Quasi-in-situ Observation and SKPFM Studies on Phosphate Protective Film and Surface Micro-Galvanic Corrosion in Biological Mg-3Zn-xNd Alloys

doi:10.1007/s40195-023-01642-4

Abstract

Abstract:

The phosphate protective film and micro-galvanic corrosion of biological Mg-3Zn-xNd (x = 0, 0.6, 1.2) alloys were investigated by scanning and transmission electron microscopy, quasi-in-situ observation, scanning Kelvin probe force microscopy (SKPFM) and electrochemical tests. The results revealed the Mg-Zn-Nd phases formed in Mg-3Zn alloy contained with Nd. Adding Nd resulted in a significant decline in the cracks of the phosphate protective film and micro-galvanic corrosion of alloys, which were recorded by quasi-in-situ observation. In addition, the Volta potential difference of Mg-Zn-Nd/α-Mg (~ 188 mV) was lower than MgZn/α-Mg (~ 419 mV) and Zn-rich/α-Mg (~ 260 mV), and the corrosion rates of alloys markedly decreased after the addition of 0.6 wt% Nd. The improvement in corrosion resistance of Nd-containing alloys was mainly attributed to the following: (i) the addition of Nd reduced the Volta potential difference (second phases/α-Mg); (ii) the phosphate protective film containing Nd₂O₃ deposited on the surface of the alloys, effectively preventing the penetration of harmful anions.

Key words: Magnesium alloys, Quasi-in-situ observation, Phosphate protective film, Scanning Kelvin probe force microscopy (SKPFM), Micro-galvanic corrosion

Zhaochen Yu, Kaixuan Feng, Shuyun Deng, Yang Chen, Hong Yan, Honggun Song, Chao Luo, Zhi Hu. Quasi-in-situ Observation and SKPFM Studies on Phosphate Protective Film and Surface Micro-Galvanic Corrosion in Biological Mg-3Zn-xNd Alloys[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(4): 648-664.

Figures/Tables 19

Table 1 Chemical components of the experimental alloys (wt%)

Alloys	Zn	Nd	Mg
Mg-3Zn	2.86	0	Bal.
Mg-3Zn-0.6Nd	2.83	0.57	Bal.
Mg-3Zn-1.2N d	2.79	1.18	Bal.

Table 2 Comparison of composition and concentration of simulated body fluid (SBF)

Fig. 1 SEM results of Mg-3Zn-xNd alloys: a Mg-3Zn alloy; b Mg-3Zn-0.6Nd alloy; c Mg-3Zn-1.2Nd alloy

Fig. 2 Phase morphologies and the corresponding EDS mapping results of Mg-3Zn-0.6Nd alloy a-d and Mg-3Zn-1.2Nd alloy e-i

Fig. 3 TEM images a, c of Mg-3Zn-1.2Nd alloy and corresponding EDS analysis b, d

Fig. 4 2D surface topography, 3D surface topography, 3D surface potential map and the relative potential curve on the A-B line obtained from SKPFM results of Mg-3Zn alloy a-d and Mg-3Zn-0.6Nd alloy e-h

Fig. 5 Optical microscope images of quasi-in-situ corrosion morphology of Mg-3Zn-xNd (x = 0, 0.6, 1.2) alloys: a-d Mg-3Zn alloy; e-h Mg-3Zn-0.6Nd alloy; i-l Mg-3Zn-1.2Nd alloy

Fig. 6 SEM images of the quasi-in-situ corrosion observation area and the corresponding EDS mapping of the Mg-3Zn-xNd (x = 0, 0.6, 1.2) alloys before and after corrosion: a-d Mg-3Zn alloy; e-h Mg-3Zn-0.6Nd alloy; i-l Mg-3Zn-1.2Nd alloy

Fig. 7 Potentiodynamic polarization curves of Mg-3Zn-xNd(x = 0, 0.6, 1.2) soaked in SBF at 37 °C: a 0 h; b 24 h

Table 3 Electrochemical parameters from the potentiodynamic polarization curves of Mg-3Zn-xNd (x = 0, 0.6, 1.2) alloys

Specimen	E_corr (V_SCE)	i_corr (μA/cm²)
Mg-3Zn	− 1.789	276.8
Mg-3Zn-0.6Nd	− 1.619	140.042
Mg-3Zn-1.2Nd	− 1.635	180.548
Mg-3Zn (24 h)	− 1.436	157.805
Mg-3Zn-0.6Nd (24 h)	− 1.131	12.35
Mg-3Zn-1.2Nd (24 h)	− 1.179	29.87

