A Research On Corrosion Fatigue Behavior Of Austenitic Stainless Steel In Seawater

The offshore floating reactor has great development potential and is expected to become the main energy supply for offshore resource exploration and mining platforms and marine vessels.China has already included the development of offshore floating reactors into the 13th Five-Year Plan.Austenitic stainless steels 304,316 and 321 are the commonly used materials in the nuclear power field and petrochemical industry,which own high strength,good corrosion resistance and good processing properties,and they are expected to become structural materials for offshore floating reactors.When equipments are operated in marine environment,they may directly contact the seawater directly or the chlorine-containing steam invades the surrounding environment of the equipments,so that the structural material of the equipment is in a high concentration of chloride ion environment.On the other hand,it may withstand the slap of seawater or the periodic oscillation of the material itself.Under the joint action of stress and corrosive medium,the structural material is prone to corrosion fatigue failure.In this paper,the direct current potential drop method is taken as the core test method.The corrosion fatigue crack initiation behavior of 304 stainless steel in seawater and the corrosion fatigue crack propagation behavior of 304,316 and 321 stainless steel in seawater are studied to evaluate their seawater corrosion resistance.In the study of corrosion fatigue crack initiation,the crack initiation curves of304 stainless steel at different stress levels in seawater were measured,and the crack propagation path and fracture morphology were microscopically characterized.The pitting pit growth and short crack propagation are performed modeling calculations.It is found that the stress level has a great influence on the corrosion fatigue crack initiation life,and the effect of corrosion on the crack initiation life changes with the change of stress level.In general,when the stress level drops,the material has a longer crack initiation life.The microscopic morphology indicates that the corrosion fatigue crack initiation mechanism of 304 in normal temperature seawater can be explained by the pitting-stress concentration model.The calculation of the growth stage of the pit has a large error,mainly because the model of the size and number of pits is not fine enough.The accuracy of the calculation of the short crack propagation stage is high,and the formula for obtaining the short crack life is N_{c f}=3.11×10_Cen^{10 1}(?σ_eqv^1.38-74.02^1.38)^-2.In the study of corrosion fatigue crack propagation,the crack growth rates of three austenitic stainless steels 304,316 and 321 were measured under different loading pa-rameters,environmental medium and temperature.The results show that the fatigue properties of three austenitic stainless steels 304,316 and 321 decrease in different degrees in normal seawater environment,which is manifested in the increase of crack growth rate.This is because the hydrogen embrittlement in seawater environment pro-motes the crack propagation.However,different materials have different resistance to corrosion fatigue crack propagation due to differences in composition and structure,and the performance of 321 seawater corrosion resistance is better than 304 and 316.Loading parameters such as loading ratio,loading frequency and stress intensity factor have an important influence on the corrosion reaction at the crack tip region.This force-chemical synergetical effect determines the crack growth rate.The Paris fatigue model has high applicability to austenitic stainless steels 304,316 and 321 in normal temperature seawa-ter.If the parameters are fitting well,it can predict the crack growth rate of the fatigue crack steady-state propagation stage.