Understanding the corrosion behaviour of the magnesium alloys EV31A, WE43B, and ZE41A

Abstract

The corrosion rates of the Mg alloys immersed in 3.5 wt% NaCl solution saturated with Mg(OH)2 were in the following increasing order: WE43B (0.23 mm/y) < EV31A (0.88 mm/y) < pure‐Mg (1.6 mm/y) < ZE41A (8.5 mm/y). The average corrosion rate for WE43B was somewhat lower than the intrinsic corrosion rate of Mg as shown by high‐purity Mg, attributed to (a) no corrosion acceleration by the small second phase particles, and (b) a more protective surface film. The high corrosion rates of ZE41A were attributed to the presence of a coarse semicontinuous T‐phase, which served as strong cathodic sites.

Corrosion of Mg alloys EV31A, WE43B, and ZE41A in chloride‐ and sulfate‐containing solutions saturated with magnesium hydroxide

“…Table presents the data on the immersion tests in 0.1 M NaCl and 0.1 M Na 2 SO 4 both saturated with Mg(OH) 2 at 23 ± 2°C for all the alloys and the three test periods (7, 14, and 21 days). The data include the specific volume of evolved hydrogen per area per day V H (ml·cm −2 ·day −1 ), the corrosion rates calculated from the hydrogen evolution volume P H , and the mass loss P W (mm/year), using the following equations: …”

“…Tables and present average corrosion rates evaluated from the mass loss P AW , and the evolved hydrogen, P AH , for the specimens immersed in 0.1 M Na 2 SO 4 , 0.1 and 0.6 M NaCl solutions all saturated with Mg(OH) 2 . For comparison, data were included in Tables and for the alloys tested in 0.6 M NaCl saturated with Mg(OH) 2 from the previous study . The data presented in Table for the alloys tested in 0.6 M NaCl solution saturated with Mg(OH) 2 represent the average corrosion rates P AH calculated from the evolved hydrogen measured during the electrochemical testing for 7 days as well as the results of the previous study .…”

“…Figure presents (a) SEM secondary and (b) backscattered electron microstructures of ZE41A, which contained several phases in the α‐Mg matrix: (i) coarse semicontinuous eutectic T‐phase (Mg 7 Zn 3 RE) within which there were blocky intermetallic particles rich in Zr and Zn, marked by solid arrows in Figure b; (ii) intermetallic particles within the grains also mainly containing Zr and Zn marked by dashed arrows; and (iii) numerous submicron particles of various morphologies with a possible composition of Mg 5 (Ce,Zn). Both blocky and intermetallic particles could be either ZnZr 2 or Zn 2 Zr 3 …”

“…Commercial Mg alloys like EV31, WE43, and ZE41 contain second phases, which typically increase the corrosion rate because the second phases have higher, nobler potentials and are cathodic to the matrix unless the second phase can form a continuous corrosion barrier . In contrast, the second phase may have little influence if the difference in corrosion potential is small . However, typically, the second phase accelerates corrosion by microgalvanic interaction with the matrix when present as isolated particles or in small volume fractions …”

“…Soltan et al compared the corrosion of EV31A‐T6, WE43B‐T6, and ZE41A‐T5 in 0.6 M (3.5 wt%) NaCl, and showed that WE43B had the lowest corrosion rate (evaluated by weight loss, P W = 0.23 mm/year) followed by EV31A (0.88 mm/year), pure Mg (1.6 mm/year), and ZE41A (8.5 mm/year). The WE43B corrosion rate was somewhat lower than the intrinsic corrosion rate for Mg ( P W = 0.4 mm/year), attributed to (a) Y rich second‐phase particles not participating in the corrosion, and (b) the formation of a somewhat more protective surface film. EV31A had a corrosion rate somewhat higher than that of WE43B, attributed to the second‐phase particles, which served as strong cathodic sites outweighing the contribution of the alloying elements (Nd, Gd, and Zn) to the protectiveness of the surface film.…”