Tomasz TAŃSKI1, L.A. DOBRZAŃSKI2, Krzysztof LABISZ3 1, 2, 3 Silesian University of Technology, Division of Materials Processing Technology and Computer Techniques in Materials Science, Institute of Engineering Materials and Biomaterials, Konarskiego st. 18a, 44-100 Gliwice, Poland tomasz.tanski@polsl.pl

Corrosion behaviour of die-casting magnesium alloys

Summary: In this paper there is presented the corrosion behaviour of the cast magnesium alloys as cast state and after heat treatment. Pitting corrosion resistance of the analysed alloys was carried out using the potentiodynamic electrochemical method (direct current), based on a anodic polarisation curve. On the basis of the achieved anodic polarisation curves, using the Tefel extrapolation method near to the corrosion potential, the quantitative data were determined, which describe the electrochemical corrosion process of the investigated alloys: value of the corrosion potential Ecor (mV), polarisation resistance Rp (kΩ/cm2), corrosion current density icor (µA/cm2), corrosion rate Vp (mm/year) as well the mass loss Vc (g/m2).

1. INTRODUCTION Contemporary materials should be characteristic of the high mechanical properties, physical and chemical, as well as technological ones, to ensure the long time and reliable use. The above mentioned requirements and expectations regarding the contemporary materials are met by the non-ferrous metals alloys used nowadays, including also the magnesium alloys [1-6]. The demand for the magnesium cast alloys is mainly connected with the development of the automotive industry (Table 1).

Table 1. Producers of Mg alloy component and applications on car models [1-6]

Component Producers and car models Engine block BMW: lighter, more powerful and durable six-cylinder inline

combustion engine. The world’s first engine block made of Noranda’s patented alloy AJ62 (Mg-Al-Sr)

Steering wheel frame Ford (Ford Thunderbird, Cougar, Taurus, Sable), Chrysler (Chrysler Plymouth), Toyota, BMW (MINI), Lexus (Lexus LS430)

Seat frame GM (Impact), Mercedes-Benz (Mercedes Roadster 300/400/500 SL), Lexus (Lexus LS430)

Instrument panel GM, Chrysler (jeep), Ford, Audi (A8), Toyota (Toyota Century)

Wheel rims Toyota (Toyota 2000GT, Toyota Supra), Alfa Romeo (GTV), Porsche AG (911 Serie)

Cylinder head Dodge (Dodge Raw), Honda Motor (City Turbo), Alfa Romeo (GTV), AutoZAZ-Daewoo (Tavria, Slavuta, Daewoo-Sens), Honda, BMW, Ford, Isuzu, Volvo Motors (LCP), Chrysler

Clutch case AutoZAZ-Daewoo (Tavria, Slavuta, Daewoo-Sens), Volvo Motors (LCP), Alfa Romeo (GTV)

Transmission case AutoZAZ-Daewoo (Tavria, Slavuta, Daewoo-Sens), Volvo Motors (LCP), Porsche AG (911 Serie), Volkswagen (Volkswagen Passat), Audi (A4,A6), Mercedes-Benz

Lower crankcase Chrysler (jeep), Alfa Romeo (GTV), GM (Oldsmobile), McLaren Motors (F1-V12)

Cylinder block (without liners and main bearing heads)

GM (Pontiac Gran AM, Corvette)

Intake manifold GM (V8 North Star motor), Chrysler Air intake system BMW (V8 motor) Steering link bracing GM (LH Midsize) Oil pump body McLaren Motors (F1-V12) Camshaft drive chain case Porsche AG (911 Serie)

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For example, General Motors in their big cars (Savana & Express) use 26,3 kg of magnesium cast alloys, and in smaller cars (Safari, Astro) – 165 kg, Ford F – 150 – 14,5 kg, VW Passat and Audi A4 and A6 from 13,6 to 14,5 kg, Alfa Romeo – 9,3kg. A further demand for magnesium casts is expected, of up to 50 kg per each car. It is mainly because of the fact that the magnesium casts have got a low density (1700-1900 kg/m3), and at the same time, their mechanical properties are similar to the aluminium casting alloys [6-9].

