Abstract: An improved scanning micro reference electrode (SMRE) technique controlled by micro-computer was used to study the pitting initiation in early stage of 18/8 stainless steel in chloride containning solution. It is shown that there exsists a critical potential E_r which characterizes the breakdown of passive film and initia- tion of the micropit corrosion in an early stage of pitting corrosion. The potential value of E_r is usually 100—400 mV less than the pitting potential E_p which corre- sponds to rapid increase of anodiccurrent in the polarization curve. The value of E_r is related closely to the surface states of specimens. On increasing the content of Cl~- and H~+ ions in solution, E_r will become more negative. when the potential is controlled in the vicinity of E_r, the micropit cannot grow to a macropit corrosion generally.
The new concept of “unstable micropitting” was suggested and the physical significance of the characteristic potential of E_r and E_p was expounded according to the experimental facts. Furthermore, the mechanism of local breakdown of passive film and pitting initiation of 18/8 stainless steel in chloride containing solution was also discussed in this paper.