Effect of nanosilica type on protective properties of composite ceramic coatings deposited on steel 316L by sol–gel technique

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Journal of Non-Crystalline Solids 354 (2008) 1786–1795 www.elsevier.com/locate/jnoncrysol

Effect of nanosilica type on protective properties of composite ceramic coatings deposited on steel 316L by sol–gel technique Jacek Grzegorz Che˛cmanowski, Bogdan Szczygieł

*

Wrocław University of Technology, Faculty of Chemistry, ul. Wybrze_ze Wyspian´skiego 27, 50-370 Wrocław, Poland Received 3 November 2006 Available online 1 October 2007

Abstract Trilayer SiO2 coatings were obtained from sol modified with nanosilica differing in its specific surface and hydrophilic and hydrophobic properties. Each of the successively deposited coating layers was sintered at ever lower temperatures: 300, 250 and 200 C. Examinations under a JSM 5800LV Joel scanning electron microscope showed that each SiO2 coating includes a smooth, uncracked, thin layer covering the entire steel 316L basis and an outer layer made up of grains forming clusters of different size and density depending on the nanosilica used. The corrosion resistance of the coatings in Ringer’s solution was evaluated on the basis of polarization studies. A comparison of the passive region widths, the passive region current intensities, the cathode current densities at potential ESCE = 750 mV, the anode region potentials corresponding to a current density of 2 lA/cm2, the through-coating porosity and the polarization resistance values shows the coatings obtained from sol modified with hydrophilic nanopowder to be superior. As regards the tested hydrophobic powders, additive R972 (SBET = 110 m2/g) ensures good protective properties. If hydrophobic silica with larger specific surface (R974,  R812, SBET with respectively 170 and 260 m2/g) is used, the deposited coatings only slightly improve the protecting properties of steel 316L. The deposited coatings are uncracked. Even at their highest compactness, the surface grains and grain clusters, do not form a tight outer shell as evidenced by, for example, the poor protective properties of the coatings produced from sol modified by hydrophobic silica  R812. The latter coatings’ protective properties are the poorest at the highest grain compactness. If hydrophobic nanosilica with small specific surface (130 and 150) is used the resulting layers have low grain compactness and good protective properties (probably smooth thicker layers directly coat the steel basis).  2007 Elsevier B.V. All rights reserved. PACS: 81.20.Fw; 82.45.Bb; 81.65.Kn; 81.15.z Keywords: Corrosion; Scanning electron microscopy; Powders; Silica; Sol–gel, aerogel and solution chemistry

1. Introduction Surgical implants are usually made from three kinds of materials: austenitic stainless steels, cobalt–chromium alloys and titanium and its alloys. The main requirements which the materials must meet include: high mechanical strength, high corrosion resistance in body fluids and biocompatibility of the materials and the corrosion products. The use of titanium and its alloys is limited by metalosis *

Corresponding author. Tel.: + 48 71 320 33 98; fax: +48 71 328 04 25. E-mail addresses: [email protected] (J.G. Che˛cmanowski), [email protected] (B. Szczygieł). 0022-3093/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.08.056

(i.e. the passing of metal ions into the biological environment [1–3]) and fretting corrosion [4]. The products of corrosion of such metals as: nickel, cobalt and chromium can accumulate in the body and act as carcinogens [5]. This process can be reduced by modifying the surface of austenitic steels, e.g. with ceramic coatings (SiO2, TiO2 or TiN). The material obtained in this way shows better biotolerance [6]. Its use becomes economically justified against titanium and its alloys. In the literature on the subject one can find reports from researches aimed at increasing the resistance of metal elements to electrochemical and high temperature corrosion by the sol–gel method employing coatings containing single

J.G. Che˛cmanowski, B. Szczygieł / Journal of Non-Crystalline Solids 354 (2008) 1786–1795

