Stress Corrosion Behavior Of T91 Steel In Static Lead-Bismuth Eutectic With Oxygen Control

The surface of materials can form a protective oxide scale in Lead Bismuth Eutectic(LBE)with proper oxygen concentrations.However,the oxygen concentration and the stress can destroy the protection of the oxide scale,and it can not control the Liquid Metal Embrittlement(LME)any more,which endangers the safe operation of the reactor.To study the stress corrosion behaviors of T91 steel,one of the candidate structural materials for lead-based reactor in high-temperature LBE,the oxygen concentration and the stress as key factors are proposed in this thesis.The main research contents and conclusions are as follows:1)The pre-exposed experiments of T91 steel in stagnant LBE with different oxygen concentrations(i.e.10-5 wt%,10-6 wt%and1O-7 wt%)at 550℃ for 500 h were conducted first.Then the slow strain rate test of pre-exposed T91 steel in LBE at 400℃ with 10-5 s-1 was carried out.The results showed that the thicknesses of oxide scales were reduced from 24.6 μm to 8.5 μm,the ductilities of the specimens were reduced from 21.9%to 19.9%,and the LME became more obvious with the decline of oxygen concentration.The analysis suggested that the oxide scales were instability under the lower oxygen concentration and lost protective to the sample surface.When LBE contacted directly with the matrix,the Pb and Bi atoms reduced the cohesive strength of interatomic bonds and accelerated the LME.2)The stress corrosion experiment for small tapered conical specimen of T91 in LBE with an oxygen concentration of 10-6 wt%at 550 ℃ under 0～180 MPa was carried out.The results showed that the cracks kept extending to the inner layer with the stress increasing,and ran through the whole(Fe,Cr)3O4 ultimately.In addition,the thicknesses of oxide scales also increased rapidly,and reached to 43.8 μm when the stress was]80 MPa.According to the mechano-chemical coupling model,the tensile stress did not affect the oxide scales directly,but it affects the internal stresses of oxide scales via changing the creep behavior of the matrix.The number of the defects and the velocity of diffusion increased with the tensile stress increasing,which enhanced the growth of oxide scales.But when the internal stress reached to critical value,so the oxide scale would crack or fall off.The above results are helpful to service behavior prediction of T91 in LBE and to the safety analysis and design of LBE cooled fast reactors.