We characterized the corrosion scale that formed on coupons under all experimental conditions using XRD (Table ). As expected, hydroxypyromorphite formed in the presence of orthophosphate, while hydrocerussite (Pb3(CO3)2(OH)2) was identified in scale from all sample coupons. Massicot (𝛽-PbO) was also universally present, but the intensities of the ( ) and ( ) peaks at 29.1 and 30.3°, respectively, were not consistent with the standard pattern. This may have been due to preferential orientation of crystallites on the coupon surfaces. Aluminum was not identified in any crystalline mineral forms by XRD, and the experimental patterns representing coupons exposed to 20 and 500 µg Al L -1 were similar (Figure ). Moreover, aluminum was not detectable by XPS in the top few nanometers of corrosion scale exposed to the high level of aluminum (0.5 mg Al L -1 ) (Figure ). Thus it is likely that the aluminum content of scale was relatively low, although XPS detection limits for light elements (e.g., Al) in a heavy element matrix (e.g., Pb) tend to be above 1 atomic percent. The low surface concentration of aluminum is consistent with our interpretation that aluminum acted primarily by promoting particulate lead formation and limiting the activity of orthophosphate in solution. Moreover, the mineralogy of the scale, as determined by XRD, was predictable without considering the aluminum concentration.