Novel composite films prepared by sol–gel technology for the corrosion protection of AZ91D magnesium alloy
Introduction
Magnesium alloys have a number of advantageous physical and mechanical properties that make them attractive materials for many industrial applications, such as automotive, portable devices, aircraft, military equipment, orthopedic equipment, diving gear and sport goods. However, one of the main reasons limiting or even preventing larger scale use of magnesium alloys for various applications is their high corrosion susceptibility [1], [2], [3], [4], [5]. Therefore, improving the corrosion protection of Mg alloys without the loss of high strength/weight ratio is a real challenge that can lead to a break-through in many industrial areas. In recent years, a number of surface technologies have been employed to improve their corrosion resistance including chemical conversion coating [6], [7], anodizing [8], electrochemical plating [9], [10], [11], sol–gel process [12], [13], [14], [15], [16], [17], etc.
Among all the techniques, the sol–gel process has been demonstrated to be an effective and environment-friendly route to prepare films on metallic substrates at low cost [18], [19], [20], [21]. Some authors [12], [13], [14], [15], [16], [17] have reported the anticorrosion resistance of sol–gel thin films on magnesium alloys. Generally, the sol–gel process is widely used for the production of effective films starting from alkoxide precursors including Si, Al, Zr, Ce, etc., they are expensive and the process of hydrolysis and polycondensation reaction is not easy to control. While the sols using inorganic salts precursors such as Ce own a relatively low pH [22], [23], [24], [25] and magnesium alloys would be eroded by the sol, leading to poor coating adhesion and subsequent corrosion. In previous works [26], we developed a novel approach to prepare sol–gel films. In the novel sol–gel process, appropriate additive was used to stabilize and disperse uniformly the inorganic salts, the sols with appropriate pH could be applied directly on magnesium alloys. Compared with the traditional sol–gel process, the novel sol–gel technology offers important advantages such as lower cost, more environment-friendly and simpler application procedures easily adaptable within industry.
In recent years, composite films [15], [27], [28], [29] are of great interest in corrosion protection. Due to the structure limits of individual film, defects originated in individual film form a path for the corrosive medium to penetrate the substrate. Composite films decouple the defects on each individual film and the formation of such corrosive paths is minimized. In addition, film materials can be chosen to combine desired properties like low chemical reactivity and diffusion barriers.
The present work has two objectives: (i) To prepare Ce and Mg films on AZ91D Mg alloys using the novel sol–gel technologies, and study the characters of the both xerogels through TG–DTA, XRD and FT-IR measurements. (ii) To obtain suitable composite films on AZ91D Mg alloys via combination of single Ce and Mg films. The surface morphology and the corrosion resistance of the films were evaluated using SEM and electrochemical tests, respectively.
Section snippets
Substrates
Die-casted AZ91D magnesium alloy with a chemical composition (wt.%) of Mg–9.02Al–0.49Zn was used in this study. All samples were cut to the size of 18 mm × 22 mm × 4 mm and were polished with carborundum (SiC) papers up to 2000 grit. The polished samples were degreased by acetone in an ultrasonic bath for 10 min and dried in a steam of air.
Sol preparation
The ethanol and celloidin were added in a volume ratio 3:1, the concentration of cerium nitrate (Ce(NO3)3·6H2O) and magnesium nitrate (Mg(NO3)2·6H2O) was 0.13 M and
Thermal analysis
Fig. 3a and b displays the TG–DTA curves for Ce and Mg xerogels, respectively. The TG–DTA curves of the Ce sol (Fig. 3a) present two main steps of thermal decomposition: (i) An obvious mass loss (70 wt.%) between 60 °C and 150 °C assigned to the desorption of the water and ethanol, as well as the decomposition of celloidin and Ce(NO3)3·6H2O. The associated exothermic peak can be attributed to the total thermal effect which represents exothermic effect. The exothermic effect include the burning out
Conclusions
In the view of the results presented in this work the following conclusions can be drawn:
The novel Ce/Mg composite films were successfully prepared on AZ91D Mg alloy substrate via sol–gel process. Compared with the traditional method, this novel process using inorganic salts as precursors was lower cost, more environment-friendly and simpler application procedures easily adaptable within industry.
TG–DTA curves indicated the stable range of Ce sol was 180–350 °C, and that of Mg sol was 250–350 °C.
Acknowledgements
The authors thank the supports of the Natural Science Foundation of Chongqing, China (CSTC. 2005BB4055) and High-Tech Cultivation Program of Southwest Normal University (No. XSGX06).
References (30)
- et al.
Surf. Coat. Technol.
(2009) - et al.
Corros. Sci.
(2005) - et al.
Mater. Sci. Eng. A
(2004) - et al.
J. Alloys Compd.
(2002) - et al.
Prog. Org. Coat.
(2003) - et al.
Electrochim. Acta
(2007) - et al.
Surf. Coat. Technol.
(2005) - et al.
J. Alloys Compd.
(2009) - et al.
Surf. Coat. Technol.
(2004) - et al.
Thin Solid Films
(2005)
Surf. Coat. Technol.
Electrochim. Acta
Prog. Org. Coat.
Thin Solid Films
Prog. Org. Coat.
Cited by (25)
Magnesium alloys as extremely promising alternatives for temporary orthopedic implants – A review
2023, Journal of Magnesium and AlloysDopamine self-polymerized sol-gel coating for corrosion protection of AZ31 Mg Alloy
2023, Colloids and Surfaces A: Physicochemical and Engineering AspectsA novel sol-gel coating via catechol/lysine polymerization for long-lasting corrosion protection of Mg alloy AZ31
2023, Colloids and Surfaces A: Physicochemical and Engineering AspectsCoating of hydroxyapatite and substituted apatite on dental and orthopedic implants
2019, Handbook of Ionic Substituted HydroxyapatitesApplication of novel sol–gel composites on magnesium alloy
2019, Journal of Magnesium and AlloysCitation Excerpt :This fact shows that the cathodic reaction of the corrosion process was more efficiently inhibited by application of the composite silica films [43,60]. It has been frequently reported in the literatures [26,30,31,39,44] that the negative shift in the corrosion potential is not necessarily associates with increasing in the corrosion current density. In this case, the negative shift in the Ecorr may be ascribed to the changes in chemical composition at the interfacial region due to the formation of TiOSi and ZrOSi bonds.