Electrochemical and In Vitro Behavior of Nanostructure Sol-Gel Coated 316L Stainless Steel Incorporated with Rosemary Extract
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- Motalebi, A. & Nasr-Esfahani, M. J. of Materi Eng and Perform (2013) 22: 1756. doi:10.1007/s11665-012-0448-0
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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.
Keywordscorrosiongreen corrosion inhibitornanostructure sol-gel coatingrosemary extractstainless steel 316L
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.
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
Pull-off adhesion test in dry conditions
Bond strength, MPa
18.8 ± 1.1
0.05% Rosemary extract
21.2 ± 1.1
Potentiodynamic Polarization Curves
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
0.012% Rosemary extract
0.025% Rosemary extract
0.05% Rosemary extract
0.1% Rosemary extract
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.
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 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.
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
Rp, kΩ cm2
0.05% Rosemary extract
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
Rp, kΩ cm2
0.05% Rosemary extract
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.
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.
This research has been supported financially by Najafabad Branch, Islamic Azad University.