Corrosion Science 53 (2011) 296–301

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Corrosion Science

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Silane–parylene coating for improving corrosion resistance of stainless steel316L implant material

Monika Cieslik a,b, Klas Engvall c, Jinshan Pan d, Andrzej Kotarba a,⇑a Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Polandb Institute of Metallurgy and Materials Science, PAS, W. Reymonta 25, 30-059 Kraków, Polandc Swerea KIMAB AB, P.O. Box 55970, SE-10216 Stockholm, Swedend Div. of Surface & Corrosion Science, Dept. of Chemistry, Royal Institute of Technology, Drottning Kristinas väg. 51, SE-100 44 Stockholm, Sweden

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 April 2010Accepted 9 September 2010Available online 17 September 2010

Keywords:A. Stainless steel 316LA. Metal implantA. SilaneA. ParyleneC. Two-layer coatingB. EIS

0010-938X/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.corsci.2010.09.034

⇑ Corresponding author. Tel.: +48 12 6632017; fax:E-mail address: kotarba@chemia.uj.edu.pl (A. Kota

The corrosion resistance of a two-layer polymer (silane + parylene) coating, on implant stainless steelwas investigated by microscopic observations and electrochemical measurements. Long term exposuretests in Hanks solution revealed that the coating of 2 lm can be successfully used for corrosion protec-tion. However, the addition of H2O2, simulating the inflammatory response of human body environmentcauses a dramatic destruction of the protective coating. Analysis of the experimental data in terms of cir-cuit models enables proposing a deterioration mechanism. OH� radicals formed at the metal surfaceattack the polymer, thus the deterioration starts from the metal/polymer interface and progress towardsthe outward surface.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Stainless steel (SS) is a widely used orthopaedic implant mate-rial for internal fixation because of its mechanical strength and thecapability to bend and shape the implant to create a custom fit inthe operating room [1]. The main advantage of the SS material is itslow cost in comparison with other metallic implant materials suchas titanium or cobalt alloys. However, upon prolonged contact withhuman tissue (37 �C, pH 5.4–7.4) corrosion phenomena takes placeon the SS surface resulting in the undesirable release of transitionmetal ions, such as chromium, iron and nickel [2,3]. This corrosionand release of ions may lead not only to mechanical failure of im-plant but also to local pain and swelling in the near implant region.Additionally, the presence of metal ions in the organism causes thehistopathological changes in detoxication organs (liver, kidney,spleen) and even induction of tumours [4]. Therefore, the implantsneed specific surface finishing in order to minimizing the adverseeffects. In recent years, different laboratory centres have been elab-orating protections against metal ion release in the human body.Many examples are described of investigations on metal implantscovered by different coatings [5,6]. Polymer coatings are widelyapplied in many industrial areas to adjust properties of the finalproducts. This kind of approach can be also used for fabrication

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+48 12 6340515.rba).

of protective coatings on medical devices. One of the polymersused today for medical devices is parylene (poly-para-xylylene)due to its excellent biocompatibility and possibility to form a thin,continuous and inert film [7]. Pre-treatment with the organic si-lane A174 prior to parylene coating is the recommended surfacepreparation. Basically, silane is used as an adhesion promoter ow-ing to its intermediate character and thus can serve as an electro-static glue between inorganic (metal surface) and organic(parylene coating) interfaces [8]. Apart from being an effective gluethere are also some reports on biocompatibility of A174 silanecoatings [9]. On the other hand, parylene, which can be easily vac-uum-deposited, is widely used for demanding medical applications[10]. This kind of polymer is stable (�200 and 150 �C), highlydielectric (minimum 276 kV/mm), low density (q = 0.705 g cm�3)material and forms non-reactive, biostable and pinhole-free coat-ing. A crystal-clear parylene layer has a very low potential for trig-gering an immune response and is highly resistant to corrosiveconditions of body fluids environment. The film also forms aneffective barrier against channelling of contaminants from a coatedsurface to the surrounding area [11].

