Journal of Materials Engineering and Performance

, Volume 22, Issue 6, pp 1756–1764

Electrochemical and In Vitro Behavior of Nanostructure Sol-Gel Coated 316L Stainless Steel Incorporated with Rosemary Extract

Authors

  • Abolfazl Motalebi
    • Department of Materials Science and Engineering, Najafabad BranchIslamic Azad University
    • Department of Materials Science and Engineering, Najafabad BranchIslamic Azad University
Article

DOI: 10.1007/s11665-012-0448-0

Cite this article as:
Motalebi, A. & Nasr-Esfahani, M. J. of Materi Eng and Perform (2013) 22: 1756. doi:10.1007/s11665-012-0448-0

Abstract

The corrosion resistance of AISI 316L stainless steel for biomedical applications, was significantly enhanced by means of hybrid organic-inorganic sol-gel thin films deposited by spin-coating. Thin films of less than 100 nm with different hybrid characters were obtained by incorporating rosemary extract as green corrosion inhibitor. The morphology, composition, and adhesion of hybrid sol-gel coatings have been examined by SEM, EDX, and pull-off test, respectively. Addition of high additive concentrations (0.1%) did not disorganize the sol-gel network. Direct pull-off test recorded a mean coating-substrate bonding strength larger than 21.2 MPa for the hybrid sol-gel coating. The effect of rosemary extract, with various added concentrations from 0.012 to 0.1%, on the anticorrosion properties of sol-gel films have been characterized by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests in simulated body fluid (SBF) solution and has been compared to the bare metal. Rosemary extract additions (0.05%) have significantly increased the corrosion protection of the sol-gel thin film to higher than 90%. The in vitro bioactivity of prepared films indicates that hydroxyapatite nuclei can form and grow on the surface of the doped sol-gel thin films. The present study shows that due to their excellent anticorrosion properties, bioactivity and bonding strength to substrate, doped sol-gel thin films are practical hybrid films in biomedical applications.

Keywords

corrosiongreen corrosion inhibitornanostructure sol-gel coatingrosemary extractstainless steel 316L

Introduction

Titanium, cobalt alloys, and 316L stainless steel fulfill the mechanical requirements of orthopedic prosthesis and fixation devices, while they cannot offer the required biocompatibility and corrosion resistance in the physiological medium (Ref 1, 2). The AISI 316L stainless steel is often used for temporary devices in orthopedic surgery as plates nails, etc. However, crevices between pieces are frequent for this kind of implants, as well as electric contact between pieces and consequently high risk of galvanic corrosion. Moreover, the presence of Cl in human body stimulates localized corrosion (Ref 3), especially dangerous for stainless steel. Corrosion provokes the release of significant quantities of Fe, promoting the formation of fibrous tissue and necrosis around implants (Ref 4, 5).

One of the prospective candidates for corrosion and oxidation protection is sol-gel-derived thin films. Sol-gel technology can offer various ways to prepare functional coatings with different properties (Ref 6). In the last years, the interest for sol-gel coatings turns around hybrid organic-inorganic coatings, with higher thickness than the inorganic ones and different properties determined by composition and processing conditions. They represent a new class of covalently bonded materials, combining the excellent mechanical properties of the ceramic component with the flexibility, transparency, and adhesion of organic substances. Hybrid systems are obtained by the structural incorporation of organic groups. The inclusion of modified alkoxysilanes such as vinylalkoxysilanes to precursor solutions is the simplest way to incorporate organic groups in SiO2 coatings (Ref 7).

