Development of Sacrificial Magnesium-Based Coatings by Vapor Deposition for the Corrosion Protection of Magnesium Alloys

Open Access
- Author:
- Nunez, Michael
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 13, 2014
- Committee Members:
- Mark William Horn, Thesis Advisor/Co-Advisor
Barbara Shaw, Thesis Advisor/Co-Advisor
Elzbieta Sikora, Thesis Advisor/Co-Advisor - Keywords:
- Magnesium
Corrosion
Thin-Film
PVD - Abstract:
- The corrosion properties of pure magnesium (Mg), AZ31, AZ61, AZ91, WE43, and EV31 bulk alloys were analyzed in a solution of 0.1 M sodium chloride (NaCl) adjusted to a pH of 7 with sodium hydroxide (NaOH). Open circuit potential (OCP) measurements, polarization resistance, electrochemical impedance spectroscopy, and potentiodynamic polarization were conducted on all bulk samples. WE43 exhibited the lowest OCP and the lowest corrosion rate of all the bulk alloys in 0.1 M NaCl solutions that were open to air. AZ91 had the lowest corrosion rate of all the bulk alloys in deaerated solutions. Pure Mg, Mg-Sn, and Mg-Zn-Sn thin films were deposited via electron beam physical vapor deposition (EBPVD) and biased target deposition (BTD), with the objective of obtaining an alloy with a lower OCP than all the bulk Mg alloys. Pure Mg films deposited via BTD had the lowest OCP, of about -1.9 V vs. SCE. The pure Mg film deposited via EBPVD also had a low corrosion rate, of about 20.2 mpy in deaerated solutions, compared to bulk pure Mg (53.36). The OCP of the Mg-Sn and Mg-Zn-Sn alloys were shifted towards more positive values with increasing Sn concentration. An increase in the corrosion rate was also observed with increasing Sn concentration, due to microgalvnic effects caused by larger volume fractions of the Mg2Sn phase with higher Sn concentrations. Mg-Sn films deposited via BTD had lower corrosion rates than Mg-Sn films deposited via EBPVD. Pure Mg films deposited via BTD had very porous structures that were detrimental to corrosion. Extreme localized corrosion in the form of filiform corrosion was observed in pure Mg films deposited via BTD.