The primary chemical reactions at a magnesium alloy surface during corrosion are water reduction, wherein water is split to evolve hydrogen, and oxidation of Mg to Mg2+. Recently, germanium and arsenic have been reported to have a significant impact on the kinetics of the cathodic water reduction reaction by inhibiting the recombination of hydrogen atoms and subsequent evolution of H2 gas from the Mg surface.(1, 2) Density functional theory (DFT) has provided support for these observations indicating unfavorable enthalpy changes for adsorbed hydrogen atom diffusion toward several metals and metalloids alloyed on a magnesium surface.(3) The DFT results also showed that the reaction enthalpy of diatomic hydrogen evolution was unfavorable on a dilute Mg-metalloid surface.
Ion implantation into the surface of metals has been known to improve wear and corrosion resistance, alter conductivity and optical properties, and improve thermal oxidation resistance. For example, the corrosion resistance of 1070 and 52100 steels has been improved by implanting Ta or Ti. Although lesser studied than some substrates, implantation into Mg and Mg alloys has been examined for corrosion and wear in implantable medical devices.(4-6) Implantation of Y into Mg and of Al into AZ31 is known to increase open circuit potential (OCP) and co-implantation of Cr and O is known to form a protective oxide film.
This study examines the role of ion implantation in forming protective, corrosion resistant magnesium surface alloys. Germanium and arsenic ions were implanted at 1532 keV into high-purity Mg samples using a National Electrostatics 5SDH-2 positive ion accelerator at doses sufficient to achieve 1-micrometer thick doped layers with peak concentrations of 0.01-0.3 atomic percent at approximately 1.5 micrometers beneath the surface. The depth of implantation and concentration were verified using Rutherford backscattering spectrometry, and were consistent with what had been expected from the ion implantation simulation program SRIM. Energy dispersive spectroscopy, and x-ray photoelectron spectroscopy were used to characterize surface chemistry. Surface morphology was characterized using scanning electron microscopy, tunneling electron microscopy, and atomic force microscopy. Higher concentrations of Ge and As were both observed to roughen the alloy surface, generating a thick MgO surface film, whereas low doses had a minimal effect. The corrosion behavior was evaluated using potentiodynamic polarization and immersion in quiescent NaCl. In order to de-convolute the effect of surface oxide thickness from the chemical effect of the alloying addition, Mg was also implanted into the Mg substrate as a reference.
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