Atom probe characterization of corroded Alloy 600

Poster presented at M&M 2013:

Internal oxidation is believed to be the main cause for primary water stress corrosion cracking in nickel-based Alloy 600. In order to elucidate the corrosion mechanism, we used atom probe tomography (APT) to investigate the near-surface nanostructure, within grains and at grain boundaries, of an Alloy 600 sample corroded in hydrogenated steam at 480oC to mimic the behavior in service (hydrogenated water at ca. 320-330oC). The composition of the alloy is reported in Table 1. The samples were prepared from a mill-annealed tube that was solution-annealed at a higher than usual temperature to grow the grain size, and were oxidized for 96h at a calculated (effective) partial pressure of O2 very close to the dissociation pressure of NiO.

Specimens for APT from the near-surface of the sample were prepared with a FIB/SEM using the conventional lift-out procedure.  APT experiments were performed on two different Cameca LEAP 3000X HR, in laser-pulsing mode (200kHz, 532nm, 0.6–0.8nJ), at a base temperature of 60K, and an average detection flux of 5x10-3 ion/pulse. Datasets containing 5–30 M ions were acquired and reconstructed using the IVAS software suite. When possible the reconstruction was calibrated based on the pole figure and interplanar spacing.