Study Of Corrosion And Stress Corrosion Behavior Of High Strength Steel In Simulated Deep-Sea Environment

High-strength steel has the advantages of high strength,good toughness,excellent corrosion resistance,and relatively low cost,so it is widely used in ships and marine engineering equipment.At present,deep-sea engineering has increasingly strong demand for high-strength steel,and related research work and basic data on the impact of deep-sea environmental factors on its corrosion performance and service behavior are still lacking and urgently need to be supplemented.Therefore,this paper takes a certain marine high-strength steel(hereinafter referred to as S25 steel)as the research object,and investigates the hydrostatic pressure(0.10 MPa,10 MPa and 20 MPa),dissolved oxygen(3.5 mg/L,4.5 mg/L,5.5 mg/L).L and 6.5 mg/L)and temperature(5℃,25℃ and 35℃)on the corrosion behavior and service behavior of S25 steel.Taking into account the difficulty of obtaining basic corrosion data under deep sea conditions,the author also used COMSOL software to simulate the corrosion of the load-bearing specimen under hydrostatic pressure.The results of weightless corrosion in laboratory simulation deep-sea experiments show that the corrosion rate of S25 steel under high hydrostatic pressure(10 MPa and 20 MPa)is roughly twice that of low hydrostatic pressure(0.10 MPa);the corrosion rate decreases with the immersion time.However,the corrosion rate drops faster under high hydrostatic pressure conditions.The polarization curve test results show that the hydrostatic pressure mainly reduces the self-corrosion potential of S25,which makes the corrosion resistance of S25 steel worse.In the later stage of corrosion,the cathodic reaction rate will be accelerated,and a passivation-like step will be produced at 20 MPa.The composition of the corrosion products shows that hydrostatic pressure promotes the transformation of S25 steel corrosion products,and under high hydrostatic pressure,it is beneficial to generate more corrosion productsα-Fe OOH,but the corrosion product layer is thinner.The results of the weightless corrosion test of 1 year immersion in the real sea show that the corrosion rate of S25 steel decreases with the increase of water depth,which proves that the influence of hydrostatic pressure on the corrosion behavior of S25 steel is not an independent factor,but is also related to other environmental factors;Under the conditions of1200 m and 2000 m water depth,the corrosion products of the real sea samples did not containβ-Fe OOH,but contained more stable α-Fe OOH.According to the morphology of corrosion products,it is found that both dissolved oxygen content and temperature will accelerate the formation of corrosion products of S25 steel,and these two environmental factors and applied stress mutually promote the corrosion reaction.However,the influence mechanism of the two is different.The polarization curve test results show that when both the dissolved oxygen content and the applied stress increase,the self-corrosion current density of S25 steel will increase,and the phenomenon of pseudo passivation will occur;while the temperature and applied stress increase,Will make the self-corrosion potential of S25 steel negatively shift.When the hydrostatic pressure increases and coupled with external stress,the corrosion products on the surface of S25 steel will increase,and the increase of hydrostatic pressure will cause serious corrosion of the stress concentration parts.However,the polarization curve test shows that the increase in hydrostatic pressure leads to an increase in the cathodic reaction rate of S25 steel and inhibits the anodic reaction,while increasing the applied stress will increase the anode dissolution rate.Therefore,the influence of hydrostatic pressure and applied stress has the effect of mutual restraint,and its harm is not as great as the damage of high dissolved oxygen,high stress and high temperature and high stress to the material.Further through simulation,it is found that the current density distribution of S25 steel is different when the surface stress state of S25 steel is different.As the stress increases,the maximum current density expands from the center line to both sides.It can provide reference and scientific basis for predicting the location of stress corrosion initiation.