1 Key Laboratory of Electrical Materials and Infiltration Technology of Shaanxi Province, School of Materials Science and Engineering, Xi'an University of Technology, Xi'an 710048 2 Shaanxi Key Laboratory of Optoelectronic Functional Materials and Devices, School of Materials and Chemical Engineering, Xi'an Technological University, Xi'an 710032
Cite this article:
Zhongming ZHANG,Kai YU,Weiwei REN,Ying MA,Chunjie XU,Zengzhe XI. MICROSTRUCTURE OF DIRECTLY EXTRUDED Mg-1Zn-1Ca ALLOY AND ITS CORROSION BEHAVIOR IN SBF SOLUTION. Acta Metall Sin, 2015, 51(8): 985-992.
The as-extruded Mg-1Zn-1Ca (mass fraction, %) alloys was fabricated successively by alloying, homogenization treatment and hot extrusion. The corrosion behavior of the alloy in simulated body fluid (SBF) solution was evaluated by electrochemical test and immersion test. The microstructure and morphology of corrosion product were observed by OM and SEM. Compositions of corrosion layer and different phases were investigated by EDS analysis. Fourier Transform infrared spectroscopy was also conducted to identify the functional groups in the corrosion products and XRD was also used to determine the phase constitutes of the corrosion products. The results show that Mg-1Zn-1Ca alloy consists of three phases, i.e. a-Mg, Mg2Ca and Ca2Mg6Zn3. After immersion in SBF solution for 72 h, the corrosion products is composed of HA (Ca10(OH)2(PO4)6), CaCO3, MgCl2 and Mg(OH)2. During the stage of immersion, the high active Mg2Ca phases act as the anode and corrode first, so they protect the around a-Mg substrate; the Ca2Mg6Zn3 phases are the lowest active, so they accelerate the corrosion of around α-Mg substrate. The corrosion resistance of as-cast Mg-1Zn-1Ca alloy are better than as-extruded alloy.
Fund: Supported by Shaanxi Provincial Science and Technology Plan Project (No.2010K10-08), Scientific Research Project of Education Department of Shaanxi Province (No.2013JK0906) and Research Project of Key Laboratory of Electrical Materials and Infiltration Technology of Shaanxi Province (No.ZSKJ201302)
Fig.1 OM images of as-cast Mg-1Zn-1Ca alloy at low (a) and high (b) magnification
Fig.2 OM images of directly extruded Mg-1Zn-1Ca alloy along cross-sectional (a) and longitudinal (b) direction
Fig.3 XRD spectrum of as-cast Mg-1Zn-1Ca alloy
Fig.4 SEM images of secondary phases in intragranular (a) and along grain boundaries (b) for as-cast Mg-1Zn-1Ca alloy
Area
Mg
Ca
Zn
A
67.10
13.90
19.00
B
69.30
26.80
3.90
C
85.37
12.47
2.16
D
87.10
4.13
8.77
Table 1 EDS analysis of second phases in as-cast Mg-1Zn-1Ca alloy corresponding to areas A~D in Fig.4 (atomic fraction / %)
Fig.5 XRD spectrum of directly extruded Mg-1Zn-1Ca alloys
Fig.6 Polarization curves of as-cast and directly extruded Mg-1Zn-1Ca alloys
Alloy
Corrosion potential V
Corrosion current density mA/cm2
Corrosion rate mm/a
As-cast
-1.6749
7.7251
4.2857
Directly extruded
-1.6372
5.9425
3.2021
Table 2 Electrochemical parameters of as-cast and directly extruded Mg-1Zn-1Ca alloys in simulated body fluid (SBF) solution
Fig.7 SEM images of corrosion products on surface of directly extruded Mg-1Zn-1Ca alloy at low (a) and high (b) magnification (Inset in Fig.7b shows the enlarged view of fluffy precipitate)
Area
C
O
Na
Mg
P
Ca
Cl
A
6.28
61.93
0.50
9.12
12.88
9.29
-
B
8.92
61.44
-
25.41
1.26
1.29
1.67
C
5.31
63.25
-
10.14
11.30
10.01
-
Table 3 EDS analysis of directly extruded Mg-1Zn-1Ca alloy surface containing corrosion products after immersion in SBF solution for 72 h corresponding to areas A~C in Fig.7 (atomic fraction / %)
Fig.8 FT-IR spectrum of corrosion products on surface of directly extruded Mg-1Zn-1Ca alloy after immersion in SBF solution for 72 h
Fig.9 XRD spectrum of corrosion products of directly extruded Mg-1Zn-1Ca alloy after immersion in SBF solution for 72 h
Fig.10 Corrosion morphologies of as-cast (a) and directly extruded (b) Mg-1Zn-1Ca alloy after immersion in SBF solution for 72 h and removal of corrosion product
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