Pitting corrosion of 304 stainless steel in secondary water supply system

The effect of NaClO solution on pitting behavior of 304 stainless steel was investigated by electrochemical tests and composition analysis. The results show that the open circuit potential (OCP) shifts to the positive direction with the increase of [ClO−], indicating that the passivation film formed on the sample surface is faster, but, has no effect on pitting potential (Eb) in the first stage (< 24 h). After 24 h immersion, the pitting occurs because Cl− is detected in the wake of the decomposition of ClO− for its thermodynamics instability. In the later stage, the [Cl−] increases with the increase of [ClO−], correspondingly resulting in lower pitting resistance. X-ray photoelectron spectroscopy (XPS) reveals that the Cr/Fe atomic ratio in the passivation film decreases with the increase of [ClO−], because Cr(III) is oxidized to Cr(VI) and dissolves in the solution, which further weakens the pitting resistance of 304 stainless steel. The electrochemical impedance spectroscopy (EIS), combined with the Mott-Schottky analysis (MSA) was employed to verify the properties of the passivation film related to pitting.

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