From its inception more than 40+ years ago, the one-dimensional pencil electrode experiment (1-D pit) has allowed investigators to investigate 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 fact, 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 phenomena 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 area of the front (which fixes the current density). These two parameters are easily measured in the 1-D pit experiment. In this presentation, we will introduce a method for measuring both the depth of the active front in crevice corrosion and the active area. The method is similar to that which Pickering introduced years ago,[4] we record video of a propagating crevice in the optical microscope. In this technique the crevice is formed by a set of washers, a piece of acrylic that acts as a crevice former and the metal specimen, nickel alloy 625. The potential of the assembly is controlled potentiostatically using a traditional three electrode set up and the current recorded as afunciton 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.

A typical result is presented in Figure 1 which shows the curent density of the active crevice front as a function of crevice depth. As seen in this figure, the crevice propagates from the deepest regions of the crevice towards the crevice mouth. this initial propagation direction is a result of an increase in the concentration of metal salts ahead of the active front and a corresponding decrease in pH. It can also be seen in this figure that at distances greater than 1 cm from the mouth, the crevice is able to propagate at current densities less than 1 mA/cm². Once the active front reaches a distance of 1cm, an exponential increase in curent density is reorded. This increase is attributed to the IR* mechanism.[4]

With the knowledge of the current density and distance from the crevice mouth we are able to use a simplification of the Nernst-Plank equation to calculate the concenrtation of species in solution. As might be anticipated from Figure 1, the concetration of metal salts increases with increasing current density. As such, concentration near the mouth is greater than at the tip, the saturation limit being reached at a distance of approximately 0.4cm from the mouth.

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

1. JW Tester, HS Isaacs, JES, 122, 1438, 1975.
2. RC Newman, HS Isaacs, JES, 130, 1621, 1983.
3. J Srinivasan, RG Kelly, JES, 163, C768, 2016.
4. K Cho, HW Pickering, JES, 138, L56, 1991.

Figure 1 Current density as a function of distance inside an alloy 625 crevice.