The Study Of Pitting Corrosion Mechanism Of 304 Stainless Steel In NaCl Solution

In this work,the corrosion behavior of 304 stainless steel in different acid solutions was studied by Tafel curve,electrochemical impedance and three-dimensional video microscope.The kinetic information of pitting was measured,and the mechanism of pitting growth was discussed.The relationship between the growth rate of pitting and metal ion concentration was established.The existing form of iron ions around the pit was proved by the chromogenic experiment,and the cathodic reactions in different stages of the growth of pits were analyzed by electrochemical test and thermodynamic conditions.The main research contents and results are as follows:According to the chemical composition of 304 stainless steel,a series of concentration of simulation solutions for pit growth were prepared.The relationship between the corrosion behavior and metal ion concentration during pitting corrosion was obtained by potentiodynamic polarization.The results show that there are two stages of active dissolution of metals inside the pit.When the metal ion concentration is lower than1.5 M(based on Fe²⁺concentration),304 stainless steel can maintain passivation state and corrosion current density less than 20mA/cm².When the metal ion concentration is in the range of 1.5 M⁴ M,the metals inside the pit show active dissolution behavior,and the corrosion rate increases linearly with the increase of the concentration of metal ions.Different concentration of Fe Cl₂ and Fe Cl₃ solution were used to simulate environment in pit,and the role of metal ions,especially iron ions,inside pit was discussed.Results show that the effect of Fe²⁺and Fe³⁺on the corrosion of 304 stainless steel is different at the same p H and Cl^-concentration,compared with the corrosion in HCl-NaCl solution system.Both Fe²⁺and Fe³⁺can result in positive shift of corrosion potential,but Fe²⁺reduce the corrosion current density of the sample,while Fe³⁺increase the corrosion current density.In Fe Cl₃ solution,as the reduction of Fe³⁺accelerates the cathodic reaction and consumes electrons,the self-corrosion potential of 304 stainless steel is shifted positively,while in Fe Cl₂ solution,the positive shift of potential is because the increase of Fe²⁺concentration can inhibit the dissolution of metal anode and lead to the accumulation of positive charge.The growth of pits and the diffusion process of corrosion products were monitored by chromogenic experiment,and Fe³⁺was proved to exist around pits.It was found that the iron ion concentration around the pit could reach 0.3 M or more.The corrosion behavior of 304 stainless steel in 0.1 M iron ion solution with different Fe³⁺/Fe²⁺ratio was studied by potentiostatic polarization curve and thermodynamic conditions analyst.The results show that there were two effects on the pit growth of the conversion from Fe²⁺to Fe³⁺.On the one hand,part of the Fe³⁺hydrolysis further reduces the p H of the surrounding solution,resulting in the acceleration of metals;on the other hand,Fe³⁺can reduce on the electrode surface and accelerate the cathode process of 304 stainless steel pitting corrosion.The decrease of the p H value of the solution and the reduction of Fe³⁺together lead to the acceleration of the corrosion rate of 304 stainless steel.This finding is of great significance to improve the pitting growth mechanism.The corrosion behavior of 304 stainless steel in Fe Cl₃ acid solution was studied by potentiodynamic polarization and 3D video microscope.The results show that at the same p H and Cl^-concentration,the self-corrosion potential and current density of 304 stainless steel increased with the increase of Fe³⁺concentration.When 304 stainless steel is corroded in acidic Fe³⁺solution,the corrosion potential of stainless steel is shifted positively due to the existence of Fe³⁺reduction reaction,which makes it difficult to carry out H⁺reduction on the electrode surface.When the content of Fe³⁺is sufficient,the reduction of Fe³⁺occurs preferentially in the cathodic reaction of 304 stainless steel corrosion,rather than the depolarization of H⁺.Therefore,in the study of Pitting Behavior of stainless steel,the effect of Fe³⁺around the pitting hole should be fully considered.