A Perspective on the Repassivation Potential and a Novel Calculation Thereof

• Published in: Meeting abstracts (Electrochemical Society) Vol. MA2022-02; no. 11; p. 706
• Main Authors: Skelton, Rebecca, Kelly, Robert G
• Format: Journal Article
• Language: English
• Published: The Electrochemical Society, Inc 09.10.2022
• ISSN: 2151-2043; 2151-2035
• DOI: 10.1149/MA2022-0211706mtgabs

Corrosion can occur on metallic surfaces in nearly any humid environment, leading to either early failure or costly repairs. Specifically, localized corrosion, such as pitting, can be difficult to identify at an early stage and can act as crack nucleation sites. Determining under which conditions pitting will take place is therefore important for maintenance and design, yet non-trivial from an electrochemical perspective. The pitting potential (E pit ) and repassivation potential (E rp ) have long been used in literature to determine a material’s susceptibility to pitting, with the latter acting as the lower threshold at which pitting can occur. A lower threshold of pitting can often be useful for conservative estimates, making E rp a popular parameter. However, a consensus on how to measure and define E rp has yet to be defined. Conventional cyclic potentiodynamic polarization (CPP) scans to measure E rp are shrouded in controversy, as values were found to be dependent on the scan rate and turn-around current density, unless sufficient charge densities had passed. Even once a plateau of E rp is reached at sufficient charge densities, there is debate as to if values are too conservative at that point. Two recent pitting frameworks have proposed alternative measurements of E rp , using both 1-D pit and 3-D bulk electrodes. Breakthroughs and limitations from both of these theories will be considered. In this work, the various methods of measuring and defining E rp will be discussed to provide clarity between different notation and to highlight possible paths going forward. In addition, an alternative calculation of E rp is proposed which is less conservative yet harmonized with the critical pit stability product (ix) crit , as both thresholds should represent the same lower limit of pitting. The proposed equation to calculate E rp includes dependencies on conventional parameters, such as the anodic Tafel behavior within a pit (b a ), the transition potential (E T ), and the critical percent saturation of metal salts ( f ). Validation is provided through both results found in literature and the use of Finite Element Method (FEM) modeling in conjunction with the maximum pit model.