Study On Volta Potential And Micro-zone Corrosion Behavior Of Deformed Structure In High Strength Steel

In this paper,20 CrMnTi high strength steel is taken as the main research object,and ultrasonic surface rolling process(USRP)is adopted to form a gradient strengthened layer with high residual compressive stress,which causes fundamental changes in electrochemical properties and produces local corrosion behavior different from that of matrix.The surface morphology was observed by scanning electron microscopy(SEM)and dislocation density was measured by transmission electron microscopy(TEM).The grain refinement of the gradient deformation layer was characterized by EBSD technology,and the electrochemical behavior of the surface structure was characterized by polarization curve.A scanning Kelvin probe force microscopy(SKPFM)was used to measure the Volta potential of the gradient strengthened layer,and analyzes the relationship between dislocation density,residual stress and Volta potential to reflect the electrochemical nobility of the gradient layer.Therefore,the feasibility of SKPFM technology to characterize 20 CrMnTi high strength steel gradient strengthened layer was theoretically proved by using the relatively mature copper material in the electron work function to investigate the influence of SKPFM technology test parameters and sample surface state on the Volta potential.Specific research results are as follows:(1)The surface grain of 20 CrMnTi steel was refined after the ultrasonic surface rolling treatment.With the increase of rolling pressure,the refining degree was enhanced and dislocation density increased.In the weak corrosion environment,the corrosion rate of surface structure accelerates with the increase of rolling pressure,both reflected by increased passive current density and decreased anodic passive range.(2)The Volta potential is affected by the test parameters and the surface state of the sample.The Volta potential decreases with the increase of the distance between the sample and the probe,reaching a maximum at 10 nm.After the copper is deformed,dislocation density is increased and Volta potential difference is decreased(bias voltage is applied to the sample).The decrease amplitude increases with the increase of deformation amount.However,after chemical erosion,the accumulated stored energy of deformation is released,resulting in the increase of Volta potential.The type of passive film affects the Volta potential of the sample surface.After 20 CrMnTi steel is oxidized with concentrated nitric acid,the Volta potential difference decreases(bias voltage is applied to the tip)and hence decreases the reactivity.However,after the immersion of 3.5% Na Cl solution and the etching of picric acid solution,the Volta potential increases and hence the corrosion reactivity increases.(3)SKPFM is an effective method to characterize the electrochemical nobility of the gradient strengthened layer.In the plastic deformation zone of the gradient deformation layer,the accumulation of dislocation density leads to the reduction of the electron work function,hence,the Volta potential difference increases and local fluctuations occur,leading to the enhancement of the electrochemical reaction trend and hence,the local corrosion activity.Plastic deformation amount decreases gradually along depth from surface,resulting in the decreased Volta potential difference.In the elastic deformation zone,the residual tensile stress enhanced the electronic activity and reduced the electron work function,leading to the higher Volta potential difference than the non-deformation zone.But the influence of residual stress on Volta potential is much less than that of dislocation density.