A Study on Corrosion Behaviors for Life-Prediction of Structural Materials in Lead Fast Reactors

Abstract

A corrosion study has been made in this thesis in order to predict corrosion behaviors based on mechanistic understanding for typical materials of construction, including ferritic / martensitic stainless steel, T91 and HT9, as well as austenitic stainless steels, SS316L. Corrosion tests were performed at temperatures up to 600℃ in order to understand the corrosion behavior in lead-bismuth eutectic liquid. Based on test results, the process of corrosion at temperatures up to 600℃ is understood, as follows: 1. The formation of duplex oxide layers (outer magnetite layer and inner Fe-Cr spinel) and their growth with time 2. Removal of outer magnetite accompanied with lead penetration 3. The fracture of inner Fe-Cr spinel (corrosion barrier) due to mechanical stresses with oxide growth 4. Re-passivation of Fe-Cr spinel by oxidation 5. The failure of Fe-Cr spinel layer by Cr-depletion along grain boundaries 6. The start of dissolution: LBE penetration into metal substrate through grain boundaries In order to avoid active corrosion, two criteria based on relationship between Cr-depletion and passivity of Cr-oxide are proposed as followings

 Allowable operation temperature limit for studied materials predicted by thermochemical calculation of re-passivity considered chemical composition of structural materials  Operation time to loss of structural integrity by oxide failure is obtained from corrosion test result. Conventional steels forming Cr-oxide as protect layer can give good performance in lead or lead alloy at temperatures up to about 470℃ before oxide layer grows by about 44μm for ferritic / martensitic steels (T91 and HT9) and about 25μm for austenitic stainless steels (SS316L). Al-containing steels or Si-containing steels forming Al-oxide or Si-oxide as corrosion barrier should be applied to lead or lead alloy at temperatures above 470℃ due to their excellent corrosion resistance. Considered operation conditions of under developing LFR, such as MYRRHA, corrosion problem can be overcome by proven ferritic / martensitic steel and austenitic stainless steels. Furthermore, Al-containing steels or Si-containing steels can be commercialized by methods of coating or functionally graded composite in the near future.