Nuclear Microprobe Analysis for Determination of Element Enrichment Following Magnesium Dissolution

Abstract

With significant increases in the production and utility of magnesium (Mg) in the past decade, Mg-alloys remain an attractive material for weight reduction in several industries, 1 in addition to substantial exploration as electrode materials in primary and secondary batteries. 2‐3 In such cases, the unambiguous determination of factors that play a role in corrosion/electrochemistry of Mg are of critical importance. The influence of impurities on the corrosion of Mg has been well documented since the early 20 th century, 4 with tolerance limits for a number of elements in Mg proposed. 5 In particular, the influence of deliberate alloying additions of low levels of transition metals (iron, manganeseandzirconium)oncorrosionofMghavebeendocumented by systematic studies. 6 Furthermore, the comparison of the electrochemistry of pure Mg specimens with low (at commercial levels of ∼40 ppmw) and ultra low levels (≤ 1 ppmw) of Fe were also recently presented. 7 Such studies add to the evidence that impurity elements, nominally of low solubility, 8‐10 influence the corrosion electrochemistry of Mg. In spite of this, at least two key aspects with respect to the in-service performance of Mg remain under researched. The first of these includes the detection and analysis of impurity elements on the Mg surface, and the study of possible enrichment of impurity elements on Mg during dissolution; both aspects are worthy of elaboration. Regarding the analysis of impurity elements on Mg surfaces, this is a particularly challenging task for the common methods nominally used in corrosion related works. Nominally, impurity concentrations are in the parts per million range. For example, commercial purity Mg will nominally contain < 100 ppmw Fe, which is below < 0.01% on the basis of weight %, and even lower on the basis of atom %. The analysis of such low levels of Fe with accuracy is not readily possible by methods such as X-ray photoelectron spectroscopy or Auger electron spectroscopy, which require concentrations approaching 1% (which is ∼100 times larger than the typical Fe impurity content) for accurate detection. Similarly, the signal to noise ratio, and large interaction volume, from energy dispersive X-ray spectroscopy are also prohibitive. In fact, even imaging of, and evidence of, impurity Fe (which is known to be present from ICP analysis of chemically dissolved metals) using Field Emission Gun-Scanning Electron Microscopy (FEG-SEM) is elusive. Site-specific Transmission Elec