Lean duplex stainless steels (DSS) combine good corrosion resistance with improved mechanical properties compared to austenitic counterparts, making them key candidate materials for cost savings through weight reduction designs. However, the occurrence of localised corrosion and environmentally assisted cracking (EAC) is not fully understood, and is still one of the most challenging problems for service applications. The purpose of this PhD project is to provide a better mechanistic understanding of the nucleation and propagation of EAC in lean DSS. The occurrence of pitting corrosion as a precursor of EAC is assessed, followed by an in-situ study to observe EAC nucleation and growth by X-ray Computed Tomography (X-ray CT), with a final chapter exploring the application of Diffraction Contrast Tomography (DCT) for observing hydrogen related microstructure changes, in-situ, during cathodic charging. The nucleation and growth of pitting corrosion in lean DSS were characterised via X-ray CT and cross-sectional SEM observation. All pits grew into semi-ellipsoid shapes, with large fractions of un-dissolved austenite grains remaining inside pits in heat-treated sample. The results revealed that lacy metal covers with holes formed by selective dissolution of the ferrite phase affects the pitting corrosion kinetics. The occurrence of EAC has been observed in heat-treated lean DSS via X-ray CT and cross-sectional microstructure analysis. The crack mainly propagated along ferrite-ferrite grain boundaries, with crack arrest observed at the austenite phase. Reduced microstructure resistance is attributed to hydrogen embrittlement effects. Laboratory-based DCT was applied to detect microstructural changes induced via hydrogen charging. DCT techniques for the in-situ study of dual phase material exposed in an aqueous environment was implemented. Changes of diffraction spot positions were observed after hydrogen charging, which cannot be explained by sample movement. The DCT work reported in here outlines the pathway to successfully continue this project. The work reported in this PhD thesis therefore contributes to a far better understanding of localised corrosion and associated EAC behaviours of lean DSS microstructures.
|Date of Award
|1 Aug 2021
- The University of Manchester
|Dirk Engelberg (Supervisor) & Timothy Burnett (Supervisor)