A High-Throughput Artificial Pit Technique to Measure Kinetic Parameters for Pitting Stability

Abstract

A cyclic electrochemical technique was developed which enables rapid, high-throughput extraction of important kinetic parameters from one-dimensional artificial pit (lead-in-pencil) electrode experiments. This approach combined methods for pit initiation and propagation with kinetic measurements at a series of pit depths to measure two key kinetic parameters -the pit stability product under a salt film (i·x) saltfilm and the repassivation potential E rp -from a single experiment. The entire sequence was cycled and automated for efficient, high-volume data collection and analysis for the case of stainless steel in neutral chloride solution. This method offered advantages in the extraction and interpretation of data from a wide range of pit depths, permitting statistical analysis of the kinetic parameters required for determining the critical conditions for pit stability and repassivation.

Evaluating the Critical Chemistry for Repassivation at the Corroding Surface Using Mass Transport Model-Based Artificial Pit Experiments

“…Another means of assessing the Figures 4a and 4b respectively. These data support the estimate of the critical value of surface concentration obtained by Srinivasan and Kelly 37 because the E OCP rises slowly with time (as shown in Figure 4a) once the pit has diluted to at least 55%. This rise is indicative of the repassivation of the steel surface.…”

“…The time required for the potentiostat to scan from the lowest potential that permits a salt film is present (i.e., E T ) Figure 3. Estimation of concentration of metal cations at the corroding surface as the 316L pit transitions from stability to repassivation (replotted from Srinivasan and Kelly 37 ). The data points represented as squares are the measured E rp values at each pit depth.…”

“…Anodic kinetics data were extracted via Tafel extrapolation of the polarization scans corresponding to the lowest surface concentration where active dissolution was observed to occur. The repassivation potential E rp , as measured in previous work, 26,27,36,37 is used in combination with these data to estimate the anodic Tafel slope at the critical surface concentration. From Figure 10, the pH at which the oxide nucleates in a pit solution at the critical surface concentration of f = 55% of saturation is calculated to be 2.65.…”

“…As mentioned in the introduction, although E rp is defined as the potential below which no pitting proceeded, the actual measurement of the value depends on a user-specified current density -a value that has ranged from 1 μA/cm 2 to 100 μA/cm 2 . 19,57,59,60,[75][76][77] The value used in the studies by Srinivasan et al [26][27][28]37 (and referred to in this work) was 30 μA/cm 2 , based on current densities similar to those employed by Tsujikawa et al 57,75,76 and Sridhar and coworkers, 19,78 as well as the ASTM G-192 standard. 79 The validity of this specification and its sensitivity to the individual anodic and cathodic kinetics can be examined using Mixed Potential Theory under the constraints of the critical pH condition as shown in the next section.…”

“…The schematic of this mass transport model is shown in Figure 1; the model itself has been discussed in detail by Srinivasan et al 29,37 This solution yields an expression for surface concentration as a function of time and pit depth (d) for a given value of the diffusion coefficient (D). The dilution time (t f ) for a metal cation at the corroding surface from a saturated condition was then calculated for different values of the surface concentration (indicated by f, the fraction of concentration at saturation) using the following expression:…”