For the carbon black NP, the efficiency smoothly slowed by a factor of ~3 then increased almost back to the initial value as the NP lost roughly five percent of M initial . The passivation transition occurred as the NP lost an additional ~2% of M initial , after which the NP continued to very slowly etching for ~5 hours, resulting in little additional mass loss. The graphite NP etching efficiency also initially dropped, then recovered, then passivated, however, the efficiency recovered to a value roughly five times greater than the initial efficiency, passivation set in only after ~35% mass loss, and the transition to passivation also involved substantial rate fluctuations. In contrast, the Si and Hf NPs etched smoothly losing roughly ~25% of M initial over two hours with little fluctuation in efficiency, then passivated, abruptly in the case of Hf, with little additional mass lost over the subsequent two hours.