PNNL

Abstract

Stress-corrosion crack growth has been investigated for nickel-base alloy 182 weld metal in simulated PWR primary water at 325oC as a function of dissolved H2 concentrations from 1.5 to 50 cc/kg. Stable and reproducible crack-growth rates were measured by direct-current potential drop at a constant K of ~35 MPavm and a H2 concentration of 29 cc/kg. After correction, this average propagation rate was determined to be ~7x10-9 mm/s. This was followed by a series of exposures at lower H2 concentrations. A peak in crack growth was observed at intermediate H2 levels apparently related to the Ni/NiO phase stability transition. Some differences in initial SCC response were seen at lower H2 concentrations depending on the prior test condition (whether in the NiO stable regime at 1.5 cc/kg or in the Ni-metal stable regime at 29 cc/kg). Average propagation rates increased from ~4x10-9 mm/s at 1.5 cc/kg H2 to ~2x10-8 mm/s at 5 cc/kg H2 and to ~3x10-8 mm/s at 12 cc/kg H2. These rates must be considered approximate due to the uneven crack front and the limited crack extensions (3.5 to 70 µm) at each condition, however higher propagation rates (5-10X) are clearly indicated at the intermediate H2 levels near the expected Ni/NiO transition.