oxides as well as double or mixed oxides: SiO2 [7], Al2O3 [8], ZrO2 [9], CeO2 [10] as well as double or mixed oxides: TiO2–SiO2 [11], Al2O3–SiO2 [12], ZrO2–CeO2 [13], CeO2– TiO2 [14,15] have been studied. The coatings’ protective properties have been evaluated on the basis of mainly electrochemical polarization measurements [10,11,14,15]. It has been shown that thanks to their barrier action the deposited coatings protect stainless steels against acids and chloride solutions. In highly corrosive media, thick coatings are able to provide good protection. But thicker than 0.3–1 lm inorganic coatings produced using the sol–gel technique are highly susceptible to cracking [16]. The degree of cracking depends on the kind and composition of coating and the heat treatment procedure. Cracks may appear as a result of changes taking place in the basis itself (e.g. the release of chromium carbide) or due to a large difference between the thermal expansion coefficient of the basis and that of the coating. Coatings obtained from gels subjected to the action of ultrasounds and from aged gels are less susceptible to cracking [17]. Cracks in coatings being in service in aggressive water media are particularly harmful. Cracks which appear during high temperature oxidation are less dangerous [16]. In order to obtain coatings without cracks the multiple-application technique (multilayer coatings) is used [18] or gel shrinkage reducing materials are added to the gel [16,19]. The undesirable feature of coatings produced by the sol– gel technique is their brittleness and lack of elasticity: under plastic deformation they crack easily and delaminate. Coatings produced from organically modified ceramics [20]: ormosils (organically modified silicates), ormocers (organically modified ceramics) or ceramers (ceramic polymers) have no such drawbacks (brittleness and lack of elasticity). The above materials are referred to as inorganic–organic composites [21]. The obtained layers are much thicker (without a danger of cracking) than inorganic coatings produced by the sol–gel technique, but unfortunately they can be in service only up to 300 C. The properties of ceramic coatings produced by the sol– gel technique depend on many factors. One of them is sol gelation time: the longer it is, the quicker a multilayer coating can be obtained [22]. If sol viscosity changes slowly with time, then the sol–gel transition reactions also must be slow. In such conditions the successive layers of multilayer coatings are deposited from sols whose properties are similar. Generally, there are several methods proposed for improving the dispersion of nanoparticles into the corresponding organic matrix, e.g. mechanical mixing, sol–gel process, in situ synthesis, ultrasonic dispersion, surface modification of nanoparticles [23].

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When preparing sols, ultrasounds are used to mix the constituents. Homogenization by ultrasounds favorably affects the hydrolysis of the alcoholates used as the oxide ceramics precursors (e.g. tetraethoxysilan – TEOS) [24,25]. Gels obtained in this way are called sonogels [25]. Their density is almost twice higher and their specific surface is by 50% larger than that of gels produced using standard mixing. Sonogels have smaller size pores and the lattice structure of SiO2 sonogels is different from that of gels obtained through typical homogenization [25]. Using ultrasounds one may obtain nanostructures [26] as well as crack-free coatings or coatings with only a few cracks [27]. The higher the sol’s viscosity, the greater the roughness of the coating and the lower its tightness [18]. If the viscosity increases as a result of the progressing hydrolysis and polycondensation, the defects arising in the layer can be caused by the formation of a lattice structure in the coating solution. If the layer peels off the steel basis, this is probably due to the fact that the adhesion forces are weaker than the cohesion forces in the gelling film [18]. At a low viscosity one can obtain up to 0.4 lm thick crack-free coatings [7]. 2. Experimental 2.1. Test methodology This paper presents the results of research on the effect of the type of SiO2 nanopowder – mainly its water wettability, specific surface (BET) and grain size – on the protective properties of ceramic SiO2 coatings produced by the sol–gel technique. Nanopowder made by Degussa was used. The coatings were deposited on low-alloy stainless steel 316L [28] whose composition is given in Table 1. The steel shows good corrosion resistance in steam and in weak organic acids [29] but it undergoes pitting corrosion in chloride containing solutions. It has been shown that as a result of their barrier action the coatings protect stainless steels against acids and chloride solutions (e.g. physiological solutions). It is recommended to use steel 316L in an environment containing chloride ions. The steel is also resistant to elevated temperatures. Steel 316L specimens were in the form of cylinders 14.8 mm in diameter and 1.0 mm thick. Before depositing the coatings the steel’s surface was ground with abrasive paper grade 400, 600 and 800, washed with distilled water and dried and degreased in an ultrasonic washer in two stages: for 45 min in tetrachloroethane (C2Cl4) and for 15 min in anhydrous ethyl alcohol.

Table 1 Chemical composition of steel 316L (wt%) Component

C

Cr

Ni

Mo

Mn

Si

Cu

V

S

P

Fe

Content (wt%)

