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

Pitting behaviors of 304 stainless steel in 3.5%NaCl solution were investigated by potentiodynamic polarization,electrochemical impedance spectroscopy,immersion and three-dimensional video microscope,mechanisms of pitting were studied.The main research concentrations and results are as follows:The growth mode of steady pitting was simulated,the influence of cathode area on the pitting steady growth rate of 304 stainless steel in 3.5%NaCl solution was quantitatively analyzed.The results showed that when the external cathode area in NaCl solution existed,the dissolution rate 304 stainless in FeCl₃ solution was accelerated due to the galvanic effect of stainless steel immersed in different kinds of solution at the same time,and the reaction of the cathode was mainly the hydrogen evolution reaction on the side of the FeCl₃ solution.When the ratio of cathode area to anode area ratio was less than 64,the anodic dissolution rate increasing with the increasing of the cathode area,and the increment of anode corrosion rate tended to be gentle when the area ratio of the anode and anode area was up to 64,which was about 2.2 times that without the external cathode area.When pitting corrosion occured naturally in 304 stainless steel in 3.5%NaCl solution,the dissolution rate of the metal at the bottom of the pit increased by 2.2 times.FeCl₃ solution was used to simulated pit environment of 304 stainless steel in 3.5%NaCl solution,by studying the corrosion behavior of 304 stainless steel in different concentrations of FeCl₃ solution,the mechanism of the how pitting grows of 304 stainless steel in the 3.5%NaCl solution was investigated.The results showed that when the concentration of FeCl₃ solution was 0.1 M¹.0 M,with the increase in the solution concentration,the open circuit potential of 304 stainless steel shifted to negative,the corrosion tendency and corrosion current density both increasing.When the concentrate was 0.1 M⁰.3 M,the passivation film of 304 stainless steel was more stable,and the corrosion current density rises slowly with the increased of the concentration,When the concentrate was more than 0.5 M,the passivation film was easier to broken.The growth rate of corrosion current density increased with increasing concentration;When the pitting occured of 304 stainless steel in 3.5%NaCl solution,as the concentration of FeCl₃solution was 0.1 M⁰.3 M,pitting corrosion stopped growing.As the concentration of the solution was 0.5 M².5 M,and the growth rate of pitting increased with the concentration of FeCl₃ solution.When the concentration of FeCl₃ solution was 3 M³.5M,the corrosion state was the most comprehensive corrosion and the corrosion rate reached the maximum.The effect of different surface flow velocity for 304 stainless steel in 3.5%NaCl was studied.The results showed that:when the surface velocity is less than 293.07cm/s,with the increasing of surface solution flowing velocity,the mass transfer rate increased,the metal ion and the degree of hydrolysis in pit decreased,the resistance of stable pitting occur and grows both enhanced,and width to depth radio of pit decreased.Moreover,as the flowing rates increased,the corrosion current density decreased,and the pits were more easily passivised.Stability of passivation film was enhanced.When used FeCl₃ to simulate the bottom environment of pit of 304 stainless steel in 3.5%NaCl solution,the surface state of the sample and the corrosion current density in the FeCl₃ solution of 3 M do not change with the shift of the flowing velocity.