Oxidation Study of Incoloy 800H Tubes Exposed to Super-Heated Steam and Supercritical Water

Abstract

Incoloy 800H, a high-nickel austenitic alloy, has great merit for use in Supercritical Water (SCW). The development of the CANada Deuterium Uranium (CANDU) nuclear Supercritical Water Reactor (SCWR) was the driving force behind this research to investigate the oxidation performance of Incoloy 800H for use as fuel cladding.The oxidation behaviour of Incoloy 800H was investigated in Super-Heated Steam (SHS) (625 o C, 0.1 MPa) for a duration of 500 hours and, again, for 1,000 hours, and in SCW (625 o C, 26.14 MPa) for 500 hours using novel tube specimens. Four specimen configurations were prepared to have a combination of Open-Ends (OE) or Closed-Ends (CE) with either a Bare Surface (BS) or a NiCrAlY Coated Surface (CS). The OE and CE specimens experienced different through-thickness stress distributions due to their testing environment, which allowed comparison and characterization of their performance with regard to oxidation and stress.Standard mass and dimension change measurements performed on the test specimens were observed to be unreliable for characterizing the relative oxidation between different test environments and specimen configurations. Detailed metallographic studies were made with the aid of optical microscopy, Scanning Electron Microscopy (SEM), Energy Dispersion Spectrometry (EDS), and X-Ray Diffraction (XRD), in addition to mass and dimensional measurements. Finite Element Analysis (FEA) was performed to determine the effect that specimen configuration and testing environment have on the stress distributions within the specimens.Testing showed that BS specimens in SHS and SCW produced sufficiently similar oxidation behaviours and oxide scales; spallation of the outer Fe-rich oxide layer was believed to have occurred. The CS specimens formed a thin and discontinuous oxide under both SHS and SCW. The CE/BS tube yielded and collapsed when exposed to SCW with iv cracks and 'folds' vulnerable to oxidation forming on the collapsed surfaces. The NiCrAlY coating provided sufficient strength to reinforce the CE/CS tube preventing its collapse in SCW conditions.From the perspective of oxidation performance, it has been determined that SHS (625 o C, 0.1 MPa) is a suitable medium for investigating the oxidation behaviour of Incoloy 800H in SCW (625 o C, 26.14 MPa) and would provide comparatively conservative results.The coating affords the tube additional protection from both oxidation and stress, providing the potential of extending the operating life of the Incoloy 800H tubes. v I am grateful for the support and expertise of