0.03

17.28

14.80

2.8

1.96

0.19

0.07

0.035

0.03

0.024

Remainder

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Trilayer coatings were deposited from sol modified with silica in the form of nanopowder. The immersion technique was used. The sol was homogenized and then the steel specimens were immersed in it for 2 min to stabilize the equilibrium at the steel basis-coating solution interface. The rate emersion was set at 10 mm/min. The coated specimens were dried in the open air for 24 h and placed in a cold oven. The oven heated up at a rate of 2 deg./min. Each successive layer was sintered for 1 h at ever lower temperatures: 300, 250 and 200 C. The topography of the coatings produced by the sol–gel method was imaged using scanning electron microscopy. Photographs of the specimens of the coated steel under different magnifications were taken by a scanning electron microscope (SEM) JSM 5800LV made by Joel. The microscope was equipped with an X-ray microanalysis system JSIS 300 Oxford (EDS). Corrosion resistance tests were conducted in Ringer’s solution (NaCl 8.60 g/dm3, KCl 0.30 g/dm3, CaCl2 0.48 g/dm3). Electrochemical DC measurements consisted in registering polarization curves in a conventional threeelectrode system. The measuring setup consisted of a measuring vessel, a Schlumberger SI 1286 potentiostat and a PC. Prior to measurement the specimen was kept in Ringer’s solution for 10 min. Then it was polarized, starting from potential ESCE = 1000 mV, in the anode direction at a rate of 1 mV/s. 2.2. Sol preparation TEOS ((C2H5O)4Si) was used as the precursor for producing SiO2 coatings. The chemical composition of the sol is shown in Table 2. The tetraethoxyliane was diluted in anhydrous ethyl alcohol (at a constant 1:4 TEOS:ethanol ratio) and homogenized in the ultrasonic washer for 75 min. Then the other components: nitric acid and acetic acid were added to the solution. After each component was introduced, the solution was stirred by ultrasounds for 25 min. The precursor on the surface of steel 316L was hydrolyzed as a result of the reaction with water (moisture) in the air [30]. The sol’s viscosity remained stable for a few months. At a sol viscosity of 2–3 cP thin layers (with a thickness of 100–300 nm) were obtained [31–33]. The coating solution was modified with ceramic powder by adding 6.5 mg of silica per each 15.0 mL of the sol. The properties of the Degussa powders used are given in Table 3. Symbol ‘R’ designates ceramic powder with hydrophobic properties. No ‘R’ in the description means that the silica is hydrophilic.

Table 2 Chemical composition of prepared sol Component

TEOS

C2H5OH

H2O

HNO3(63%)

CH3COOH

Content (wt%)

52.940

46.822

–

0.100

0.138

Table 3 Characteristics of SiO2 nanopowder added to sol Specimen symbol

Specific surface (BET) (m2/g)

Bulk density, (g/dm3)

Powder diameter (nm)

AEROSIL130 AEROSIL150 AEROSIL300 AEROSIL380 AEROSILR972 AEROSILR974 AEROSILR812

130 ± 25 150 ± 15 300 ± 30 380 ± 30 110 ± 20 170 ± 20 260 ± 30

30 30 30 30 – – –

19 14 7 7 16 12 –

3. Results 3.1. Microscopy The topography of the SiO2 coatings deposited using the sol–gel technique was determined through microscopic examinations. The steel 316L specimens were photographed under different SEM (Joel JSM 5800LV) magnifications. In order to ensure good electrical conductivity, the specimens were coated (in a vacuum duster) with conducting carbon (from graphite electrodes) and gold. The trilayer SiO2 coatings deposited on steel 316L and sintered at successive temperatures: 300, 250 and 200 C are made up largely of spherical grains of SiO2 (Fig. 1(a)). Examinations under a magnification of 50,000· reveal that the grains (up to 1 lm in diameter) are clusters of smaller silica particles with distinct grain boundaries (Fig. 1(b)). The SiO2 coatings, in places where grains occurred and between the grains, were analyzed qualitatively and quantitatively through EDS microanalyses. Similar analyses were carried out for the uncoated steel 316L specimen. The results (the averages for five specimens) are shown in Table 4. Large amounts of Cr, Fe, Ni and Mn occurred in the intergrain areas where also more silica than in uncoated steel 316L was found, indicating a ceramic coating. The analysis of grains on the specimen’s surface showed fewer steel components, but more silica. Also a comparison of the topography of the steel specimens to be coated and that of the coated specimens showed that the entire surface of the steel basis was covered with a ceramic coating. As a result of the grinding and polishing, the steel specimen’s basis is rough and scratches (Fig. 2). There are no scratches after a coating is deposited by the sol–gel technique (e.g. Fig. 1). Figs. 3 and 4 show the topography of SiO2 coatings produced from sols containing different types of hydrophilic and hydrophobic nanosilica. The addition of hydrophilic nanosilica with a relatively small specific surface: 130 m2/g (Fig. 3(a) and (b)) and 150 m2/g (Fig. 3(c) and (d)) results in coatings characterized by a very similar topography. In both cases, clusters of various size SiO2 grains irregularly spherical in shape are visible. If finer nanosilica (AEROSIL 300; SBET = 300 m2/g) is used, the SiO2 clusters observed under the microscope are smaller, but they are more numerous and cover the

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Fig. 2. Topography of steel 316L prepared for depositing SiO2 coating (magnification 10,000·).

face (SBET = 170 m2/g) (Fig. 4(c) and (d)), in which case the grains have a spheroidal shape. High density of small size grains and the presence of clusters made up of spheroidal silica are observed when AEROSILR812 (SBET = 260 m2/g) is the filler (Fig. 4(e) and (f)). 3.2. Electrochemical measurements

Fig. 1. Topography of trilayer SiO2 (without nanosilica) coating on steel 316L under microscopic magnification of: (a) 5000· and (b) 50,000·.

