Comparative study of corrosion in physiological serum of ceramic coatings applied on 316l stainless

Advanced ¡I;/aleria/s Research Vol 68 (2009) pp /52-/58 On/ine availab/e sinee 2009/Apr/27 al \\ \1'\I ,scienl¡fí, nt!1 t (2009) Trans Tech Pub/icalions, Swit;:er/and ' doi ,' 1O4028/www.scienlific.netíA}v/R68152

Comparative study of corrosion in physiological serum of ceramic coatings applied on 316L stainless steel substrate M. 1. Espitia-Cabrera,1 ,a H. D. Orozco-Hernández,2.b M. A. Espinosa­ Medina,3.CL. Martínez,4d and M. E. Contreras-García 2.e Facultad de Ingeniería Química, Universidad Michoacana de San Nicolás de Hidalgo, C.U.

Edificio "E", Morelia, Mich., México.

2 Instituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicolás de Hidalgo,

C.U. Edificio "U", P.O. Box 888, Morelia, Mich. , México.

3 Instituto Mexicano del Petróleo Eje central Lázaro Cárdenas Norte 152, C.P. 07730, México, D.F.

México.

4 Centro de Ciencias Físicas, UNAM, Cuernavaca, Mor., México.

1

a

ieumih~yahoo.com.mx , b orozcohd@hotmail.com, e maespin@imp.mx, Img@proteccionycorrosion,com, e eucontre@umich.mx

Keywords: ceramic films, alumina, zirconia, corrosion , Hank's solution.

Abstract: In recent years the use of ceramic coatings to reduce metallic corrosion has been greatly improved . It has been proved by several studies that coating of Ti0 2, Zr02, Si0 2, and other oxides or mixed oxides provide efficient protection against the corrosion of stainless steel in different media. In this work, alumina and zirconia films were obtained by electrophoretic deposition to investigate their use as protective coatings of 316L-Stainless steel for prostheses and dental implants. These were characterized by scanning electron microscopy (SEM), grazing incidence X­ ray diffraction (GID), and atomic force microscopy (AFM). The corrosion behaviour in Hank's solution at room temperature was studied using a potentiodynamic polarization technique. The electrochemical measurements showed a high corrosion resistance of 316L-SS coated by both types of ceramic film. The best behaviour was presented by the alumina coating. The morphologies of the corroded films showed that the alumina and zirconia films presented low damage with little pitting corros ion compared with the uncoated 316L-SS.

1. Introduction

There are many methods of depositing ceramic coatings, including plasma spray [1], physical vapour deposition [2] , cathodic deposition [3], and sputtering. Compared with these methods. electrophoretic deposition (EPO) is the most appropriate for preparing homogeneous coating Jayers on large areas [4], as there is l ittle restriction on the shape of the deposition substrate and it is a low­ cost technique [5]. Ceramic coatings improve the corros ion resistance [6], heat resistance, and abrasion resistance of metals. Ceramic coatings withstand hard environmental conditions, and their applications have become increasingly important, especially when ceramics are considered for critical components io' chemical processes where materials are subjected to high temperatures and aggressive chemical envjronments [7].

In this work, zirconia and alumina films were deposited by EPO on 316L-SS substrate at room temperature . The 316L-SS shows important characteristics for use as a ceramic support: its thermal expansion is close to that of the ceramic material [8] and it is relatively low-cost compared with platinum alloys, which are commonly used by other authors to study the EPO method [3]. The 316L-SS is used as it is a sufficiently biocompatible material , but it is susceptible to chlorine attack in physiological media. The zirconia and alumina are known to have excellent technological nronf':rti f': <; <;11c,h ~<; c hf':mi c~ 1 inertness, thermal stahilitv. mechanical stren2:th. and wear resistance.

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Zirconia and alumina films are biocompatible materials used in human prostheses [9]. Zirconia film has been used as an interlayer between 316L-SS and hydroxyapatite coatings to prov ide adherence of hydroxyapatite to the substrate [1 Ol In this work, alumina coating corrosion behaviour is compared with zirconia coating corros ion behaviour in a Hank ' s solution medium at room temperature using electrochemical techniques. The Hank 's solution is a chlorine medium that simulates phys iological serum [IIJ.

