Strategies of metal corrosion protection
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Strategies of metal corrosion protection Su‑Il Pyun1 Received: 15 July 2020 / Accepted: 22 September 2020 © Springer Nature Switzerland AG 2020
Abstract The present article involves the electrochemical fundamentals of overvoltage/polarization of charge transfer and preceding diffusion, short-circuited corrosion cells, the mixed potential theory underlying submicrocells, differential de-aeration cells, and finally introduction of two methods, namely potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS), of determining corrosion current density icorr and polarization resistance Rp to make strategies to raise the cathodic hydrogen overvoltage/anodic overvoltage with two examples: a sacrificial anode and a green corrosion inhibitor. Carl Wagner’s pioneering creative work on the mixed potential theory is addressed with particular emphasis that even nowadays constitutes the principle (classical paradigm) of both the linear polarization and the Tafel extrapolation methods. From the critical assessment of corrosion inhibitors, some unexplained questions are extracted and proposed as formidable future challenges to address in the electrochemistry of corrosion inhibition. Keywords Mixed potential theory · Cathodic hydrogen overvoltage · Charge transfer resistance · Diffusion resistance · Maximum limiting diffusion current density · Differential de-aeration cell · Exchange current density
Introduction Corrosion cells can be classified into two groups, depending upon the effective area of anodic and cathodic sites in size and dimension: (a) the local cell/element (microcell) with effective electrode areas of a fraction of a square millimeter, e.g., a second phase serves as a possible effective cathodic site such as inclusion impurities for Zn, cementite in a ferrite matrix of unalloyed steel, graphite in gray cast iron, grain interior, and pit/crack surface etc.; (b) the submicrocell of much smaller size with its anodic and cathodic sites being spatially and temporarily randomly distributed over the surface of a metal. In the latter case, it is just appropriate to regard the corroding metal as one mixed electrode on which anodic and cathodic reactions occur simultaneously.
* Su‑Il Pyun [email protected] 1
Corrosion and Interfacial Electrochemistry Research Room, R101, Research Building, Munji-Campus of Korea Advanced Institute of Science and Technology, #193 Munji‑Ro, Yuseong‑Gu, Daejon 305‑701, Republic of Korea
This will serve as a useful premise to develop the mixed potential theory. The mixed potential theory first pioneered and developed by Wagner [1] from the submicrocell does not contradict the conventional local cell model, but the former rather supplements the latter. Wagner’s mixed potential theory is nowadays still valid and is one basis for the charge transfer kinetics of modern corrosion science giving a low field approximation, while the Tafel’s law [J Tafel (1905) Z Physik Chem 50: 641] forms another basis giving a high field approximation; both these approxi
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