Table 4 Chemical composition of SiO2 coating on steel 316L (Fig. 1) Element

Si Cr Mn Fe Ni Au Mo

Element content (wt%) Grains

Intergrain area

Steel 316L

5.17 8.31 0.71 26.42 4.93 54.46 –

3.28 11.78 1.08 37.05 6.97 39.54 –

0.57 18.18 1.50 66.37 11.58 – 1.81

entire area of the specimen (Fig. 3(e) and (f)). If powder AEROSIL 380 (380 m2/g) is used, small size SiO2 grains are densely distributed, but clusters larger than 2 lm also occur (Fig. 3(g) and (h)). When hydrophobic nanosilica is added to the sol, the surface topography is somewhat different than when hydrophilic nanosilica was the additive. It is becomes apparent that no large (exceeding 1 lm) clusters of SiO2 grains occur on the surface. For powder R972 (SBET = 110 m2/g) the grains and the clusters have irregular shapes (Fig. 4(a) and (b)) in contrast to the coatings obtained from the sol with nanopowder R974 having much larger specific sur-

The polarization curves for steel 316L covered with trilayer ceramic SiO2 coatings modified with hydrophilic powder with different specific surface (BET) are shown in Fig. 5. Ringer’s solution was the measurement medium. From the polarization curves iE = 750 mV (the cathode current density at randomly selected potential ESCE = 750 mV) and Ei = 2 lA/cm2 (the anode region potential corresponding to a current density of 2 lA/cm2) were determined [34]. Since potential Ei = 2 lA/cm2 is located in the region of a rapid increase in current, it was assumed, in accordance with other authors’ interpretation [35–38], that it is the corrosion pits nucleation potential. The differences in the shape of the cathode curves for the steel covered with the different coatings are slight. But in a wide range of potentials the polarization curves for coated steel 316L show much lower cathode current values in comparison with uncoated steel 316L. Similar observations can be made for the anode branch of the polarization curve. The presence of a coating markedly reduces the current density corresponding to a given potential. The type of hydrophilic SiO2 nanopowder in the prepared sol has only a slight influence on the shape of the anode curve. Pit nucleation potentials Ei = 2 lA/cm2 for coated steel 316L are higher than potentials Ei = 2 lA/cm2 for the uncoated steel. The highest Ei = 2 lA/cm2 occurs for the steel specimen with a coating deposited from sol containing AEROSIL130. This value diminishes in the following order: AEROSIL150, 300 and 380, i.e. with increasing powder specific surface (and with decreasing powder grading). The cathode–anode transition potentials (EK  A) of the coated specimens are by about 250 mV more electronegative than for uncoated steel 316L. The passive region’s

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Fig. 3. Surface topography of trilayer SiO2 coatings on steel 316L, deposited from sol containing hydrophilic nanosilica: (a, b) AEROSIL130; (c, d) AEROSIL150; (e, f) AEROSIL300 and (g, h) AEROSIL380.

width depends on the specific surface of the SiO2 powder in the coating. It exceeds 1500 mV for nanopowder AERO-

SIL130 which has the smallest specific surface. The larger the specific surface, the smaller the passive region.

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Fig. 4. Surface topography of trilayer SiO2 coatings on steel 316L, deposited from sol containing hydrophobic nanosilica: (a, b) AEROSILR972; (c, d) AEROSIL R974 and (e, f) AEROSIL R812.

Fig. 6 shows the polarization curves for steel 316L with trilayer coatings deposited from sol containing hydrophobic SiO2 powder (R972, R974 and R812). In this case, the lowest current densities in the whole range of potentials (and so the highest polarization), both in the cathode and anode region, occur for steel 316L covered with a coating containing nanosilica R972 with the smallest specific surface. Potential EK  A of the specimen with nanopowder  R972 was the lowest while that of specimens with  R974 and R812 was higher than the cathode–anode transition potential of uncoated steel 316L. Similarly as for the coatings with hydrophilic powder, the sol modifying nanopowder-passive region width relationship is also distinct for hydrophobic powder. The pas-

sive region for powder R972 is the widest (about 1100 mV). For powders with larger specific surface (170 and 260 m2/g) the polarization of the electrode processes and the passive region width decrease. The effect of the type of powder on the protective properties of the deposited coatings can be assessed by comparing the polarization curves determined in Ringer’s solution for steel 316L with trilayer coatings deposited from sol modified with hydrophilic or hydrophobic nanosilica (Fig. 7). For the coatings containing hydrophilic powder the cathode–anode transition potentials are lower than those (EK  A) for the specimens with hydrophobic powder. The polarization of the electrode processes is similar in both types of coatings. As regards the passive region

