196 Int. J. Microstructure and Materials Properties, Vol. 4, No. 2, 2009 The influence of Mg 17 Al 12 phase volume fraction on the corrosion behaviour of AZ91 magnesium alloy Andrzej Kiełbus* and Grzegorz Moskal Silesian University of Technology 40 019 Katowice, Poland Fax: +40 32 603 4400 E-mail: andrzej.kielbus@polsl.pl E-mail: grzegorz.moskal@polsl.pl *Corresponding author Abstract: The microstructure and the corrosion behaviour of AZ91 magnesium alloy in the as-cast and after heat treatment conditions were investigated. The microstructure of the cast alloy consisted of an α-mg phase matrix with a continuous and discontinuous β phase (Mg 17 Al 12 ) at the grain boundaries. The β phase had a twofold influence on corrosion: as a barrier (high volume fraction) and as a galvanic cathode (small volume fraction). After the T4 treatment, the β phase dissolved in the matrix and after the T6 treatment, discontinuous precipitation of the β phase along the grain boundaries was observed. The microstructural examinations were conducted using light microscopy. The corrosion behaviour was investigated in a 3.5% solution of NaCl. The aim of this paper is to present the results of investigations on the microstructure of the AZ91 magnesium alloy after heat treatment. Keywords: magnesium alloys; AZ91; microstructure; Mg 17 Al 12 phase; corrosion. Reference to this paper should be made as follows: Kiełbus, A. and Moskal, G. (2009) The influence of Mg 17 Al 12 phase volume fraction on the corrosion behaviour of AZ91 magnesium alloy, Int. J. Microstructure and Materials Properties, Vol. 4, No. 2, pp.196 206. Biographical notes: Andrzej Kiełbus received his MS and PhD degrees from the Silesian University of Technology in Poland. Dr. Kiełbus is a member of the Polish Materials Society. He has coauthored 100 publications in the areas of magnesium alloys, die-casting, steels for power plant elements, corrosion and degradation of steels after long-term service. Presently he is an Assistant Professor in the Department of Materials Science, of the Silesian University of Technology, teaching materials science and research methods. Grzegorz J. Moskal received his MS and PhD degrees from the Silesian University of Technology in Poland. Dr. Moskal is a member of TMS and ASM International. He has coauthored 80 publications in the areas of coatings for gamma Ti-Al alloys, TBC systems, corrosion and oxidation of gamma Ti-Al alloys, and lifetime and degradation assessment of power plant pressure elements. Copyright 2009 Inderscience Enterprises Ltd.
The influence of Mg 17 Al 12 phase volume fraction on the corrosion behaviour 197 1 Introduction The corrosion resistance of magnesium is particularly low when its alloy contains some specific metallic impurities or when it is exposed to the action of aggressive electrolytes which contain chloride ions. The magnesium oxide that forms on the surface of Mg alloys usually guarantees sufficient protection for the substrate under normal atmospheric environmental conditions (Song and Atrens, 1999). In the majority of cases, corrosion of magnesium alloys begins from local corrosion. Song and Atrens (1999) suggest that an alloy s morphology depends on its chemical composition and the environmental conditions. There are two principal causes of the low corrosion resistance of magnesium alloys. The first is connected with internal galvanic corrosion, which results from the presence of secondary phases and impurities. The other ensues from the formation of a quasi-stable hydroxide layer on the alloy surface, which layer is far less stable than the protection layers formed, e.g., on aluminium. Hanawalt et al. (1942) found that the chemical elements that constitute magnesium alloys have different influences on the corrosion processes taking place in the alloys. There are some chemical elements whose effect is definitely negative. It was found that iron, nickel, copper and cobalt, with concentrations lower than 0.2at.%, very strongly accelerate the corrosion rate, whereas silver, calcium and zinc in a range from 0.5 to 5.0at.% have a gentle effect on the corrosion rate. Other elements, such as aluminium, tin, cadmium, manganese and silicon, show a very small effect or none at concentrations up to 5at.%. Individual phases in Mg-Al alloys have a distinct influence on corrosion resistance, since the majority of the alloying elements influence the corrosion after secondary phases have been formed. A high content of aluminium in the AZ91 alloy is connected with a significant fraction of the β-mg 17 Al 12 phase, which is distributed along grain boundaries and forms a cathodic area in relation to the matrix. This phase shows passivity over a wide range of the ph value. It is also inactive in relation to solutions containing chlorides and acts as a corrosion barrier for the α-mg matrix (Lunder et al., 1993). The high corrosion resistance of the β-mg 17 Al 12 phase is connected with the formation of a passive layer on its surface. There is, however, an opposite opinion, according to which the corrosion rate grows for alloys containing the β-mg 17 Al 12 phase. Removal of this phase from an alloy will reduce the probability of microgalvanic cell formation (Avedesian and Baker, 1999). A considerable influence on the corrosion resistance of magnesium