Effect of melt temperature on the oxidation behavior of AZ91D magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres

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1 available at Effect of melt temperature on the oxidation behavior of AZ91D magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres Hukui Chen Department of Chemistry, Baoji University of Arts and Science, Baoji , China ARTICLE DATA Article history: Received 12 December 2008 Accepted 30 May 2010 Keywords: Magnesium melt Oxidation 1,1,1,2-tetrafluoroethane Temperature ABSTRACT The oxidation behaviors of AZ91D magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres at temperatures between 660 C and 760 C have been studied. The experimental results show that with the increase of melt temperature, the oxidation rate of molten AZ91D magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres increased and the oxidation kinetics changed from parabolic law to linear law. On the other hand, the amount of MgF 2 in the oxide film formed on AZ91D decreased, and the amount of MgO increased. The effect of melt temperature on the oxidation behaviors is primarily related to the relative content of MgO and MgF 2 in the film, as well as the diffusion rate and the evaporation rate of magnesium through the film Elsevier Inc. All rights reserved. 1. Introduction Molten magnesium and its alloy are commonly protected from oxidation and burning by the use of protective atmosphere containing SF 6 in melting and casting operations. However, the high greenhouse effect of SF 6 has caused the magnesium industry to look for its alternatives. In recent years, 1,1,1,2- tetrafluoroethane (HFC-134a) has been considered a most promising alternatives to SF 6, and much research has been carried out on the oxidation behaviors of molten magnesium and magnesium alloy in the atmosphere containing HFC-134a [1 3]. It has been found that the concentration of HFC-134a and melt temperature have a great effect on the oxidation of molten magnesium and its alloy in the atmosphere [4,5]. The influence of HFC-134a concentration on the oxidation of the melts has been discussed [6,7]. However, the effect of melt temperature on the oxidation of this alloy in atmospheres containing HFC-134a is still not clear. In the present study, we investigated the effect of melt temperature on the oxidation of AZ91D magnesium alloy in HFC-134a/air atmospheres and elucidated the relationship between melt temperature, the oxidation behavior and the structure of oxide film, which will benefit to better understanding the protective mechanism of HFC-134a for AZ91D magnesium alloy melt. 2. Experimental The main materials for the present study were AZ91D magnesium alloy and HFC-134a gas. The chemical composition of AZ91D magnesium alloy in weight percent is 9.06%Al, 0.70%Zn, 0.30%Mn, %Ni, %Fe, 0.025%Si, %Cu and Mg balance. The chemical composition of HFC-134a gas in weight percent is HFC-134a 99.8%, HCl %, and H 2 O 0.001%. The samples, cylinders of 3 mm height and 50 mm diameter, were sectioned from the central part of the as-cast ingot. Before each experiment, they were manually polished with grade 320 SiC paper and kept in a vacuum desiccator. Oxidation tests were performed in a thermo-gravimetric measuring instrument, which consisted of a recording electronic balance, a resistance furnace with sealing cover, a magnesia crucible which was hung into the resistance furnace from the bottom of the electronic balance with the platinum Tel.: ; fax: address: hkchen7115@yahoo.com.cn /$ see front matter 2010 Elsevier Inc. All rights reserved. doi: /j.matchar

2 895 Fig. 1 Curves of weight gain versus time of molten AZ91D magnesium alloy oxidized in 1%HFC-134a/air at different temperatures. silk and a gas system which provides a certain amount of gas mixtures of air and HFC-134a to the hot chamber. Before use, the crucible was heated until constant weight was achieved. Prior to oxidation, the air and HFC-134a, which were dried and purified by passage through columns of CaCl 2 and silica-gel desiccant, were mixed in the required proportion and then continuously fed into the hot chamber at 500 ml/min. After purging the gas mixture inside the chamber for at least 1 h, the sample was placed in the crucible that had been hung in the chamber and then heated to the desired reaction temperature at a rate of 100 K/min. The weight gains of the sample were continuously measured by an electronic balance with an accuracy of 0.1 mg in the atmosphere of air containing 1% HFC-134a at C for time intervals up to 150 min. After the oxidation treatment, the surface morphology and chemical composition of the oxide scale were investigated using a scanning electron microscope (SEM) equipped with an energy dispersive spectroscopy (EDS) system. The phase composition of the oxide scale was identified by X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS). Fig. 2 Surface film morphologies of AZ91D alloy after exposure to the atmosphere of air containing 1%HFC-134a at (a) 660 C, (b) 720 C, (c) 760 C and (d) 800 C.

