TIME-RESOLVED RRDE APPLIED TO PITTING OF FE-CR ALLOY AND 304-STAINLESS-STEEL

The electrochemistry of pit generation-repassivation sequences of Fe-C r and 304 stainless steel has been investigated in a sulphuric acid/Na Cl mixture by time resolved RRDE experiments. The pitting (disc) curre nt, I-D, and the ring current, I-R, are recorded. The actual flux of c ations, Fe2+, produced by the pit corrosion is restituted by fast Four ier transform (FFT) processing of I-R With the complex collection effi ciency. Simultaneous AC modulation of the disc potential was newly int roduced in order to follow the impedance during pitting and repassivat ion and the AC response of I-R. It is found that in both materials the iron oxidation products are fully dissolved at any time during the pi t life. Cr and possibly Ni (304 SS) are responsible for the salt layer precipitation. According to AC data, drastic changes in the charge ex change between the pit site and the outer solution take place around t he midrange of each pitting event. After apparent pit repassivation, s hown by the pitting current, the pit remains actively corroding. Mass transfer limitation of the pit activity in unsupported electrolyte con ditions is likely to explain the pit impedance. The technique is thoug ht to bring valuable information on the pit micro-electrochemistry and on the nature of individual events involved in prepitting and pitting noise.