Table 4 Modification of Nd to alloys under different conditions

Alloys	Conditions	E_corr (V_SCE)	i_corr (μA/cm²)	η (%)	References
Mg-2Zn-0.6Zr	Hank’s	− 1.509	20.27	-	[10]
Mg-2Zn-0.6Zr-0.2Nd	Hank’s	− 1.494	16.21	20
Mg-2Zn-0.2Mn	SBF	− 1.687	369.8	-	[11]
Mg-2Zn-0.2Mn-0.6Nd	SBF	− 1.535	50.23	86.4
Zn-5Al	3.5% NaCl	− 1.142	2.181	-	[27]
Zn-5Al-0.06Nd	3.5% NaCl	− 1.117	1.061	51.4
Mg-9Al	3.5% NaCl	− 1.472	86	-	[28]
Mg-9Al-0.25Nd	3.5% NaCl	− 1.535	43.4	49.5
Mg-2Zn-0.2Mn	Kokubo	− 1.633	287.59	-	[30]
Mg-2Zn-0.2Mn-0.6Nd	Kokubo	− 1.502	36.32	87.3
Mg-2Zn-0.2Mn-1.2Nd	Kokubo	− 1.588	58.21	79.7
Mg-2Zn-0.2Mn-1.8Nd	Kokubo	− 1.611	93.65	67.4
Mg-3Zn	SBF	− 1.789	276.8	-	Present study
Mg-3Zn-0.6Nd	SBF	− 1.619	140.042	49.4
Mg-3Zn-1.2Nd	SBF	− 1.635	180.548	34.7
Mg-3Zn (24 h)	SBF	− 1.436	157.805	-
Mg-3Zn-0.6Nd (24 h)	SBF	− 1.131	12.35	92.1
Mg-3Zn-1.2Nd (24 h)	SBF	− 1.179	29.87	81.1

Fig. 8 EIS test results of Mg-3Zn-xNd (x = 0, 0.6, 1.2) immersion in SBF and equivalent fitting circuit of the EIS test: a 0 h; b 24 h; c equivalent fitting circuit

Table 5 Electrochemical parameters of Mg-3Zn-xNd (x = 0, 0.6, 1.2) alloys obtained by the fits of the experimental EIS data

Specimen	R_s (Ω·cm²)	R_ct (Ω·cm²)	CPE_dl(F/cm²)		R_f (Ω·cm²)	CPE_f(F/cm²)
Specimen	R_s (Ω·cm²)	R_ct (Ω·cm²)	C_1-_T	n₁	R_f (Ω·cm²)	C_2-_T	n₂
Mg-3Zn	32.2	58.69	2.96 × 10⁻⁵	0.87	13.03	5.41 × 10⁻³	0.92
Mg-3Zn-0.6Nd	24.33	145.5	4.06 × 10⁻⁵	0.81	24.88	2.8 × 10⁻³	0.86
Mg-3Zn-1.2Nd	19.09	73.13	2.73 × 10⁻⁵	0.87	30.20	5.51 × 10⁻³	0.77
Mg-3Zn (24 h)	32.84	209.7	3.5 × 10⁻⁵	0.78	16.76	6.5 × 10⁻⁵	0.78
Mg-3Zn-0.6Nd (24 h)	114.4	4271	4.25 × 10⁻⁵	0.66	3204	1.6 × 10⁻³	0.64
Mg-3Zn-1.2Nd (24 h)	125.2	2726	4.4 × 10⁻⁵	0.69	2073	1.7 × 10⁻³	0.66

Fig. 9 Hydrogen evolution volume a and weight loss rates b of Mg-3Zn-xNd (x = 0, 0.6, 1.2) alloys after immersed in SBF for 24 h

Fig. 10 Surface corrosion morphology images and corresponding EDS mapping of tested alloys immersed in SBF for 24 h: a, b Mg-3Zn alloy; c, d Mg-3Zn-0.6Nd alloy; e, f Mg-3Zn-1.2Nd alloy

Fig. 11 Longitudinal images and the corresponding EDS mapping of tested alloys immersed in SBF for 24 h: a Mg-3Zn alloy; b Mg-3Zn-0.6Nd alloy; c Mg-3Zn-1.2Nd alloy; d locally enlarged longitudinal image; e EDS result

Fig. 12 XRD results of corrosion products produced by test samples (Mg-3Zn, Mg-3Zn-0.6Nd) after immersion in SBF for 24 h

Fig. 13 XPS analysis of the surface corrosion films on Mg-3Zn-0.6Nd alloy: a Ca 2p region; b O 1s region; c P 2p region; d Nd 3d region

Fig. 14 Corrosion mechanism of Mg-3Zn-xNd (x = 0, 0.6,1.2) alloys in SBF: a, b Mg-3Zn alloy; c, d Mg-3Zn-xNd alloy

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