Rudolph A. Marcus formulated it between 1956 and 1965 and received the Nobel Prize in Chemistry in 1992 for it. The theory relates the rate constant of an electron transfer to two quantities: the free energy of the reaction and the reorganisation energy, which is the energy required to rearrange the solvent and the coordination sphere around the reacting species before the electron can move. One of its counterintuitive predictions, the inverted region where a more favourable reaction becomes slower, was confirmed experimentally decades after it was proposed.
Why a corrosion model needs it
Corrosion is electron transfer at the interface between a metal and an electrolyte. Three classical relations cover different parts of the problem: Nernst gives the thermodynamic driving force, Arrhenius gives the temperature dependence, and Marcus gives the kinetics of the transfer step itself. A model carrying only the thermodynamics can say that a reaction is possible; it cannot say how fast it will proceed under the conditions the asset is actually running.
One term of a model, not a model
Marcus theory does not predict a corrosion rate on its own. The observed rate also depends on flow regime, deposits and biofilm, surface condition, and trace species such as hydrogen sulphide that change the interface chemistry at concentrations of a few parts per million. In CorrosionAI it is one of the equations embedded in the physics layer, combined with the rest and with the measured history of the asset.