Electron Microscopy of Oxide Formed on Nickel Alloy X-750 in Simulated Boiling Water Reactor Environment

Doctoral thesis, 2017

The environment of a nuclear reactor is hostile and the materials used have to withstand harsh and corrosive working conditions. Nickel alloy X-750 has been extensively used in the nuclear industry for the last 30 years. The good corrosion properties and the high strength at elevated temperatures made this material a good choice for springs, bolts, guide pins and spacer grids in boiling water reactors (BWRs). The focus of this thesis is on the spacer grid corrosion. A spacer grid is a metal lattice that holds the nuclear fuel in position. It is critical to limit the corrosion of the spacer grids so that the fuel is held stable for safety purposes and also since corrosion and dissolution of the alloy can lead to spread of radiation in the closed water system and consequently an increased risk of radiation exposure for maintenance workers at nuclear plants. A BWR operates at 286°C under a pressure of 70 bar, and the radiation induces radiolysis of the water, making it corrosive. Significant corrosion of alloy X-750 has been observed under these conditions. In order to study the high temperature water corrosion of alloy X-750, this aggressive environment was simulated in an autoclave system. In this thesis, the connection between the corrosion performance and the effect of pretreatments and the impact of iron content of the alloy is studied. The evolution of the oxide growth was studied by exposing the specimens in a temporal range from 2 h to 840 h. The samples have been studied with a set of complementary characterization techniques (mainly electron microscopy, but also X-ray diffraction and atom probe tomography), and focused ion beam milling has been used for sample preparation. Pre-oxidation treatment results in the formation of a thin multi-layer oxide, which improves the corrosion properties of the alloy, leading to contained dissolution of the metal. The oxide formed is composed of an outer layer of trevorite (NiFe2O4), a Cr-rich middle layer which is Ni-chromite in the outer part and chromia with some titania in the inner part. Of these oxides only trevorite is stable in BWR water and when comparing the 5 wt% Fe alloy with 8 wt% Fe, the latter performs better (i.e. dissolves and oxidizes less) as a result of a higher quality (thicker and compact) trevorite layer created by pre-oxidation. Since the pre-oxide layer could be lost during operation, it is important to know what will happen to the corrosion of alloy X-750 in this condition. In order to answer this question, the oxide evolution of non-pre-oxidized alloy X-750 has also been studied by comparing samples exposed for different times. During exposures in an autoclave, trevorite layer consisting of blocky crystals forms on the surface by re-precipitation of the dissolved metal. At the same time underlying metal is oxidized, forming a nano-grained and porous oxide layer. It has been shown that the dissolution rate and the growth rate of the oxides decrease with time, indicating that the formed oxides offer some degree of protection. Moreover, the alloy with higher Fe content corrodes less when exposed to longer exposure time, which makes it a preferable candidate for reactor operations. This thesis constitutes a step on the way to understand BWR corrosion of alloy X-750, which eventually will make it possible to develop improved spacer grid materials.