Improvement of the properties of AZ91D magnesium alloy by treatment with a molten [AlCl.sub.3]-NaCl salt to form an Mg-Al intermetallic surface layer.
Keywords Magnesium alloy, Intermetallic compounds, Coatings, Corrosion
Magnesium is an attractive metal because of its low density and its relative abundance in earth's crust and seawater. Thus, magnesium and its alloys are particularly suitable for weight-saving constructions in aircraft industry, vehicle production, and portable electric devices. (1) However, poor corrosion resistance and low wear resistance of these alloys often limit their application. To improve the corrosion resistance of magnesium alloys, various surface modification methods have been proposed and tried, (2-5) such as laser and ion beams, physical vapor deposition (PVD), chemical vapor deposition (CVD), chemical conversion, anodization, electrochemical plating, organic coating, etc. All these surface modification methods aim at improving the corrosion resistance of magnesium alloys.
Some of the above-mentioned methods have been claimed to be able to offer good corrosion protection to magnesium alloys. Nevertheless, all these techniques have some inevitable disadvantages. (6-12) For example, laser, ion beams, PVD, and CVD are expensive. Chemical conversion, anodization, and plating face environmental pressure: and an organic coating normally needs a chemical conversion or anodized coating to prime the substrate first. The environmental and health risks associated with the use of [Cr.sup.6+] ions make it likely that the use of such coatings will be severely restricted in the near future. It is, therefore, necessary lo investigate other nontoxic coating processes with equal or even better corrosion protection. (13)
Based on the understanding of the role of the [beta]-phase ([Al.sub.12][Mg.sub.17]) (12), (14-16) in reducing the corrosion rate if the volume faction of the phase in the "skin" of a magnesium alloy is high enough, Song in 1997 (17) pointed out that an alloy would become very resistant to corrosion without sacrificing its mechanical properties if it was designed to have a skin mainly consisting of the [beta]-phase. It was reported that when magnesium alloys are immersed into the molten Na[BF.sub.4] melting point of which is about 657 K. lower than that of magnesium, magnesium fluorides can be formed on the surface of the alloys. (18) However, the film shows a porous microstructure.
The electrodeposition of aluminum on magnesium alloy in molten aluminum chloride (Al[Cl.sub.3])-l-ethyl-3-methylimidazolium chloride (EMIC) ionic liquid have been reported. (19) Molten salt surface treatment of a protective alloying layer on the AZ91D alloy, which has never been published to the best of our knowledge, is attempted in this study. Moreover, the improvement in corrosion resistance will be investigated for the first lime. Al element was chosen for the protection layer on the Mg alloy due to lour main factors. (19) (1) Being a lightweight metal. Al does not increase the overall density significantly. Moreover, the corrosion resistance of Al and its alloys is satisfactory. (2) A coated Mg alloy can generate a continuous Mg-Al intermetallic compound near the surface. It has been confirmed that this compound possesses good corrosion resistance. (12), (14-16) (3) Good recycling it materials can be maintained because Al is a primary alloy element for the AZ series Mg alloys. (4) Al[Cl.sub.3]-NaCl molten salt has; i low melting point of 154[degrees]C.
AZ91D magnesium alloy (Table 1) was used in the as-received condition. The AZ91D magnesium alloy ingot was cut into [PHI] 20 mm x 5 mm circle specimens and polished using up to [1000.sup.#] SiC paper. The specimens were washed with deionized water and acetone, and then dried.
The salt mixture for this study was selected based on the requirements of high corrosion resistance and a low melting point below 300[degrees]C. The salt mixture of 50% NaCl-50% [AlCl.sub.3] (molar ratio) meets these requirements. NaCl and anhydrous [AlCl.sub.2] were melted at 300 C under argon gas (99.999%) followed by HCl gas purification to remove oxide ions; argon was flushed again to remove dissolved HCl gas. The moisture and oxygen content were maintained in low level. Then the samples were immersed into the molten mixed salts for 7 h.
Table 1: Chemical composition of the AZ91D alloy (in wt%) Al Mn Ni Cu Zn Ca Si K Fe Mg 9.1 0.17 0.001 0.001 0.64 <0.01 <0.01 <0.01 <0.001 Balance
To study the effectiveness of molten salt treatment. the cross section morphology and microstructure of the treated specimen were observed with a scanning electron microscope (SEM). X-ray diffraction (XRD) technique was used for analyzing formed intermetallic compounds. Corrosion properties were studied in 5% NaCl solution by using the continued immersion lest and electrochemical polarization testing. The electrochemical polarization testing was performed using a SI-1287/SI-1260 electrochemical measuring system. A high density graphite electrode and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. In the polentio-dynamic polarization test, the potential was scanned from -1.9 V to -0.8 V (vs SCE) with a sweep rate of I mV/s. In addition, the measurement of microhardness of the intermetallic compounds and the substrate were conducted using a HX-1000 microhardness tester on the cross section of a coated coupon under a load of 25 g. The loading time was 30 s.