alloys is shown by the casting technology, which causes the occurrence of differences in the microstructure (Song et al., 1999). For instance, the surface layer of a die-cast AZ91 alloy shows an almost ten times better corrosion resistance than the core. This may result from a high fraction of the β-mg 17 Al 12 phase and its continuous morphology around the α-mg phase grain. If the matrix grain is very fine and the precipitations fraction not too low, the β-mg 17 Al 12 phase forms a near-continuous network of precipitates. Thus, corrosion of the β-mg 17 Al 12 phase is completely blocked by the corrosion products on the phase surface and, in consequence, it is strongly inhibited. Instead, the matrix undergoes corrosion much more easily. In the latter case, when matrix grains are large, the precipitates of the β-mg 17 Al 12 phase are clustered and the distance between them
198 A. Kiełbus and G. Moskal is large, matrix corrosion is not blocked either by the precipitates of the β-mg 17 Al 12 phase or by the corrosion products formed. The presence of the β-mg 17 Al 12 phase just accelerates the corrosion of the α-mg phase (Song and Atrens, 2003). There are few publications concerning the influence of ageing on the corrosion resistance of a die-cast AZ91 alloy. Song et al. (2004) found that ageing at a temperature of 230 C for 36 h has an insignificant influence on corrosion resistance. However, ageing at a temperature above 230 C considerably reduces the corrosion resistance during a salt test. The causes of this phenomenon have not been explained yet. The corrosion rate of a die-cast AZ91 alloy subjected to ageing at 160 C falls with the time of ageing in the initial period of treatment and then, after 45 h, it grows. The dependence between the corrosion rate and ageing time may be connected with changes in the microstructure and local changes in the chemical composition during ageing. In the initial period of the process, precipitates of the β-mg 17 Al 12 phase are distributed along grain boundaries, which makes them act as a corrosion barrier. This induces an initial decrease in the corrosion rate. In a later period of the ageing process, a reduced concentration of aluminium in the matrix makes the α-mg phase more active, thus increasing the corrosion rate (Ko et al., 2003). The aim of this paper is to present the results of investigations on the microstructure of the AZ91 magnesium alloy after heat treatment. 2 Materials and methods The material for the research was an AZ91 alloy cast into a sand mould. The casting technology included melting the alloy in a 250 kg induction furnace with the application of modification with SPEFINAL T 200 as well as refining with Emgesal Flux 200. The casting temperature was 740 C. The chemical composition of the analysed alloy is shown in Table 1. After casting, heat treatment (solution and ageing treatment) was performed. In order to protect the specimens against oxidation, the process was conducted in an argon atmosphere. The solution and ageing treatment parameters are presented in Table 2. Table 1 Chemical composition of the AZ91 alloy in wt-% Mg Al Zn Mn Si Cu Fe Be Balance 9,15 0,6 0,24 0,03 0,01 0,01 0,0001 Table 2 Parameters of the heat treatment Solution treatment Ageing treatment Designation Temperature ( C) Time (h) Cooling Temperature ( C) Time (h) Cooling O As-cast P1 415 1 air P20 (T4) 415 20 air P48 415 48 air P20S1 415 20 air 168 1 air P20S20 (T6) 415 20 air 168 20 air P20S48 415 20 air 168 48 air
The influence of Mg 17 Al 12 phase volume fraction on the corrosion behaviour 199 The microsections for structural examination were subjected to grinding with sandpaper of 250 to 1200 granulation. Next, they were mechanically polished with the use of diamond pastes. For etching, two reagents were used with the following chemical compositions: 10 ml fluoric acid (48%) + 90 ml water 2 ml fluoric acid + 2 ml nitric acid + 96 ml water. The microstructural observations were performed with an Olympus GX+70 light microscope. Quantitative examination of the β phase fraction and of the solid solution α grain size was conducted using the MET-ILO automatic image analysis programme invented by Janusz Szala from the Department of Materials Science, Silesian University of Technology in Poland. An evaluation of the AZ91 alloy corrosion resistance was performed based on polarisation curves recorded at room temperature in a 3.5% NaCl solution. The measuring procedure commenced with recording the open-circuit potential values (E OCP ) and the potentiostatic curve. Next, recording of the potentiodynamic polarisation current was performed with changing of the potential at a rate of 10 mv/min from a 300 mv potential, more cathodic than E OCP, until a value of 500 mv was reached in relation to the Normal Hydrogen Electrode (NHE). 3 Research results 3.1 Microstructure of the AZ91 alloy after casting The cast AZ91 alloy was characterised by a solid solution α structure with α + discontinuous β areas and a continuous β (Mg 17 Al 12 ) phase at the grain boundaries. Moreover, the occurrence of Laves phase in the form of Mg 2 Si and precipitates, probably of the MnAl 4 phase, was proved (Figure 1). The average volume fraction of the β phase was V V = 28.69% and the mean area of the solid solution grain size was Ā = 3400 µm 2. The microanalysis results of the chemical composition of individual phases are shown in Table 3. The mapping of Mg, Al and Mn can be seen in Figure 2. Figure 1 Microstructure of the as-cast AZ91 alloy solid solution α + continuous and discontinuous Mg 17 Al 12 intermetallic phase (left) and Mg 2 Si precipitates (right)
