Corrosion Behavior of AZ31 Magnesium Alloy in Highly Alkaline Environment

doi:10.1007/s40195-017-0549-8

Abstract

Abstract:

Industrial components made with a magnesium-aluminum alloy AZ31 are often used in diverse engineering applications where component weight, strength, and durability are critical. Similarly as other Mg alloys, however, the AZ31 is susceptible to corrosion in alkaline environments. In this work, corrosion of commercial grade AZ31 alloy plate was examined in potassium hydroxide (KOH) using immersion and potentiodynamic studies. The results suggest that the concentration of Al and Mn in the alloy may govern the kinetics of micro-galvanic corrosion and the initiation of corrosion pits. Further, trace amounts of Ni in the AZ31 alloy were seen to enable formation of Ni(OH)₂ surface layer, which may have further accelerated alloy corrosion due to its cracking and void coalescence. The effect of pH on the corrosion behavior of AZ31 was studied with reference to Pourbaix diagrams.

Key words: Magnesium alloy, AZ31, KOH

Somi Doja, Lukas Bichler, Simon Fan. Corrosion Behavior of AZ31 Magnesium Alloy in Highly Alkaline Environment[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(4): 367-375.

Figures/Tables 14

Table 1 Composition of AZ31 plate (wt%)

	Mg	Al	Zn	Mn	Fe	Ni	Cu
Matrix	95.49	2.99	1.2	0.2	0.08	0.01	0.03
Al-Mn precipitates	11.02	39.98	0.36	48.49	0.09	0.02	0.04

Fig. 1 SEM-BSE images of AZ31 alloy: a at low magnification, b at higher magnification showing the Al-Mn precipitates

Fig. 2 SEM-BSE images of surface of AZ31 alloy after 9 days of exposure to 45 wt% KOH: a revealing pits on the surface, b with corrosion products still attached to the pits

Fig. 3 SEM-BSE images of the surface of AZ31 alloy after 18 days of exposure to KOH electrolyte: a revealing pits on the surface, b with corrosion product still attached to the pit

Table 2 Composition of corroded surface (wt%)

	O	Mg	Al	K	Mn	Fe	Ni	Cu	Zn
Matrix	33.29	63.1	1.21	0.84	0.53	0.22	0	0.01	0.79
Gray area near the pit	30.02	41.32	0.89	14.36	12.02	0.79	0	0.18	0.41

Fig. 4 SEM-XEDS map in the vicinity of a pit observed in AZ31 exposed to KOH after 18 days of exposure

Fig. 5 SEM-BSE cross-sectional images of corroded surface after immersion for 3 a, 6 b, 9 c, 12 d, 15 e, 18 f days

Fig. 6 SEM line scan across a corroded surface after 18 days of immersion in 45 wt% KOH electrolyte

Fig. 7 Elemental mapping by SEM-XEDS of the cross section of the corroded surface after 18 days of immersion in 45 wt% KOH

Fig. 8 XRD results for surface of AZ31 immersed for 18 days in 45 wt% KOH electrolyte

Fig. 9 Cross-sectional view of the corroded AZ31 sample after 18 days of immersion in 45 wt% KOH electrolyte

Fig. 10 Change of concentration of during immersion test as a function of exposure time evaluated by SEM-XEDS

Fig. 11 Potentiodynamic polarization curve of AZ31 in 45 wt% KOH electrolyte

Fig. 12 Nyquist and the equivalent circuit obtained from EIS analysis for AZ31 sample immersed in 45 wt% KOH

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