A One Dimensional Crevice Experiment for Determining the Critical Factors Contributing to Crevice Corrosion Repassivation
Authors: Lillard R. Scott , Mehrazi Shirin 
Published: December 2020

From its inception more than 40+ years ago, the one-dimensional pencil electrode experiment (1-D pit) has allowed investigators to study factors on pitting corrosion such as ohmic drop, concentration of species as a function of pit depth and the stability of salt films.[1,2] In recent work, Srinivasan and Kelly have developed a 1-D transport model of repassivation for pencil electrodes made of SS 316L.[3] Unfortunately, for the phenomenon of crevice corrosion, the understanding of these critical factors (that no doubt control crevice propagation and repassivation) are far less understood. The primary reason for this has been the inability to simultaneously measure the location of the active crevice front (e.g. its depth) and the active area (which fixes the current density). These two parameters are easily measured in the 1-D pit experiment.

Previously, we introduced a method for measuring both the depth of the active front in crevice corrosion and the active area.[4] The method, similar to that which Pickering introduced years ago,[5] records video of a propagating crevice in the optical microscope. The crevice (RCA) is formed by an alloy 625 washer covered by an acrylic washer that acts as a crevice former. The potential of the assembly is controlled potentiostatically using a traditional three electrode set up and the current is recorded as a function of time. A stereo microscope equipped with a digital camera is used to record images of the initiation and propagation of crevice corrosion. Image processing software is used to quantify the area of the active front. We calculate the actively corroding area, crevice current density and wall potential (Eapp-IR) all as a function of time.

In this presentation, we will present the results from our crevice repassivation study on alloy 625. We use the THE method (ASTM G192) to analyze the transition to repassivation in the 625 RCA. Once initiated, the crevice was allowed to propagate for various times, equivalent to increasing charge passed. After a fixed time, the applied potential was decreased in a stepwise fashion. At a critical potential, forward movement of the active corrosion front towards the mouth (e.g. new damage area) stopped, though crevice propagation continued. Transport calculations reveal that this potential may be associated with salt film dissolution, consistent with the transition potential (ET / Esat) reported for one dimensional pitting experiments. The results are discussed in terms of traditional crevice corrosion models (such as Oldfield-Sutton[6] and IR*[5]) and more recent models for pit growth stability (Li Scully Frankel[7]). A mechanism for crevice growth and the transition to repassivation that is similar to that believed to controlled pit propagation and repassivation is proposed.

Acknowledgments

Work on this project was made possible by a grant from the US DoD TCC #1000000149, program directors Gregory A. Shoales, USAFA, and Daniel Dunmire, Corrosion Policy Oversight.

References

JW Tester, HS Isaacs, JES, 122, 1438, 1975.

RC Newman, HS Isaacs, JES, 130, 1621, 1983.

J Srinivasan, RG Kelly, JES, 163, C768, 2016.

RS Lillard, S Mehrazi, DM Miller, JES, 16
7, 2020.

5. K Cho, HW Pickering, JES, 138, L56, 1991.

6. JW Oldfield, WH Sutton, Br. Corr. J., 13, 1978.

7. T Li, JR Scully, GS Frankel, JES, 166, 2019.