Ternary alloying additions and multilayering as strategies to enhance the galvanic protection ability of Al-Zn coatings electrodeposited from ionic liquid solution

In recent years, there have been a large number of studies showing that compositionally modulated or multilayer coatings have significantly higher corrosion resistance than monolithic equivalents, in a diverse set of environments. While electrodeposition is an attractive processing method for multilayer coatings, electrodeposition of the aluminum-zinc coatings that would be natural candidates for galvanic corrosion coatings presents several unique challenges. In particular, co-deposition of aluminum and zinc involves a large electrochemical gap between the two elements, low solid solubility, and strong tendency for formation of zinc dendrites. Here we examine the effects of Mn and Zr additions during the electrodeposition of aluminum-zinc from AlCl3/1-ethyl-3-methylimidazolium chloride solution. Aluminum-zinc electrodeposits, with and without addition of Mn or Zr, are prepared and characterized to determine the range of compositions and microstructures that can be produced. The addition of Zr, but not Mn, results in decreased grain size, surface roughness, and compositional homogeneity, as well as the emergence of a nanocrystalline or amorphous phase at low deposition rates. The Al-Zn-Zr electrodeposits also exhibit improved corrosion resistance, as measured by cyclic voltammetry and immersion testing in 50 mM NaCl solution, with additional improvement for multilayered coatings which use the nanocrystalline deposit as a base layer.