Modeling of SCC initiation and propagation mechanisms in BWR environments

During operation of mainly BWRs’ (Boiling Water Reactors) excursions from recommended water chemistries may provide favorite conditions for stress corrosion cracking (SCC). Maximum levels for chloride and sulfate ion contents for avoiding local corrosion are therefore given in respective water specifications. In a previously published deterministic 288 °C – corrosion model for Nickel as a main alloying element of BWR components it was demonstrated that, as a theoretically worst case, bulk water chloride levels as low as 30 ppb provide local chloride ion accumulation, dissolution of passivating nickel oxide and precipitation of nickel chlorides followed by subsequent local acidification. In an extension of the above model to SCC the following work shows that, in a first step, local anodic path corrosion with subsequent oxide breakdown, chloride salt formation and acidification at 288 °C would establish local cathodic reduction of accumulated hydrogen ions inside the crack tip fluid. In a second step, local hydrogen reduction charges and increasing local crack tip strains from increasing crack lengths at given global stresses are time stepwise calculated and related to experimentally determined crack critical cathodic hydrogen charges and fracture strains taken from small scale SSRT tensile tests pieces. As a result, at local hydrogen equilibrium potentials higher than those of nickel in the crack tip solution, hydrogen ion reduction initiates hydrogen crack propagation that is enhanced with increasing global stresses. In accordance with respective experimental literature data it is shown that decreasing chloride and increasing pH levels of the primary bulk water at 288 °C reduce the total crack propagation rates including anodic path corrosion as well as hydrogen cracking. It is also demonstrated that crack propagation rates can be significantly suppressed by hydrogen water chemistry (HWC) that leads to reduction of bulk surface corrosion potentials. As a conclusion the extended SSC-model for nickel supplies quantitative insight into the frequently controversially discussed high temperature SCC mechanisms of a basic alloying element of BWR components.