Corrosion Inhibition of Mg Alloy AZ31 By Selenite (SeO32-)
Authors: Feng Zhiyuan , Hurley Belinda , Buchheit Rudolph 
Published: February 2020

The study of Mg corrosion inhibition by selenium/selenite is very limited. The first relevant work was done by Guy and Whitby in the 1930s1. That work used an acidic selenite-based coating bath to convert Mg alloy surfaces for increased corrosion resistance. It was found that the selenite can react on Mg alloys surfaces to form a protective film. After that report, there was no work reported until Kaminski showed evidence of corrosion inhibition in free corrosion exposure experiments2. The present study follows the work of Kaminski and develops a more comprehensive characterization of Mg corrosion inhibition by selenite using electrochemical and surface analytical methods. Results from these studies support the distinctive qualities of Se films and provide insights on the corrosion inhibition mechanism and inform technological approaches for using this corrosion inhibitor in practical applications.
Corrosion inhibition was characterized using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) at three different concentrations of Na2SeO3, which ranged from 1 mM to 50 mM in a base solution of aerated 0.1 M NaCl. After 8 hours immersion in NaCl/inhibitor solution, anodic polarization showed that at low inhibitor concentration the corrosion current density was decreased to less than 10-6 A/cm2. EIS indicated that a low concentration of sodium selenite provided better corrosion protection, as evidenced by an increase in polarization resistance. During these characterizations, a black or red film formed on substrates after immersion tests. A red film was observed at high selenite concentrations, and some trapped transparent crystals were also noted. Several repeat experiments indicated that after three days immersion, nearly all samples were free of obvious corrosion defects. Raman spectroscopy was employed to analyze the surface chemistry. Amorphous selenium tended to form at low inhibitor concentrations, and the film formed was black. Crystallized selenium only formed at high inhibitor concentrations, and the film formed was red. Transparent insoluble crystals were trapped in the red film. Raman spectroscopy showed that these crystals were insoluble MgSeO3.
Overall, the corrosion inhibition by selenite is better than that conferred by vandate, but not as good as that conferred by chromate under similar experimental conditions. Among the various film morphologies observed, the amorphous black film provided better corrosion protection than the crystallized red film, as evidenced by electrochemical testing results. In this presentation, details from electrochemical characterization and surface analysis will be discussed in the context of corrosion protection mechanism and prospects for use in a technological application.
References
1. D.B. Guy, L. Whitby, Process for protecting magnesium and its alloys against corrosion, (1934). https://www.google.com/patents/US1961030.
2. D. Kaminski, Corrosion Inhibition of Magnesium Alloys and Influence of Atmospheric Carbon Dioxide, The Ohio State University, 2016. https://books.google.com/books?id=-VBEnQAACAAJ.