200 A. Kiełbus and G. Moskal Table 3 Chemical composition of identified AZ91 alloy phases Chemical element (at.%) Phase Mg Al Zn Si Mn α phase 89,41 9,75 0,84 β phase 63,39 35,65 0,69 Mg 2 Si phase 62,39 37,61 MnAl 4 phase 1,37 78,17 20,47 Figure 2 The SE image and the distribution of Mg, Al, Mn in microareas of the AZ91 magnesium alloy SE Mg Al Mn
The influence of Mg 17 Al 12 phase volume fraction on the corrosion behaviour 201 3.2 Microstructure of the AZ91 alloy after solution treatment After solution treatment followed by air-cooling, the β phase dissolved in the matrix. However, the occurrence of a few β (Mg 17 Al 12 ) precipitates along the grain boundaries and MnAl 4 phases in the solid solution α matrix was shown. A 1-hour solution treatment of the AZ91 alloy at a temperature of 415 C caused a considerable (fivefold) decrease in the β phase quantity compared to the initial state. The mean area of the solid solution α grain equals Ā = 4400 µm 2 and is slightly higher compared to the initial state (Figure 3). Extension of the solution treatment time to 20 h brings about an even more considerable decrease in the β phase quantity (to 2.19%), whereas the grain size grows to A = 6050 µm 2. An even longer solution treatment time leads to further reduction of the β phase volume fraction (V V = 1.44%); however, it does not influence the solid solution grain mean area A = 5900 µm 2 (Figure 4). Figure 3 Microstructure of the AZ91 alloy after solution treatment at 415 C/1 h/air: (a) LM, (b) SEM (a) (b) Figure 4 Microstructure of the AZ91 alloy after solution treatment at 415 C/48 h/air: (a) LM, (b) SEM (a) (b)
202 A. Kiełbus and G. Moskal 3.3 Microstructure of the AZ91 alloy after ageing The ageing treatment applied after solution treatment with air-cooling caused precipitation of a discontinuous β phase. Furthermore, the β phase (Mg 17 Al 12 ) precipitated at the solid solution grain boundaries (Figure 5). It was found that both 1 h and 20 h of ageing at a temperature of 168 C after prior solution treatment from a temperature of 415 C/20 h did not influence the number of β phase precipitates (V V = 1.59% and V V = 1.93% respectively) (Table 4). The mean area of the solid solution α grain did not change, either (A = 5900 µm 2 and A = 5500 µm 2 respectively). Only a duration as long as a 48-h holding at 168 C had a significant influence on the growth of the number of β phase precipitates (V V = 10.69%). A grain growth to A = 7200 µm 2 was observed as well (Figure 6 and Table 4). Figure 5 Microstructure of the AZ9D1 alloy after solution treatment at 415 C/20 h/air: (a) ageing at 168 C/20 h/air, LM; (b) ageing at 168 C/48 h/air, LM (a) (b) Table 4 Assessment of phases area fractions and the mean area of the solid solution grain in the AZ91 alloy microstructure after heat treatment Heat treatment Phase O P1 P20 P48 P20S1 P20S20 P20S48 V V (Mg 17 Al 12 ) (%) 28,69 5,66 2,19 1,44 1,59 1,93 10,69 Ā (α) (µm 2 ) 3400 4400 6050 5900 5900 5500 7200 Figure 6 Microstructure of the AZ9D1 alloy after solution treatment at 415 C/20 h/air: (a) ageing at 168 C/20 h/air, LM; (b) ageing at 168 C/48 h/air, LM (a) (b)
The influence of Mg 17 Al 12 phase volume fraction on the corrosion behaviour 203 3.4 Examination of corrosion resistance As a result of the tests performed in a 3.5% solution of NaCl, potentiodynamic curves of AZ91 were obtained, which are presented in Figure 7 in the form of semilogarithmic graphs. No passive behaviour can be observed for all specimens. At first we noted that the shape of the polarisation curves was almost the same and had a similar course for all the specimens of the alloy investigated, the current values being quite similar and relatively high. An electrochemical analysis shows that the highest corrosion potential was determined for a specimen in the initial state but that the values of E cor were similar. The magnitude of the cathodic current density increased with an increased volume fraction of the β phase. The Tafel slope was almost the same, whereas the I cor values were higher for specimens after heat treatment. It was found that the highest corrosion resistance in a 3.5% NaCl solution is shown by a material in its initial state (specimen O i cor = 0.042 ma/cm 2 ) and a material after 48 h of ageing (specimen P20S48 i cor = 0.053 ma/cm 2 ), whereas the lowest corrosion resistance is characteristic of a material after 48 h of solution treatment (specimen P48 i cor = 0.230 ma/cm 2 ). Figure 7 Polarisation curves of the AZ91 magnesium alloy in 3.5% NaCl 4 Summary The subject of the research carried out was an evaluation of the influence of the heat treatment process on the microstructure and hardness of the AZ91 alloy. It was found that immediately after casting, the AZ91 alloy had a solid solution α structure with α + discontinuous β areas and a continuous β (Mg 17 Al 12 ) phase at the grain boundaries. After solution treatment, a reduction in the number of β phase precipitates was observed.
