Effect of Microstructure on Corrosion Behavior of Mg-Sr Alloy in Hank’s Solution

doi:10.1007/s40195-018-0750-4

Abstract

Abstract:

Mg-Sr alloy has been studied as a potential biodegradable material with excellent bioactivity to promote the bone formation. However, its degradation behavior needs to be well controlled to avoid the negative effect, which is important for future application. Therefore in this study, the microstructure and its effect on corrosion behavior of an Mg-1.5Sr alloy were investigated. The microstructures of the alloy under different processing procedures were characterized by both optical and scanning electron microscopes. The corrosion performance was studied in Hank’s solution using immersion, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. The results showed that the grain size and the amount and distribution of β-Mg₁₇Sr₂ had obvious effects on the corrosion behavior of Mg-Sr alloy. The smaller the grain size was, the more the protective surface layer formed on Mg-Sr alloy, and the higher the corrosion resistance was. For the as-cast Mg-Sr alloy, the network-like second phases precipitated along the grain boundaries could not hinder the corrosion due to their own corrosion cracking accelerating the intergranular corrosion. However, the refinement of second phases increased the corrosion resistance of the as-extruded alloy. After solution treatment at 450 °C for 5 h, the grains in the alloy did not grow much and β-Mg₁₇Sr₂ phases homogenously distributed in the alloy, resulting in the increase in corrosion resistance. However, after aging treatment, large amount of precipitated second phases increased the galvanic corrosion of the alloy, accelerating the development of corrosion.

Key words: Mg-Sr alloy, Microstructure, Corrosion, Extrusion, Heat treatment

Jia-Hui Dong, Li-Li Tan, Yi-Bin Ren, Ke Yang. Effect of Microstructure on Corrosion Behavior of Mg-Sr Alloy in Hank’s Solution[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(3): 305-320.

Figures/Tables 17

Table 1 Heat treatments for as-extruded Mg-Sr alloy

Sample	Solution treatment		Aging treatment
Sample	Temperature (°C)	Time (h)	Temperature (°C)	Time (h)
T4-450 °C	450	5	-	-
T4-560 °C	560	5	-	-
T6-10 h	560	5	200	10
T6-24 h	560	5	200	24
T6-40 h	560	5	200	40

Fig. 1 Microstructures of as-cast Mg-Sr alloy: a optical micrograph, b higher magnification showing braid shape bands within grain, c SEM micrograph, d EDS line scan analyses

Fig. 2 Optical micrographs of as-extruded Mg-Sr alloy under different heat treatments shown by transversal and longitudinal sections: a1 and a2 before heat treatment, b1 and b2 T4-450 °C, c1 and c2 T4-560 °C, d1 and d2 T6-10 h, e1 and e2 T6-24 h, f1 and f2 T6-40 h

Fig. 3 a Average grain size and b volume fractions of β phase in cast and extruded Mg-1.5Sr alloy under different heat treatments

Fig. 4 XRD patterns of a as-cast, as-extruded Mg-Sr alloy and b under different heat treatments

Fig. 5 SEM micrographs of as-extruded Mg-1.5Sr alloy under different heat treatments shown by transversal and longitudinal sections: a1 and a2 before heat treatment, b1 and b2 T4-450 °C, c1 and c2 T4-560 °C, d1 and d2 T6-10 h, e1 and e2 T6-24 h, f1 and f2 T6-40 h

Fig. 6 Variation of pH value for as-cast and as-extruded Mg-1.5Sr alloy during immersion in Hank’s solution for 9 d

Fig. 7 SEM micrographs showing corroded surface of Mg-1.5Sr alloy immersed in Hank’s solution for 3 d (a1-c1) and 9 d (a2-b2): a1 and a2 as-cast alloy, b1 and b2 transversal section as-extruded alloy, c1 and c2 longitudinal section of as-extruded alloy

Fig. 8 SEM micrographs showing corrosion morphologies of β phase and cross sections of Mg-1.5Sr alloy exhibiting the depth of corrosion pit after 3-d immersion in Hank’s solution: a1 and a2 as-cast alloy, b1 and b2 transversal section of as-extruded alloy, c1 and c2 longitudinal section

Fig. 9 a Variation of pH and b weight loss rate for extruded Mg-1.5Sr alloy under different heat treatments immersed in Hank’s solution as a function of immersion time

Fig. 10 Optical micrographs showing corrosion morphologies of extruded Mg-1.5Sr alloy under different heat treatments after 14-d immersion: a before heat treatment, b T4-450 °C, c T4-560 °C, d T6-10 h, e T6-24 h, f T6-40 h

Fig. 11 Potentiodynamic polarization curves of Mg-1.5Sr alloy: a as-cast and as-extruded alloy, b solution-treated alloy, c solution followed by aging alloy

Table 2 Ecorr, βc and Icorr obtained from Tafel fitting for differently treated Mg-1.5Sr alloy in Hank’ s solution

Sample	E_corr (V vs. SCE)	I_corr (μA cm^-2)	β_c (V dec^-1)	Corrosion rate (mm y^-1)
As-cast	- 1.740	10.468	0.308	0.236
Ex-t	- 1.630	4.951	0.226	0.112
Ex-l	- 1.640	6.617	0.260	0.149
T4-450 °C	- 1.600	4.731	0.215	0.107
T4-560 °C	- 1.640	6.775	0.235	0.153
T6-10 h	- 1.650	7.702	0.201	0.174
T6-24 h	- 1.650	9.855	0.231	0.222
T6-40 h	- 1.690	14.115	0.194	0.318

Fig. 12 EIS measurement results of Mg-Sr alloy: a and b Nyquist plots acquired at 1-h immersion in Hank’s solution, c equivalent circuit for EIS data fitting, d fitted Rtp and Rfp (ZRe: real part of impedance; Zim: imaginary part of impedance; R: impedance)

Table 3 Fitting results of Mg-Sr alloy immersed in Hank’s for 1 h

Sample	R_s (Ω cm^-2)	R_tp (Ω cm^-2)	CPE1 (F cm^-2)	n ₁	R_fp (Ω cm^-2)	CPE2 (F cm^-2)	n ₂	L (H cm^-2)
As-cast	12.92	7689	2.20E-05	0.7425	62.19	1.02E-05	0.6977	10.60
Ex-t	15.57	1.47E + 04	1.76E-05	0.7657	113.00	3.09E-05	0.3911	17.69
Ex-l	12.02	1.28E + 04	1.61E-05	0.7362	91.92	3.58E-05	0.5732	18.36
T4-450 °C	13.14	1.56E + 04	1.66E-05	0.7662	110.5	1.43E-05	0.6578	29.72
T4-560 °C	14.58	9449	1.90E-05	0.7689	87.42	1.28E-05	0.6793	18.58
T6-10 h	17.1	8958	2.02E-05	0.7523	57.65	1.87E-05	0.6049	23.40
T6-24 h	10.09	7637	2.01E-05	0.7681	87.16	1.69E-05	0.652	21.63
T6-40 h	20.52	6750	2.41E-05	0.7288	43.50	4.46E-07	0.3635	25.78

Fig. 13 Optical micrographs showing that grain boundaries could not stop corrosion process after 10 h immersion

Fig. 14 Schematic diagrams for corrosion mechanism of Mg-Sr alloy: a as-cast alloy, b as-extruded alloy

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