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10

2

10

1

10

0

10

-1

10

-2

10

-3

10

-4

i , μA/cm2

i , μA/cm2

1792

316L R

130

R

150

R R

-1000

-500

0

500

1000

10

2

10

1

10

0

10

-1

10

-2

316L

300 380

10 1500

-3

-1000

-500

0

ESCE , mV

R

130

R

380

R

R972

500

1000

1500

ESCE , mV

Fig. 5. Polarization curves determined in Ringer’s solution for steel 316L with trilayer SiO2 coatings deposited from sol modified with hydrophilic nanopowders differing in their specific surface.

Fig. 7. Polarization curves determined in Ringer’s solution for steel 316L with trilayer SiO2 coatings deposited from sol modified with hydrophilic or hydrophobic nanopowders.

2

R

10

130

1250 1

Ei = 2μA/cm2 , mV

i , μA/cm2

10

0

10

-1

10

316L -2

10

-3

R

R972

R

R974

R

R812

1000

R

R972

R

150

750 R

300

R

R

380

R974

500 R

R812

10

-1000

-500

0

500

250

1500

ESCE , mV

0.1

Fig. 6. Polarization curves determined in Ringer’s solution for steel 316L with trilayer SiO2 coatings deposited from sol modified with hydrophobic nanopowders differing in their specific surface.

width, the coatings with hydrophobic powder show superiority. The values of iE = 750 mV and Ei = 2 lA/cm2 read off the polarization curves are shown in Fig. 8. Coatings with low current intensity iE = 750 mV and high potential Ei = 2 lm/cm2, i.e. ceramic coatings with hydrophilic nanosilica, have the best protective properties. The coating with powder AEROSIL130 seems to be the best. Also the polarization resistance (Rp) and through-coating porosity (P) were determined from the polarization curves. The results are shown in Fig. 9. Through-coating porosity was defined as a ratio of the cathode current density for the coated specimens (iE = 750 mV) to the current density for the uncoated specimens (i 0 E = 750 mV) at potential ESCE = 750 mV [39–42] 

P ¼ ðiE¼750mV =i0E¼750mV Þ 100%:

316L

ð1Þ

The through-coating porosity represents the relative intensity of cracks and other kinds of untightness extending

1

10

iE = - 750 mV , μA/cm2 Fig. 8. Comparison of electrochemical parameters read off polarization curves determined in Ringer’s solution for ceramic coatings containing SiO2 nanopowder.

down to the basis and it is, in a way, a measure of the coating’s porosity. An analysis of the results shows that the through-coating porosity of all the coatings containing hydrophilic nanosilica is very low. Coating permeability sharply rises (from 2% to 70%) when hydrophobic nanosilica is used. It is the higher, the larger the powder’s specific surface. The results correspond to a large extent to the polarization resistance magnitude. The uncoated specimen made of steel 316L shows the lowest Rp in Ringer’s solution. 4. Discussion An EDS analysis of steel 316L with an unmodified SiO2 coating showed a larger amount of silicon both within and outside the grain in comparison with uncoated steel 316L, which is indicative of thin ceramic coatings. The precursor on the surface of steel 316L was hydrolyzed as a result of

J.G. Che˛cmanowski, B. Szczygieł / Journal of Non-Crystalline Solids 354 (2008) 1786–1795

Through-coating porosity P , %

60

40

20

0 130

150

300

380

R972

R974

R812

Sample

600

Rp , kohmcm2

500

400

300

200

100

0 316L

130

150

300

380

R972 R974 R812

Sample Fig. 9. Comparison of through-coating porosity (P; (a)) and polarization resistance (Rp; (b)) for steel 316L with coatings deposited from sol containing hydrophilic (AEROSIL: 130, 150, 300, 380) and hydrophobic nanosilica (R972, R974, R812).

the reaction with water (moisture) in the air [30]. The interface involving silica surface plays on important role in the adsorption process, and the surface characteristics of the adsorbate determine the nature of bonding between adsorbate and adsorbent [43]. Zhang and Grischkowsky [44] found that adsorption of water on hydrophilic silica was found the adsorbed water remains in submonolayer form.

