The Effect of Microalloying Additions in Aluminum Alloys on Intermetallic Electrochemistry and Dissolution Behavior
Authors: Campbell Daniel Orion , Buchheit Rudolph 
Published: February 2020

Advances in aluminum alloy processing have enabled more reliable control of low-level alloying additions. Small alloying additions, often referred to as “microalloying” have been reported to affect strengthening mechanisms, intergranular corrosion resistance, stress corrosion cracking resistance, and other properties [1,2]. In some cases, notable improvements in resistance to corrosion and stress corrosion cracking have been reported.
Microstructural characterization has shown that some low-level alloying additions can be dissolved into intermetallic compounds that form during natural or artificial aging. This raises the possibility that alloying alters intermetallic electrochemistry and intermetallic dissolution behavior. For example, MgZn2 dissolves Cu as aging time of 7XXX alloys increases [3]. Potentiodynamic polarization measurements of bulk synthesized Mg(Zn,Cu)2 with Cu concentrations of up to 17 wt.% show that dissolution kinetics are significantly ennobled and the breakdown potential of the phase is increased. Similar effects are observed when Cu is added to Al3Mg2. Zn additions to Al2CuLi result in an isomorphic substitution of Zn for Cu in the structure of the intermetallic compound and a decrease in the reactivity of the phase. The changes in intermetallic electrochemical behavior due to microalloying have not yet been linked to overall alloy corrosion rates or modes and detailed characterization of the relationships between alloy chemistry, alloy processing, intermetallic chemistry and dissolution behavior and overall alloy corrosion behavior are warranted.
In this presentation, the effect of microalloying on the dissolution behavior of MgZn2, Al2CuMg is reported. Bulk crystals of pure MgZn2, Al2CuMg and were synthesized then remelted with low-level alloying additions (< 1 at.%) followed by heat treating to form the desired phases. Transition and rare-earth metals typically have low solubility in Al, resulting in these elements incorporating into existing intermetallics or forming alternate phases. These alternate phases may also result in suppressing the formation of Al2CuMg and MgZn2 by arranging into more energetically favorable phases. Zn additions do not readily incorporate into bulk Al2CuMg, instead forming particles of Al0.38Cu0.46Mg0.16 containing 1-5%Zn and increasing the corrosion rate of the particle by an order of magnitude. Conversely, Cu additions to MgZn2 showed significant incorporation of Cu on Zn sites in the MgZn2 structure, as well as more noble corrosion behavior. Furthermore, this project explores the effect of other elemental alloying additions whose microstructural effects have been studied in literature, such as Ag and Ce, to see what corrosion effects may also take place.
Phase identification was based on structural and chemical characterization. Powder x-ray diffraction (XRD) was used to confirm structure and energy dispersive spectroscopy (EDS) was used to characterize chemistry. Electrochemical characterization was carried out using open circuit potential and potentiodynamic polarization measurements performed with an electrochemical microcell.
In this presentation, the effect of Cu microalloying on the structure and electrochemistry of MgZn2 and Zn microalloying on Al2CuMg will be summarized. How microalloying affects dissolution rates and modes of corrosion will be described. How changes in intermetallic electrochemistry affect alloy corrosion behavior will also be discussed.
References:

Carroll, Mark C. (2001). Improvements to the Strength and Corrosion Resistance of Al-Mg-Mn Alloys of Near-AA5083 Chemistry (Doctoral Dissertation).

Li, J. F., N. Birbilis, D. Liu, Y. Chen, X. Zhang, C. Cai. Intergranular corrosion of Zn-free and Zn-microalloyed Al–xCu–yLi alloys. Corrosion Science, 2016. 105: 44-57.

Fang, Xu, M. Song, K. Li, D. Yong. Effects of Cu and Al on the crystal structure and composition of η (MgZn2) phase in over-aged Al–Zn–Mg–Cu alloys. Journal of Materials Science, 2012. 47: 5419-27.