Most investigations on coatings used for medical devices aim atincreasing the corrosion resistance of metal implants. The corro-sion resistance and biocompatibility are the two fundamentalproperties of metal implants. At the same time, these propertiesdetermine the interaction between the metal surface and humanbody from an electrochemical and a biological point of view. The

M. Cieslik et al. / Corrosion Science 53 (2011) 296–301 297

electrochemical methods are generally used for monitoring theprocesses taking place on metal implant surfaces in artificial bio-logical environments. To simulate inflammatory reactions causedby an implanted foreign object in a human body, an artificial bodyfluids with the addition of hydrogen peroxide are used for in vitrostudies [12]. These kinds of experiments, involving electrochemicalmeasurements, are in good agreement with their biocompatibilityresponse obtained from in vivo experiments [13].

The aim of the present study is to address the electrochemicalbehaviour of a two-layer (silane + parylene) polymer coating onSS (316L) and to investigate the protective function during the longterm exposure to body fluids of Hanks solution and Hanks solutionwith the addition of H2O2.

Fig. 2. Cross section image of polymer coating on SS obtained with the use of aconfocal microscope. The thickness of the uniform two-layer (silane + parylene)coating is ca. 2 lm.

2. Experimental

2.1. Test materials and solutions

Samples of SS 316L grade (Fe – base, C – 0.03 wt.%, Cr –16.82 wt.%, Ni – 10.02 wt.%, Mn – 1.26 wt.%, Mo – 2.07 wt.%, Si –0.46 wt.%, N – 0.04 wt.%, P – 0.02 wt.%), cold rolled using highlypolished rolls and bright annealed (BA) surface finishing, were sup-plied by Swerea KIMAB AB. The lower carbon type (316L) of stan-dard 316 grade was used as this material reduces the risk ofintercrystalline corrosion, which was identified as a problem inthe early days of the application in orthopedics biomaterials [1,2].

Prior to this analysis, coated samples were cut in square-shapedcoupons of 20 � 20 mm2 and with a thickness of 0.8 mm. The sur-faces of the samples were cleaned and pickled and coated by thefollowing procedure. Firstly, the metal surface was covered by amonolayer of silane A174 using a dipping method (solution:0.5 vol.% of a-methacryloxypropyltrimetoxy silane in 50% water/50% iso-propanol, immersion time: 30 min, drying process: highpressure argon at 65 �C for 30 min). In the second stage, the coatingwith parylene N, Fig. 1, was applied via chemical vapour deposition(CVD) by Para Tech Coating Scandinavia AB (dimer – di-p-xylylenewas used as a precursor, decomposition to monomeric form at650 �C, spontaneous deposition and polymerization at room tem-perature). Such a procedure leads to the formation of a surface pro-tective coating with a sandwich-like structure. The steel substrateis covered by a very thin silane layer (�0.2 lm), which is used as anadhesion promoter, and a relatively thick (�2 lm) protective par-ylene coating. As can be inferred from Fig. 2, the parylene outerlayer on freshly prepared samples is smooth and uniform.

All electrochemical investigations were performed in Hankssolution, which is an artificial salts mixture, usually used in com-bination with naturally occurring body substances (blood serum,tissue extracts) and/or more complex chemically defining nutri-tive solutions for culturing animal cells. In the present experi-ments, the commercial Hanks solution of Aldrich analyticalgrade (NaCl – 0.8 g/L, CaCl2 – 0.02 g/L, MgSO4�7H2O – 0.02 g/L,KCl – 0.04 g/L, KH4PO4 – 0.01 g/L, NaHCO3 – 0.127 g/L, Na2H-PO4�2H2O – 0.01 g/L, Glucose – 0.2 g/L, distilled water, and pH7.4) was used [14]. Additionally, to simulate the conditions ofan inflammatory response, where hydrogen peroxide is formed,in selected experiments, 100 mM H2O2 was added to the Hanks

Fig. 1. Parylene-N polymer.

solution. All the measurements were carried out at least twotimes to check the reproducibility of the results.