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Bandosz [45] suggested that the complex nature of silica surface is due to its microporosity and the present of surface hydroxyl group. The particle size could also play a very important role in the properties of organic–inorganic and inorganic–inorganic composites [46–49]. Effect of particle size in the nanoscale on the properties nanocomposites is scarcely involved, especially on the epoxy/nanosilica composites [50]. The specific surface area, i.e. primary particle size, and the corresponding order of aggregation of primary particles are key parameters affecting many physicochemical properties of the powders and aqueous suspensions of nanosilicas. Aggregation of primary particles decreases with decreasing specific surface area (or increasing average size of primary particles). The Vnano, Snano, Vmes and Smes values increase with increasing SBET value [51]. The macroscopic examinations under magnifications up to 50,000· (Figs. 1–4) showed that all of the coatings were devoid of cracks. This is probably owing to the use of: a low Rh value [52] (the higher the Rh value, the larger the critical thickness of the coating and the higher the heat treatment temperature), acetic acid as the chelating agent [53] (which contributes to the formation of thicker layers) and homogenization of sols by ultrasounds (whereby a nanostructure can be obtained) [26]. The mild conditions of sintering at a low rate of temperature increase (2 deg./ min.) are conducive to the formation of highly elastic [54] coatings devoid of cracks [55]. The lower the number of cracks in the coating [56], the smaller the surface area of anodic regions. It is a well-known fact that a reduction in the ratio of anodic to cathodic surface areas results in a more refined surface [57] and in a decrease in the corrosion rate. Regardless of the type of SiO2 nanopowder, its addition to the sol increases the compactness of the coatings in comparison with coatings deposited from the coating solution alone. The compactness of the coating’s external layer increases with the nanopowder’s specific surface. In the case of both hydrophilic and hydrophobic powder, SiO2 grains join to form clusters. When a hydrophilic powder is used, the grains are much larger. The gently rising branch of the anodic curve for the uncoated steel (Fig. 5) is probably the result of the simultaneous nucleation and repassivation of micropits and oxygen release. The critical temperature of pitting corrosion, i.e. the temperature below which no pitting corrosion of steel 316L occurs, is about 20 C [36,37]. The polarization experiments were conducted at a temperature of 25 C, which means that there was a probability of pitting corrosion initiation. This is corroborated by the polarization curves for uncoated steel 316L in Ringer’s solution at a temperature of 37 C [38], showing a sharp increase in current density at a potential of about 0.3 V relative to the saturated calomel electrode. The produced SiO2 coatings show a different degree of corrosion resistance in Ringer’s solution depending on the type of nanosilica used to modify the sol. It follows from the anode polarization curves that the passive region

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is the widest in the steel 316L specimens covered with coatings containing nanosilica having the smallest specific surface (130 and R972). The width decreases as the silica’s SBET increases. This applies to both hydrophilic and hydrophobic powders. But the coatings with hydrophilic powder seem to be superior as regards the passive region width and the current density in the passive region. Also a comparison of iE = 750 mV (the cathode current density at ESCE = 750 mV) and Ei = 2 lA/cm2 (the anode region potential corresponding to current density 2 lA/cm2) shows the coatings with hydrophilic nanosilica to have a better corrosion resistance. As regards hydrophobic nanosilica additives, good results are obtained when R972 is used. Hydrophobic silica with larger specific surface (R974, R812) only slightly improves the protective properties of coatings on steel 316L. Yuan et al. [23] found that the epoxy coatings embedded with nanosilica have improvement in corrosion resistance, and the nanocomposite coatings with nanosilica particles at 53 and 79 nm have better corrosion resistance than those at 27 and 173 nm. This probably has much higher adhesion strength than the latter. Fig. 8 shows parameters: iE = 750 mV and Ei = 2 lA/ cm2 in the current–potential system. The location of the points provides information about the quality of the protective coatings. The coatings in the figure’s upper left corner are likely to have the best protective properties, whereas the ones in the lower right corner – the worst. According to Fig. 8, the coatings with hydrophobic or hydrophilic nanopowders have protective properties. The silica additive caused mainly changes in cathodic current density iE = 750 mV while the hydrophilic silica added to the coating solution affected the pit nucleation potential. The determined polarization resistances (Fig. 9) are a measure of the corrosion rate of steel 316L. A decrease in the corrosion rate (and so an increase in polarization resistance Rp) results in better biotolerance of the material [6]. Currently there exist several quantitative electrochemical methods of determining ceramic coating porosity due to the presence of cracks, perforations, capillaries and the like passing through the cross section to the basis metal. Porosity in relation to corrosion is expressed by a ratio of the pore surface to the total surface of the specimen [39]. Porosity in this sense can be called coating permeability. In English this kind of porosity is referred to as through-coating porosity or connected porosity [39–41]. A comparison of the obtained Rp values and the specimens’ permeabilities shows that the wettability of the powder by water as well as its specific surface and particle diameter are crucial factors. This must be taken into consideration when coatings produced from sols modified with nanopowders having different physicochemical properties are dried. Hydrolysis and condensation reactions were determined by the rate of water penetration into the layer through its defects (pores, capillaries, etc.). The hydrophobic silica additive probably caused the screening

