The corrosion of commercially pure magnesium (Mg) and AZ31B-H24 with simultaneous measurements of electrochemical impedance (EIS) and hydrogen gas evolved over a 24 h immersion period was studied in solutions of three chloride concentrations. The corrosion rate was determined from the Stern-Geary approach. The integral electrochemical-based mass loss was compared to the gravimetric mass loss and inductively coupled plasma optical emission spectrometry (ICP-OES) solution analysis of the total Mg concentration released. The use of ICP-OES to support the other assessment methods has not been previously reported. Assuming Mg dissolves as Mg 2+ , there was agreement using these four unique measures of Mg corrosion. The integration of the polarization resistance (R P ) over time, as evaluated from EIS at the low frequency limit incorporating full consideration of the pseudoinductive impedance behavior of Mg, provided excellent correlation to the cumulative mass loss, ICP-OES solution analysis, and volume of hydrogen collected for commercially pure Mg and reported for the first time for AZ31. The choice of using the Tafel slope in the Stern-Geary approach, as well as the subsequent comparison of results to corrosion rate data in the literature, are discussed.
The corrosion rate of Mg indicates a strong crystallographic dependence in chloride-containing, alkaline environments that correlates inversely with oxide film thickness. In contrast, a different crystallographic orientation dependency is observed initially during open circuit corrosion in non-chloride containing, near neutral pH buffered, chelating environments such as Tris(hydroxymethyl)aminomethane (TRIS) and Ethylenediaminetetraacetic (EDTA) which minimize air-formed MgO oxides. The origins of the differences in the rates of the coupled corrosion processes as a function of crystallographic orientation were investigated utilizing electrochemical impedance spectroscopy (EIS) and Raman vibrational spectroscopy. High frequency constant phase elements (CPEs) were exploited to determine oxide thicknesses as a function of crystal orientation. In unbuffered NaCl, open circuit corrosion was faster on basal planes while lower corrosion rates were observed on low index, prismatic and pyramidal planes. This variation in rate with crystallographic orientation in 0.6 M NaCl was interpreted to depend on the MgO and Mg(OH) 2 film thicknesses as a function of orientation. In particular, crystal planes with a lower MgO/Mg(OH) 2 thickness corroded at higher rates as assessed in-situ by EIS and suggested by Raman spectroscopy. The crystallographic orientation dependence of corrosion seen initially upon exposure in the buffered neutral pH environments corresponded with crystal facet surface energy for bare Mg but this trend disappeared with exposure time and Mg(OH) 2 films were then detected.
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