2. Experimental Procedure Alumina and zirconia were deposited on 316L-SS plates with areas of 1.0 cm2 . The substrate was previously SiC powder polished and washed with acetone in an ultrasonic bath . The volume of the electrolytic cell was 100 mi . For the alumina deposition , the solution was 0.045 M of AI(N03h and for the zirconia, the solution was 0.05M of ZrOCh·8H20 . The deposition conditions were explained in a previous work [1 1] . After deposition , the samples were thermall y treated at 580 O c for two hours. The ceramic electrodeposited film morphologies were observed by SEM using a Philips XL microscope, the chemical composition was determined using EOS , and the topologies of the films were observed by AFM using a Questant SPM model Q250 45695143-072 microscope, using tapping mode. The area scanned for the AFM characterisation was 40 11m2. The phases present in a sample of corroded zirconia coating were determined using GID with a Brucker 0-8 model Advance e-e diffractometer scanning from 25 ° to 95 ° at arate of Io/min using a Co X-ray tube. The corrosion behav iour of the 3 16L-SS when uncoated. coated with zirconia, and coated with alumina in Hank 's solution, a simulated body fluid environment, at room temperature was evaluated by an electrochemical polarisatioo technique with an ACM Instruments Gillac potentiostat. A three­ electrode cell was employed. using a platinum wire with a constant area as a counter electrode, and a saturated calomel electrode (SCE) was used as a reference electrode. The polarisation sweep tests were developed. applying a sweep rate of 1 mV/s within the range potential of -300 to +500 mV versus an open circuit potential (OCP).

3. Results and discussion Figures I (a) and 2(a) show the SEM micrographs of the alumina and zirconia films respectively . In both samples, the surface was covered completely and homogeneously. The observed cracks are due to the thermal treatment and they are only on the surface. Figures I(b) and 2(b) show the corresponding EOS spectra of the samples. In both films the compositions correspond to the formulated films. For the alumina film. the coating was homogeneous and opticalJy transparent, making it possible to observe the stainless steel structure (Figure 1(a».

Fe

(b)

er Fe

°.1 .... " • .

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~I.-'" '" '" ~_"-----~_

'-o . ,'

3

I

2

Figure l.

(a) SEM micrograph of alumina film

00

,

5 Energ y (k.V)

•O

316L-SS, and (b) EOS analysis of the coating.

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Fe

(b) ~

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Cr

~

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100

200

300

400

500

600

700

800

900

1000

Energy (keV)

Figure 2.

(a) SEM micrograph ofZr02 films on 316L-SS, and (b) EOS analysis ofthe coating.

According to the results of the AFM anal ysis, ceramic films were found to have an adequate thin arrangement which would allow their use as a coating on a nanometric scale. A roughness depth ranging between l and 2 11m has been found to be beneficial for mechanical and corrosion resistance characteristics in human body implants [15]. The zirconia coatings showed surface characteristics that are expected to contribute to and improve hardness, wear resistance, and biocompatibility.

Surface topography is also an important property that determines the quality of implants. For instance, the femoral head implant should be as smooth as possible, while an increase in the surface roughness could be advantageous for an implant system that needs to adhere to bone tissue [12-14].

However, increased roughness could be deleterious to the corrosion resistance of the implant coating [14]. Figures 3(a) and (b) present the alumina coating: (a) shows the two-dimensional perspective (20: 40 x 40 11m2 sample size) and (b) shows the three-dimensional perspective (3D; 40 x 40 x 2.84 11m 3 sample size) as weJl as an overview where one can see the arrangement of particles. The darker sides illustrated in Figures 3(a) and (b) represent deeper areas of scanning where the needle penetrates 300 nm deep into the sample's surface. The lighter areas on light yellow also represent the scanning height.

Figure 3.

AFM images of alumina film: (a) 20 and (b) 3D.

The average height reported for the alumina film has a value of 1.542 11m . The presented topography of these fill'nS was uniform and smooth and indicates that they were formed by well packed nanometric agglomerates with grain sizes from 100 nm to 700 nm. The 20 AFM image of the zirconia coating showed a different arrangement (Figure 4).

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Figure 4. AFM image of zirconia film; 2D. lt could be suggested that this type of arrangement consists of a continuous deposit with accumulations in lines that follow the grain boundaries of the 316L-SS substrate, originating from the surface defects of the substrate that resulted in this particular arrangement. However, the Zr02 coating presented a film thickness of 2.907 flm. which seems to be very high, but some areas were very smooth, as presented in a previous work [16]. According to the remaining resuJts of the AFM analysis, ceramic thin films have an adequate arrangement which enables them to be used as continuous protecti ve coatings.

The X-ray results are shown in Figure 5. The X-ray diffraction pattern of the zirconia coating after the corrosion test shows the characteristic

A = Chromium Iron Nickel T = Tetragonal zirconia Fe= Iron,Syn-Fe H = Hematite Na= Halite

IA

A.T

A,T

Figure 5. X-ray diffraction pattern ofzirconia coating on stainless steel substrate showing the characteristic XRD peaks of the tetragonal phase (marked "T") , XRD peaks of the tetragonal phase of zirconia, which appear in addition to peaks of the substrate and a small peak of halite that is present in the surface due to the electrolyte solution drying. The tetragonal zirconia phase was obtained in similar temperature conditions aboye 550 oC [17. 18].

