PITTING CORROSION DYNAMICS AND MECHANISMS OF 304 STAINLESS STEEL IN 3.5%NaCl SOLUTION

Pitting rate of a single pit and pitting mechanisms of the 304 stainless steel in 3.5%NaCl solution were investigated by utilizing electronic speckle pattern interferometer (ESPI), electrochemical noise (EN) and three-dimensional video microscope. The results show that under 0.05 V polarization, the pitting corrosion process can be divided into four stages: drastic fluctuations of the current noise occured at 740 s, which means the passivation film was breaking, thus it can be concluded that the span of pitting incubation period is 740 s; a speckle occurred on the ESPI image at 750 s, this the span of initiation period of the steady pitting is about 10 s; the growth rate of the pit increased during 750-780 s, which indicates that the pit corrosion is in active dissolution period; since then, the growth rate of the pit declined rapidly which means the pit was repassivated. After 793 s, the growth rate of the pit raised again as secondary pits emerged. The pit image was observed and its volume was measured by three-dimensional video microscope, and the results were in agreement with those which were obtained by corrosion product concentration analysis. Some secondary pits were found in the bottom of the pit in three-dimensional reconstruction images.