The Electrochemical Characters Of Metastable Pitting Of 316L Stainless Steel

Potentiostatic and potentiodynamic polarization methods were employed to study the behavior of metastable pitting on 316L stainless steel in NaCl solutions. The distribution of Em (the potential at which the first metastable pit emerged) and Eb (the potential at which the stable pit emerged) and the relationship between them were studied by potentiodynamic method. The influences of some factors, such as the concentration of Cl- ions, potential and pH value, on characteristic parameters of metastable pitting were also studied by both methods. In addition, extreme value distribution of peak currents and the relationship between growth velocity K and peak current Im were studied.Both Em and Eb obeyed normal distribution. With the increase of the concentration of Cl- ions, both Em and Eb reduced with lgCNaCl in a way of linearity. Because the concentration of Cl- ions had a little effect on (Em - Eb) value, we could get the Eb value by measuring Em value.Generally speaking, with the increases of concentration of Cl- ions and potential, the number of metastable pit nucleation N, growth velocity K and peak current Im had a tendency to incease, and the life-time of metastable pits had a tendency to decrease. The results had some difference in potentiostatic and potentiodynamic polarization methods. PH value, however, had a little effect on the metastable pitting parameters.When the constant potential was lower than Eb, the highest peak currents Im in different time range followed extreme value distribution. For stated Im, with the decreaseof concentration of Cl- ions and potential, the probability of which the peak current value of mestable pits didn't exceed Im value would increase, which resulted in the probability of which metastable pits turned into steady pits decreased.The outline of the distribution of K-I seemed as the shape of a bell, resembling Gauss distribution, which showed that there was an optimal growth range when metastable pits turned into steady pits. When metastable pit growth velocity was in this range, the peak currents were most propitious to increase. This velocity range approached the steady growth velocity. With the increase of the concentration of Cl-ions and constant potential, the highest peak currents Im which emerged on metastable pits might increased.