The surface chemistry of 3-mercaptopropyltrimethoxysilane films deposited on magnesium alloy AZ91
Introduction
Magnesium and its alloys have excellent mechanical properties with a specific density that is 1/4 that of steel and 2/3 that of aluminum. These properties make it a desirable light weight construction material for a number of applications. Unfortunately, these materials are also highly susceptible to corrosion, particularly in the presence of aqueous solutions [1]. This has limited their use in the automotive, aerospace and biomedical industries, where exposure to solutions containing ions such as chloride, sulphate and phosphate is unavoidable. In order to improve the corrosion resistance of magnesium and its alloys, protective coatings can be applied [1]. However, the development of protective coatings on high aluminum content magnesium alloys has proven to be a challenge for these materials [1]. This is due to both their multi-phase surface microchemistry and rapid surface oxidation. In particular, the formation of intermetallic (Mg17Al12) species at the grain boundaries, results in a non-uniform surface which greatly complicates the coating process [1].
Organosilanes are ambifunctional molecules with the general structure (R′O)3Si–R–X [2]. Where R is an alkyl chain, R′ is typically CH3 or C2H5 and X can be a number of different functional groups such as S–H, –NH2, –OH and –COOH. The X group is typically chosen for its ability to interact with a specific organic coating or molecule that will be subsequently deposited on top of the organosilane layer.
These molecules are commonly used as adhesion promoters between inorganic substrates and organic coatings.
The alkoxysilane part of the molecule can covalently bond with a variety of mineral or metal surfaces through complex hydrolysis/condensation reactions where Si–O–Metal bonds are ultimately formed. These reactions are typically done by immersing the substrate in an organic solvent that may also contain small amounts of water.
In order to bond to the surface, organosilanes must undergo both hydrolysis and condensation reactions. These reactions are shown below:RSi(OR′)3 + 3H2O → RSi(OH)3 + 3R′OH — HydrolysisRSi(OH)3 + MOH → RSi(OH)2–O–M + H2O — Condensation.
It is commonly believed that organosilanes can only undergo condensation reactions with a metal surface after hydrolysis of the molecule in solution has occurred. However, some researchers have speculated that the silanes may initially hydrogen-bond with the substrate and then subsequently form covalent bonds upon curing [3]. The rate of hydrolysis depends on several factors, such as the functional group (X) on the alkyl chain, the availability of water and the pH of the coating bath [4], [5]. In general, the rate of hydrolysis has been shown to increase at lower pH while the condensation reaction is favoured at higher pH [4]. The condensation reaction can occur either between the organosilane molecules themselves or with the substrate surface. The net reaction, a sum of the hydrolysis and condensation steps, is dependent on the availability of water, the nature of the surface to be grafted, the chemical functionality of the silane and the pH of the original solution [5]. The quality of the coating produced is also affected by factors such as curing and solution concentration. For example, it has been found that curing of the film after deposition leads to better cross-linking between the organosilane molecules. Furthermore; the solution concentration has been shown to have a significant influence on the film thickness [3].
Although organosilanes are most commonly used as adhesion promoters, these coatings have also been shown to improve formability [7], enhance corrosion resistance [8], [9], increase scratch resistance [7] and improve wear resistance [10] of a variety of different materials. They have become of considerable interest for treating aluminum alloys because they have been shown to provide both corrosion protection and improved adhesion [6].
Although the interaction of these molecules with materials such as silicon, aluminum and steel has been studied, relatively little has been reported on their interaction with magnesium alloy surfaces [11], [12], [13]. Both Zucchi et al. and Khramov et al. reported that organosilane based coatings positively impact the corrosion resistance of magnesium alloys. Zucchi et al. further proposed that the Si–O–Mg interaction should be strong due to the strong affinity of the silanol (–Si–O–H) functional group for basic oxides. However, Mitchell et al. reported that the chemical structure of the organosilane played a significant role in the stability of the coatings produced and that coating thickness plays a critical role in the protective properties of the organosilane film.
The purpose of this study was to investigate:
- 1.
The influence of pH, solution concentration and deposition time on organosilane film formation on AZ91 magnesium alloy substrates.
- 2.
The influence of the substrate surface microchemistry on the coating uniformity.
- 3.
The nature of the chemical interaction at the organosilane/magnesium substrate interface.
The organosilane of interest in this study was 3-mercaptopropyltrimethoxysilane (MPTS). This organosilane is of particular interest due to the strong interaction of thiols with silver [14]. This could be of benefit for the use of organosilanes as a pre-treatment prior to electroless plating on magnesium alloys. The chemical structure of the organosilane used in this study is shown in Fig. 1.
Section snippets
Sample preparation
Prior to coating, Mg alloy AZ91 plates (Lunt Technologies Inc.) were cut into 1 inch by 1/2 inch coupons and were sequentially polished to a 1 micron surface finish with diamond suspension polishing oil purchased from Buehler. After polishing, the samples were ultrasonically degreased in acetone followed by methanol and finally deionized water in order to remove any excess oil. In order to remove residual organics and to increase the concentration of hydroxyl groups at the magnesium alloy
Influence of coating bath pH
The influence of coating bath pH was investigated for pH 4, 7 and 10 solutions. The surface chemistry and morphology of the films deposited from each solution were studied by a combination of ATR-FTIR and SEM/EDS. Fig. 2 shows the ATR-FTIR spectra obtained for organosilane coatings deposited from pH 4, 7 and 10 solutions. Characteristic peaks for MPTS were observed on the magnesium alloy coupons at all 3 pH values. The low intensity peak at 2543 cm− 1 is characteristic of the S–H stretching band
Conclusions
The influence of coating bath parameters, substrate microstructure and pre-treatment on the deposition of MPTS onto magnesium alloys has been discussed. The surface chemistry of the magnesium/MPTS interface has also been studied. These studies have shown that MPTS coatings are uniformly deposited on all phases of the AZ91 magnesium alloy surface at a pH of 4. The concentration of the MPTS coating bath was found to have a significant influence on film thickness while deposition time was shown to
Acknowledgements
The authors gratefully acknowledge the financial support of the Ontario Centers of Excellence, Meridian Technologies Inc., the Canada Foundation for Innovation and the Ontario Research Fund. We would also like to thank Mr. Mark Biesinger of Surface Science Western for his help with the X-ray photoelectron spectroscopy studies.
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