Effects of Macrosegregation and Microstructure on the Corrosion Resistance and Hardness of a Directionally Solidified Zn-5.0wt.%Mg Alloy

Abstract

This study aims to analyze the influence of macrosegregation on microstructure evolution, and of microstructure length scale on the corrosion resistance of a Zn-5.0wt.%Mg alloy casting. The analyzed samples were taken along the length of castings unidirectionally solidified in unsteady state conditions of heat extraction. Microstructure characterization, microhardness, linear polarization and electrochemical impedance spectroscopy (EIS) tests were performed. A unique type of microstructure was observed, characterized essentially by a morphology typified by idiomorphic MgZn 2 crystals in a "complex eutectic mixture" [coexistence of stable (Zn+Mg 2 Zn 11 ) and metastable (Zn+MgZn 2 ) eutectics]. The correlation between thermal and microstructural parameters, permitted experimental growth laws, correlating the evolution of the lamellar eutectic spacing with the cooling rate to be established. Vickers microhardness and electrochemical corrosion tests showed that more refined microstructures associated with higher experimental cooling rates, are related to a better set of higher hardness and corrosion resistance.

The Effect of Sn Addition on Zn-Al-Mg Alloy; Part I: Microstructure and Phase Composition

“…Different aspects of the alloy’s microstructure were investigated. The main phases identified were in the general agreement with the published data as follows: η(Zn), α(Al), Mg 2 Zn 11 , MgZn 2 [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 20 , 21 , 22 , 23 , 24 ] and Mg 2 Sn [ 26 , 27 , 32 , 33 ].…”

“…Different aspects of the alloy's microstructure were investigated. The main phases identified were in the general agreement with the published data as follows: η(Zn), α(Al), Mg2Zn11, MgZn2 [7][8][9][10][11][12][13][14][15][16][17][18][20][21][22][23][24] and Mg2Sn [26,27,32,33]. In general, the vol.% of all other microstructure components outside of η(Zn) is decreasing with the addition of Sn into the investigated alloys.…”

“…Mg 2 Zn 11 and MgZn 2 are the most common intermetallic phases in the currently investigated Zn-based systems. MgZn 2 is reported to be less ductile, while both are reported to be slightly less ductile compared to Mg 2 Sn [ 18 , 19 , 20 , 29 , 35 , 36 ]. The possibility to replace Mg x Zn y phases by Mg 2 Sn and thus reduce the overall volume content of intermetallic phases in a Zn 1.6 wt.% Al 1.6 wt.% Mg based alloy would indicate an interesting research path for further extensive investigation of corrosion and mechanical properties of such alloys.…”

“…Mg is useful especially for increasing the galvanic protection offered by a coating at cut-edges and mechanically damaged spots. Additional alloying may further improve the properties of Zn-Al-Mg alloys as reported by several literature sources [15][16][17][18][19][20][21][22][23][24][25]. Unfortunately, there is still lack of information about Sn addition into these systems [26].…”

The Effect of Sn Addition on Zn-Al-Mg Alloy; Part II: Corrosion Behaviour

“…These also include Zn-Al-Mg-based coatings such as Magizinc (MZ) with Zn 1.6 wt.% Al and 1.6 wt.% Mg. It is widely used in the coating industry including steel sheet production for building, energetics, and the automotive industry [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ].…”