And this temperature was not enought to crystall ize the alumina coating that was found amorphous

by X-ray diffraction. The diffractogram is not shown, only the characteristic X-ray diffraction peaks ofthe 316L-SS were obtained.

The polarization responses of the uncoated and coated samples in the Hank solution are shown in Figure 6.

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Potentiodynamic polarization curves of the coated and uncoated specimens in the Hank's solution .

The uncoated 316L-SS sample showed a control mechanism in a reduction process using a 9.3E-4 2 mA/cm limited density current, and presented a pote ntial value of E corr of -212 m V vs SCE, demonstrating more activity than th e coated samples. In this comparative study the alumina coating showed the more positive OCP. folJowed by the z irconia coating which was more active by less than 90 mV (more negative) . However. the zirconia coating showed less i corr (1.578E-05 mA/cm 2) . and this coating system had a pseudo-passivity behaviour which started at 60 mV after the OCP in 2 the anodic direction reached 1.403E-05 mA/ cm of current passivation. This coating showed an additional response of the 316L-SS material: a reduction controlled mechanism of more positive 2 potential, showing a limited current density for 6.412E-05 mA/cm . In similar behaviour. the alumina coating presented a mix of controlled mechanisms in the reduction and activation processes. Table 1 presents the electrochemicaJ parameters obtained of the polarisation curves. The alumina and zirconia ftlms' morphologies showed low damage with little pitting corros ion compared with the uncoated 316-SS .

• . d o f t he po 1ansatlon Tabl e I Electroc h emlca parameters o btalne curves ; Igure 6

sample SS316L Zr203-coating Ah03-coating

E corr

Ba

Be

lcorr.

(mV vs SCE) -212 +209 +295

(mV) 420 137 90

(mV) 104 80 50

(mA/cm ) 5.675E-04 1.578E-05 1.754E-05

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(mA/cm ) 9.333 E-04 6.412E-05 1. J04E-04

(mA/cm

2

)

-­ 1.403E-05

--

The characteristic morphologies of the deposited layers of ceramic coatings are shown in the images of SEM and AFM characteri sation. and the protective response of these coatings is also evident from the potentiodynamic measurements. Good results for similar coating systems produced using an electrolytic method were reported by Giacomelli et al . [19]. who evaluated electrolytic Zr02 coatings deposited on a NiTi alloy and tested in an artificial saliva environment.

This method of deposition has presented good morphoJogical characteristics which depend on the deposition time and of their thicknesses. which are important variables in the coating adhesion. In this way , the zirconia coatings obtained by EPD, have good protective behaviour against corrosion. This 'result is comparable with the results obtained by Balamurugan el al. who obtained sol-gel coatings of zirconia prepared from sono-catal ysed soIs and deposited using a dip­ coating technique on 316L-SS which had good protective behaviour against corrosion of a metallic implant surface [20]. Al so. we found that the z irconia and alumina films evaluated here acted as geometric blocking layers, indicating improved corrosion resistance of the material , as reported previously Giacomelli el al. [19]. Furthermore. the observed homogeneous morphologies of the coatings are important characteristics. as can be seen in Figures l. 2 . and 3. due the physical barrier

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which separates the corrosive environment from the metallic surface. The second characteristic of the coatings is the low electrochemical activity in this environment: the excellent response observed in the coatings evaluated here illustrated that characteristic, which has also been reported, under other conditions, previously [11,20].

4. Summary

Zirconia and alumina nanostructured ti Ims obtained by electrophoretic deposition did not present film cracks, and these ceramic layers covered the substrate uniformly. These films demonstrated very high quality, although the alumina coating remained amorphous and the zirconia coating was crystalline. Both ceramic coatings showed better corrosion resistance behaviour in the Hank solution than did the uncoated 316L-SS. The alumina coating showed the lowest active potential.

However, better corrosion protective behaviour was presented by the zirconia coating, which showed passivity behaviour in a narrow potential range of the OCP , and had lower anodic corrosion density with a corrosion mechanism controlled by the reduction process, which was improved by the coating' s resistive characteristic.

Acknowledgements

The authors express their thanks to Wilian Cauich Ruiz. Daniel Aguilar, Luis Mariano Hernández, and Ecar Leana Morales for their technical support. This work was financially supported by the Universidad Michoacana de San N icolás de Hidalgo through the Cle. References

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Advances in Semiconducting Materials 10A028/www.scientific.netlAMR.68

Comparative Study of Corros ion in Physiological Serum of Ceramic Coatings Applied on 316L Stainless Steel Substrate 10A028/www.scientific.netlAMR.68.152

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