The morphology and microstructure of the molten salt alloying layer are shown in SEM photograph (Fig. 1). In Fig. 1, A is the AZ91D substrate and B is the Mg-Al intermetallic compounds. The compositions of the intermetallic compounds (B) were analyzed using XRD. Figure 2 shows an XRD pattern of the surface of molten salt treated specimen; it was found that the surface layer of treated specimen consisted of [epsilon]-Al-Mg, [Al.sub.12][Mg.sub.17], and [beta]-Al-Mg intermetallic compounds.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
The molten salt treated specimen and the untreated AZ91D specimen were immersed in 5 wt% NaCl solution. The results are listed in Table 2. The untreated AZ91D specimen suffered from severe localized corrosion after being immersed in NaCl solution for <2 h. For the molten salt treated specimen the first pitting appeared after exposure to NaCl solution for 36-72 h. This suggests that the molten salt diffusion coating significantly enhanced the corrosion resistance of AZ91D.
Table 2: Pitting corrosion of untreated and treated AZ91D specimen exposed to 5 wt% NaCl solution Specimen Untreated Treated specimen specimen Corrosion test Immersion Immersion Time to the first observed pitting 2 h 36-72 h
The above difference in the corrosion resistance between the molten salt treated and untreated AZ91D can be further confirmed by potentiodynamic tests. Polarization curves, corrosion potentials, and corrosion current densities are shown in Fig. 3 and Table 3. respectively. From Table 3, it can be seen that the corrosion potential of the molten salt treated specimen was 80 mV higher than that of the bare AZ91D specimen and the corrosion current densities of the treated specimen was decreased one order of magnitude compared to the untreated one. According to the electrochemical principles, the former would have less corrosion tendency than the latter. This is correspondent to the results of the polarization curve. In Fig. 3, the corrosion potential of the treated-specimen increased compared to the untreated specimen. On the other hand, the molten salt treated specimen shows passivation in the corrosion process. The treated specimen has a lower corrosion rate than the untreated one.
[FIGURE 3 OMITTED]
Table 3: Corrosion potential ([E.sub.corr], V) and corrosion current densities ([I.sub.corr]/area, A/[cm.sup.2]) in 3.5% NaCl solution Material [E.sub.corr] (V) [I.sub.corr]/area (A/[cm.sup.2]) Untreated AZ91D specimen -1.3892 1.5866 x [10.sup.4] Molten salt diffusion- -1.3096 1.7067 x [10.sup.-5] treated specimen
The microhardness test
Microhardness is an indicator for wear resistance that is also one of the important properties of a coating in practice. The low hardness HV (60-80) of AZ91D, together with its unsatisfactory corrosion resistance, has limited the practical applications of AZ91D. Figure 4 and Table 4 show the microhardness changes of the surface alloying layer and the substrate after molten salt treatment. In the surface alloying layer, the Mg-Al intermetallic compounds microhardness value was up to 330 HV, the microhardness value of the area near the Mg-Al intermetallic compounds was about 116 HV and the values of microhardness decreased with the depth increasing from surface coating to the substrate. The microhardness value of the Mg-Al intermetallic compounds in the alloying layer was much higher compared to HV 60 of AZ91D substrate.
[FIGURE 4 OMITTED]
Table 4: The results of microhardness test Test point (Fig. 4) Microhardness values (HV) 1 (AZ91D magnesium substrate) 60-80 2 (Near the substrate) 85 [+ or -] 7 3 (Near the Mg-Al intermetallic compounds) 116 [+ or -] 18 4 (Mg-Al intermetallic compounds) 330 [+ or -] 24
The cross section observations of molten salt treated specimen by SEM (Fig. 1) show that an alloying layer had been formed near the surface of the substrate. The XRD (Fig. 2) showed that the surface alloying layer consists of [epsilon]-AlMg, [Al.sub.12][Mg.sub.17], and [beta]-AlMg.
Rolland et al. proposed that Al[Cl.sub.3] species formed a complex ion, Al[Cl.sub.4.sup.-], in Al[Cl.sub.3]-NaCl molten salt system (20):
Al[Cl.sub.3] + NaCl [right arrow] NaAl[Cl.sub.4] (1)
Substitution reaction-related Al deposition occurs during dipping the substrates in molten NaAl[Cl.sub.4].
