Biodegradation Behavior of Coated As-Extruded Mg–Sr Alloy in Simulated Body Fluid

doi:10.1007/s40195-019-00892-5

Abstract

Abstract:

As-extruded Mg-Sr alloy, a kind of promising biodegradable biomedical material, was coated using micro-arc oxidation and also using a phosphate conversion coating. The corrosion behaviors were investigated using Hanks’ solution. The corrosion of the as-extruded Mg-Sr alloy became more serious with increasing immersion time; that is, the corrosion pits became more numerous, larger and deeper. The micro-arc oxidation coating and the phosphate conversion coating were effective in improving the corrosion resistance of the as-extruded Mg-Sr alloy. The micro-arc oxidation coating was much more effective. Moreover, the as-extruded Mg-Sr alloy and the coated as-extruded Mg-Sr alloy exhibited lower corrosion rates than the as-cast Mg-Sr alloy and the corresponding coated as-cast Mg-Sr alloy, indicating that the corrosion properties of the coated samples are dependent on their substrates. The finer microstructure of the substrate of the as-extruded condition corroded much slower. The corrosion resistance of the coated Mg-Sr alloy depended on the coating itself and on the microstructure of the substrate.

Key words: Mg alloy, Micro-arc oxidation coating, Phosphate conversion coating, Biodegradation, Corrosion resistance

Ming-Chun Zhao, Ying-Chao Zhao, Deng-Feng Yin, Shuo Wang, Yong-Ming Shangguan, Chao Liu, Li-Li Tan, Ci-Jun Shuai, Ke Yang, Andrej Atrens. Biodegradation Behavior of Coated As-Extruded Mg–Sr Alloy in Simulated Body Fluid[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(10): 1195-1206.

Figures/Tables 12

Fig. 1 Optical micrograph a, XRD pattern b of as-extruded Mg-1.5Sr alloy, and optical micrograph c of original as-cast Mg-1.5Sr alloy

Fig. 2 Surface and cross-sectional morphologies of MAO coating (a surface, b cross section) and Sr-P coating (c surface d cross section)

Fig. 3 EDS spectra of surface and cross section for MAO coating (a surface, b cross section) and Sr-P coating (c surface, d cross section)

Table 1 Chemical composition of the surface and the cross section for the coatings

Coating		Mg	O	F	P	K	Ca	Sr
MAO surface	wt%	32 ± 2	31 ± 2	15 ± 1	16 ± 1	4.3 ± 0.2	2.5 ± 0.2
MAO surface	at.%	28 ± 2	41 ± 3	17 ± 1	11 ± 1	2.2 ± 0.1	1.6 ± 0.1
MAO cross section	wt%	42 ± 2	29 ± 2	8.5 ± 0.3	18 ± 1	1.4 ± 0.1	1.2 ± 0.1
MAO cross section	at.%	37 ± 2	40 ± 2	9.5 ± 0.2	13 ± 1	0.8 ± 0.1	0.6 ± 0.1
Sr-P-surface	wt%	1.1 ± 0.1	30 ± 1		18 ± 1			51 ± 3
Sr-P-surface	at.%	1.5 ± 0.1	60 ± 3		20 ± 1			19 ± 1
Sr-P-cross section	wt%	1.2 ± 0.1	25 ± 2		21 ± 1			53 ± 3
Sr-P-cross section	at.%	1.6 ± 0.1	54 ± 3		24 ± 1			21 ± 1

Fig. 4 XRD patterns of a MAO coating, b Sr-P coating

Fig. 5 Potentiodynamic polarization curves a, Nyquist plots b, pH variation c, corrosion rate calculated by mass loss d of different coatings and as-extruded Mg-1.5Sr substrate immersed in Hank’s solution

Table 2 Electrochemical parameters fitted from the polarization curves of as-extruded condition

As-extruded condition	I_corr (μA/cm²)	E_corr (V/SCE)	C (mm/y)
Mg-1.5Sr substrate	4.1	- 1.64	0.09
Sr-P coating	1.5	- 1.58	0.03
MAO coating	0.5	- 1.73	0.01

Fig. 6 Potentiodynamic polarization curves a, corrosion rate calculated by mass loss b of different coatings and as-cast Mg-1.5Sr substrate immersed in Hank’s solution

Table 3 Electrochemical parameters fitted from the polarization curves of as-cast condition

As-cast condition	I_corr (μA/cm²)	E_corr (V/SCE)	C (mm/y)
Mg-1.5Sr substrate	5.6	- 1.69	0.13
Sr-P coating	1.7	- 1.59	0.04
MAO coating	0.8	- 1.65	0.02

Fig. 7 Cross-sectional morphologies of MAO coating (a 7 d, b 14 d) and Sr-P coating (c 7 d, d 14 d) after immersion

Fig. 8 EDS spectra after immersion for MAO coating in a cross section for 7 d; b cross section for 14 d; c cross section close to substrate for 14 d and Sr-P coating in d cross section for 7 d; e cross section close to substrate for 7 d; f cross section for 14 d

Table 4 Chemical compositions of the coatings after immersion of 7 and 14 days

Coating		Mg	O	F	P	K	Ca	Sr
MAO: cross section-7 d	wt%	36 ± 3	27 ± 2	13 ± 1	20 ± 2	1.2 ± 0.1	2.2 ± 0.1
MAO: cross section-7 d	at.%	32 ± 3	37 ± 4	14 ± 1	14 ± 1	0.7 ± 0.1	1.7 ± 0.1
MAO: cross section-14 d	wt%	40 ± 4	28 ± 3	7.3 ± 0.4	23 ± 3		2.9 ± 0.1
MAO: cross section-14 d	at.%	36 ± 4	38 ± 3	8.4 ± 0.4	16 ± 1		1.6 ± 0.1
MAO: cross section-14 d Close to the substrate	wt%	51 ± 4	42 ± 4		7.2 ± 0.3
MAO: cross section-14 d Close to the substrate	at.%	42 ± 3	53 ± 4		4.7 ± 0.3
Sr-P: cross section-7 d	wt%	2.2 ± 0.1	21 ± 2		20 ± 2			57 ± 4
Sr-P: cross section-7 d	at.%	3.3 ± 0.1	49 ± 3		24 ± 3			24 ± 2
Sr-P: cross section-7 d Close to the substrate	wt%	56 ± 4	44 ± 4
Sr-P: cross section-7 d Close to the substrate	at.%	46 ± 3	54 ± 4
Sr-P: cross section-14 d	wt%	2.6 ± 0.1	23 ± 2		22 ± 1			53 ± 4
Sr-P: cross section-14 d	at.%	3.7 ± 0.1	50 ± 4		25 ± 2			21 ± 2

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