Although the originally developed sol-gel-derived pure inorganic or hybrid organic-inorganic coating formulations have been introduced as promising treatments for long-term protection of various metals against atmospheric corrosion, their corrosion protection performance is limited when integrity of the coating is compromised. To improve corrosion protection properties of the coating when it is mechanically damaged, the incorporation of active corrosion inhibitors into the coating is needed. Organic corrosion inhibitors are promising candidates, as they appear to be compatible with hybrid coating material that can be loaded with inhibitors by adding the inhibitor into application solution prior to cross linking and film formation. Some authors (Ref 8-11) have incorporated organic inhibitors with the purpose of obtaining a self-healing effect in organic-inorganic hybrid coatings. The inhibitors incorporated in the film should migrate and precipitate producing a passivating effect where a defect was originated. The known hazardous effects of most synthetic organic inhibitors and the need to develop cheap, nontoxic, and eco-friendly processes have now urged researchers to focus on the use of natural products. Natural organic compounds such as rosemary extract are biodegradable and do not contain heavy metals or other toxic compounds, and are abundant in nature (Ref 12). Yee (Ref 13) determined the inhibitive effects of rosemary extract on four different metals—aluminium, copper, iron, and zinc, each polarized in two different solutions, that is, sodium chloride and sodium sulfate. Ouariachi et al. (Ref 14) also reported the inhibitory action of Rosmarinus officinalis oil as green corrosion inhibitors on C38 steel in 0.5 M H2SO4. Thus rosemary extract is a potential candidate to be used as an inhibitor dopant in the hybrid sol-gel film.

In this work, hybrid organic-inorganic thin films obtained by sol-gel were studied as a barrier against corrosion and ion diffusion of AISI 316L, with the aim of improving their behavior as biomaterial. The hybrid organic-inorganic sols were developed from polymerization and hydrolytic polycondensation of vinyltrimethoxysilane (VTMS) and rosemary extract as a green inhibitor. The coated steel was evaluated in physiological conditions through polarization tests, electrochemical impedance spectroscopy (EIS), and in vitro tests.

Experimental Methods

Hybrid sol-gel coatings were applied onto AISI 316L stainless steel plates by the spin-coating method. Sols were prepared in alcoholic medium through polymerization, hydrolysis, and polycondensation of vinyltrimethoxysilane (VTMS, Merck) as precursor. Inhibitor-doped sol-gel films were prepared by addition of rosemary extract to the sol. The inhibitor content of the hybrid organic coatings ranged from 0 to 0.1 wt.%. The rosemary was incorporated as an aqueous solution of rosemary extract (Gol Darou-Iran). VTMS was polymerized to 20-mers (PVTMS) with tertiary-butyl peroxide as an initiator using refluxing for 2 h at 423 K in flowing nitrogen (Ref 15, 16). An ethanol solution of PVTMS was mixed with an aqueous solution of rosemary extract and calcium acetate and refluxed for 2 h at 393 K. The molar ratio PVTMS/EtOH/H2O/Ca:1/8/9/0.05 and a concentration of 2 vol.% of polymers were used in sols.

Thin films were deposited on mechanically polished AISI 316L pieces of 12 mm diameter and 4 mm thickness. Substrates were degreased and cleaned in an ultrasonic bath and rinsed in ethanol. The coatings were obtained at room temperature using a spin rate of 4000 rpm, dried at room temperature for 24 h, and heat treated for 72 h at 333 K in electric furnace. One layer coating was prepared on AISI 316L.

The coating integrity (bubbles, microcracks, blisters, and scales) was evaluated by scanning electron microscopy (VEGA//TESCANE). Elemental chemical analysis of the coating was performed by energy dispersive X-ray spectroscopy (EDX) connected to the SEM. UV-vis reflection spectra and FTIR were measured with a (JASCO, V-570, Rev. 1.00) spectrometer. The effect of the green inhibitor on the adhesion of the coating to the substrate was determined by pull-off tests performed under dry conditions. Samples in dry conditions were sandwiched in an alignment jig between 25 mm diameter aluminum cylinders utilizing an epoxy adhesive (UHU Epoxy Adhesive, Germany). A 2 h curing at 373 K was allowed at a pressure of 30 kPa and the resulting specimens were then subjected to tensile testing in a tensile machine (Model H25KS, Hounsfield, UK) at a cross-head speed of 1 mm/min. Reported adhesion strength values are averaged over five measurements.

Electrochemical tests were conducted at room temperature in SBF solution using an electrochemical unit (Model PARstat 2273). Bare AISI 316L was used as blank. A three-electrode cell was employed using a graphite of convenient area as counter electrode and a saturated calomel electrode (SCE, Radiometer Copenhague) as reference electrode. Potentiodynamic tests were conducted from the −0.25-0.7 V versus OCP, with a scan rate of 0.001 V/s. EIS test was performed in a frequency range of 100 kHz to 10 mHz with a sinusoidal AC voltage of 10 mV amplitude. This test was performed after 1 h and 21 days of immersion in the electrolyte. Impedance fitting was performed using the Zview software.