2.2. Methods

The thickness and uniformity of the two-layer coating wereevaluated with the use of laser confocal microscopy (OlympusLEXT OLS3100 scanning microscope with the laser line 408 nmLD Laser/Class 2). This technique allows for optical sectioning ofopaque specimens by acquiring in-focus images from selecteddepths and thus facilitates the observations of the two-layer layerprofile. The morphology of the coated surface was analysed byscanning electron microscope E-SEM XL30 (FEI company). To avoidthe destruction of the polymer coating an electron beam energy of0.8 keV was used, resulting in magnification of 3000�.

For the electrochemical measurements, open circuit potential(OCP) and electrochemical impedance spectroscopy (EIS) were per-formed using a typical three electrode set-up described in detailelsewhere [15]. The three-electrode electrochemical cell used con-sisted of a sample work electrode, a Pt mesh counter electrode anda KCl-saturated Ag/AgCl reference electrode (SSE). The electrodeswere connected to a 1287 Electrochemical Interface coupled witha Solartron 1250 Frequency Response Analyzer. A computer withCorrWare and ZPlot Software (Solartron) was used for data acqui-sition and analysis. During the experiments, the exposed samplearea was �1 cm2. The measurements were performed using250 mL solution at room temperature in ambient condition. Priorto the EIS measurement, the OCP was recorded for 1 h to ensurea stable potential of the sample. All impedance measurementswere performed at the open circuit potential with alternating cur-rent amplitude of 50 mV and the applied frequencies range variedfrom 0.01 Hz to 10 kHz with an acquisition rate of 10 points perdecade. The EIS measurement was performed at the OCP and re-peated after 1, 3, 7 and 9 days, respectively, to monitor the changeof the coated sample, i.e., changes occurring in the coating and atthe interface between the stainless steel and the coating. Theimpedance spectra of the samples were analysed in Nyquist andBode plots.

3. Results and discussion

The typical results obtained from OCP measurements are pre-sented in Fig. 3. The initial period observed in the experimentslasted for about 10–15 min and was related to the stabilizationof the experimental conditions. After this initial period the signalsstabilized and the EIS analysis were carried out. The higher OCP

Fig. 3. OCP measured vs. saturated silver electrode (SSE) as a function of exposuretime of the two-layer polymer coating on SS exposed to Hanks solution and Hankssolution with H2O2, respectively, showing stabilization of the OCP after approxi-mately 10 min.

Fig. 4. Nyquist plots of impedance spectra of the two-layer polymer coating onstainless steel after different exposure times in Hanks solution (a) and Hankssolution with H2O2 (b).

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level for the Hanks solution with H2O2 addition is indicative of theoxidation power of H2O2.

In Fig. 4a and b, the EIS results from the two-layer coated SSsamples exposed to Hanks solution and Hanks solution with theH2O2 addition are shown. At the beginning of both experiments(after 1 h), a straight line on the Nyquist plots revealed the sameelectrochemical characteristics. This linear trend is prevailed upona prolonged time for the two-layer coated SS samples exposed tothe Hanks solution (Fig. 4a). This indicates a high impedance valuecharacteristic for a good barrier coating on the steel. After 3 days ofexposure, a small drop in the impedance was observed showingthat the electrolyte started to penetrate into the coating. However,as can be noticed in Fig. 4a, after a longer exposure time the coat-ing maintained its protective properties. However, in the case ofHanks solution with 100 mM H2O2 addition, the impedance re-sponse drastically changed after one day. In the Nyquist diagram,two semicircles appeared and became clearer after longer expo-sure, see Fig. 4b. With increased exposure time, the diameter ofthe semicircles (especially the first semicircle at the high fre-quency) becomes smaller, indicating the decrease of the corrosionresistance with time.

The Bode plots, shown in Fig 5, present changes in impedancevalues with different exposure times for Hanks solution (Fig. 5a)and Hanks solution with the H2O2 addition (Fig. 5b).

