Effect Of Alternating Stress On Corrosion Electrochemical Behavior Of E690 Steel

In this work,the electrochemical parameters of E690 steel in 3.5wt% NaCl solution under different frequencies and stress ratios were measured.The effects of frequency and stress ratio on the corrosion electrochemical behavior were investigated.Combined with microanalysis methods such as Ultra-depth-of-field 3D Microscope,Scanning Electron Microscope(SEM)and Energy dispersive X-ray Spectroscopy(EDS),the mechanism of alternating stress on the electrochemical behavior was discussed.Based on this,the effects of alternating stress on cathodic protection behavior were investigated.The conclusions can be drawn as followed.(1)The elastic alternating stress affects the corrosion rate of E690 steel by generating active sites on surface.The density of active sites is positively correlated with frequency f.When f?1.0Hz,the corrosion rate increases with increasing frequency.When f>1.0Hz,the corrosion rate is controlled by the oxygen diffusion process and not changes with frequency because the active sites cannot all participate in the electrochemical reaction.(2)There are two different rules for the influence of the alternating stress ratio R on the corrosion process.When R?0.6,the stress fluctuation factor dominates,and the corrosion rate decreases with the increase of R.When R>0.6,the average stress level dominates,which causes the corrosion rate to increase,and stabilizes when R?0.8,which is equivalent to the result when subjected to constant stress at the same level.(3)The initiation of pitting corrosion on the surface of E690 steel can be promoted by alternating stress.Most of the corrosion pits are near Al2O3 inclusions.The tendency to form corrosion pits is positively related to the average stress level.(4)For E690 steel,the safe potential range of cathodic protection is not changed with alternating stress.With alternating stress,the cathodic polarization potential of the oxygen reduction zone is not enough to suppress the formation of corrosion pits.The effectiveness of cathodic protection is reduced.The effects of alternating stress on the polarization resistance at the cathode potential is different in the oxygen reduction zone and the hydrogen reduction zone.The mechanism is worthy of further study.