In vitro tests were performed at 310 K, by immersion of coated steels in SBF, with the Kokubo composition (Ref 17, 18): it has an inorganic ion composition similar to that of human blood plasma. Formation of biological like mineral phases (specifically, apatite phases) on the sample surface was monitored by infrared spectroscopy FTIR (JASCO, V-570, Rev. 1.00) and scanning electron microscopy (VEGA//TESCANE).

Results and Discussion

Non-electrochemical Results

Sols were transparent and colorless before addition of rosemary extract, turned to slightly transparent yellow after the treatment. Hybrid coatings after the thermal treatment appeared transparent, homogeneous, and defect-free with a faint yellowish color. The SEM technique was used in order to examine the structure of the PVTMS thin films with various contents of rosemary extract. Figure 1 shows SEM micrograph of doped and undoped PVTMS thin films on stainless steel substrate. The coatings appear homogeneous and crack-free, although a secondary phase as a spots aggregate (smaller than 500 nm diameter) can be observed into the doped thin film. Elemental chemical analysis by EDX was performed on both coatings, showing a different distribution of elements. The EDX analysis displayed in Fig. 1 confirms the present of carbon in the agglomerates and incorporation of the inhibitor into the coating.
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Fig. 1

SEM images of (a) undoped PVTMS thin film and (b) doped PVTMS thin film with 0.1% rosemary extract. EDX patterns were shown in SEM image

SEM micrographs of cross section and plane view of the coated sample are shown in Fig. 2. SEM observations reveal the formation of a defect-free and highly adherent film on the steel substrate which leads to the improvement of corrosion resistance of 316L stainless steel. The thickness of the coating is around 94 nm. Also Fig. 2(b) shows the nanostructure of the coatings, with particles of 25-35 nm sizes.
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Fig. 2

SEM micrographs for stainless steel 316L coated with PVTMS thin film: (a) cross-sectional view, (b) plane view

UV-visible reflectance spectra of doped and undoped PVTMS thin films are shown in Fig. 3. The absorbance of the coating was increased with incorporation of rosemary extract in all UV-visible which is an evidence of the presence of the inhibitor in the coating.
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Fig. 3

UV-vis reflectance spectra of (a) undoped PVTMS thin film and (b) doped PVTMS thin film with 0.1% rosemary extract

The data reported in Table 1 indicates that the green inhibitor provided an important increase of the coat-substrate bond strength compared to the undoped PVTMS thin films. Thus the green inhibitor conferred to the coating an adhesive strength exceeding its cohesive strength. The marked increase of coating adhesion caused by the presence of rosemary extract has not been investigated in detail so far and further studies are needed to elucidate the interfacial reactions involved. In summary, this can be attributed to chemical reaction between rosmarinic acid as a component of rosemary extract and (Fe) on stainless steel surface.
Table 1

Pull-off adhesion test in dry conditions

Sample

Bond strength, MPa

Detached area

No inhibitor

18.8 ± 1.1

0

0.05% Rosemary extract

21.2 ± 1.1

0

Electrochemical Results

Potentiodynamic Polarization Curves

Potentiodynamic polarization measurements carried out on doped/undoped PVTMS thin films and their comparison with the bare metal are presented in Fig. 4. The inhibition efficiency (IE%) was calculated using the following equation:
$$ {\text{IE}}\% = \left[ {\left( {I_{0} - I} \right)/I_{0} } \right] \times 100 $$
where I0 and I are corrosion current densities of PVTMS thin films without and with different concentrations of the inhibitor, respectively. The electrochemical parameters obtained from polarization measurements such as corrosion current density (Icorr), corrosion potential (Ecorr), cathodic Tafel slope (βc), anodic Tafel slope (βa), and inhibition efficiency (IE%) are given in Table 2.
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Fig. 4

Potentiodynamic polarization curves for PVTMS thin films in SBF with various rosemary extract contents and their comparison with the bare metal

Table 2

Kinetic parameters of undoped and doped hybrid sol-gel thin films and their comparison with the bare metal in SBF solution at 310 ± 1 °C

