Abstract
Electrochemical polarization measurements were applied to Cu-added ultra low carbon steels aged at 773 K for duration ranging from 9×10 to 5×106 s to investigate size effect of copper precipitation particles on passivation of the steel. The anodic polarization curve measurements in 0.5 kmol/m3 H2SO4 solution revealed that the passive current density Ipass showed almost no variation until the aging process reached the overaged stage in which the Vickers hardness of the steel began to decrease. Thereafter, the Ipass increased monotonously with increasing aging time in the overaged stage. This increase in Ipass due to aging was considered to result from the preferential anodic dissolution of grown copper particles. Thus, the corrosion resistance of the Cu-added ultra low carbon steel scarcely deteriorated even if the size of copper particles increased with aging time before the maximum hardness was reached. These results reflected that there was a critical nanometer size of copper particles as a nanoscopic galvanic action beyond which a good protective passive film could not be formed.
