Fluoride Accumulation into SUS 304 Passive Film and Selective Dissolution of Iron Component

Abstract

Changing the timing of fluoride ion addition, passivation behavior at constant potentials and anodic polarization one for SUS 304 stainless steel were investigated in sodium sulfate aqueous solutions of pH 2-13, along with ICP analysis of solutions and XPS one of passive film. No pitting corrosion occurred for SUS 304 in fluoride solutions, but accelerated dissolution in the transpassive region and slight increase of passive current were observed in lower pHs. It was verified that fluoride ion dissolved preferentially iron component of SUS 304. This iron dissolution occurred immediately after polarization start in originally F^- mixed solutions and after F^- addition when being prepassivated in F^- free solutions. XPS analysis revealed fluoride incorporation into the passive film formed in originally F^- mixed solutions. It should be noted that no fluoride incorporation was observed in the film even though F^- was added in the course of passivation. At the same time, chloride incorporation was not also detected in the film, independent of the timing of Cl^- addition.