of water in the layer whereby the degree of hydrolysis and condensation was reduced. Evaporation of the liquid phase from the gelating layer was hindered and as a result the degree of gelation of the layer containing more unevaporated liquid phase in comparison with the coatings with the hydrophilic powder additive was lowered. During the sintering of SiO2 with hydrophobic silica the above factors contributed to increased permeability in comparison with that of the coatings containing the hydrophilic powder. The deposited coatings do not show cracks. Even at their highest compactness, the grains and grain clusters forming the exterior of the coatings do not constitute a tight layer. This is evidenced by, for example, the very poor protective properties of the coatings deposited from sol modified with hydrophobic silica R812 . In the case of the latter coatings, their protective properties are the worst at the highest grain compactness. Whereas the use of hydrophilic nanosilica with small specific surface (130 and 150) results in coatings with low surface grain compactness and good protective properties. Probably there are smooth thicker layers directly on the steel basis in this case.

5. Conclusions Trilayer SiO2 coatings deposited (using the sol–gel technique) from sol modified with nanosilica and sintered successively at: 300, 250 and 200 C show a high degree of surface development. Besides the very thin film which covers the whole steel 316L basis, they are made up of grains forming clusters varying in their size and compactness depending on the type of nanosilica used. The evaluation of the through-coating porosity and polarization resistance of the coatings confirmed most of the above conclusions. As concerns the corrosion protection of steel 316L in Ringer’s solution, favorable results are obtained when the sol is modified with hydrophobic nanosilica. From among the tested hydrophobic powders additive R972 (SBET = 110 m2/g) ensures good protective properties. The coatings deposited from sols modified with nanosilica AEROSILR972 or AEROSIL380 show the highest polarization resistance. The worst results were obtained for powder AEROSILR812, in which case the through-coating porosity was the highest and its polarization resistance the lowest. Hence the type of nanosilica does have an effect on P and Rp. The powder’s water wetting power, its specific surface and particle diameter have been found to be the most important factors.

Acknowledgement This research was financed by the Polish Committee for Scientific Research (Project No. 3 T09B 025 28).

J.G. Che˛cmanowski, B. Szczygieł / Journal of Non-Crystalline Solids 354 (2008) 1786–1795

References [1] K.H. Kramer, in: H. Stallforth, T. Revell (Eds.), Materials for medical engineering, Euromat 1999, vol. 2, Wiley-Vch, 1999, p. 5. [2] J. Breme, Mem. Sci. Rev. Metall. 10 (1988) 625. [3] D.R. Haynes, S.D. Rogers, S. Hay, et al., J. Bone Joint Surg. [Am] 75A (1993) 825. [4] H.S. Dobbs, J. Mater. Sci. 17 (1982) 2398. [5] J. Black, J. Bone Joint Surg. B 70B (1988) 517. [6] Y. Okazaki, Y. Ito, Adv. Eng. Mater. 5 (2) (2000) 278. [7] O. De Sanctis, L. Gomez, N. Pellegri, C. Parodi, A. Marajofsky, A. Duran, J. Non-Cryst. Solids 121 (1990) 338. [8] R.G. Biswas, J.L. Woodhead, A.K. Bhattacharaya, J. Mater. Sci. Lett. 16 (1997) 1628. [9] M. Atik, M.A. Aegerter, J. Non-Cryst. Solids 147 (1992) 813. [10] R.G. Biswas, R.D. Sanders, J. Mater. Eng. Perform. 6 (1998) 727. [11] M. Atik, N.P. De Lima, M.A. Aegerter, L.A. Avaca, J. Appl. Electrochem. 25 (1995) 142. [12] M. Atik, N.P. De Lima, L.A. Avaca, M.A. Aegerter, Ceram. Int. 6 (1995) 403. [13] R. Di Maggio, L. Ferdizzi, S. Rossi, P. Scardi, Thin Solid Films 286 (1996) 127. [14] A. Nazeri, P.P. Trzaskoma-Paulette, D. Bauer, J. Sol-Gel Sci. Technol. 3 (1997) 317. [15] P.P. Traskoma-Paulette, A. Nazeri, J. Electrochem. Soc. 4 (1997) 1307. [16] M. Guglielmi, J. Sol–Gel Sci. Technol. 8 (1997) 443. [17] G.J. Brinker, Sol–gel science, The Physics and Chemistry of Sol–gel Processing, Academic Press, San Diego, 1990. [18] J.-W. Lee, Ch-W. Won, B.-S. Chun, J. Mater. Res. 8 (12) (1993) 3151. [19] G. Balasubramanian, D.D. Dionysion, M.T. Suidan, V. Subramanian, I. Bandin, J.-M. Laine, J. Mater. Sci. 38 (4) (2003) 823. [20] H. Schmidt, J. Non-Cryst. Solids 100 (1988) 65. [21] J. Głuszek, In_zynieria Materiałowa 5 (130) (2002) 351. [22] W. Beier, A.A. Goktas, G.H. Frischat, J. Non-Cryst. Solids 121 (1990) 163. [23] J. Yuan, S. Zhou, G. Gu, L. Wu, J. Mater. Sci. 40 (2005) 3927. [24] U. Wellbrock, W. Beier, G.H. Frischat, J. Non-Cryst. Solids 147&148 (1992) 350. [25] M. Ramirez-del–Solar, L. Esquivias, J. Zarzycki, J. Non-Cryst. Solids 121 (1990) 84. [26] P. Colomban, J. Mater. Res. 13 (4) (1998) 803. [27] M. Atik, J. Zarzycki, C. R’KHA, J. Mater. Sci. Lett. 13 (1994) 266. [28] B.D. Craig (Ed.), Handbook of Corrosion Date, ASM International, 1990.

