Corrosion Fatigue Crack Initiation Behaviors And Mechanisms Of E690 Steel In Simulated Seawater

In this work,corrosion fatigue crack initiation(CFCI)behavior and mechanism of E690 steel in simulated seawater were inverstigated.We first studied the micromechanism of CFCI and initial propagation by stress-controlled corrosion fatigue test followed by a series of characterization measurement.Then the electrochemical behavior evolution during corrosioin fatigue process was monitored by methods of in-situ electrochemical noise,electrochemical impedence spectroscopy and so on.The effect of cathodic potential on the CFCI mechanism was also studied by S-N curves under different cathodic potentials combined with fast and slow scanning-rate polarization curves.After that,synergistic effect between hydrogen and cyclic stress as well as the consequent fatigue damage accumulation.At last,with consideration of the above results,a model based on corrosion kinetics and equivalent intial flaw size theory was proposed for predicting the CFCI life in fast and economical way.Finally the conclusions can be drawn as followed.(1)The CFCI and initial propagation mechanism of E690 steel in simulated seawater transformed along with cyclic stress level enhanced.The transformation occurred at the range of 0.8?0.95 ?p0.2 peak stress.When the peak stress is below this range,the anodic dissolution(AD)at grain boundaries(GBs)was stronger than the mechanical effect,inducing that cracks initiated from GBs preferentially.Furthermore,cracks initiated from the parent austenite grain boundaries(PAGBs)with probility of 68.4%and from the bainite lath boundaries(BLBs)with probility of 31.6%.When the peak stress was close to or above the proof stress,cracks initiated from pits and propagated along the original direction in shortest way by splitting the bainite laths,finally resuting in quasi-cleavage morphology.(2)The electrochemical behavior during corrosion fatigue process of E690 steel was influenced by cyclic stress.Cyclic stress impeded the formation of stable corrosion product layer and promoted the interface electraochemical reactions.The charging and discharging of electric double layer caused by cyclic stress produced the non-faradic cyclic current response.And whether its amplitude was linearly related to the peak stress or not can be used to jugde the sample surface deformed or not.The variation of electrochemical noise indexes indicated that the inclination to localized corrosion increased with cycles increased.Along with the peak stress increased,transformation from uniform corrosion to pitting corrosion occurred at the range of 0.8?0.95 ?p0.2,inducing the transformation of CFCI behavior and mechanism.(3)The variation of CFCI life and mechanism with cathodic potential varied with the peak stress.When the peak stress was close to or above the proof stress,the CFCI life first increased then decreased along with cathodic potential decreased and reached the maximum at about-850 mV.In range of OCP?-900 mV,the CFCI mechanism was controlled both by AD and hydrogen-induced localized plastic deformation(HILPD).In the range below-900 mV,AD was almost inhibited and HILPD became the controller.When the peak stress was far below the proof stress,the CFCI mechanism was controlled only by AD and the CFCI life always increased with cathodic potential decreased and quickly reached 107 cycles in range of-850?-950 mV.Moreover,both AD and HILPD would promote the crack initiating and propagating from PAGBs and BLBs.When they were both inhibited,the transgranular share increased in crack propagation,resulting in quasi-cleavage-dominated fracture morphology.(4)Hydrogen existed in E690 steel mainly as non-diffusible hydrogen.The non-diffusible hydrogen content was expressed as a function of hydrogen charging current density and peak stress.The synergy between hydrogen and cyclic stress was affected by peak stress.When the peak stress was far below the proof stress,the synergy was weak and proper hydrogen enhanced the toughnesss of E690 steel.When the peak stress was close to or above the proof stress,the synergy was strong and decreased the resistance to corrosion fatigue:hydrogen promoted the CFCI through hydrogen embrittlement and HILPD respectively as hydrogen charging current density was greater or less than 25 mA·cm-2.Moreover,the concentration degree of plastic deformation didn't reflect the fatigue damage cumulation induced by hydrogen embrittlement.(5)A model based on corrosion kinetics and equivalent initial flaw size theory was proposed for predicting the CFCI life of E690 steel in simulated seawater with considering the different CFCI mechanisms under high and low peak stresses.The curves predicted by the model fit the experimental data well.However divation under low peak stress still existed due to ignorance of the effect of stress concentration on corrosion kinetics.