3 896 MATERIALS CHARACTERIZATION 61 (2010) Results Fig. 1 shows the weight gain versus time curves of molten AZ91D alloy in 1%HFC-134a/air atmospheres at temperatures of 660, 720, 760 and 800 C. At the temperatures of 660 C, a slight weight increase, following a parabolic law, was observed, which indicated that the oxidation of molten AZ91D alloy was slow at the temperature. When the temperature increased to 720 C, a relatively low weight increase, following the parabolic law approximately, revealed that the oxidation was faster than at 660 C. Further increase of the temperature to 760 C made the alloy more prone to oxidation. As the temperature increased up to 800 C, the oxidation was accelerated and obeyed a linear law. The oxidation rate was greater than that at other temperature. These results indicate that an increase in melt temperature led to an increase of the oxidation rate of AZ91D alloy in 1%HFC-134a/air atmospheres and a change of the oxidation kinetics from parabolic law to linear law. Fig. 2 shows the SEM pictures of the oxide surface films formed on molten AZ91D alloy in 1%HFC-134a/air atmospheres at the above temperature range. At 660 C, a dense and compact protective oxide layer could be seen, although the surface was not very even, and there were few white grains (These grains were proved to be MgO particles by EDS analysis) in the film [Fig. 2a]. At 720 C, it can be seen from Fig. 2b that the oxide layer was still compact and protective, but contained a lot of white grains. At 760 C, the number of the white grains obviously increased [Fig. 2c], suggesting that the oxide film did not protect the AZ91D melt sufficiently. At 800 C, the porous and loose oxide layer was observed [Fig. 2d], which indicated that the oxide film did not protect the melt at all. These results revealed that the increase of melt temperature resulted in the deterioration of the protective effect of the oxide film for molten AZ91D alloy, that is, the acceleration of the oxidation of the melt. In order to explore the underlying reason causing the oxidation behaviors of AZ91D magnesium alloy in HFC-134a/ air atmospheres to change with melt temperature, EDS and XRD were used to analyze the element composition and oxidation products in the oxide scale formed on AZ91D in air containing 1%HFC-134a. The EDS analysis results of the film at different temperature are listed in Table 1. It is seen that the film contained Mg, F, O, C and Al elements. As the melt temperature increased, the content of fluorine element in the film decreased, and the content of oxygen element increased. The XRD analysis results of the film at 660 C and 760 C are shown in Fig. 3. As can be seen, the film was mainly composed Table 1 Element concentration of the surface films formed on AZ91D after exposure to air containing 1% HFC-134a at different temperatures for 2.5 h (at.%). Element Mg F O C Al 660 C C C C Fig. 3 X-ray diffraction patterns of the oxide film formed on AZ91D after oxidation in the atmosphere of air containing 1% HFC-134a at (a) 660 C and (b) 760 C. of MgF 2, Mg and C. However, MgO was surprisingly not detected at all. The Mg peaks may be caused by the X-ray radiation penetrating the thin film into the substrate metal. Considering the extremely high affinity of magnesium with oxygen, in order to determine the presence of MgO in the film formed in air containing 1%HFC-134a, the film at 660 C was also examined by XPS, and the results are shown in Fig. 4. As shown in Fig. 4a, there mainly existed F, O, Mg, A1 and C elements in the film. The Si element may be contaminant introduced in the process of preparation and analysis of the sample. This is in agreement with the EDS analysis results above. From Fig. 4b, it can be seen that Mg was present in two chemical states. The binding energy peak at 51.60±0.2 ev was assigned to MgF 2, the binding energy peak at 49.90± 0.2 ev was MgO. Since the intensity of the peak at 51.67±0.2 ev was greater than that of the peaks at 49.90±0.2 ev, the content of MgF 2 in the film was higher than that of MgO. From Fig. 4c, it can be seen that F was present in two chemical states. The binding energy peak at ±0.2 ev was attributed to MgF 2, the binding energy peak at ±0.2 ev was AlF 3, and the content of MgF 2 was much greater than that of AlF 3 in the film. In Fig. 4d, the binding energy peak at 75.74±0.2 ev was the

4 897 Fig. 4 XPS spectra of surface film formed on AZ91D after exposure to air containing 1% HFC-134a at 760 C (a) survey, (b) Mg2p, (c) F1s, (d) Al2p and (e) C1s. peak of AlF 3 and the peak at 73.43±0.2 ev was Al 2 O 3. Here the content of AlF 3 was much greater than that of Al 2 O 3. Fig. 4e showed that C was present in three chemical states. The lower binding energy peak at ±0.2 ev was attributed to simple substance of carbon and the peaks at ±0.2 ev and ± 0.2 ev resulted from CO and CO 2.COandCO 2 may come from the adsorption of CO and CO 2 in covering gas on the surface of MgF 2 and MgO film. CO and CO 2 could be negligible due to their very low content in the film. According to XPS analysis results, it is understood that MgO do exist on the surface film formed in air containing 1%HFC- 134a. The reason for the absence of MgO peaks in XRD results in the film may be that either MgO is amorphous or the amount of MgO is too little to be detected by XRD. Based on the results of EDS, XRD and XPS analysis, it can be concluded that the oxide film formed on AZ91D in HFC-134a/ air atmospheres mainly consisted of MgF 2, MgO, C and small amounts of AlF 3. As the melt temperature increased, the

