Corrosion Characterization of Physically Vapor Deposited Magnesium Alloys - Toward a Biodegradable Battery

Open Access
Author:
Naccarelli, Anthony James
Graduate Program:
Engineering Science and Mechanics
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 12, 2013
Committee Members:
  • Barbara Shaw, Thesis Advisor
Keywords:
  • corrosion
  • thin-film
  • magnesium
  • PVD
Abstract:
The corrosion rates of bulk magnesium, AZ31, AZ61, and AZ91 specimens were determined using polarization resistance, electrochemical impedance spectroscopy (EIS), and cathodic polarization. Compositions of the bulk alloys were determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Thin-film magnesium, magnesium-zinc, magnesium-aluminum, and magnesium-aluminum-zinc specimens were created using a dual-gun electron beam physical vapor deposition (EB-PVD) system. Polarization resistance and electrochemical impedance spectroscopy (EIS) were used to determine the corrosion rates of bulk and thin-film specimens. Also, anodic and cathodic polarization were performed on the bulk specimens. Film thickness was measured with a profilometer. The compositions of the thin-film samples were determined using energy-dispersive X-ray spectroscopy (EDS). Film morphologies were characterized with a field-emission scanning electron microscope (FE-SEM). Magnesium-aluminum-zinc alloys with 19.4-20.6 weight percent aluminum and 1.4-1.7 weight percent zinc were found to have the lowest corrosion rates of any specimen (bulk or thin-film) tested during this work. The magnesium and magnesium-zinc thin-films had columnar morphologies while the magnesium-aluminum and magnesium-aluminum-zinc films had more dense morphologies. The corrosion rate of the thin-films was observed to decrease with increasing aluminum content, assuming the apparent area was equal to the true exposed area. The addition of zinc to magnesium films was observed to decrease corrosion rate. However, increasing zinc concentration did not substantially affect the corrosion rate. Bulk and thin-film magnesium-aluminum-zinc alloys with similar aluminum and zinc concentrations were observed to have similar corrosion rates. The thin-film magnesium-aluminum-zinc specimens showed open circuit potential (OCP) values over 100 mV more negative (active) than bulk magnesium-aluminum-zinc alloys with similar aluminum and zinc concentrations.