Sample

βa, mV/dec

βc, mV/dec

Ecorr, V/SCE

Icorr, μA/cm2

% IE

Bare metal

150

160

−0.284

3.471

No inhibitor

138

150

−0.145

0.704

0.012% Rosemary extract

104

137

−0.136

0.600

14.8

0.025% Rosemary extract

98

127

−0.0536

0.055

92.2

0.05% Rosemary extract

78

114

−0.0417

0.0533

92.4

0.1% Rosemary extract

76

113

−0.040

0.0540

92.3

Clearly, in comparison with the corrosion potential (Ecorr) of the bare steel substrate (−0.284 V), Ecorr was increased by applying the PVTMS coatings. Additionally, Ecorr was further enhanced by adding the inhibitor (rosemary extract) to the PVTMS film, reaching (−0.040 V) for the coating containing 0.1% inhibitor. This increase represents a nobler electrode potential being achieved, thus indicating the improvement of corrosion resistance of used steel by applying the rosemary-doped PVTMS coatings.

Corrosion current density is commonly utilized as an important parameter to evaluate the kinetics of corrosion reactions. The corrosion rate is normally proportional to the corrosion current density measured through polarization. In this study, the bare steel substrate dissolved far more quickly than any coated systems. By examining the current density at the same polarized potentials, a significant reduction of dissolution current due to applying PVTMS coating can be observed. Moreover, the reduction in anodic current densities became more significant by doping PVTMS coatings with rosemary extract and was proportional to the inhibitor content in the applied coatings. The coating with 0.1% inhibitor depicted a pseudo-passive behavior with the lowest anodic current density. This represents the lower corrosion rate of the coated systems, and is easily interpreted as shielding protection of the substrate by barrier coatings. It is also observed that the inhibition efficiency increases with increasing concentration of the inhibitor content in the coating. Highest inhibition efficiency was obtained for coating with 0.1% rosemary. It has to be mentioned that inhibition efficiency obtained by adding 0.05 and 0.1% rosemary was not large, which can be set as threshold amount for the added inhibitor.

Figure 5 shows the scanning electron micrographs for the bare metal, doped/undoped PVTMS thin films after potentiodynamic polarization in a SBF solution. With comparison of SEM micrographs at the same magnifications between doped and undoped PVTMS thin films, detected doped films have a smooth surface and are crack-free. After electrochemical tests were carried out on doped PVTMS thin film, no localized corrosion was detected, but for bare metal and undoped PVTMS thin film, localized corrosion was observed in the form of small cracks and pits of different size. According to Fig. 5(a) and (b), the region surrounding some of the localized corrosion were damaged, and this may indicate a preferential localized attack. This localized attack can lead to delamination and lifting of the coating from the metal surface. Moreover, localized corrosion is known as the most dangerous type of corrosion for metallic orthopedic implants, since it is eventuated to permeate noxious component from metal structure to body environment that occasionally lead to death. Therefore utilized inhibitor in micro structure of coat can increase the corrosion resistance for this form of corrosion.
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Fig. 5

SEM images of (a) the bare metal, (b) undoped, and (c) doped PVTMS thin films after potentiodynamic tests in SBF solution

EIS Measurements

EIS measurements are particularly useful in long time tests because they do not perturb the system dramatically, and it is possible to monitor the gradual change of the coating-metal system over time.

The typical Nyquist and Bode plots, i.e., impedance and phase angle plots of doped/undoped PVTMS thin film and their comparison with the bare metal after 1 h and 21 days, are shown in Fig. 6(a)-(c).
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Fig. 6

(a) Nyquist, (b) Bode plot, and (c) phase angle plots of undoped and doped PVTMS thin film with 0.05% rosemary extract after soaking in SBF for 1 h and 21 days and their comparison with the bare metal

