Corrosion resistant alloys (CRAs) are used as materials of construction for canisters containing nuclear waste and have been proposed as waste forms for certain HLWs (e.g., pyrochemical metals and isolated 99Tc). CRAs are Fe- or Ni-based alloys that achieve their superior corrosion properties through the development of a thin protective surface oxide film, called a passive film. Unfortunately, CRAs are susceptible to rapid attack in the form of localized corrosion such as pitting, crevice corrosion, and stress corrosion cracking (SCC) under conditions where the passive film breaks down locally. Improved CRAs could alter the nature and performance of any storage or disposal facility. The metals projects will study CRAs and use Integrated Computational Materials Engineering (ICME) to develop new and improved CRAs. This has never before been attempted.
Project M1: Tailored Alloying in Corrosion Resistant Alloys (CRAs) and Development of New CRAs.
The goal is to develop the fundamental understanding underlying the corrosion resistance of CRAs through first principles, thermodynamic, and kinetic modeling, as well as experimentation. Approaches to increasing corrosion resistance through alloying will be addressed and the opportunities associated with the novel and tailored compositions and structures possible in Bulk Metallic Glasses and High Entropy Alloys will be investigated. The modeling will adopt DFT approaches and CALPHAD thermodynamic modeling to describe and predict the alloy microstructure as functions of composition and processing. The stability of passive films will be modeled using CALPHAD and localized corrosion will be addressed using a kinetic model informed by first principles calculations. Experiments will be aimed at a) providing guidance to the modeling efforts, b) testing alloy systems recommended by the modeling activities, and c) identifying the controlling factors and mechanisms governing the corrosion behavior of novel CRAs. Some of the experimental and simulation approaches will be common with the other two materials classes. The approach for this project is summarized in Figure 1.
Project M2: Atmospheric Pitting and Cracking of Stainless Steels.
This project will address the chemical and electrochemical attributes enabled by deliquescing environments on salt-contaminated SS during exposure in humid conditions; controlling factors in pit formation and transition to SCC; and development of cracks from pits generated by various exposure and metallurgical conditions. This project is tied to M1 by including the study of the new alloys developed in that project. The intent in starting examination in stainless steels is to select an alloy that will crack in a reasonable amount of time enabling the team to study the mechanism(s) driving atmospheric pitting, pit-to-crack transition, and H embrittlement. The project will also have a stronger focus on modeling. Specifically, results on stainless steel will be incorporated into DFT and Bayesian probabilistic models to enrich the understanding of the mechanisms driving pit-to-crack transition, hydrogen embrittlement, and possible failure in CRAs that will be investigated later in the project.