Abstract
The passivation behavior of SM400B carbon steel was investigated in 0.1 mol dm−3 carbonate solutions. The existence of carbonate in the solution reduced the critical current density for passivation, icrit, by virtually an order of magnitude in the pH range of 7.5 to 10.0 at 298 K. The critical potentials for passivation at the reduced icrit were within the potential-pH range where FeCO3 is thermodynamically stable. These results showed that carbonate accelerated the passivation tendency. Depassivation pH in the carbonate solution was determined to be 7.5 at 298 K by comparison between icrit and the diffusion-limiting current density for dissolved oxygen of 8.14 ppm. The depassivation pH is considerably lower than that of 9.3 in a solution that does not contain carbonate. The passivation tendency was enhanced with increasing temperature from 298 K to 363 K. The free corrosion potential, Ecorr, of polished specimens in the aerated carbonate solution at 298 K stayed in an active dissolution potential range in the early stages after immersion. They were, however, suddenly ennobled to the passive region after a long incubation time at pH≥8.0. The time dependency of Ecorr could be described from the viewpoint of electrochemical kinetics.
