No direct Fe-P bonds formed between the surfaces and TCP molecules in any of the NEMD simulations, which indicates that iron phosphate, rather than iron phosphide, is formed on the ferrous surfaces, consistent with observations in previous studies. More P-O bonds are formed than are broken in the TCP molecules, leading to an overall increase in the number of P-O bonds, as shown by the solid blue lines in Figure . This is due to the formation of P-O-P linkages between the phosphorus atoms, leading to polyphosphate growth. This process occurs mainly through nucleophilic substitution reactions, whereby P-O-R bonds in TCP (where R is a cresyl group) are replaced by P-O-P bonds. The total increase in the number of P-O bonds at the end of the 1 ns simulation is similar (40-50 bonds) for the α-Fe(110), Fe 3 C(010), and Fe 3 O 4 (001) surfaces. This is because both the P-O cleavage and P-O formation reactions are accelerated by the more reactive surfaces (α-Fe(110)), as shown by the steeper dashed blue lines in Figure . The total number of P-O bonds at the end of the simulation is also not sensitive to the temperature and pressure, as shown in the ESI (Figure ). For the Fe 3 O 4 (001) surface, nucleophilic attack from the surface oxygen atoms could increase the rate of P-O dissociation in the TCP molecules. The variation in the number of P-O sur f bonds formed between phosphorus atoms in the TCP molecules and oxygen atoms in the Fe 3 O 4 (001) surface is shown in the ESI (Figure ). At low temperature and pressure, the number of P-O sur f bonds formed (Figure ) is much less than the total number of P-O bonds formed (Figure ), suggesting that this process plays a negligible role in nucleophilic substitution reactions. However, at high temperature and pressure, the number of P-O sur f bonds increases, such that it they represent a significant proportion (>10 %) of the total P-O bonds formed.