The Nyquist plots of uncoated/PVTMS steel at the immersion times 1 h and 21 days are characterized by a depressed semicircle, while the plots of coated steel in the presence of the inhibitor at the soaking time of 1 h as well as 21 days present a depressed semicircle with a long tail at the low frequency region. The tail is inclined at an angle of 45° to the real-axes at very low frequency. This behavior indicates that the diffusion process of ions takes place on the coated specimen after the addition of rosemary-extract inhibitor. The Bode plot for the PVTMS coated steel (doped and undoped) show higher impedance magnitudes at low frequency than the plain steel in test solution. Nevertheless, these values tend to decrease after 21 days immersion indicating the decrease in polarization resistant of both coated systems with and without inhibitor. However, phase angle plots of uninhibited/inhibited PVTMS coated steel were different after 21 days, this gives an indication to the different corrosion behavior of both systems, moreover, more precise information about the behavior of the studied system can be obtained from phase angle diagrams. The PVTMS coated steel show the formation of new phase angle at low frequencies after 21 days immersion which is an indication to coating delamination taking place at coating/steel substrate due to the water uptake. This feature was not observed for the doped sample, but rather a shift towards lower frequency, indicating that the major protection effect is due to the inhibitor added to the coating. The presence of the inhibitor increases the impedance and changes the other aspects of the behavior. These results support the results of polarization measurements that the inhibitor improved the protection behavior of the coating.

For the interpretation of the electrochemical behavior of a system from EIS spectra, an appropriate physical model of the electrochemical reactions occurring on the electrodes is necessary. The electrochemical response to impedance tests for the coated materials under consideration was best simulated with the equivalent circuit depicted in Fig. 7. This widely accepted scheme has been deduced to represent the electrochemical behavior of metal covered with an unsealed porous film (Ref 19, 20). The equivalent circuit consists of: a solution resistance Rs of the test electrolyte, a capacitance Cdl, and polarization resistance Rp for defects in the coatings, and a capacitance Ccoat and Rcoat for the remainder of the coating layer regarded as intact (non-defective).
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Fig. 7

Equivalent circuits for (a) undoped PVTMS and (b) doped PVTMS thin film with 0.05% rosemary extract

The inhibition efficiency of undoped and doped PVTMS thin film with optimum rosemary extract content and of the bare metal after 1 h and 21 days immersion in SBF solution, respectively, was evaluated by Rp and Cdl values of the impedance. Values are given in Tables 3 and 4, respectively
$$ C_{\text{dl}} = 1/\left( {R_{\text{p}} \times { 2}\Uppi \, F_{ \hbox{max} } } \right), $$
where Cdl and Rp and Fmax are double layer capacity, polarization resistance, and frequency maximum, respectively.
$$ {\text{IE\% }} = \left( {R_{ 2} - R_{ 1} } \right)/R_{ 2} \times 100 $$
Data presented in Tables 3 and 4 show that the values of Rp increase with adding inhibitor to the coating, while Cdl tend to decrease. A large Rp is associated with a slower corroding system (Ref 21). Furthermore, a better protection provided by an inhibitor is associated with a decrease in Cdl (Ref 21). The decrease in Cdl, which results from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer (Ref 22). It follows from the data in Tables 3 and 4 that Cdl is decreased upon adding inhibitor to the coating. These results suggest that rosemary extract enhanced the corrosion protection of the applied coating on steel. Inhibition efficiencies obtained from Tafel extrapolation and impedance methods agree well.
Table 3

Impedance parameter of undoped and doped hybrid sol-gel thin film with 0.05% rosemary extract and their comparison with the bare metal in SBF solution at 310 ± 1 °C after 1 h soaking

Sample

Rp, kΩ cm2

Cdl, μF/cm2

% IE

Bare metal

16.397

61

No inhibitor

52.463

48

0.05% Rosemary extract

436

13.9

88

Table 4

Impedance parameter of undoped and doped hybrid sol-gel thin film with 0.05% rosemary extract and their comparison with the bare metal in SBF solution at 310 ± 1 °C after 21 days soaking

Sample

Rp, kΩ cm2

Cdl, μF/cm2

% IE

Bare metal

16.397

61

No inhibitor

49.924

53

0.05% Rosemary extract

340

18.1

85.3

Inhibition Mechanism

The rosemary contains some tannic acid and the main chemical composition of rosemary extract is rosmarinic acid (is a phenolic compound (Ref 23)) which contains polyphenolic compounds and readily form complex with di- and trivalent metal ions (Ref 24, 25). The inhibitor action of this compound could be explained due to the formation of complexes in the form of chelate with iron ions in the solution. The Fe3+ ion is coordinated with the phenolic groups in the terminal side in each molecule taking phase as shown Fig. 8. The adsorbed layer acts as a barrier between the metal surface and aggressive solution leading to a decrease in the corrosion rate.
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Fig. 8

Chemical structure of the complex formed

The presence of more than one active centre in the chemical composition of rosmarinic acid forces the rosemary extract to be horizontally oriented at the metal surface, which increases the surface coverage and consequently increase the inhibition efficiency.

