Effect of Mn Modification on the Corrosion Susceptibility of Mg-Mn Alloys by Magnesium Scrap

doi:10.1007/s40195-020-01058-4

Abstract

Abstract:

The microstructure and anti-corrosion behavior of Mg-Mn alloys by magnesium scrap have been investigated in this study. The results show that the size of the Fe-rich particles in magnesium scrap decreases but the quantity increases with the Mn addition. Although the presence of Mn-containing Fe-rich particles with unique symbiotic structure can effectively weaken the micro-galvanic corrosion, the presence of more free Fe (Fe-rich particles) does not necessarily lead to severe corrosion of the alloy. The corrosion susceptibility of Mg-Mn-Fe alloy primarily depends on the solubility of iron in the Mg matrix, while it can be significantly reduced by suitable Mn addition. Besides, the tolerance limit of the Fe impurity can be expressed as Fe_max = 0.0083 Mn (relative to the iron solubility). Only when the Fe/Mn ratio is below 0.0083 can the alloy have excellent corrosion resistance, with the corrosion rate changing in the scope of 0.38 ± 0.09 to 0.54 ± 0.15 mg/cm ² day and i_corr from 3 to 9 × 10 ^-4 A/cm ².

Key words: Magnesium scrap, Mn modification, Fe impurity, Corrosion susceptibility, Corrosion resistance

Dong-Dong Gu, Jian Peng, Jia-Wen Wang, Zheng-Tao Liu, Fu-Sheng Pan. Effect of Mn Modification on the Corrosion Susceptibility of Mg-Mn Alloys by Magnesium Scrap[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(1): 1-11.

Figures/Tables 12

Table 1 Chemical composition of the Mg-Mn-Fe alloy samples (wt%)

Alloys	Mn	Fe	Si	Mg
1# (Mg-0Mn-0.023Fe)	0	0.0236	0.0179	Bal
2# (Mg-0.1Mn-0.023Fe)	0.1090	0.0232	0.0188	Bal
3# (Mg-0.3Mn-0.020Fe)	0.2780	0.0201	0.0203	Bal
4# (Mg-0.5Mn-0.018Fe)	0.5030	0.0182	0.0171	Bal
5# (Mg-1.0Mn-0.018Fe)	0.7610	0.0172	0.0206	Bal
6# (Mg-2.0Mn-0.010Fe)	1.4890	0.0079	0.0200	Bal

Fig. 1 SEM images and EDS results of Fe-rich particles: a, c, e 1# alloy, b, d, f 4# alloy

Fig. 2 EDS element line scanning spectrums of Fe-rich particles: a 1# alloy; b 4# alloy

Fig. 3 Scanning electron micrographs showing typical corrosion morphology of the Mg-Mn-Fe alloy samples after immersion in 3.5 wt% NaCl solution for 48 h with the corrosion products removing: a 1#, b 2#, c 3#, d 4#, e 5#, f 6#, g high magnification of the marked area A in a, h high magnification of the marked area B in c, i j high magnification of the marked area C and D in d, k high magnification of the marked area E in e

Fig. 4 Optical images showing the macro-corrosion morphology evolution on the surfaces of the Mg-Mn-Fe alloy samples during immersion in 3.5 wt% NaCl solution

Fig. 5 SEM of corrosion morphologies and EDS results of alloy samples in 3.5 wt% NaCl solution for 1 h: a 1# alloy; b 4# alloy

Fig. 6 a Potentiodynamic polarization curves, b EIS spectra of alloys in 3.5 wt% NaCl solution

Fig. 7 Changes in iron solubility, the free iron, the total iron and corrosion performance of the Mg-Mn-Fe alloy samples

Table 2 Iron content and corrosion performance of the alloy samples

Alloy	Mn (wt%)	The total iron (ppm)	Iron solubility (ppm)	The free iron (ppm)	Corrosion rate (mg/cm² day)
1#	0	236	180	56	144.88 ± 5.22
2#	0.109	232	105	127	70.09 ± 3.60
3#	0.278	201	75	126	13.79 ± 1.17
4#	0.503	182	42	140	0.40 ± 0.12
5#	0.761	172	30	142	0.54 ± 0.15
6#	1.489	79	20	59	0.38 ± 0.09

Fig. 8 Corrosion performance vs. the iron content and Fe/Mn ratio of Mg-Mn-Fe alloy samples

Fig. 9 Fe-Mn solubility limits and the iron tolerance limit. The red numbers adjacent to the data points are the corrosion rates of the Mg-Mn-Fe alloy samples

Fig. 10 Schematic illustrations of a Mg-Fe, b Mg-Mn-Fe alloy during the corrosion process

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