn e pplied to metl sustrte through two techniques: dip-coting or spin-coting [11, 12]. The proper selection of synthesis prmeters nd the wy of prepring mterils is significnt in successful sol gel synthesis. These dvntges mke the sol gel process one of the most pproprite technologies for preprtion of thin, nnostructured films. The sol gel process gives possiilities to otin cotings with unique diffusion, rsion, dielectric etc. properties [17]. The preprtion of coting y sol-gel method is n estlished method to produce homogeneous cotings with uniform thickness elow 2μm [20]. Sol gel technology hs een significntly improved in the pst 20 yers. The sol gel cotings re widely investigted in the re of optics [21], nnoiocomposites [22], iomterils [23], self-clening mterils [24] nd others [25]. Different sol-gel cotings hve een developed to increse the corrosion resistnce of metls, lthough the pure inorgnic films hve limittions such s micro crcks, residul porosity nd thickness limittions [26, 27]. Cotings with good pssivity, insulting properties nd good triologicl properties show high corrosion resistnce in ggressive eenvironment. Some ppers [28-31] showed the fesiility of otining homogeneous nd free of crcks sol-gel cotings on steel sustrtes. Other works [32] hve demonstrted increses in the corrosion resistnce y using zirconi or silic cotings. The im of this work is the otining of protective SiO 2 lyers y sol-gel method (dip coting technique) on stinless steel pltes nd investigtion their surfce morphology, phse composition nd chemicl stility in slty solution of NCl. 2 Experimentl prt The preprtion procedure of SiO 2 films y the sol gel method ws s follows: tetrethoxysilne (TEOS) wss dissolved into ethnol nd stirred for 1 h. The mixture of ethnol, wter nd hydrochloric cid ws dded to the TEOS solution nd lso followed y stirring for 2 hours in order to otin 0.5M nd 2 M solutions. The mole rtio H 2O:TEOS ws 3.7. The smple otined from 0.5 M solution were denoted A, while the smple deposited from 2M solution were denoted B. The stinless pltes AISI 316 were used s sustrtes. The sustrtes were clened ultrsoniclly in ethnol nd cetone. The films were immersed in the corresponding solution nd withdrwn t constnt rte of 3 cm/min nd then were dried consequently t 60 C nd t 90 o C. The smples A nd B wre heted t 300 o C. These steps were repeted 3 times. In order to study the effect of therml tretment temperture on the protection properties, nother smple denoted C produced from 2M solution ws treted t 500 o C for 1h. The phse composition of the smples ws studied y X-ry diffrction (XRD) with CuKα-rdition (Philips PW 1050 pprtus). The verge crystllite sizes of the films were estimted ccording to Scherrer s eqution: The morphology of the cotings ws investigted y mens.of Scnning electron microscope PHILIPS SEM 515, (Belgium), operting t 20kV voltge. In order to study their chemicl 2

3 resistnce the investigted smples s well s uncoted steel (reference smple) were exposed to the ction of slty solution of 3.5% NCl t 25 C (EN ISO10289/2006) in the course of 346 hours.the temperture of solution nd the ir temperture were controlled y clirted thermometers. It ws determined the mss weight loss fter 256 nd 346 hour. 3 Results nd discussion XRD nlyses reveled tht ll smples re morphous,since no sygnls of crystllized SiO 2 were oserved. Severl reserch groups were lso otined morphous silic cotings thermlly treted t 500 o C nd 600 o C [33,34] The SEM photogrphs of the sustrte nd silic coted steel efore nd fter corrosion test e shown in Figures 1-3. Fig.1. SEM photogrphs of re steel sustrte ) efore nd ) fter corrosion test. Fig.2. SEM photogrphs of smple A: ) efore nd ) fter corrosion test. Fig.3. SEM photogrphs of smple C: ) efore nd ) fter corrosion test. 3