1795

[29] Z. Szklarska –Smialowska, Pitting corrosion of metals, NACE, 1986. [30] A.R. Conde DiGiampaolo, M. Puerta, H. Ruiz, J.L. Olivares, J. NonCryst. Solids 147&148 (1992) 467. [31] J. De Damborenea, N. Pellegri, O. De Santis, A. Duran, J. Sol-Gel Sci. Tech. 4 (1995) 239. [32] O. De Santis, L. Gomez, N. Pellegri, A. Duran, Surf. Coat. Tech. 70 (1995) 251. [33] N.D.S. Mohallem, M.A. Aegerter, J. Non-Cryst. Solids 100 (1988) 526. [34] T. Hayashi, T. Yamada, H. Suito, J. Mater. Sci. 18 (1983) 3137. [35] R. Qvarfort, Corros. Sci. 28 (2) (1988) 135. [36] Z. Szklarska-Smiałowska, in: H. Isaacs, U. Bertocci, J. Kruger, Z. Smialowska (Eds.), Advances in localized corrosion, NACE, Houston, Texas, 1990. [37] R. Qvarfort, Corrosion Sci. 29 (8) (1989) 987. [38] J. Głuszek, J. Masalski, P. Furman, K. Nitsch, Biomaterials 18 (11) (1997) 789. [39] Y. Arata, A. Ohmori, C.-J. Li, Thin Solid Films 156 (1988) 315. [40] J.P. Celis, D. Drees, E. Maesen, J.R. Roos, Thin Solid Films 224 (1993) 58. [41] M.F. Lizandier, E. Lanza, A. Sebaoun, A. Giroud, P. Guiraldenq, Wear 153 (1992) 387. [42] B. Matthes, E. Broszeit, J. Aromaa, H. Ronkainen, S.-P. Hannula, A. Leyland, A. Matthews, Surf. Coat. Tech. 49 (1991) 489. [43] S.K. Parida, S. Dash, S. Patel, B.K. Mishra, Adv. Colloid Interf. Sci. 121 (2006) 77. [44] J. Zhang, D. Grischkowsky, D. Opt. Lett. 29 (9) (2004) 1031. [45] T.J. Bandosz, J. Colloid Interf. Sci. 193 (1997) 127. [46] S. Zhou, L.M. Wu, W.D. Shen, G.X. Gu, J. Mater. Sci. 39 (2004) 1539. [47] S. Zhou, L.M. Wu, J. Sun, W.D. Shen, J. Appl. Polym. Sci. 88 (2003) 189. [48] A.J. Kinloch, A.C. Taylor, J. Mater. Sci. Lett. 22 (2003) 1439. [49] L.S. Schadler, R.W. Siegel, NanoStruct. Mater. 12 (1999) 507. [50] T. Naganuma, Y. Kagawa, Compos. Sci. Tech. 62 (2002) 1187. [51] V.M. Gun’ko et al., J. Colloid Interface Sci. 289 (2005) 427. [52] T.J. Garino, Mater. Res. Soc. Symp. Proc. 180 (1990) 497. [53] B. Yao, L. Zhang, J. Mater. Sci. 34 (1999) 5983. [54] H. Schmidt, G. Rinn, R. Nass, D. Sporn, in: C.J. Brinker, D.E. Clark, D.R. Ulrich (Eds.), Mater. Res. Soc., Pittsburgh, 1988. pp. 743. [55] J. Schlichting, J. Non-Cryst. Solids 63 (1984) 173. [56] M.J. Paterson, D.G. McCollouch, P.J.K. Paterson, B.B. Nissan, Thin Solid Films 311 (1997) 196. [57] J.M. West, Basic Corrosion and Oxidation, Ellis Horwood Publishers, Chichester, 1986.