204 A. Kiełbus and G. Moskal Its volume fraction fell with the extension of the solution treatment time from 28.69% for the material in its initial state (cast) to 1.44% for the material after 48 h of solution treatment. Also, an increase in the solid solution α grain size was observed from 4400 µm 2 for a material after a 1 h solution treatment to 5900 µm 2 for a material after a 48 h solution treatment (Figure 8). Figure 8 Influence of the solution treatment time on the β phase quantity and the solid solution α grain size Application of the ageing treatment caused precipitation of the β phase along the solid solution grain boundaries. The β phase volume fraction is not large and amounts to V V = 1.93% (1 h ageing) and V V = 1.59% (20 h ageing). Only an extension of the ageing time to 48 h caused a significant (fivefold) increase in the β phase quantity, V V = 10.69%. Moreover, as the ageing time was prolonged, an increase was observed in the solid solution grain size from 5900 to 7200 µm 2 (Figure 9). An analysis of the corrosion investigation results showed that, as the quantity of the β phase decreases and the solid solution α grain size increases, the corrosion current density grows as a result of prolongation of the solution treatment time, and the material thus shows lower resistance to the action of a corrosive environment. With the extension of the ageing time, an increase in the β phase quantity occurs and the corrosion current density is reduced, the material being characterised by higher resistance to the corrosive environment (Figure 10).
The influence of Mg 17 Al 12 phase volume fraction on the corrosion behaviour 205 Figure 9 Influence of the ageing time on the β phase quantity and the solid solution α grain size Figure 10 Influence of the β phase quantity on corrosion resistance of the AZ91 alloy To recapitulate the above findings, one can affirm that, as the Mg 17 Al 12 phase quantity increases, the corrosion resistance of the AZ91 magnesium alloy increases as well.
206 A. Kiełbus and G. Moskal Acknowledgements The present work was supported by the Polish Ministry of Science and Information Society Technologies under Research Project No. 3 T08C 060 28. References Avedesian, M.M. and Baker, H. (1999) Magnesium and magnesium alloys, in ASM Speciality Handbook. Hanawalt, J.D., Nelson, C.E. and Peloubet, J.A. (1942) Corrosion studies of magnesium and its alloys, Trans. AIME, Vol. 147, pp.273 299. Ko, Y.J., Yim, C.D., Lim, J.D. and Shin, K.S. (2003) Effect of Mg 17 Al 12 precipitate on corrosion behavior of AZ91D magnesium alloy, Materials Science Forum, Vols. 419 422, pp.851 856. Lunder, O., Nisancioglu, K. and Hansen, R.S. (1993) Corrosion of die cast magnesium aluminium alloys, SAE Technical Paper Series No. 930755. Song, G. and Atrens, A. (1999) Understanding magnesium corrosion a framework for improved alloy performance, Advanced Engineering Materials, No. 1, pp.11 33. Song, G. and Atrens, A. (2003) Understanding magnesium corrosion, Advanced Engineering Materials, Vol. 3, No. 12, pp.837 858. Song, G., Atrens, A. and Dargusch, M. (1999) Influence of microstructure on the corrosion of die cast AZ91D, Corrosion Science, Vol. 41, pp.249 273. Song, G., Bowles, A.L. and St. John, D.H. (2004) Corrosion resistance of aged die cast magnesium alloy AZ91D, Materials Science and Engineering A, Vol. 366, pp.74 86.