Determination of E
crit Using 1-D Pit Approach and Its Relationship to the Repassivation Potential of 3-D Pits
Authors: Georges Mariana , Frankel Gerald S. 
Published: December 2024

Localized corrosion is a major concern in industry, as it can compromise components made from corrosion-resistant alloys, ultimately leading to failures, reduction in the lifetime of equipment, downtime in production, and safety issues. Understanding the mechanisms underlying pitting corrosion is paramount to minimizing this form of attack and developing mitigation approaches. Cyclic potentiodynamic polarization (CPP) is a common electrochemical test used to evaluate the susceptibility of materials to pitting corrosion. An important parameter that can be extracted from this technique is the repassivation potential (E
rp), which has been used as a measure of resistance to localized corrosion in long-term applications and also for assessing SCC susceptibility. Another interesting approach for evaluating the kinetics of localized corrosion is the 1-dimensional (1-D) pit test, which consists of a thin wire sample embedded in epoxy resin. This setup generates a real pit environment at the material surface as the wire dissolves and provides additional parameters that cannot be directly extracted from CPP curves.

In this study, 1-D pit tests were used to evaluate the corrosion behavior of SS316L at different temperatures and to identify the critical conditions for repassivation. The potential was scanned downward from the mass-transport limit region at different potential scans rates () for pits with different depths to allow for precise determination of the repassivation potential and the critical concentration, C
crit, associated with pit growth stability at each temperature. Tailored values of were needed because was found to affect the shape of the downward polarization curves of 1-D pits of varying depth. For small pit depths, a scan rate of 20 mV/s is suitable for measuring E
rp, which is associated with a sharp decrease in current density caused by passive film formation at the bottom of the pit. For deeper pits, however, lower scan rates are needed to allow enough time for dilution of the electrolyte inside the pit cavity, providing an accurate measurement of E
rp from the sharp drop in current.

The critical pit bottom potential at repassivation, E
crit, can be determined from E
rp by correcting for the ohmic potential drop caused by current flow in the pit. Values of E
crit measured using the same deviate from the expected logarithmic dependence of E
crit on pit depth [1] for depths higher than 100 mm. However, if a tailored is used, i.e. scan rates that decrease with increasing pit depth, then accurate E
crit values can be obtained as indicated by the logarithmic relationship over a 10x variation in pit depth. The E
crit obtained from 1-D pit tests combined with the equations of mass transport for different pit geometries enable the prediction of E
rp for hemispherical 3-D pits in bulk samples, which is approximately the pit geometry commonly seen after cyclic potentiodynamic polarization (CPP) tests. A good correlation was found between E
crit from 1-D pit experiments and E
crit from CPP curves. The effect of pit cover was also taken into consideration in the ohmic potential drop calculations for bulk 3-D pits. This methodology provides predictions for repassivation potential, enhances the understanding of the variability in E
rp values obtained from the CPP technique, and improves the reliability of this parameter.

This work was supported by the US Office of Naval Research under Grant No. N00014-23-1-2202.

Reference

Li, J.R. Scully, G.S. Frankel, Localized Corrosion: Passive Film Breakdown vs. Pit Growth Stability: Part III. A Unifying Set of Principal Parameters and Criteria for Pit Stabilization and Salt Film Formation, Journal of The Electrochemical Society, 165 (2018) C762-C770.