Anti-corrosion performance of a new silane coating for corrosion protection of AZ31 magnesium alloy in Hank's solution
Highlights
► New ormosil coating for improved corrosion protection of bioresorsable Mg alloys. ► New equivalent circuit model for interpretation of the EIS data. ► Surface conditioning is a key step for improved corrosion resistance of the coating.
Introduction
Because of their mechanical and physical properties, various magnesium alloys have been considered as interesting biometallic materials for degradable orthopaedic implants [1], [2]. Magnesium and its ions are essential to human metabolism and are naturally found in bone tissue [2]. It has also been reported that magnesium-based implants can stimulate the development of a hard callous at fracture sites [3].
Currently, the main limitation for the use of magnesium and its alloys as metallic implants is the fast degradation rate in the physiological environment and risks of excess of Mg2+ ions or H2 release in the body. In order to decrease the corrosion rate to biocompatible levels and to enhance the mechanical properties of magnesium, some alloys such as AZ31, AZ91, WE43, WZ21 [3], [4], [5], [6], [7] were introduced for biomedical applications and suitable biocompatibility has been reported. There are several Mg alloys that have been tested as bio implants with reduced toxicity and ability to enhance cell proliferation. One of the most used is AZ31, despite the presence of aluminium. Other Mg alloys are also very promising such as some rare-earth containing ones. Nevertheless, the high corrosion rate of these alloys in physiological solution is still the main drawback; hence some surface treatments are required to decrease the corrosion rate in such media.
Surface pre-treatments and application of coatings are therefore essential to delay the corrosion activity. Simple surface treatments such as immersion in HF have been proposed to improve corrosion resistance. However, in general, the protective performance achieved by this treatment is low [8], and much better results can be obtained combining it with polymeric coatings. It was found that this combination enhances adhesion and the corrosion resistance, as reported elsewhere [9].
It has been demonstrated that the most effective method to delay corrosion activity on Mg alloys is the application of surface coatings. Different coatings and various application procedures have been suggested, such as plasma spray [10], anodising [11] and sol–gel processes [12].
Thin hybrid organic–inorganic sol–gel coatings are an attractive approach to overcome the problem of corrosion [13] in many metallic materials [12], [14]. Glycidoxypropyltrimethoxysilane (GPTMS) is one of the most applied silanes and it is used as binder in paints or as filler in inorganic and composite materials. Underhill et al. [15] have studied the deposition of GPTMS on aluminium and Montemor et al. [16] have used the bis-[triethoxysilylpropyl] tetrasulfide silane (BTESPT) to increase corrosion protection of AZ31. Some authors propose to use a combination of silanes such as glycidoxypropyltrimethoxysilane (GPTMS), tetraethylorthosilicate (TEOS), tetramethoxysilane (TMOS), 3-aminopropyltrimethoxysilane or 3-(2-aminoethyl) aminopropyltrimethoxysilane, to achieve better corrosion protection [17].
This paper demonstrates the protective performance of new formulations of silane based coatings modified with diethylenetriamine (DETA), as cross linker, on the corrosion resistance of AZ31 magnesium alloys immersed in Hank's solution. This modified silane coating was applied by dip coating and the effect of the surface conditioning prior to coating application on the overall performance of the coated system was investigated by electrochemical impedance spectroscopy. Moreover, new equivalent circuit models are proposed to interpret the EIS data and allow concluding that a combined hydrofluoric acid etching and silane coating provide a significant delay of the corrosion activity.
Section snippets
Experimental procedure
The substrate material used for the present investigation was commercial AZ31 magnesium alloy, cut in coupons having dimensions of 30 mm × 25 mm × 2 mm that were mechanically polished with SiC paper. A set of coupons was pre-treated by immersion in 12% HF for 15 min, while others were treated electrochemically in order to promote the growth of a passive film. For this purpose, the samples were immersed in 0.1 M NaOH (pH 12) and were polarised for 5 h at a constant potential of 0 V, which is in the
Results and discussions
Fig. 1a and b shows the Bode plots of blank AZ31 (mechanically polished coupons) immersed for 8 h in Hank's solution. This solution contains chloride ions, which are known as very aggressive for Mg alloys, enhancing its corrosion activity [20]. The impedance spectra reveal a small increase of impedance values during the first hours of immersion and a sudden drop after 8 h of immersion. The early increase, observed during the first 4 h of immersion is probably due to the formation of a compact and
Conclusions
In this study, a new modified glycidoxypropyltrimethoxysilane (GPTMS) coating proposed for corrosion protection of AZ31 magnesium alloy is reported and the effects of different pre-treatments before silane coating formation were investigated.
The SEM results revealed that neither the thickness nor the morphological features are influenced by the surface pre-treatment. The thickness of the silane coating is approximately 2 μm and the coating comprises two distinct regions: on outer porous layer
Acknowledgement
The authors acknowledge the financial support of the project from ERA MNT/0001/2009.
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