Re-defining the kinetics of redox reactions on passive metal surfaces
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REVIEW PAPER
Re-defining the kinetics of redox reactions on passive metal surfaces Digby D. Macdonald 1
&
Jie Qiu 1
Received: 24 June 2020 / Revised: 25 July 2020 / Accepted: 27 July 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract It is known that the kinetics of redox reactions occurring on the surfaces of passive metals depend upon the properties of the passive film, ostensibly due to quantum mechanical tunnelling (QMT) of electrons and holes between the metal and the redox couple at the barrier layer/solution (bl/s) interface. In this paper, the tunnelling probability is used to inter-convert the exchange current densities for the redox reactions occurring at the bl/s interface and on the hypothetical bare metal surface. We review our previous work on combining QMT theory with the point defect model (PDM), which provides an analytical expression for the bl thickness as a function of voltage. By combining QMT theory and the PDM, we derive a modified form of the generalized ButlerVolmer equation that requires as input only the kinetic parameters for the redox reaction on the hypothetical bare surface and parameters contained in the PDM. The application of the theory is illustrated with reference to the corrosion of carbon steel in concrete pore solution, to calculating the corrosion potential of, and crack growth rate in, sensitized type 304 SS in boiling water reactor (BWR) coolant circuits, and the use of hydrogen oxidation on platinum to determine the thickness of the bl as a function of voltage and temperature. This illustrates a new, powerful technique for probing the formation of passive films on metal surfaces. Keywords Redox reactions . Electrochemical kinetics . Quantum mechanical tunnelling . Corrosion rate . Corrosion potential . Crack growth rate
Introduction The role of quantum-mechanical tunnelling of charge carriers through the barrier oxide layer on a passive metal, in determining the kinetics of redox reactions, such as the hydrogen electrode reaction (HER) and the oxygen electrode reaction (OER) on a passive metal when in contact with an aqueous environment, is explored theoretically. Because the corrosion potential is the single most important parameter in determining the rates of corrosion and stress corrosion cracking in a steel, the theory is extended to describe corrosion of carbon steel in concrete synthetic concrete pore solution [SCPS, sat. Ca(OH)2 + NaOH to yield pH = 13.5 at 25 °C] and to calculate the corrosion potential of, and crack growth rate in, sensitized type 304 SS in boiling water (nuclear) reactor (BWR) primary coolant circuits.
* Digby D. Macdonald [email protected] 1
Department of Nuclear Engineering, University of California at Berkeley, Berkeley, CA 94720, USA
The exchange current density for the cathodic reaction occurring on the passive metal surface is given as the product of the exchange current density on the bare metal surface and the quantum mechanical tunnelling probability (QMTP) of charge carriers through the barrier laye
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