Liquid phase formation of alkyl- and perfluoro-phosphonic acid derived monolayers on magnesium alloy AZ31 and their chemical properties
Graphical abstract
This study demonstrates preparation and estimation of chemical properties of phosphonic acid derived self-assembled monolayer prepared on magnesium alloy by liquid phase.
Highlights
► Phosphonic acid derived self-assembled monolayers (SAMs) were formed on magnesium alloy by liquid phase method. ► Chemical bonding states between the SAMs and magnesium alloy were investigated using X-ray photoelectron spectroscopy. ► Chemical stability and corrosion resistance of the SAMs on magnesium alloy increased with an increase in the alkyl chain length.
Introduction
Magnesium is one of the light metals and has excellent physical and mechanical properties such as low density, good electromagnetic shielding, and high strength/weight ratio [1], [2]. In addition, it is the eighth most abundant element in the Earth’s crust by mass. Thus, magnesium and its alloys are expected to be applied to various transportation equipment industries such as automobiles, railways, and aerospace [3], [4], [5]. Magnesium is the eleventh most abundant element by mass in the human body, so its ions are essential to all living cells, where they play a major role in manipulating important biological polyphosphate compounds like ATP, DNA, and RNA. Thus, magnesium alloys are also expected to be applied to potential biodegradable bone implant materials due to their biodegradability in the bioenvironment [6], [7], [8], [9]. However, a great issue to apply magnesium alloys to these applications is the rapid corrosion of magnesium alloys in chloride containing solutions including environmental humidity and human body. Therefore, it is very important to improve the corrosion resistance of magnesium alloys. An effective mean to achieve this is appropriately to control interface regions between the magnesium alloys and environment by surface modification using ultrathin organic layer. Self-assembled monolayer (SAM) is a potential candidate for controlling the interfaces because they have their ease of manufacture without external aid and strong chemical bonding to the substrate. Typical SAMs, such as alkanethiols reacted with gold and alkylsilanes with silica, have been studied extensively as demonstrated by numerous reports in the literature [10]. In addition, the studies of the properties of n-alkanoic acids reacted with oxidized aluminum [11], [12] and magnesium alloys have been reported [13].
Phosphonic acid derived SAMs are believed to create a more robust modification layer compared to those created from chemisorbed carboxylic acids, especially after annealing to maximize the number of P–O bonds to the metal oxide lattice [14], [15]. Thus, the interactions between phosphonic acid and metal oxides have been studied on engineering materials surfaces such as steel [16], titanium [17], [18], [19], [20], [21], tantalum[22], [23], [24], copper [25], zirconium [19], [26], iron [21], [27], aluminum [21], [25], [28], [29], and silicon [26], [30], [31]. Giza et al. investigated tailoring of oxide chemistry on aluminum by means of low-pressure water and argon plasma surface modification to influence the kinetics of the self-assembly process of octadecylphosphonic acid monolayers [32]. They concluded that the hydroxyl-rich surface led to significantly accelerated adsorption kinetics of the phosphonic acid on the aluminum oxides. Hoque et al. performed the comparison studies of perfluorodecyldimethylchlorosilane (PFMS), octadecylphosphonic acid (ODP), decylphosphonic acid (DP), octylphosphonic acid (OP), and perfluorodecylphosphonic acid (PFDP) from view points of chemical stability and robustness [33]. They showed the stability against warm nitric acid of ODP/Al SAM to be the most stable followed by PFDP/AS, DP/Al, PFMS/Al, and OP/Al SAM. Ito et al. prepared octadecylphosphonic acid (ODPA) on GaN substrates and characterized them from the viewpoint of chemical stability [34]. They showed that as-prepared ODPA monolayers were desorbed from the GaN substrate by soaking in an aqueous solution, whereas ODPA monolayers heated at 160o exhibited suppressed desorption in acidic and neutral aqueous solutions. It is known that the interaction or chemical bonding states change as being monodenate [22], [26], [29], bidenate [17], [24], or tridenate [35], [36], depending on the materials surfaces. Various interactions between phosphonic acid and metal oxides have been widely investigated; however, the preparation and chemical properties of phophonic acid on magnesium alloys have been not yet investigated.
In this paper, we report the preparation of alkyl- and perfluoro-phosphonic acid SAMs on magnesium alloy. The chemical and anticorrosive properties of the prepared SAMs on magnesium alloys were characterized using X-ray photoelectron spectroscopy (XPS), contact angle measurements, atomic force microscopy (AFM), and electrochemical measurements.
Section snippets
Experimental procedures
Magnesium alloy AZ31 (composition: 2.98% Al, 0.88% Zn, 0.38% Mn, 0.0135% Si, 0.001% Cu, 0.002% Ni, 0.0027% Fe, and the rest is Mg) with a thickness of 1.5 mm was used as the substrate. Prior to the preparation of phosphonic acid SAM, all the magnesium alloy substrates were polished using some SiC emery papers and alumina suspension containing alumina nanoparticles with a diameter of 50 nm by buffing machine, resulting in the mirror finished surface. The substrates were then ultrasonically cleaned
Preparation and characterization of alkylphosphonic acid derived monolayers
Phosphonic acids are known to interact strongly with transition metal oxides such as tantalum oxide (Ta2O5) [22] and titanium oxide (TiO2) [18], [43]. The phosphonic acids attach to the metal oxides (which are basic hydroxyl surfaces) via acid–base interactions that anchor the molecules to the surface. Thus, it is expected that the phophonic acid would attach to the MgO or Mg(OH)2 surface.
The wetting properties of the SAM modified magnesium alloy provide a simple assay of the quality of the
Conclusions
Alkyl- and perfluoro-phosphonic acid derived SAMs were successfully formed on Mg alloy by liquid phase method. The chemical and anticorrosive properties of the prepared SAMs on magnesium alloys were characterized using contact angle measurements, XPS, AFM, and electrochemical measurements. In the case of alkylphosphonic acid SAMs (OP, DP, and ODP), the advancing and receding water contact angles increased with an increase in the preparation time. The maximum advancing and receding water contact
Acknowledgment
This research was supported by the Adaptable and Seamless Technology Transfer Program through Target-driven R&D (A-STEP) from Japan Science and Technology Agency and was carried out by the joint research program of the EcoTopia Science Institute, Nagoya University.
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