Electrochemical monitoring of ageing embrittlement of ferritic steel for high temperature reactor pressure boundary component

Electrochemical method was studied to apply it for the nondestructive evaluation of ageing embrittlement of 2 1/4 Cr-1Mo steel for the reactor pressure vessel of the high temperature gas-cooled reactor (HTGR) which is under construction in Japan. Isothermal ageing were conducted at temperatures from 400 deg. C to 500 deg. C for up to 50000 h for this study. Electrochemical test specimens with dimensions of 10x10x5 (mm) were taken from the undamaged portion of Charpy V-notch impact bars. In the anodic polarization curves measured in 55% Ca (NO3)2 solution, the embrittled materials show the higher secondary peak of the current density in the passive potential region. This characteristic increase in the current density corresponds to intergranular segregation of phosphorus which prevents the Formation of passive layer at grain boundaries, and the degree of the increment can be correlated with the shift of Charpy ductile to brittle transition temperature (DBTT). It is also suggested form the comparison of materials isothermally aged with and without applied stress that the enhancement of phosphorus segregation due to an applied stress is only recognized after 3000 h ageing and is diminished with increasing ageing time. (author). 13 refs, 14 figs, tab