The metastable pitting of mild steel in bicarbonate solutions

The metastable pitting behavior of mild steel in 0.5 M NaHCO3 solutions containing different concentrations of Cl− was studied with potentiodynamic and potentiostatic polarization methods. Current fluctuations occurred before stable pitting, with the characteristic of quick increase and slow decrease. The occurring potential for metastable pitting Em obeys normal distribution, and decreases with the increase of chloride concentration. Potential has no apparent effect on the frequency of fluctuations. In potentiostatic test, when potential is above Em and far below the pitting breakdown potential Eb, the current fluctuations remain for a period then disappear, resulting in pits with diameters of microns. When potential is close to Eb, the polarization current may finally rises continuously after a certain time of fluctuations, and metastable pitting is replaced by stable pitting.