Experiments Based on One-Dimensional Diffusion Modeling to Determine Critical Factors in Pitting

Critical conditions for stable pitting were first formulated by Galvele¹, considering mass transport in a one-dimensional pit, with the parameter (i*x) (pit stability product²) describing steady-state kinetics. Beck³, Alkire and coworkers⁴, and Newman and coworkers^5,6 established artificial pits as the experimental tool to examine these kinetics. An associated descriptor of critical conditions is the critical potential^1,7–9. Given the quasi-steady state obtained under a salt film^6,10,11 for a corroding pit, this potential was taken as the potential at which this film was first stable, the transition potential E_T⁶. However, pitting was observed to proceed stably at potentials below E_T^12–14, calling into question its validity as the critical potential as well as the salt film as a critical surface condition.

Long-term experiments oriented towards establishing a potential lower bound for propagating pits showed that the repassivation potential E_rp^15–18 below which no new pits nucleate and any existing pits repassivate, could be a critical potential. E_rp would describe the same conditions as the critical pit stability product (i*x)_crit, a fraction of (i*x) measured under a salt film. Studies to date have focused on determining either the critical potential^15,18–20 or the critical pit stability product^2,13,21,22, and have not provided a definitive quantitative connection between the two and to other associated critical factors like surface concentration (C_S¹²) and pH.  This work presents a combined experimental and modeling framework that relates these factors. Artificial pit experiments were performed using 316L stainless steel wires in sodium chloride solution. Experimental estimates related to the critical factors include (i*x)_saltfilm and E_rp. Experimental results were fed into a one-dimensional diffusion model. A graphical method for determining C_S at the transition between pitting stability and repassivation was developed, comparing a) Δt, the difference between t_act (time taken to scan to E_rp from E_T) and t_f (time taken for the C_S to dilute to different fractions of saturation), and b) E_rp (Fig. 1).

Experiments were also designed to obtain anodic kinetics at different C_S. The time required for a corroding surface under a salt film to dilute to various C_S values was calculated from the mass transport model developed. Pits were grown under a salt film to predetermined depths and then allowed to dilute under open circuit to various C_S. Rapid scanning to anodic potentials was subsequently carried out once each C_S was achieved, so that the anodic kinetics at that particular C_S could be determined, removing the need for separate experiments in simulated pit chemistries. Results showed that there was a change in type of behavior and not merely in degree for C_S < 50% of saturation (Fig. 2), indicating that the critical C_S is lower than previously considered estimates, which has implications on damage estimation models based on anodic stability. Estimates of the critical pH associated with this transition was also obtained based on the influence of local cathodic reactions^23,24. A single quantitative framework was therefore developed relating key parameters describing pitting stability and repassivation. 

References

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