The impedance level, for uncoated samples exposed to theHanks solution, at low frequency limit is close to 105–106 X cm2

[16] whereas, for coated samples it is close to 109–1010 X cm2. Thisdemonstrates that during the whole experiment, with the expo-sure time of 9 days, the coating remains protective. However, after3 days, a small decrease in the impedance, probably due to a lim-ited penetration of the coating by ions from the electrolyte, canbe noticed.

In contrast, for samples exposed to the Hanks solution with theH2O2 addition (Fig. 5b), the impedance level at medium (101 Hz)and low frequencies (10�2 Hz) is much lower compared to thatwithout H2O2. The impedance at low frequency limit dropped dras-tically in the initial period, and decreases by 3 orders of magnitudeafter 9 days of exposure, approaching that of an uncoated sample.It implies that after a relatively short time, the electrolyte pene-trates into the coating, and gradually the steel surface becomes

available for the electrolyte and corrosion may occur as a conse-quence [17,18].

The evolution of the EIS spectrum with time of exposure in theHanks solution with the H2O2 addition is typical for degradation ofpolymer coatings on metal surfaces [19]. The commonly reportedinterpretations are also applicable here. Initially the coating actslike a barrier layer and the spectrum appears like a straight linein the Nyquist plot. The impedance response is dominated by acapacitance due to the dielectric property of the polymer coating.In this case, the simple equivalent circuit (Fig. 6a), consisting of aparallel resistance (Rfilm) and capacitance (Cfilm), in serial connec-tion with a solution resistance (Rs), can be used for spectra fitting.At the same time, a constant phase element (CPE) was introducedto mimic a non-ideal dielectric behaviour because the semicirclesin the Nyquist plots are depressed due to roughness and heteroge-neity of the surface, or other effects that causes uneven current dis-tributions on the electrode [20]. For the CPE used in the fitting,CPE-T has the similar physical meaning as capacitance C, whilethe exponential factor CPE-P is only a fit parameter with no phys-ical meaning. When the CPE-P is close to 1 (for pure capacitance), itis common in practice to use CPE-T value as an approximation ofcapacitance.

Fig. 5. Bode plots showing the impedance data of the two-layer polymer coating onstainless steel after different exposure times in Hanks solution (a) and Hankssolution with H2O2 (b).

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The capacitance can be related to the thickness of the coating,and the film resistance may be too high (>109 X cm2) to be accu-rately determined. After 1 day of exposure, the spectrum exhibitdiffusion impedance at low frequencies, and the equivalent circuit

Fig. 6. Equivalent electrical circuits (a–c) representing the electrochemical response for3 days, respectively) of exposure to Hanks solution with H2O2.

2 (Fig. 6b) can be used for spectra fitting, the high frequency semi-circle originated from the resistance (Rfilm) and capacitance (Cfilm)of the polymer coating while the diffusion impedance can be de-scribed by a Warburg diffusion element (Wo). After 3 days of expo-sure, the spectra show clearly two semicircles, the high frequencysemicircle is the response from the coating, while the low fre-quency semicircle is the response from the electrochemical dou-ble-layer at the interface between the metal and coating. In thiscase, the equivalent circuit 3 (Fig. 6c) can be satisfactorily usedfor the spectra fitting. As can be seen in Table 1 summarizing thedata from spectra fitting, the coating capacitance (CPE2-T) in-creased slightly with time after 3 days of exposure, indicatingsome water uptake into the coating. Meanwhile the coating resis-tance decreased with time, due to the ingress of ions from the solu-tion to the metal surface. The level of the double-layer capacitance(CPE1-T) on several tens of lF/cm2 and the charge transfer resis-tance (Rct) around 106 X cm2 are typical for uncoated stainlesssteel exposed to a aqueous salt solution [21]. The values obtainedfrom the spectra fitting are reasonable for such a coating systemand the degradation of the coating, which justify the interpretationof the EIS spectra.

