The Transition from Short- to Long-Term Marine Corrosion of Carbon Steels: 2. Parameterization and Modeling

Previously, it was suggested that in the bi-modal model for corrosion loss as a function of exposure period the instantaneous corrosion rate increases by a factor of about 4 through the transition phase from mode 1 to mode 2. Recent experimental observations (from Part 1 [Melchers and Jeffrey, Corrosion 78, 5 (2022): p. 415]) and other experimental observations covering a range of corrosion influencing parameters closely support this value. The reason for it is examined through a model of the development of corrosion through the transition zone, based on interpretations of the data presented in Part 1. It is shown that pitting plays a crucial role in lowering local pH conditions and permitting transitioning of the corrosion process from oxygen reduction (in mode 1) to hydrogen evolution (in mode 2) as the thermodynamically possible cathodic reaction. In both cases, the rates of corrosion are controlled by diffusion considerations, not electrochemical kinetics. This also permits a theoretical basis for the observed ratio of around 4. Further, eventual long-term corrosion, usually observed as close to a linear trend in time, is proposed as representable a system in quasi-static equilibrium, involving both the hydrogen evolution reaction at the corrosion interface and loss of ferrous ions from the external face of the rust layers.