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 pot...