3Mg + 2NaAl[Cl.sub.4] [right arrow] 2Al + Mg[Cl.sub.2] + 2NaCl (2)
The Gibbs free energy of reaction (2) will follow with the formula of [DELTA]G = -461997 + 5.071T which was calculated using thermodynamic data. When the value of [DELTA]G is negative, the reaction (2) might occur. The temperature range is determined by the limiting condition:
[DELTA]G = -461997 + 5.071T < 0.
where T < 91105.7 K.
The Gibbs free energies of reaction (2) within the temperature range of 523~673 K is negative, which indicates that the reaction (2) might occur at the temperature from 523 to 673 K. Merge the reactions (1) and (2). the equivalent reaction can be obtained.
3Mg + 2[Al.sup.3l] [right arrow] + 3[Mg.sup.2k] (3)
The formation of the alloying coating may contain the following steps. Aluminum deposition causes Al atoms to diffuse into the AZ91D substrate, beginning with the contact interface between the magnesium surface and aluminum particles. Once the atoms of aluminum cross the interfacial barriers, they become part of a Mg-Al solid solution and continuously diffuse inward into the substrate due to concentration gradient. With increasing concentration of aluminum elements, aluminum would react with the magnesium first on the surface and then form Mg-Al intermetallic compounds. The processes of reaction, diffusion, and solid solution continuously occur during the treatment process.
Corrosion protection mechanism
Since the alloying layer is rich in Mg-Al intermetallic compounds (Fig. 2), improved corrosion resistance by this alloying layer can be expected. The results obtained from the continued immersion test (Table 2) and the potentiodynamic measurements (Fig. 3 and Table 3) demonstrate that the molten salt treated specimen has higher corrosion resistance compared with bare AZ91D magnesium alloy. Obviously, this result from the alloying layer formed on the surface of AZ91D magnesium alloy by molten salt method. The alloying layer was an effective corrosion barrier to decrease the corrosion rate for AZ91D magnesium alloy when exposed to 3.5% NaCl solution.
As shown in Figs. 1 and 2, the surface alloying layer consisted of three Mg-Al intermetallic compounds. Continuous Mg-Al intermetallic compounds act as corrosion barrier. Thus, both the molten salt treated specimen and the bare specimen seem to have a similar corrosion rate at the beginning when exposed to the test solutions. However, once the Mg-Al intermetallic compounds of the molten salt surface alloying layer are entirely exposed to the NaCl solution as an integrate corrosion barrier, the propulsion of the corrosion front is much slower. This could get confirmations from the experimental results (Fig. 3). At the beginning part of polarization curves, two specimens showed near-current densities. With the experiment proceeding, the difference of current densities became obvious. The polarization curve of the molten salt treated sample appears passivation in corrosion process. The molten salt alloying layer containing Mg-Al intermetallic compounds made the treated specimen to have better corrosion resistance than the untreated AZ91D specimen. Similar results have been seen by others. (21), (22) The study of Lunder et al. (22) shows that the [beta]-phase ([Al.sub.12][Mg.sub.17]) is inert to the chloride solution in comparison the magnesium matrix appeared to render magnesium electrochemically nobler in general. The intermetallic compound [Al.sub.8][Mg.sub.5] has similar properties in Mazurkiewicz's study. (21) Obviously, according to the experimental test result, the Mg-Al intermetallic compounds of the diffusion alloying layer had the similar role as [beta]-phase and [Al.sub.8][Mg.sub.5] in NaCl solution.
At given experimental conditions (573 K for 7 h), an alloying layer has formed on the surface of AZ91D specimen via molten salt process. The alloying layer was composed of the Mg-Al intermetallic compounds. Aluminum deposition in [AlCl.sub.3]-NaCl molten salt systems caused element Al to penetrate into the AZ91D substrate and form Mg-Al intermetallic compounds. Compared to untreated AZ91D specimen, the molten salt treated specimen has better corrosion resistance due to the formation of alloying layer. The existence of the molten salt alloying layer enhanced the surface corrosion potential and decreased the corrosion rate. The microhardness values of the alloying layer are much higher than that of the AZ91D substrate.
Acknowledgments This work was supported by the Cultural Fund for Major Program of Technology Innovation Engineering in Higher Education Institutions. China (Grant No. 707025). State Key Laboratory of Metal Matrix Composites and The Instrumental Analysis Center of Shanghai Jiao Tong University.
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M. He. L. Liu. Y. Wu. Z. Tang. W. Hu [??]
State Key Laboratory of Metal Matrix Composites.
Shanghai Jiao Tong University. Shanghai 200030. China
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