In other studies [22,23], the decline of impedance was inter-preted in terms of degradation and eventually the delaminationof the polymer coating. However, in this study, the SEM imagesdo not reveal a physical separation of the coating from the metalsurface (Fig. 7a–c). A more plausible explanation for the observedsurface changes can be related to structural degradation of thepolymer coating leading to the formation of micro-cracks of thewidth of �1 lm, clearly visible in Fig. 7c. This cracking is mostlikely due to the oxidative degradation of the polymer by H2O2,which has a negative influence on the corrosion protection of thecoating, and the reason for that will be discussed below.

Since the impedance level at the low frequency limit is morerelevant for the corrosion resistance, a summary graph of imped-ance values measured at low frequency of 10�2 Hz, where the dif-ferences for various exposure times were observed, is presented inFig. 8. Two characteristic profiles for impedance value as a functionof exposure time can be observed. In the case of SS covered withthe two-layer coating and immerse in Hanks solution, the imped-ance is stable at a high level of 109 X for the whole period of themeasurement, indicating well-protected surface. The situationchanges dramatically in the presence of H2O2. After an initial

the two-layer polymer coating on stainless steel after different periods (1 h, 1 day,

Table 1EIS fit parameters for double layer (silane + parylene N) SS samples after long term exposure to Hanks solution with 100 mM H2O2.

Fit parameters Time of exposure

1 h 1 day 3 days 7 days 9 days

Rs (X cm2) 2356 ± 235 3372 ± 337 5032 ± 503 5626 ± 562 6672 ± 667Cfilm (F cm�2) CPE2-T 1.09 ± 0.05 � 10�9 1.71 ± 0.08 � 10�9 1.55 ± 0.09 � 10�9 1.86 ± 0.05 � 10�9 2.12 ± 0.05 � 10�9

n 0.944 ± 0.009 0.883 ± 0.005 0.895 ± 0.006 0.849 ± 0.004 0.847 ± 0.002Rfilm (X cm2) 8.3 ± 0.4 � 109 2.03 ± 0.02 � 106 5.76 ± 0.05 � 105 5.25 ± 0.02 � 105 4.35 ± 0.01 � 105

Cdl (F cm�2) CPE1-T – – 3.54 ± 0.05 � 10�7 1.09 ± 0.02 � 10�6 1.114 ± 0.007 � 10�6

n – – 0.712 ± 0.007 0.602 ± 0.009 0.698 ± 0.004Rct (X cm2) – – 3.9 ± 0.1 � 106 8.4 ± 0.1 � 105 1.497 ± 0.012 � 106

Wo (X s�1/2 cm2) W1–R – 4.71 ± 0.09 � 108 – – –T – 89 ± 2 – – –n – 0.841 ± 0.004 – – –

Fig. 7. SEM images of the two-layer polymer coated stainless steel before exposure (a), after 7 days of exposure to Hanks solution (b) and Hanks solution with H2O2 (c).

Fig. 8. The effect of H2O2 on corrosion resistance (decline in impedance at 0.01 Hz)as a function of exposure time for the two-layer polymer coated stainless steelexposed to Hanks solution with and without the H2O2 addition.

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period of 1 day the impedance rapidly decreases. Starting from thesame level of 109 X cm2 it dropped to 107 X cm2 after the next2 days and finally reaching the level of uncoated SS surface (105–106 X cm2) after 9 days [24,25]. This finding is in line with theSEM observations and indicates that the cracks (Fig. 7c) practicallyreach the metal surface. Taking into account the danger of heavymetal ions release from SS surface [14], it has to be emphasisedthat the initial inflammatory response of the body is of crucialimportance for a safe and long lasting implantation.

The data from Table 1 can be also used for evaluation of thethickness (d) of the parylene N coating directly from the formula:

d ¼ ee0ACc

ð1Þ

where e is the dielectric constant of the coating (2.65), e0 is thedielectric constant of vacuum (i.e., 8.86 � 10�12 F/m), A is an areaof coating and Cfilm is a coating capacitance (for calculation assumedthat Cfilm = CPE2-T for n � 1, see Table 1). The substitution for theexperimental values for the results after 1 h of experiment, givesd = 1.68 lm which is in good agreement with the thickness directlymeasured from the confocal image of the coating cross sectionshowed in Fig. 2.