The goal of this paper is to present of the investigation results of the casting magnesium alloys in its as-cast state and after heat treatment. 2. EXPERIMENTAL PROCEDURE Pitting corrosion resistance of the analysed alloys was carried out using the potentiodynamic electrochemical method (direct current), based on a anodic polarisation curve. The corrosive agent was a 3% NaCl solution. The measurements were carried out in a three-electrode two-chamber glass electrolyser wit a volume of 150 cm3, equipped with a water jacket connected to the thermostat of the UH-4 type ensuring a regulation with accuracy of ±0,1°C. The samples as well the investigated electrodes were made from the cast magnesium alloys, the reference electrode was appropriately a platinum and calomel electrode. The measurements ware performed at the room temperature after 20 minutes from the first contact of the investigated material with the electrolyte, by a potential change rate of 120 mV/min. The surface area of the tested samples of the cast magnesium alloys was equal 0,5 cm2. The investigations have been carried out on test pieces of MCMgAl12Zn1, MCMgAl9Zn magnesium alloys in as-cast and after heat treatment states (Table 3). The chemical composition of the investigated materials is given in Table 2. A casting cycle of alloys has been carried out in an induction crucible furnace using a protective salt bath Flux 12 equipped with two ceramic filters at the melting temperature of 750±10ºC, suitable for the manufactured material. In order to maintain a metallurgical purity of the melting metal, a refining with a neutral gas with the industrial name of Emgesalem Flux 12 has been carried out. To improve the quality of a metal surface a protective layer Alkon M62 has been applied. The material has been cast in dies with bentonite binder because of its excellent sorption properties and shaped into plates of 250x150x25. The cast alloys have been heated in an electrical vacuum furnace Classic 0816 Vak in a protective argon atmosphere. The analysis of the investigated samples after the corrosion test was performed using the Zeiss SUPRA 35 scanning electron microscope with the EDAX Trident XM4 dispersive radiation spectrometer at the accelerating voltage of 20 kV.

Table. 2. Chemical composition of investigation alloy The mass concentration of main elements, %

Al Zn Mn Si Fe Mg Rest

12,1 0,62 0,17 0,047 0,013 86,9 0,15

9,1 0,77 0,21 0,037 0,011 89,8 0,072

Table 3. Parameters of heat treatment of investigation alloy

Conditions of solution heat treatment Sing the state of heat

treatment Temperature, °C Time of warming, h Way coolings

0 As-cast

Solution treatment

1 430 10 Water

2 430 10 Air

3 430 10 In furnace

Aging treatment

4 190 15 Air

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3. DISCUSSION OF EXPERIMENTAL RESULTS As a result of the performed potentiodynamic investigations the polarisation curves and anodic loops for magnesium cast alloys in as-cast state as well after heat treatment (Figs. 1, 2) were achieved. The polarisation curves of the investigated materials consist of branches of the anodic curves, which corresponds to the corrosion reaction process, as well of branches of cathodic curves, which corresponds to the hydrogenic depolarisation.

The curves show, that the investigated materials undergo pitting corrosion, which they are susceptible for, particularly in case of the cast magnesium alloys of the Mg-Al-Zn type. On the basis of the achieved anodic polarisation curves, using the Tefel extrapolation method near to the corrosion potential, the quantitative data were determined, which describe the electrochemical corrosion process of the investigated alloys: value of the corrosion potential Ecor (mV), polarisation resistance Rp (kΩ/cm2), corrosion current density icor (µA/cm2), corrosion rate Vp (mm/year) as well the mass loss Vc (g/m2) (Table 4). The anodic polarisation curve process, and the corrosion current density determine the pulping rate of the tested surfaces (Figs. 1, 2). The parameters measured during the corrosion tests, acieved for the investigated magnesium alloys, as well for the environment - where the tests were carried out, cannot be considered separately. The values like corrosion potential Ecor or corrosion current density icor, can be used for comparison of the properties of the investigated materials both in as-cast state or after heat treatment, because all the performed measurements were made in the same environmental conditions.