As shown in electrochemical results, this component via interfere in anodic reactions lead to decrease of reaction rate therefore decreased βa in polarization curves and increase of corrosion resistance. The sol-gel thin film resistance of inhibitor-doped PVTMS coating is more than one order of magnitude larger than the pore resistance of undoped PVTMS coating at the beginning of immersion in SBF. During corrosion tests new defects appear in all the coatings leading to formation of conductive pathways and decreasing pore resistance of coatings. However even after a long immersion the pore resistance of inhibitor-containing film is sufficiently higher confirming superior stability and barrier properties.

The higher corrosion protection in the case of inhibitor-doped coatings is most probably related to blocking of pores and defects by insoluble complex of rosmarinic acid with iron.

Bioactivity Evaluation

Figure 9 shows the diffuse reflectance FTIR spectra of the doped PVTMS thin film before and after 7 and 21 days reaction in vitro. As shown in Fig. 9(a), vibrational peaks are observed only for siloxane network before exposure to SBF. However, after exposure and as illustrated in Fig. 9(c), the spectrum reveals a pair of hydroxyapatite peaks. The silicon-oxygen-silicon rocking vibration peak at 475 cm−1 is diminished in the sample after reaction and replaced by the oxygen-phosphorous-oxygen bending vibrations of the hydroxyapatite PO43− groups at 598 and 566 cm−1 (Ref 26). Since the penetration depth of the infrared beam is very small (less than 1 μm), it can be assumed that the hydroxyapatite peaks arise from a surface layer formed on the thin film.
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Fig. 9

FTIR spectra of doped PVTMS thin film after soaking in SBF for 0 (a), 7 (b), and 21 (c) days

The EDS study reveals the inclusion of phosphorus in the composition of the newly formed layer after soaking in SBF 21 days (Fig. 10). The phosphorus present on the newly formed layer proceeds from the SBF solution. Figure 2 shows the EDS results for the doped hybrid sol-gel thin film before soaking in SBF. It shows an increase in the calcium content and a decrease in the silicon content. Changes in the chemical composition were accompanied by a distinct change in the morphology of the layer surface. For undoped PVTMS thin film, SEM at 50,000× (Fig. 10a) shows that this thin film has many particles on its surface proving slight formation of apatite layer. For doped PVTMS thin film, SEM at the same magnification (50,000×) indicates the presence of abundant spherical shapes with their accumulation on each other to form a bone-like apatite layer. Accordingly, these observations indicate that doped hybrid sol-gel thin film has become more bioactive than undoped PVTMS thin film.
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Fig. 10

SEM image of the surface of doped PVTMS thin film after soaking in SBF for 28 days

Conclusion

In order to improve corrosion protection for a long term, a green corrosion inhibitor (rosemary extract) has been incorporated into sol-gel matrix. SEM and EDX analyses have been used to investigate the morphology and composition of the doped sol-gel coatings. EIS measurements have been employed to model the sol-gel film/stainless steel 316L interface and to follow the corrosion process in SBF solution. According to the obtained results, hybrid organic-inorganic thin films preloaded with green corrosion inhibitor, detected smooth and crack-free coating. This coating provides a little barrier protection that with the incorporation of rosemary inhibition efficiency arrived to higher than 90%. The inhibition mechanism of rosemary extract was a mixed-type mechanism with more effect on anodic curve. The PVTMS coating incorporated with 0.05% rosemary extract produced maximum inhibition. This additive could be a prospective candidate for the development of new environmentally friendly pretreatments. Also doped PVTMS thin film on stainless steel 316L can be considered as a bioactive thin film which can provide better performance for stainless steel 316L for biomedical applications.

Acknowledgments

This research has been supported financially by Najafabad Branch, Islamic Azad University.

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© ASM International 2012