KINETICS OF PASSIVE FILM FORMATION ON SCRATCHED BARE SURFACES OF STAINLESS-STEELS IN MAGNESIUM-CHLORIDE SOLUTIONS

Kinetics of passive film formation on the bare surface of scratched st ainless steels SS in magnesium chloride (MgCl2) solutions were studied using the rapid scratching technique under potentiostatic conditions. An experimental device was designed to record data at the rate of 20, 000 points/s, with the rotating rate of the specimen at 3,000 rpm and a scratch scar length of about 4.6 mm to 4.8 mm. A new phenomenon was observed in that two peaks were seen rather than a continuous decay in the curve of current decay on scratched SS in MgCl2 solutions. Curren t decayed steeply to approximately passive current within about 2.5 ms to 3 ms after the diamond knife was moved away from the specimen. Cur rent rose again for about 2 ms to 3 ms. This phenomenon was considered to represent the processes of adsorbed layer formation on the bare su rface and transformation of the adsorbed layer into a passive film. Re sults were affected by the recording rate of experimental data, the sp ecimen rotating rate, and the scratch scar length. The kinetics of pas sive film growth were shown empirically by i(t) = I0exp(-betat) with I 0 and beta being constants. Passive film growth was controlled by ion conduction in a strong electric field, as defined by i = A exp (BV/x).