The initiation of localized corrosion of types 430 and 443J1 ferritic stainless steels was evaluated in 0.15 mol dm −3 Na 2 SO 4 solution. A liquid-phase ion gun (LPIG), a silver microelectrode covered with a silver chloride layer, was cathodically polarized to generate Cl − in the vicinity of the stainless steel polarized at 0.4 V SSE . Contact of the stainless steel surface with the Cl − -concentrated environment by the LPIG operation induced a rapid increase in anodic current flow through the stainless steel electrode after the induction period t d and consumption of the cathodic electric charge Q d by the LPIG microelectrode. Numerical modeling using the LPIG microelectrode current for t d gave a critical Cl − concentration [Cl − ] d needed for the initiation of localized corrosion. These parameters obtained by LPIG tests showed that type 443J1 stainless steel was superior to type 430 stainless steel in localized corrosion resistance. AES depth profiling of the stainless steel surfaces after the LPIG operation revealed enrichment of Cl in the outermost oxide film as well as decreased film thickness. The mechanism of degradation of the stainless steel surface due to contact with the Cl − -concentrated solution is discussed. The adsorption of Cl − on the oxide surface is thought to be a trigger of the oxide degradation. Stainless steel, an Fe-based material with a minimum of 11 wt% Cr content, has corrosion resistance, which is attributed to the formation of Cr-and/or Fe-oxide films, so-called passive films, on the surface. The resistance is related to the chemical composition of the film, which is strongly dependent on the chemical composition of the substrate. The pitting resistance equivalent (PRE) number (= wt% Cr + 3.3 (wt% Mo + 0.5 wt% W) + 16 wt% N) has been used to determine the localized corrosion resistance of stainless steel.1 The larger the PRE number of stainless steel is, the greater is the resistance of the stainless steel to localized corrosion. The PRE number is related to the contribution of elements to the resistance of a passive film on stainless steel. Alloying with Cr forms non-crystalline Cr 2 O 3 on stainless steel by a direct reaction of Cr with H 2 O and improves the resistance to localized corrosion.2 Alloying with Mo enhances the protectiveness of a passive film to Cl − by increasing oxygen affinity of the stainless steel.3 Alloying with W inhibits localized corrosion by dissolved WO 4 2− from the passive film to an aqueous electrolyte or by forming insoluble WO 3 , which enhances the stability of the passive film.4 Alloying with N decreases local pH by decreasing acidity in pits due to NH 4 + formation and promotes repassivation. 5 For ferritic stainless steel, however, the effect of N is not quite satisfied and usage of the simple PRE number (= wt% Cr + 3.3 wt% Mo) is proposed.