The Effect Of The Competetion Between Chloride Ions And Inhibition Ions On The Pitting Corrosion Under Strain

Pitting corrosion is a kind of corrosion which is difficult to be discovered and may cause serious damage. An important reason of pitting is that the surface protection film on the metal material can be broken by chloride ions. Though adding some inhibitors is useful to prevent pitting, the efficiency of inhibition is affected by the corrosive environments. And many kinds of steels are used under stress at present, so it is significant to research the pitting behavior on the competition between Cl" ions and anions under stress.The effect of strain on the pitting behavior of Q235 carbon steel and 304 stainless steel in NaCl solution with different anions was studied by potentiodynamic polarization measurement. According to the results of the potentiodynamic polarization curves, the value of Eb of Q235 carbon steel under different strain was compared, the influence of Cl" concentration on the pitting of Q235 carbon steel with and without 8% strain in 0.2 mol L-1 HCO3-+0.01 mol L-1 NaCl solution was studied and the changes of some information about metastable pitting after 8% strain were analyzed. The effect of strain on the passive stability and the passivation rate was studied using EIS, XPS and potential step method. The main conclusions are as follows:(1) In 3.5%NaCl (wt%) solution, the Eb of 304 stainless steel decreased after 8% strain, and the recults showed that 0.3 mol L-1 NaHCO3 and 600 ppm D-sodium gluconate could inhibit the pitting. But after 8% strain, the Eb with NaHCO3 in 3.5%NaCl solution increased while the Eb with D-SG in 3.5%NaCl solution decreased.(2) The Eb of Q235 carbon steel with 8% strain could be higher than the Eb of the sample without strain in NaHCO3-+NaCl solution. This result was related to the value of [HCO3-]/[Cl-] and the magnitude of stress. When the strain increased, the Eb would decreased.8% strain lead to the increase of Eb in 0.2 mol L-1 HCO3-+0.01 mol L-1 NaCl solution, but when the concentration of Cl- increased, the disparity of Eb between the sample with 8% strain and that without strain was decreased, and disappeared at 0.1 mol L-1.(3) In 0.2 mol L-1 HCO3-+0.01 mol L-1 NaCl solution, compared the results of Q235 carbon steel with different strain (0 and 8%), the metastable pits under 8% strain was easier to nucleate, and their life would be shorter and it was hard for them to have bigger diameter.(4) 8% strain caused lower impedance, smaller charge transfer resistance Rct and decreased Fe3+/Fe2+ratio in the passive film, so the stability of passive film reduced with strain. However, the content of FeCO3 in the passive film was more under 8% strain, and its passivation rate was faster than that without strain, so there were more HCO3- ions participating the process of adsorption and passivation. The phenomenon that strain caused Eb increased was attributed to that the strain promoted the anodic dissolution of Fe which in turn promoted the adsorption of HCO3- on the surface, as the result the harmful effects of Cl- on the passive film was inhibited.