Influence of HNO3 treatments on the early stages of pitting of type 430 stainless steel

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.