Effect of CO2 on Surface Film Formation in Magnesium-Aluminum Alloys
Authors: Cantonwine Sara , Buchheit Rudolph 
Published: February 2020

Interest in magnesium alloys has increased due to their high strength to weight ratios; however, pure magnesium is highly active and Mg alloys lack corrosion resistance. Alloying additions are employed in the magnesium system to increase corrosion performance, formability and strength. This study focuses on the commercially popular AZ series because of their wide range of applications and ease of accessibility. The AZ series, magnesium-aluminum-zinc alloys, have a complex microstructure comprising second phase particles at grain boundaries formed through solid-state reactions and intermetallic particles formed during solidification processing. A range of alloy compositions have been studied to develop empirical and mechanistic relationships between alloy composition and microstructure; as well as, strength, ductility, and corrosion resistance.

Electrochemical, microscopy, and surface science techniques have been used to determine relationships between microstructural composition and corrosion properties of the AZ series. Previous studies have shown discrepancies in the results from atmospheric testing to the accelerated chamber and bulk solution testing.1 Under atmospheric conditions, Mg-alloys corrode at a much slower rate than the bulk solution or chamber test methods.2,3 The lack of available CO2 results in the inability to continuously buffer the bulk solution; therefore, allowing the pH of the bulk solution to reach alkaline conditions similar to the surface/electrolyte interfacial pH.

Experiments have been completed to qualitatively measure pH using universal pH indicator and open circuit potential collection to observe any pH change indicative of film formation or corrosion product. Electrochemical data has been collected through electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization to assess the kinetics and any time dependence this protective film may have. Characterization of the surface film was done using in-situ Raman infrared spectroscopy in conjunction with X-ray Photoelectron spectroscopy. Results show that CO2 availability alters the corrosion behavior of the alloy significantly. Surface characterization techniques have revealed the presence of carbonate, CO3
2-, as well as aluminum on the surface. Results indicate a synergistic relationship between Al content and CO2 affecting surface film formation and alloy corrosion rate.

Practically, the surface film formation and interaction with the existing atmosphere must be considered in characterizing the protective action of inhibitors and coatings to determine viable methods of protection for this alloy class and other Mg-alloys. This work also sheds light on the inconsistencies between accelerated laboratory testing and atmospheric application conditions.

[1] Avedesian, M.M. and H. Baker, ASM Specialty Handbook: Magnesium and Magnesium Alloys, in Magnesium and Magnesium Alloys, M.M. Avedesian and H. Baker, Editors. 1999.

[2] Lindström, R., L.G. Johansson, and J.E. Svensson, The Influence of NaCl and CO2 on the Atmospheric Corrosion of Magneisum Alloy AZ91. Materials and Corrosion, 2003(54): p. 7.

[3] Lindström, R., J.E. Svensson, and L.G. Johansson, The Influence of Carbon Dioxide on the Atmospheric Corrosion of Some Magnesium Alloys in the Presence of NaCl. Journal of The Electrochemical Society, 2002. 149(4): p. B103.