4 Figure 1 revels the SEM imges of steel sustrte re sustrte nd fter corrosion test. The films structure hs een undergoing considerle chnges during the corrosion test. Some crcks re clerly visile on the microphotogrphs. The silic films surfce, prepred from 0.5 M TEOS solution is reltively dense with some crystl imperfections (Fig 2-). Smple A preserves the surfce structure without visile crcks nd pores fter corrosive ttck. (Fig. 2-) TEOS solution of high concentrtion leds to more flt surfce of the cotings fter tretment t 300 nd 500 o C. After corrosion test there re mny deep crcks in oth smples B nd C (Fig.3). Smple C lso revels significnt errosion of the surfce. Figure 4 represents the surfce of SiO 2 cotings fter corrosion test. On the surfce structure of the smple A no visile crcks nd pits re to e oserved fter corrosive test, ut hs some pitting effects (Fig 4-). The roughness nlysis gives the vlue R out 15 nm nd Rq 24 nm for smple A fter ttck. The corrosive ttck induces erosion of the surfce nd mny crcks in the films, otined from 2M TEOS solution. The film surfce displys significnt effect of corrosion. The roughness nlysis of smple B gives the vlue R 20 nm nd Rq 31 nm. There is ppernce of microcrcks nd crters. (Fig 4-). The erosion of the cotings ecomes stronger fter tretment t 500 o C, leding to eruption of some res of smple C. Durn et l. hve proved tht the SiO 2 cotings, deposited on stinless steel nd glvnized steel treted t 550 o C were more crcked ecuse of the elimintion of orgnic groups during the tretment reduces the plsticity of the cotings [33]. Fig. 4. AFM imges (3dimensionl) of smples A () nd C () Acccording to DTA-TG nlysis of the film A (Fig 5) exotherml nd endotherml peks re not registered. Fig. 5. DTA-TG nlysis of smple A. 4

5 The weight losses of the nd the uncoted steel nd SiO 2 cotings (smple A) fter 250 in the NCl solution re 1.81 nd 1.06 mg/cm 2, respectively. No weight chnges were oserved fter 346 hours. As cn e seen from Fig. 6 the weight loss increses up to 250 hours of the test, fter tht remins constnt. It cn e seen tht the smple A exhiit the etter protection properties in comprison to the uncoted steel. 4 C Wieght loss [mg/cm 2 ] B uncoted steel A Test durtion [h] Fig. 6. Weight loss of the SiO2 sol-gel cotings vs time. The corrosion rte is sed on the numer of illimeters (thousndths of n inch) it penetrtes ech yer. The corrosion rte ws evluted, ccording to the Stndrd - ASTMG31. The corrosion rte of uncoted steel is 0.058mm/yer, while for smples A, B nd C re 0.035; nd 0.133mm/yer, respectively. The otined results reveled tht the SiO 2 cotings, prepred from 0.5 M solution of TEOS exhiit the est protection properties. The formtion of crcks is dependent on the internl stress in the cotings [18]. This effect is result of the evportion of lrge mount of solvents nd orgnic residuls. Proly, in the smples produced from TEOS solution of high concentrtion (B nd C) during the the therml tretment the formtion of crcks nd pits re induced y the evportion of the lrger mount of intermedites (produced y hydrolysis). As results the mechnicl properties (hrdness, plsticity etc) nd the surfce chrcteristics re deteriorted. This could e reson for decresed nti-corrosion properties. The ppliction of TEOS solution of low concentrtion seemed more siutle for production iof SiO 2 cotings with enhnced corrosion resistnce, due to their surfce nd mechnicl chrcteristics. 4 Conclusions Silic sol gel cotings, otined from two different sol-gel solutions of low nd high concentrtions of TEOS nd treted t 300 o C nd 500 o C possess reltively dense surfce. The protection efficiency of silic cotings decreses with the incresing of TEOS solution concentrtion. The cotings, deposited from 2M solution showed severl microcrcks nd some erosion fter immersing in corrosive medium regrdless the tretment temperture. After corrosion ttck, the smple, prepred from 0.5 M TEOS solution treted t 300 o C revels lmost unchnged surfce structure. The corrosion rte of this coting is lower thn those of uncoted steel. 5

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