Table 4. The parameters measured during the corrosion tests

Investigation alloys

Sing the state of heat

treatment

Corrosion potential Ecor (mV)

Polarisation resistance

Rp (kΩ/cm2)

Corrosion current density icor (µA/cm2)

Corrosion rate Vp

(mm/year)

Mass loss Vc (g/m2)

Breakout potential En

(mV)

Repassivation potential Ecp (mV)

0 -1548,3 0,36 14,3 0,32 1,53 -1471,6 -1722,9 1 -1561,1 0,29 17,4 0,39 1,87 -1468,7 -1726,6 2 -1539,3 0,17 22,1 0,50 2,37 -1454,1 -1705,6 3 -1577,7 0,16 23,8 0,51 2,41 -1518,6 -1730,7

MCMgAl12Zn1

4 -1518,4 0,40 9,6 0,22 1,03 -1441,4 -1695,1 0 -1551,6 0,4 13,7 0,31 1,48 -1470 -1692,3 1 -1555,6 0,31 15,8 0,35 1,69 -1497,4 -1700 2 -1537,4 0,27 18,9 0,43 2,03 -1468,6 -1689,8 3 -1573,3 0,21 21,1 0,48 2,27 -1508,6 -1704,1

MCMgAl9Zn1

4 -1522,6 1,13 8 0,18 0,85 -1443 -1691,1 The analysis of anodic polarisation curves, potential, corrosion resistance and density as well

corrosion current confirm a slightly higher corrosion resistance of the MCMgAl9Zn1 material in as cast state compared to the MCMgAl12Zn1 samples, where the corrosion potential is equal -1551,6 mV, polarisation resistance 0,4 kΩ/cm2, and the current density in the passive range 13,7 µA/cm2

(Table 4). Similarly corrosion resistance investigations of the cast magnesium alloys were performed after heat treatment. The lowest value of corrosion current density icor, determining the smallest anodic pulping of the cast magnesium alloys with different aluminium content and connected with it best corrosion resistance after heat treatment show the MCMgAl9Zn1 alloys. Whereas the minimal resistance to the corrosion agent, which is connected to the failure propagation into the material as well to the material surface, show the MCMgAl12Zn1 alloys (Table 4, Figs. 1, 2). This is unequivocally with surface corrosion acceleration in the corrosive agent, which is showed based on the calculated value of the corrosion rate Vp as well on the mass loss Vc of the investigated samples (Table 4). The analysis of the achieved results for the alloys with 12, 9% aluminium content has confirmed the analogy of corrosion resistance increase of the materials after precipitation hardening in comparison to the as-cast alloys, and to the alloys after solution heat treatment. The lowest parameter values of the pitting corrosion process in all the investigated alloys have the samples after solution heat treatment cooled with furnace.

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The values of the breakout potential En, whichby growing pinholes on the tested samples surface occurs, and the value of the repassivation potential Ecp (Table 4), where no more active pinholes below the value exist, were both determined using the potentiodynamic curves progress. As a result of the comparison of the corrosion loops width of the investigate alloys (of En and Ecp parameters in the range, where new pinholes are not growing and in the existing ones corrosion processes can proceed), as well of the inclination angle and amplitude of the curve loops it can be conclude, that the lowest parameters (breakout potential and passivation potential) are achieved for materials after solution heat treatment cooled with furnace, whereas the best results are achieved for materials after precipitation hardened.

