Effect Of AC Stray Current On Hydrogen Permeation Behavior And Mechanical Properties Of Q235 Steel

The main substations in China are AC substations,and the grounding grid of substations suffers from the corrosion of AC stray current for a long time.Because the corrosion of AC stray current is mLuch less than that of DC stray current,there is little research on AC stray current corrosion.However,with the increase of transmission voltage level,the corrosion of grounding grid in AC substation has been broken,and it has not been reasonably explained up to now.The State Power Grid has set up a key problem of corrosion protection of grounding grid.This paper is a part of this research topic.Taking Q235 steel as the research object,the different AC electrical density was studied by means of strengthening medium method,weight loss method,electrochemical hydrogen permeation method,static tensile test and mophology analysis method,focusing on the phenomenon of hydrogen evolution in the corrosion cathode of AC stray current.The corrosion and hydrogen permeation of Q235 steel in simulated soil solution pH with different acid soils were studied.The results show that:(1)When the applied AC density increases from 20mA/cm2 to 100mA/cm2,the diffusible hydrogen concentration of Q235 steel increases from 4.74 ppm to 7.01 ppm.and the corrosion rate is linear with the current density of v=0.00307+1.865E-4i.The mechanical properties of Q235 steel have decreased greatly.The surface of the fracture surface has gradually disappeared and large-scale hydrogen-induced cracking has occurred.The degree of embrittlement increases and the trend of brittle fracture appears.(2)The frequency of AC will affect the hydrogen permeation behavior of Q235 steel.When the AC density is constant,the frequency increases from 50Hz to 1000Hz.The diffusion hydrogen concentration of Q235 steel decreases and the corrosion rate decrreases.The tensile strength and strain of Q235 steel both rise,and the fracture morphology shows the tendency of toughness and embrittlement.The degree is weakened.The effect of AC density on hydrogen embrittlement corrosion of Q235 steel is much greater than the frequency effect.(3)Under the alternating current density of 20mA/cm2,the pH of the acidic soil simulated solution increases from 3 to 6,and the corrosion rate of Q235 steel decreases.Change of corrosion process from hydrogen evolution corrosion to oxygen absorption corrosion,and the hydrogen evolution of Q235 steel cathode decreases.The diffusible hydrogen concentration decreased from 5.49ppm to 3.05ppm.The mechanical properties of Q235 steel rebounded and the tensile strength increased from 344.65MPa to 363.97MPa.Fracture dimple size decreases obviously,the number increases,and the embrittlement degree of Q235 steel decreases.(4)When the AC corrosion hydrogen permeation time increases from 2h to 15h,the corrosion amount of Q235 steel increases,and the hydrogen atoms generated by the reaction will continuously enter the defect,which increases the hydrogen absorption of the material and leads to an increase in embrittlement.There are two reasons for AC stray current corrosion accelerating fracture of Q235 steel in grounding grid.One is that it causes corrosion of Q235 steel in positive half cycle of AC,which leads to material thinning;the other is that hydrogen atoms enter into Q235 steel in negative half cycle of AC,resulting in hydrogen-induced cracking.