In general, the SS surface can be effectively protected with a2 lm thick two-layer coating – silane and parylene – against corro-sion processes in the body fluid environment. However, the prob-lem of initial degradation of the polymer in the presence ofhydrogen peroxide is now identified, although in the literatureno influence of H2O2 on parylene was reported [26]. In this studythe concentration of H2O2 added to the Hanks solution is probablytoo high to be realistic for in vivo conditions. Nevertheless, the re-sults point out the role of the metal substrate in the degradation ofthe polymer in the present of peroxide and superoxide species gen-erated by the inflammatory response. It is worth mentioning herethat the separated from SS surface parylene does not degrade whenexposed to Hanks solution even with addition of H2O2. The ob-tained data allow us to propose an overall model of the degrada-tion mechanism:

(1) At the initial stage, the first day, the electrolyte diffusionthrough the coating takes place.

(2) In contact with the metal surface and also with dissolvedFe2+ ions, the H2O2 decomposes and form hydroxyl radicals(OH�), according to the Fenton reaction [27]:Fe2þ þH2O2 ! Fe3þ þ OH� þ OH�

Fe2+ ions can be dissolved in small amount of water accumu-lated in so-called pockets at the metal–polymer interface.These pockets can be very small and the size is determinedby the number of water molecules needed to reach bulkwater properties able to dissolve ions. In the literature, such

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properties were observed for water clusters consisting ofdown to only 10–12 molecules [28]. The pockets most likelyoriginate from defects in the silane or parylene layer orimperfect adhesion between the silane and the parylenecoating.

(3) The reactive OH� radical induces, in general, so-called, autox-idation reactions [29]:OH� þH-Poly! H2Oþ PolyPoly� þ O2 ! Poly-OO�

The formed peroxyl radical, Poly-OO�, will react further inmany different ways, leading to fragmentation of the poly-mer chain as a very probable outcome.

Thus, it can be concluded that the degradation process starts atthe metal–polymer interface and progress towards the outer sur-face. Due to spontaneous decomposition of H2O2 to O2 and H2Othere is a possibility of simple antidote. A thicker coating layermay reduce the probability of H2O2 to reach the surface and thusquenching the formation of OH� radicals allowing for the practicalapplication of the investigated coating. Another possibility is theapplication of denser parylene C, with one chlorine group per re-peat unit, which is also approved for medical applications. Sincethe inflammatory reaction takes place usually in the first periodafter the implantation a multilayer coating with adjusted perme-ability and functions (prevention of H2O2 diffusion, metal ions re-lease, corrosion) can serve as a strategy here. These studies are inprogress now.

4. Conclusions

Electrochemical measurements and microscopic observationswere used for investigations of the corrosion resistance of thetwo-layer (silane + parylene) coating on implant grade stainlesssteel 316L. Long term exposure tests in simulated body fluid(Hanks solution) revealed that the layer can be successfully usedfor corrosion protection. However, the addition of 100 mM H2O2,simulating the inflammatory response of the body, causes a dra-matic destruction of the coating. This finding implies serious prac-tical consequences since parylene was considered as a stablecoating material against H2O2. The obtained experimental dataare consistent and enable to propose a deterioration mechanism,which involves formation of OH� radicals at the metal surface viaFenton reaction and their subsequent attack on the polymer. Thedeterioration starts from the metal/polymer interface and progresstowards the outer surface. The insight into the mechanism allowsfor suggestion of ways to hamper the H2O2 diffusion through thepolymer and thus preventing the degradation.

Acknowledgements

This research project was operated within the Foundation forPolish Science Ventures Programme, co-financed by the EU Euro-pean Regional Development Fund. Authors would like to thankPara Tech Coating Inc. for help in coating preparation and Mrs.A.M. Janus for microscopic observations.

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