Improved corrosion resistance of Mg alloy AZ31B induced by selective evaporation of Mg using large pulsed electron beam irradiation

doi:10.1016/j.jmst.2018.12.004

Abstract

Abstract:

Large pulsed electron beam (LPEB) irradiation was employed as a surface treatment of magnesium (Mg) alloy AZ31B to enhance its corrosion and wear resistance. Selective evaporation of Mg induced by LPEB irradiation at an energy density of 5 J/cm² for 40 cycles has led to the formation of an Al-enriched re-solidified layer with nano-grained structure consisting of Mg_3.1Al_0.9 metastable phase. The formation of such a re-solidified layer after LPEB irradiation has enabled a decrease in corrosion rate of Mg alloy AZ31B in 3.5% NaCl solution. Different equivalent electrical circuit models were proposed to account for the corrosion behavior of untreated Mg alloy AZ31B and those subjected to LPEB irradiation. A decrease in wear depth when compared to that of the untreated alloy suggests an increase in wear resistance of LPEB-irradiated Mg alloy AZ31B. Adhesive wear is the predominant mechanism of untreated Mg alloy AZ31B while abrasive wear mechanism dominates for LPEB-irradiated Mg alloy AZ31B.

Key words: Magnesium alloys, Electron beam treatment, Corrosion resistance, Microstructure, Rapid re-solidification, Wear resistance

Woo Jin Lee, Jisoo Kim, Hyung Wook Park. Improved corrosion resistance of Mg alloy AZ31B induced by selective evaporation of Mg using large pulsed electron beam irradiation[J]. J. Mater. Sci. Technol., 2019, 35(5): 891-901.

Figures/Tables 18

Table 1 Chemical compositions of Mg alloy AZ31B (wt%).

Mg	Al	Zn	Mn	Si	Ca	Fe	Cu	Ni	Others
Bal.	2.3	1.0	0.20	0.10	0.04	0.005	0.04	0.005	0.30

Fig. 1 (a) Schematic diagram of large pulsed electron beam (LPEB) experimental setup and (b) energy transfer mechanisms induced by LPEB irradiation.

Table 2 Parameters of large pulsed electron beam irradiation process.

Parameter	Value
Energy density (J/cm²)	3-10
Pulse duration (μs)	2
Dwell time (s)	10
Beam diameter (mm)	60
Irradiation pattern	2?×?2
Pitch (mm)	20
Number of cycles	10-40
Irradiation height (mm)	30
Vacuum pressure (Pa)	0.05

Fig. 2 Schematic diagram of experimental setup for (a) three electrode cell and (b) ball-on-disc wear tests.

Fig. 3 SEM images on surface of Mg alloy AZ31B before and after LPEB irradiations with varying energy density for 20 irradiation cycles: (a) bare surface, Ra ?=?0.103?μm; (b) 3?J/cm2, Ra ?=?0.217?μm; (c) 5?J/cm2, Ra ?=?0.304?μm; (d) 10?J/cm2, Ra ?=?0.369?μm.

Fig. 4 SEM images on surface of AZ31B before and after LPEB irradiation for different numbers of cycles with an energy density of 5?J/cm2: (a) 10 cycles; (b) 20 cycles; (c) 40 cycles; (d) more than 40 cycles.

Table 3 Chemical compositions of Mg alloy AZ31B before and after LPEB irradiation (at.%).

Energy density	C	N	O	Mg	Al	Mn	Zn
Bare	1.07	-	1.57	95.05	1.29	0.53	0.49
3?J/cm²	5.71	0.67	2.46	85.66	4.47	0.44	0.59
5?J/cm²	6.13	-	4.30	82.90	5.96	0.36	0.35
7?J/cm²	1.35	0.17	3.04	88.35	6.18	0.42	0.49
10?J/cm²	3.28	-	3.51	88.89	3.62	0.39	0.31

Fig. 5 SEM images on cross section of AZ31B (a) before and (b) after LPEB irradiation with 5?J/cm2 energy density and (c) depth profile of EDS analysis along tracing line.

Fig. 6 EDS spectra on raw and LPEB-irradiated surfaces of Mg alloy AZ31B corresponding to Al and Mg in terms of (a) energy density and (b) number of irradiation cycles.

Fig. 7 XRD peaks on surface of Mg alloy AZ31B before and after LPEB irradiation for a range of energy densities.

Fig. 8 Mechanism of phase transformation on Mg alloy AZ31B alloy induced by LPEB irradiation.

Fig. 9 Potentiodynamic polarization curves of Mg alloy AZ31B alloy before and after LPEB irradiation for a range of energy densities and after 40 irradiation cycles.

Table 4 Summary of polarization electrochemical parameters of bare and LPEB-irradiated Mg alloy AZ31B samples.

Energy density	E_corr(mV vs. SCE)	i_corr(nA/cm²)	v_corr(mm/year)	β_a(mV/dec)	β_c(mV/dec)	R_p(kΩ cm²)
Bare	-1595	43.5	9.95?×?10^-4	0.253	-0.204	490.3
3?J/cm²	-1570	47.1	10.8?×?10^-4	0.208	-0.263	461.6
5?J/cm²	-1421	24.6	5.63?×?10^-4	0.192	-0.275	882.8
10?J/cm²	-1596	29.6	6.77?×?10^-4	0.239	-0.235	735.1

Fig. 10 (a) Nyquist plots and (b, c) Bode plots of AZ31B alloys before and after LPEB irradiation for a range of energy densities and after 40 irradiation cycles (Z′: real part of impedance; Z": imaginary part of impedance).

Fig. 11 Equivalent circuit modeling corresponding to (a) raw and (b) LPEB-irradiated surface of Mg alloy AZ31B.

Table 5 Electrical parameters investigated from fitted lines of EIS spectra obtained on surface of Mg alloy AZ31B before and after LPEB irradiations.

Energy density	R_s (kΩ cm²)	CPE_f (nF)	R_f (kΩ cm²)	CPE_t (nF)	R_ct (kΩ cm²)
Bare	10.90	166.7	249.8	0.000151	411.7
3?J/cm²	7.472	-	-	0.843	423.9
5?J/cm²	7.853	-	-	1.192	795.3
10?J/cm²	7.985	-	-	1.211	589.7

Fig. 12 Three-dimensional surface profiles of wear tracks formed on (a) bare and LPEB-irradiated surface of Mg alloy AZ31B after 40 irradiation cycles with energy densities of (b) 3?J/cm2, (c) 5?J/cm2 and (d) 10?J/cm2.

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