Optical signatures arise from variations in both the thickness and chemical composition of the passive layer that forms dynamically during CA. To decouple the effect of chemical composition, we first performed ex-situ XPS analysis of the passive layer. The analyses were conducted on samples that were re-polished after the CA experiments to remove the thick corrosion products ejected from the pits. The samples were then left under ambient conditions for several hours, allowing a new passive layer to form via spontaneous oxidation. Although the precise ratios of chemical species may differ from those present during CA due to this reformation, the principal constituents of the passive layer remain unchanged, which should be adequate for fitting optical signatures.