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Hans Hoffmeister , Oliver Klein
Nuclear Engineering and Design, Volume 241, Issue 12, December 2011
Abstract
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.
The diagram visualizes the calculated simulation effect of pH and chloride contents on SCC rates of nickel at 288°C at a corrosion potential of -360 mV/SHE and a global stress of 100MPa. Numerical simulation of crack tip electrochemical conditions includes corrosion rates, oxide and chloride precipitation, anodic polarization and acidification leading to local cathodic hydrogen reactions. Simulation of crack tip mechanical conditions include development of local plastic strain, formation of the plastic zone ahead of the crack tip and hydrogen induced crack propagation. The link between electrochemically produced cathodic hydrogen and hydrogen induced fracture strain is experimentally provided from high temperature Slow Strain Rate Tests under cathodic hydrogen charging. Respective hydrogen ion reduction charges and fracture strains are implemented for calculation of hydrogen induced crack propagation.
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