Revisiting the Effect of Crystallographic Orientation on the Corrosion of Commercially Pure Magnesium
Authors: Bland Leslie Gail , Scully John 
Published: February 2020

For many metals and alloys, the corrosion rate has a strong dependence on crystallographic orientation [1-5]. Assessing corrosion properties using single crystal samples gives an indication of the corrosion properties of specified orientations, but is limited to a few crystal planes [3, 4, 6, 7]. An understanding of the corrosion rate as a systematic function of orientation is needed since corrosion properties can change significantly with only a few degrees difference in crystallographic orientation [1]. Conflicting results have been reported regarding the orientation dependence of Mg corrosion [8, 9]. Electron Backscatter Diffraction (EBSD) correlated with surface topography measurements, such as confocal laser scanning microscopy, now enable understanding of the dissolution behavior of a large number of crystal planes [1].
The electrochemical dissolution of Mg shows strong crystallographic dependence in chloride-containing, alkaline environments [10-12]. The surface film formed on Mg consists of a thin, nano-crystalline MgO inner layer and an Mg(OH)2 platelet layer [13-15]. The thickness of the oxide layer on Mg has been determined through several techniques, including focused ion beam (FIB) cross-section [15,16]. The outer, Mg(OH)2 layer is typically on the order of 500 nm while the MgO layer is on the order of 50-90 nm [13-16]. This work show the MgO thickness depends on crystal plane.
The origins of Mg corrosion rate with oxide thickness and crystallographic orientation were investigated utilizing electrochemical impedance spectroscopy (EIS) with measurement of impedance to low frequency (1 mHz) [17]. EIS constant phase elements were exploited to determine the oxide thicknesses on each crystal plane.
To describe the miller index of each plane, {hkil}, the variables described in Figure 1(a) were used which depicts a given {hkil} on an inverse pole figure (IPF) (Figure 1(b)). The {hkil} is defined in terms of the interplanar angles, βi. βi is the angle measured from a primary low index hexagonal plane normal to the {hkil} of interest. αi is the angle between βi and the angular distance from the {0001} towards the prismatic or the pyramidal planes on the inverse pole figure. Therefore βi and αi describe the exact crystal plane orientation in the stereographical triangle. For non-chloride containing, neutral pH environments which do not support film growth, such as tris(hydroxymethyl)aminomethane (TRIS) and ethylenediaminetetraacetic (EDTA), a limited crystallographic orientation dependence on the corrosion rate was observed. However, in unbuffered 0.6 M NaCl, the corrosion rate varied with crystallographic orientation, with the fastest corrosion rate occurring along the basal plane and the slowest along the prismatic and pyramidal planes. The corrosion rate as a function of the β{0001} angle (with β{0001}=0° corresponding to the basal plane and β{0001}=90° corresponding to the prismatic and pyramidal orientations) correlated with oxide thickness iss shown in Figure 2. Much of the variation in the corrosion kinetics with crystallographic orientation correlates with film thickness. The talk will address possible effects of film thickness on corrosion rate.

Acknowledgements

This work was funded by the Office of Naval Research Grant N000141210967 with Dr. David A. Shifler and the Office of Naval Research Multi-University Research Initiative under a subcontract from Northwestern University SP0028970-PROJ0007990.

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Figure 1