In order to record metastable pit current transients, a 904L
austenitic stainless
steel was polarized potentiostatically by applying 750 mV (Ag/AgCl)
in 1 M NaCl solution at
45, 49 and 54°C, knowing that the alloy CPT is 56°C. Stable pit
current transients were
recorded employing the same potential and test solution but at
62°C. It was found that the
metastable pit current, I, follows a power law relation ship of
I ∝
tn
during the initiation and
growth prior to repassivation. The value of n was found to vary
with temperature, however,
values of 0.5 to 1.5 were obtained in all cases. The results of
Scanning Electron Microscopy
(SEM) studies on these transients showed two types of pit
morphology: small pits associated
with the small transients, whose interior surface were not
visible, and open pit cavities with
polished surface for larger transients, recorded at temperature
closed to the CPT. Current
transients from the stable pits showed an initial shape similar
to that of stable pit. The stable
pit currents rises approximately in proportion to t3/2,
but only for a few seconds and then
continues to increase proportion to t1/2
without repassivation. The SEM
studies also revealed
that stable pits are covered with a lace-like pattern with
polished interior surface.