Synthesis and Characterization of Mg-Based Coatings for Sacrificial Cathodic Protection of Mg Alloy AZ31B
Authors: Cain Taylor W. , Melia Michael A. , Fitz-Gerald James M. , Scully John R. 
Published: February 2020

Magnesium (Mg) alloys are becoming increasingly popular for weight reduction in the automotive industry due to their low density and nominal specific strength [1]. However, the deleterious intrinsic corrosion rates of Mg alloys have hindered their widespread application. To date, protection strategies for Mg alloys has consisted primarily of barrier coatings such as Rockhard™ and Tagnite™ which have not demonstrated the ability to protect bare Mg at scratches, cracks, and other defects [2] while conversion coatings have not been shown to survive harsh working conditions and do not provide sacrificial cathodic protection or inhibitor release [3]. The drawbacks of these coating strategies provide motivation for additional research. In addition, sacrificial cathodic protection of Mg alloys has not been widely studied and presents opportunities of high interest. Recent studies have shown that sacrificial cathodic protection of Mg and its alloys is possible and shows some promise albeit complicated by cathodic corrosion of Al rich phases and large local cathodic reaction rates [4]. However, an optimal sacrificial coating has not been produced to date. As such, the overall goal of this research is to design a tunable coating which acts as (1) a corrosion barrier, (2) provides sacrificial cathodic protection, and (3) functions as a reservoir for ionic inhibitor release. Such a coating has been designed recently for aluminum alloy AA2024 using an amorphous Al-Co-Ce alloy [5].
The present work aims to study sacrificial Mg-based coatings produced by pulsed laser deposition (PLD) tuned for the Mg alloy AZ31B-H24 (Mg-3Al-1Zn). Binary and ternary coatings using rare earth elements known to decrease the OCP of Mg [6] and exhibit some ability for inhibitor release (RE = La, Gd, Y) as well as elements such as Ge and Sn [7] which poison cathodic kinetics, will be explored. Preliminary results of a Mg88Gd9Y3 based coating on AZ31B revealed a two-phase microstructure consisting of an extended solid solution Mg-rich matrix with a grain size on the order of 100 nm as well as a small fraction of Gd-Y rich particles < 1 μm in diameter. Immersion testing of PLD coated AZ31B in 0.6 M NaCl revealed an open circuit potential of -1.58 VSCE which is mildly cathodic to uncoated AZ31B (-1.56 VSCE) after a 24-hour exposure. In addition, the coating was observed to dissolve preferentially on an AZ31B surface demonstrating ability as a sacrificial anodic coating. Future testing will systematically explore the effects of alloying element concentration on the corrosion behavior of PLD coated AZ31B utilizing both AC and DC electrochemical techniques. Such an understanding will provide a framework to optimize a sacrificial cathodic coating for AZ31 based on critical factors such as open circuit potential, corrosion rate, galvanic couple potential, and galvanic couple current.

Acknowledgments:
This project is funded by the US Air Force Academy under agreement number FA7000-13-2-0020 as part of Corrosion University Pilot Program under the direction of Mr. Daniel Dunmire and by the National Science Foundation under NSF DMR#1309999 as well as the Army Research Office under agreement number W911NF-14-2-0005 with project manager Joe Labukas. The U.S. Government is authorized to reproduce and distribute reprints for government purposes notwithstanding any copyright notation thereon. The views and conclusions contained are those of the authors and should not be interpreted as necessarily representing the official policies or endorsement of the US Air Force Academy or the US Government.

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