5 898 MATERIALS CHARACTERIZATION 61 (2010) amount of MgF 2 in the film decreased, the amount of MgO increased, but the change trend of C is not clear. 4. Discussion From the results obtained in the present study, it is found that the oxidation rate of molten AZ91D magnesium alloy in HFC- 134a/air atmospheres increases with increasing the melt temperature. The main reason can be summarized as follows. First, it can be seen from the experimental results obtained above that as the melt temperature increased, the amount of MgF 2 in the oxide film formed on AZ91D decreased, and the amount of MgO increased. Since MgO (the density α is 0.81) is less compact than MgF 2 (the density α is 1.60), the increase of MgO in the film would reduce the compactness of the film, weaken the protection of the film for molten AZ91D magnesium alloy, and consequently result in the acceleration of the melt oxidation. According to the theory of chemical dynamics, the increase of melt temperature can speed up the reaction of magnesium with O 2 and F 2, a main decomposition product of HFC-13a at high temperature, resulting in the increase of the amounts of MgO and MgF 2 in the oxide film. However, it is probably because the activation energy of the reaction of magnesium with O 2 is higher than that of magnesium with F 2, the increment of the reaction rate of magnesium with O 2 with increasing temperature is greater than that of the reaction of magnesium with F 2, which lead to the amount of MgO increase with temperature but MgF 2 decrease. Second, the increase in melt temperature accelerated the diffusion of magnesium through the oxide film, and sped up the oxidation of molten AZ91D alloy. The diffusion of magnesium through MgO crystal lattice can be expressed as follows: D =1: expð 150; 000 = RTÞ where D is the diffusion coefficient of Mg in MgO crystal lattice, m 2 s. It can be seen from Eq. (1) that the diffusion of magnesium through MgO film quickens with increasing temperature. Although the diffusion rate of magnesium through the mixed film of MgO and MgF 2 may be lower than through MgO film, the change rule of the diffusion coefficient of magnesium in the mixed film with temperature is the same as magnesium in MgO film. Therefore, the increase of melt temperature accelerates the diffusion of magnesium through the oxide film, and speeds up the oxidation of molten AZ91D alloy. Third, increasing melt temperature sped up the evaporation of magnesium, and accelerated the oxidation of molten AZ91D alloy. The evaporation rate of magnesium in vacuum can be described by Arrhenius Equation: K evap =0:6exp ð25; 000 = RTÞ where K evap is the evaporation rate coefficient of Mg in vacuum, gcm 2 s 1. As can be seen from Eq. (2), the evaporation rate of ð1þ ð2þ magnesium increases with increasing temperature. In air or the not well protective atmosphere, the evaporation and the oxidation of Mg coexist. The increase in the evaporation rate will result in the increase of the chance of magnesium contact with O 2, and consequently speed up the oxidation of molten AZ91D alloy. 5. Conclusions The influence of melt temperature on the oxidation behaviors of AZ91D magnesium alloy in HFC-134a/air atmospheres has been studied and conclusions drawn as follow. (1) With the increase of melt temperature, the oxidation rate of molten AZ91D magnesium alloy in HFC-134a/air atmospheres was increased and the oxidation kinetics changed from parabolic law to linear law. (2) With the increase of melt temperature, the amount of MgF 2 in the oxide film formed on AZ91D decreased, and the amount of MgO in the film increased. (3) The main reason causing the oxidation rate increase with temperature is the increase of the ratio of MgO content to MgF 2 content in the film with temperature. Acknowledgement This work was supported by Natural Science Basic Research Plan in Shaanxi Province of China (Grant No. SJ08-ZT05). REFERENCES [1] Ricketts NJ, Cashion SP. Hydrofluorocarbons as a replacement for sulphur hexafluoride in magnesium processing. Magnesium Technology Warrendale: TMS; [2] Ricketts N, Cashion S, Bailey R. Industrial trials with the AM-cover gas system for magnesium melt protection. Proceedings of the light metals technology conference, Brisbane, Australia, September 18 20; [3] Chen HK, Liu JR, Huang WD. Characterization of the protective surface films formed on molten magnesium in air/hfc-134a atmospheres. Mater Charact 2007;58:51. [4] Cashion SP, Ricketts NJ. Replacing SF6 with the hydrofluorocarbon gas HFC-134a for magnesium melt protection. Presented at the conference of metallurgists, Toronto, Canada, August 26 29; [5] Won H, Young JK. Effects of cover gases on melt protection of Mg alloys. J Alloys Compd 2006;422:208. [6] Chen HK, Liu JR, Huang WD. 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