Research On Metastable Pitting And Related Morpology Of Pure Iron In NaNO₂+NaCl Solution

Metastable pitting behavior of pure iron in NaNO2+NaCl solutions was studied by potentiodynamic polarization, potentiostatic polarization and AC impedance. By comparison of stainless steel and carbon steel, the following issues were discussed: factors influencing the initiation and propogation of metastable pitting, the influence of polarizing on metastable pitting and surface condition and the relationship between SEM morphology and current fluctuations.As for pure iron, the main factors influencing the behavior of metastable pitting could be: the concentration of aggressive ion, surface roughness, temperature and potential ect. However, the behavior of stainless steel could be remarkably affected by one more factor, the inclusions. It caused the different pitting behaviors of the two materials.Times of polarization experiments caused the decrease in initiation number and peak current values. Em and Eb increased, otherwise. In each polarization experiment, the initiation number went up with the increase of potential. Stainless steel had more notable influence than other materials.There were quite differences among the current fluctuations of pure iron, A3 carbon steel and stainless steel. It could ascribe to the different passivation ability of materials. It was observed that the pits on the sample surface were composed of many smaller pits, which were the results of nucleation, growth and repassivation of metastable pits. The small pits were nucleated following the surface grooves left by polishing.Good relationship was observed between the number and electric quantities of the current fluctuations measured, and the number and sizes of the pits observed, which indicates that it is possible to predict the extent of corrosion according to the measured current fluctuations.