Fig. 1. Anodic curves of the magnesium cast alloys MCMgAl12Zn1: a) in as-cast state, b) after aging treatment

Fig. 2. Anodic curves of the magnesium cast alloys MCMgAl9Zn1: a) in as-cast state, b) after aging treatment

Surface morphology of the investigated samples after corrosion test performed before and after heat treatment (Fig. 4-6) show irregular shaped pinholes and numerous cracks in the surface layer of the material. The majority of the defects are present in case of solution heat treatment with furnace cooling and are coming into existence in neighborhood to the occurred precipitations, what causes discontinuity of the surface and that fore a significant mass loss. The smallest visible surface layer destruction is characteristic for magnesium cast alloys after ageing (Fig. 6b). On the surface of the samples there are present also corrosion products, which builds compact conglomerates with characteristic needle shape formed in the majority of the cases inside of the pinholes (Fig. 4b, 5b). The EDS microanalysis confirm the occurrence of corrosion products on the sample surface (Fig. 3).

Fig. 3. Spectrum of the pointwise chemical composition analysis from corrosion products area

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Fig. 4. Microstructure of the magnesium alloy MCMgAl12Zn1: a) in as-cast state, b) after cooling in the air-

after corrosion tests

Fig. 5. Microstructure of the magnesium alloy MCMgAl9Zn1: a) in as-cast state, b) after cooling in the air-

after corrosion tests

Fig. 6. Microstructure of the magnesium alloy MCMgAl9Zn1: a) after cooling with the furnace, b)-after aging

corrosion tests

4. SUMMARY Pitting corrosion resistance of the analysed alloys was carried out using the potentiodynamic electrochemical method (direct current), based on a anodic polarisation curve. The analysis of the achieved results for the alloys with 12, 9% aluminium content has confirmed the analogy of corrosion resistance increase of the materials after precipitation hardening in comparison to the as-cast alloys, and to the alloys after solution heat treatment. As a result of the comparison of the corrosion loops width of the investigate alloys as well of the inclination angle and amplitude of the curve loops it can be conclude, that the lowest parameters (breakout potential and passivation potential) are achieved for materials after solution heat treatment cooled with furnace, whereas the best results are achieved for materials after precipitation hardened.

200 µm 10 µm

10 µm 50 µm

200 µm 10 µm

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Surface morphology of the samples after corrosion test performed after and before heat treatment show irregular shaped pinholes and numerous cracks on the material surface layer. REFERENCES [1] Horst E.F., Mordike B.L.: Magnesium Technology. Metallurgy, Design Data, Application,

Springer-Verlag, Berlin Heidelberg 2006 [2] Górny Z., Sobczak J.: Non-ferrous metals based novel materials in foundry practice, ZA-PIS,

Cracow, 2005, (in Polish) [3] Fajkiel A., Dudek P., Sęk-Sas G.: Foundry engineering XXI c. Directions of metallurgy

development and Ligot alloys casting, Publishers Institute of Foundry engineering, Cracow, 2002, (in Polish)

[4] Kainem K.U.: Magnesium – Alloys and Technology, Wiley-VH, Weinheim, Germany, 2003 [5] Dobrzański L.A., Tański T., Trzaska J.: Optimization of heat treatment conditions of magnesium

cast alloys, Int. J. Materials Science Forum 638-642, 2010, pp 1488-1493 [6] Dobrzański L.A., Tański T.: Influence of aluminium content on behaviour of magnesium cast

alloys in bentonite sand mould, Int. J. Solid State Phenomena 147-149, 2009, pp. 764-769 [7] Yingwei S., Dayong S., Rongshi Ch., En-Hou H: Corrosion characterization of Mg–8Li alloy in

NaCl solution, Int. J. Corrosion Science 51/5, 2009, pp 1087-1094 [8] Lili G., Chunhong Z., Milin Z., Xiaomei H., Nan S.: The corrosion of a novel Mg–11Li–3Al–

0.5RE alloy in alkaline NaCl solution, Int. J. Journal of Alloys and Compounds 468/1-2, 2009, pp. 285-289

[9] Gaia B., Ugo B., Roberto B., Giuseppe C.: About some corrosion mechanisms of AZ91D magnesium alloy, Int. J. Corrosion Science 47/9, 2005, pp. 2173-2184

Acknowledgements Research was financed partially within the framework of the Polish State Committee for Scientific Research Project No. 4688/T02/2009/37 headed by Dr Tomasz Tański

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