Noise is a general term used to describe the fluctuating behavior of a physical variable with time. In the electrochemical process one may measure either the voltage or current fluctuation with time, so they can be called to electrochemical noises. The analysis of electrochemical noise is now considered to give useful information about the rate and the nature of electrochemical reactions taking place at the electrode. In recent studies the noise from the current transient under the potentiostatic condition is ascribe to events - film breakdown and repair : metastable pitting, although the theoretical basis of the interpretation is somewhat limited.
In this study, the current transients generated during metastable pitting of austenitic stainless steels immersed in solutions containing chloride were monitored and analyzed under potentiostatic control by electrochemical noise technique.
1. PSD analysis
The current transients generated during metastable pitting of austenitic stainless steels immersed in various conditions were monitored. With increasing chloride concentration, applied potential, and temperature, the magnitude of PSD is increasing due to the events with high current density. It is indicated that the stability of the passive film is decreasing with increasing chloride concentration, applied potential, and temperature. And the cut-off frequency in which the PSD magnitude decreases abruptly shifts to the high frequency region.
Even though the PSD slope became steep with increasing corrosivity, that relationship is not clear because of many scattered data. However, in terms of passivation time, the PSD slope approaches to the zero with increasing the passivation time. It means the magnitude of PSD is not changed through the whole frequency due to reducing the number of current spike in stable passive film.
2. Metastable pitting potential
By the current transients generated during metastable pitting of austenitic stainless steels in potentiostatic condition the critical metastable potential can be acquired. The critical metastable potential is the potential at which the metastable pits start to propagate for short period. Events become greater with increasing applied potential and reaches a maximum value - transition potential. The presence of the transition potential is due to the elimination of susceptible nucleation sites for metastable pitting in a given potential.
Like the pitting potential, the critical metastable pitting potential has the logarithmic relationship with chloride concentration. So, the Modified Point Defect Model can explain the initiation of the metastable pit. The critical amount for initiating metastable pitting, ξ'', is quite small comparing to ξ, the critical amount stable pitting.
3. Criteria for the transition from metastable to stable pitting
Using the assumption of hemispherical metastable pit shape, the metastable pit current density can be calculated. The metastable pit size distribution was analyzed in terms of the metastable pit current density as a function of the peak current in current transient. The metastable pits yielded peak currents between ~$10^{-7}$ and ~$10^{-5}$ A, metastable pit current densities at peak current between ~0.01 and ~1 A/㎠, and metastable pit radii at peak current between ~2 and ~15 mm. As the potential is increased, the metastable peak currents and pit radii at peak current both increase toward line. As potential is increased, the metastable peak currents and pit radii at peak current both increase toward $\frac{I_{pit}}{R_{pit}} = 3\times 10^{-3}$ line. This is the boundary for dividing the stable pits and metastable pits.