| The pitting corrosion of A3 carbon steel was investigated by electrochemical methods. The statistical distributions of breakdown potentials Eb and protection potentials Ep of A3 carbon steel in NaNO2+NaCl solution were studied by potentiodynamic polarization method. The influences of some factors, such as pH value, surface roughness, Cl" ions and sulfur-containing anions, on pit nucleation and growth of carbon steel were also studied by the same method. Potentiostatic polarization method was employed to study pitting current growth dynamics of carbon steel in NaNO2+NaCl solution and fluctuations of pitting current in phosphate-borate buffer solution containing C1-.Both pitting breakdown potential Eb and protection potential Ep of carbon steel obey normal distribution. pH value has a great effect on pitting when it is between 11.5 and 12. With pH value increased, pit nucleation was more difficult but pit growth was promoted. When pH value is lower than 11.5, it has less effect on pitting of carbon steel. Surface roughness also has an effect on carbon steel pitting. Rougher surface promoted nucleation of pitting, but retarded growth of the pits. Cl" also has a great effect on pitting by promoting pit nucleation. But Cl" has less effect on pit growth. The pitting current of carbon steel at fixed potential increased in the following form :I-Ip=K(t-tl)bIn alkaline solutions, CNS', SO32', S2O32" ions promoted pit nucleation, and the effect decreases in the order: S2O32">SO42"> SO32"->CNS". S2" suppressed pit nucleation. Pit propagation was promoted by S2", S2O32"and SO32' anions, and the promoting effect decreases in the order as S2"> S2O32"> SO32". CNS" and SO42" anions suppressed pit propagation, and the effect of SO42" was stronger than CNS". In solutions with different pH values, the sulfur-containing anions had different influence on pitting behavior of carbon steel. With pH value decreased, the influences of SO42" on the promotion of pitnucleation and suppression of pit propagation were strengthened but the influences of SO32" and CNS' on pitting were not affected. However, the influence of S2O32" and S2" on pitting was affected greatly by decrease of pH value. The decomposed products of S2O32" in acid solution were different from those in alkaline solution, which led to more negative Eb and promoted the influence of S2O32" on pitting. A great amount of H2S gas was produced because of the presence of S2" ions in acid solution, which resulted in more positive Eb than that in neutral solution. So pit nucleation becomes difficult.During pitting corrosion of carbon steel current fluctuations occurred in phosphate-borate buffer solution containing Cl". The fluctuations were seen above Eb, indicating the information of stable pitting. The fluctuation shows sharp rise and slow drop. The rapid rise is attributed to the breakdown of the passive film whereas the decrease is due to passive film repair. Repassivation rate can be calculated from the branch of slow decrease. The Repassivation rate follows an exponential decay form. In potentiostatic test the frequency of fluctuations decreased with the time until zero, indicating that with the time the number of active pitting sites on the surface decreased gradually and the sensitivity of each active site decayed gradually too. The peak values of the current fluctuations followed normal distribution in this experiment. The increase of potential led to increases of both background current and the peak currents. During crevice corrosion there were also current fluctuations. Compared with pitting corrosion, the potential at which the current fluctuations began to occur was more negative, and the peak current was greater and the frequency of fluctuations was less. The fluctuations also showed quick rise and slow drop and peak value obeyed normal distribution. The Repassivation rate decayed as exponential form. |
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