The effect of HNO3 surface treatment on the early stages of pitting of Type
430 stainless steel has been studied by AC impedance method. At a given lo
w potential. the Warburg impedance coefficient sigma, which is calculated f
rom impedance plots, is a maximum after wet polishing, and decreases with i
ncreasing HNO3 concentration in the solution of surface treatment. It has b
een suggested that the surface becomes more passive and the most susceptibl
e sites for pit nucleation are eliminated after HNO3 treatments, and that i
ncreasing the HNO3 concentration in treatment solution results in increasin
g the resistance of the steel to metastable pitting. The critical potential
(E-m) at which metastable pit or pits start to grow on the surface of the
steel depends on HNO3 concentration in treatment solution. The higher the H
NO3 concentration in treatment solution, the higher the E-m values, showing
that the nucleated pit or pits proceeding to metastable growth stage becom
es more difficult. (C) 2000 Elsevier Science Ltd. All rights reserved.