The Electrochemical Feature Of Pitting Early Stage On Carbon Steel In Chloride Solution

The electrochemical behaviors of early stage of pitting on A3 carbon steel in NaNO₂ and NaHCO₃ solutions were studied. The current-time measurement, polarization curve measurement and EIS were employed. SEM was also used to observe the corrosion morphology of carbon steel surface. The main results are as follows:1. The current fluctuations of A3 carbon steel in NaNO₂ + NaCl solutions were studied. The current fluctuations showed the feature of quick rise and relatively slow decay, indicating quick breakdown and slow repassivation of the passive film on the surface. The current fluctuations resulted in observable pits on sample surface. A larger pit may be composed of several smaller pits. The occurring potential for metastable pits E_m decreased with the increase of chloride concentration, indicating that Cl- promotes nucleation of metastable pits. Under potentiostatic control, the average peak current and number of fluctuations all increased when appliedpotential was raised. This indicates that the potential increase may accelerate the active dissolution and make more active sites, hence the induction rate of pitting increase.2. Metastable pitting behavior was observed for carbon steel in NaHCO3 + NaCl solution, which was represented by current fluctuations with the characteristic of quick increase and slow decrease. Em and Eb all obey normal distribution. In potentiostatic test, when potential was above Em and far below Eb, the current fluctuations remained for a period then disappeared, resulting in small pits with diameters of microns. When potential was close to Eb, the current may finally rise continuously after a certain time of fluctuations, and metastable pitting was replaced by stable pitting. Fluctuation frequency decreased with time. There is no apparent relationship between potential and the frequency of fluctuations.3 . The effects of CO32", NO2", SO42", MOO42" and PO43" anions on meatstable and stable pitting behaviors of A3 carbon steel in NaHCC}? + NaCl solutions were studied. It was found that these anions all made Eb and Em move to positive direction, indicating that they retard not only nucleation of pitting corrosion but also nucleation of metastable pitting. NO2" ions showed the strongest inhibition on nucleation of metastable pitting. The effects of these anions on nucleation and growth of metastable pits are very similar to those on stable pits.4. A larger pit on carbon steel surface may be composed of severalsmaller pits, especially in NaNO₂ + NaCl solutions, which suggests that a repassivated metastable pit may still be active site for following metastable pits to nucleate. Repeated local dissolution events around a certain active site caused accumulated corrosion damage. The pit size distribution calculated from the current fluctuations was very close to that observed with SEM, indicating that there is good correlation between current fluctuations and growth process of metastable pits. It is possible to make prediction for accumulated pitting damage of carbon steel in certain systems according to the current fluctuations and the calculation of current value. Corrosion products may pile up at pit mouth and lead the pit to an occluded state, thus metastable pitting process was replaced by stable pitting. Once the pit mouth was completely blocked up by corrosion products, the pit would stop growing.5. The pitting behaviors of A3 carbon steel in NaHCO₃ and NaNO₂ solutions were compared. It was found that many metastable pits occurred before stable pits for carbon steel in NaHCO₃ solution. While in NaNO₃ solution, a few metastable pits occurred and they would be replaced by stable pits soon. Many individual current fluctuations of carbon steel in NaBCO₃ solution leaded to many stochastic pits on surface. While in NaNO₂ solution it was found that a larger metastable pit was usually composed of several smaller pits, which was consistent with the overlapped currents on current-time curves. The potential step measurements, EIS and...