# Investigation of In Situ and Ex Situ Passivation of Pyrophoric Uranium-Niobium Alloy Powder
## 3 Results and Discussion
### Optical Analysis and Color  
The apparent color of a material can serve as a useful indicator of its oxide layer characteristics, with prior studies demonstrating the utility of the color analysis of uranium oxide powders through optical photography [33]. The thickness of the oxide layer influences color due to phase interference from light reflecting at the air-oxide and metal-oxide boundaries [34]. This phenomenon, commonly observed to cause color variations in metals such as anodized titanium [35], has been documented in both UO\({}_{2}\) thin films on unalloyed uranium [36,37] and Nb\({}_{2}\)O\({}_{5}\) thin films on niobium [38]. For U-6 Nb, the oxide layer primarily consists of a continuous UO\({}_{2}\) layer with an Nb\({}_{2}\)O\({}_{5}\) layer between it and the bulk metal [24], suggesting that interference effects generate the observed colors. While the relationship between color and oxide thickness is well established for unalloyed uranium [37], it remains unexplored for U-6 Nb. Nonetheless, the relative uniformity of the oxide layer thickness of U-6 Nb can still be inferred from its consistent coloration. Both in situ and ex situ passivation processes caused observable bulk color changes compared to the unpassivated powder. The unpassivated powder (Figure 2a) appears light silver-gray. With increasing passivation gas input during in situ passivation (Figure 2b), the powder becomes progressively darker, consistent with more UO\({}_{2}\) formation at particle surfaces. Ex situ-passivated powder (Figure 2c) also appears distinctly darker than the unpassivated powder and visually similar to the powders passivated in situ at 1-3 SLPM. Figure 2: Powder color change due to passivation: (**a**) unpassivated powder; (**b**) powder passivated in situ with 1, 2, 3, 4, 5, and 10 SLPM (standard liters per minute) of passivation gas increasing from left to right; (**c**) ex situ-passivated powder. However, apparent bulk powder color alone is insufficient to reliably predict pyrohoricity. For instance, while the ex situ-passivated powder consistently passes the sparkle test, powders passivated in situ at 1-3 SLPM occasionally fail. This discrepancy arises because combustion can be initiated by a small number of inadequately passivated particles (Video S1d), which is something that bulk characterization methods cannot effectively detect. Furthermore, it is important to note that non-pyrophoric particles do remain combustible and can still ignite if exposed to an ignition source, such as neighboring pyrophoric particles. Therefore, more localized analysis is required to resolve the individual particle passivation layer characteristics. To further investigate the differences in passivation processes, individual spheres were analyzed by optical microscopy (Figure 3). Both unpassivated and ex situ-passivated powders show minimal sphere-to-sphere variation in color, indicating uniform oxide layer thickness. In contrast, the in situ-passivated powder (5 SLPM shown in Figure 3b) displays significant variations in color without any apparent correlation to particle size, ranging from light orange to blue-green to deep violet, suggesting substantial differences in oxide thickness among spheres. While other factors, such as contamination by foreign chemical species or the presence of multiple oxide species, are known to cause color changes in metals, these alternatives can be excluded here. Contamination by outside chemical species was unlikely, given that identical feedstock materials and controlled inert atmosphere procedures were used for both methods. Although color differences can arise from differences in the ratio of the metal oxides, like in the case of Fe-Cr steel [39], differences in uranium-to-niobium oxide ratios are similarly improbable, since uranium oxide preferentially forms over niobium oxide due to its significantly lower Gibbs enthalpy of formation [24], which consistently results in a uranium-rich oxide surface.