The Electrochemical Feature Of Pitting Early Stage On Several Kinds Of Steels

The electrochemical behaviors of early stage of pitting on X70 pipeline steel, 16Mn steel, A3 carbon steel and ingot in phosphatic buffer solution were studied by means of current-time measurement, polarization curve measurement and EIS. SEM was also used to observe the corrosion morphology of carbon steel surface. The main results are as follows:1. The current fluctuations of X70 pipeline steel in Na3PO4+ NaCl buffer solution were studied. It is found that the effect of CI" ion on pitting protection potential is neglectable and PO43- ion strongly inhibits the nucleation of metastable pitting. Impurities and the surface grooves left by polishing result in the nucleation of metastable pitting. With the increase of the current potential, the nucleation frequency increases at first and then drops, while peak current increases continually. The impedance of passive film keeps unchanged when X70 pipeline steel is in passive state.2. Metastable pitting behaviors were observed for A3 carbon steel, 16Mn steel and ingot in Na3PO4 + NaCl buffer solution. It was represented by nucleation frequency with the characteristic of increases at first and then decrease for A3 carbon steel, while nucleation frequency of 16Mn steel decreases continually. With the increase of the current potential, nucleation frequency of ingot increases at first and then decrease, and finally average out.3. When compared with several kinds of steels, it shows that impurities and the surface grooves left by polishing result in the nucleation of metastable pitting for X70 steel and A3 steel, whereas it results from the surface grooves and passive film for ingot. The effect of potential is neglectable for 16Mn steel in the nucleation of metastable pitting.4. The electrochemical behaviors of passive films on X70 pipeline steel, 16Mn steel, A3 carbon steel and ingot in phosphatic buffer solution were studied by analysis of Mott-schottky plots and EIS. It is indicated that metastable and stable pitting events can be illustrated by the experimental 1/C2. There is a certain relationship between the values of transitional potential in nucleation frequency and the critical potential ionizing both levels of the Mott-Schottky plot.