The Artificial Pit Electrode as a Technique to Evaluate Critical Mass Transport and Electrochemical Parameters in Stainless Steel Pitting and Repassivation

The critical conditions of mass transport and localized chemistry required for a 1-D pit to grow stably were first examined by Galvele (1), resulting in the pit stability product (i*x) as the key parameter of interest. Experimentally determining this parameter requires a quasi-steady-state configuration (2) such as that under a salt film (3, 4). The lowest potential at which this film is stable has been termed the transition potential E_t (5). However, pitting is known to proceed stably at potentials lower than E_t (6, 7), and hence E_t cannot be used as a conservative lower bound. This means that a salt film is not critical to stability and the critical pit stability product (i*x)_crit = f*(i*x)_{salt film}, where f is some fraction of saturation. The repassivation potential E_rp, the potential below which all existing pits passivate, has been proposed as an alternative critical potential (8). In this regard, E_rp has to be examined in terms of diffusive transport in order to relate it to pitting stability. Moreover, studying the effects of experimental variables on (i*x) and E_rp enables understanding their influence on pitting stability, providing information for corrosion mitigation strategies.

            A cyclic artificial pit technique was developed to enable high-efficiency, automated data collection and analysis. This approach utilized an artificial pit constructed from stainless steel wire (diameter 50-100 μm), grown to different depths applying a high potential in neutral chloride solution for different time periods, after which a cathodic potentiodynamic sequence was employed, a variation of the method of Sridhar and Cragnolino (8). The entire sequence was looped multiple times.  The pit stability product under a salt film, (i*x)_{salt film} was obtained from the diffusion-limited dissolution current density at different depths (5), and E_rp was noted as the potential at which a significant number of current density data points were lower than 30 μA/cm² (8, 9). This approach enabled the measurement of both critical parameters from the same experiment and the direct correlation of E_rp to pit depth and diffusive transport.

            The high level of automation permitted sampling a wide region of parameter space. The large volume of data from this technique facilitated replication and a statistical treatment of (i*x) and E_rp. Data obtained were also utilized to develop a mass-transport-based model relating E_rp and the surface concentration at the edge of pit stability, based on which an estimate of the critical degree of saturation, f was obtained.

            Simultaneous examination of the effects of different variables on E_rp, salt film solubility, and kinetics was possible. As shown in Figure 1, (i*x)_{salt film} values of three stainless steels tested were the same within the limits of experimental scatter. E_rp for 17-4 PH was seen to be slightly higher than for 316L. E_rp for 17-7 PH was much lower, leading to the hypothesis that electroactive copper in 17-4 PH may contribute to interfering redox reactions (10), yielding an artificially high E_rp. Current-peak analysis (11) using data from the same experiment allowed this hypothesis to be tested.

            Studies have focused on the inhibitory effects of oxyanions on the pitting potential and therefore, pit initiation (12). However, examining the effects of these anions on pit repassivation and stability is important to understand their influence on pit growth. As shown in Figure 2, salt film solubility and repassivation potential are affected differently by the oxyanions. Preliminary analysis shows that the oxyanions may enhance the cathodic reaction in the pit solution, resulting in a higher measured E_rp. Real-time ohmic compensation could also be included in the polarization sequence inside the loop to examine the approach to E_rp, which may help understand the inhibitory mechanisms better.

 

References

1.             J. R. Galvele, J.  Electrochem. Soc., 123, 464 (1976).

2.             I. Epelboin, Zeitschrift für Physikalische Chemie (Neue Folge), 98, 215 (1975).

3.             T. R. Beck, J.  Electrochem. Soc., 120, 1317 (1973).

4.             G. T. Gaudet et al., AIChE J., 32, 949 (1986).

5.             N. J. Laycock and R. C. Newman, Corr. Sci., 39, 1771 (1997).

6.             G. S. Frankel et al., J. Electrochem. Soc., 143, 1834 (1996).

7.             R. C. Newman and E. M. Franz, Corrosion, 40, 325 (1984).

8.             N. Sridhar and G. A. Cragnolino, Corrosion, 49, 885 (1993).

9.             ASTM G192 - 08, ASTM International, West Conshohocken, PA (2008).

10.          R. H. Heidersbach and E. D. Verink, Corrosion, 28, 397 (1972).

11.          W. R. Heineman and P. T. Kissinger, Laboratory Techniques in Electroanalytical Chemistry, Marcel Dekker, Inc., New York, NY (1984).

12.          H. P. Leckie and H. H. Uhlig, J.  Electrochem. Soc., 113, 1262 (1966).