Abstract
As from frequent observations Stress Corrosion Cracking (SCC) of nuclear reactor components may be initiated and its propagation also controlled by local pitting and crevice corrosion. Deterministic modeling of local corrosion including incubation times for crevice corrosion and related anodic acidification should therefore provide a basis for component lifetime predictions. Based on previous work for room temperature (RT), the crevice corrosion at 288°C of nickel as an important base element in respective high alloyed nuclear materials is modeled by coupling anodic polarization with formation of nickel oxide and chloride calculated from the water-hydrogen-nickel-chloride heterogeneous phase equilibrium diagram. As a boundary condition, hydrogen treatment of irradiation subjected cooling water for reduction of corrosion potentials and mitigation of SCC in Boiling Water Reactors (BWR) is included. Assuming equilibrium conditions at operating temperature 288°C in a relevant component crevice the time step wise calculated development of the critical crevice solution is quantitatively shown to initiate crevice corrosion by break down of the passive nickel oxide layer followed by formation of non passive nickel chloride and the subsequent acidification of the crevice solution. The effects of bulk hydrogen fugacity, bulk levels of pH, chlorides, oxygen and different crevice widths are investigated. As a result, the additions of hydrogen and respective reduction of corrosion potentials provide significant increases in the passive layer break down times and acidification times inside the crevice. Increasing bulk oxygen levels and decreasing bulk pH reduce the calculated degradation times. Increasing bulk chloride levels between 1E-09% and 1E-05% are shown to accelerate crevice corrosion while, above this range, no significant effect of further increased chlorides was detected. For a standard 10000 h time for crevice acidification to less than pH=1 the respective chloride - hydrogen fugacity domains are evaluated including the effects of oxygen and pH. Such diagrams may be related to respective effects on stress corrosion cracking and its mitigation by hydrogen water chemistry (HWC).
