The Study Of Pitting Behaivor Of 304 Stainless Steel In 3.5% NaCl Solution

The potentiostat polarization was used to study the acceleration effect of pitting product concentration on the initiation of metastable pit and steady pit growth of stainless steel.The effects of oxygen concentration on pitting of 304 stainless steel and behavior the pitting behavior of it in low concentration simulated pits solution were investigated by using the method of potentiodynamic polarization,AC impedance spectroscopy,potentiodynamic electrochemical impedance spectroscopy and immersion method.In the initiation stage of metastable pitting,with the increase of pitting product concentration,the indicators of metastable pitting sensitivity change as follows:the pitting incubation period was significantly reduced,and the number of nucleation of metastable pitting was greatly increased,the average peak current and the average peak width were significantly increased,and the growth rate of the metastable pitting was increased.At the initial stage of metastable pitting to steady pitting growth,the growth rate of pitting volume,pitting diameter and pitting depth of a single steady pitting increased with the increase of pitting product concentration after constant potential polarization of 0.45 V_vsSCE.At the same pitting product concentration,the growth rate of pitting volume and pitting depth increases with time;the growth rate of pitting diameter decreases;the rate of width to depth decreases;and the shape of corroded hole changes from circular to conical.With the decrease of the rate of width to depth,the concentration of pitting products in the pits increases,and the corrosion rate of the pits increases.The pitting products concentration are the key factors affecting the metastable pitting initiation and steady pitting growth.The pitting corrosion behavior of 304 stainless steel in low concentration simulated pits solution was studied by using simulated pits solution which was prepared according to the main composition of 304 stainless steel.In the 0.3M¹.1M simulated pits solution,with the increase of simulated pits solution concentration,the open circuit potential of304 stainless steel decreases,the corrosion current increases,and the corrosion tendency of the samples increases.The corrosion current increases slowly when the concentration of simulated pits solution is between 0.3M and 0.7M.When the concentration of simulated pits solution exceeds 0.9M,the corrosion current of the pitting increases rapidly.The conclusions above were verified in immersion experiments.With the increase of oxygen concentration in solution,the corrosion potential shifts positively and the corrosion current increases.In NaCl solution,when the oxygen concentration is more than 0.6 mg/L,the corrosion potential is less than the equilibrium potential of oxygen,larger than the equilibrium potential of hydrogen ion,and there is no hydrogen evolution reaction in the solution.When 3mol/L FeCl₃ solution is coupled with a cathode,the increased of oxygen concentration in NaCl solution,and the cathode consumes a lot of electrons of NaCl solution,which makes the potential of FeCl₃ solution shift positively and accelerates the dissolution of metal.