Use of localised techniques to elucidate the influence of process variables on the corrosion of stainless steels
Abstract: Several factors during steel manufacture and fabrication can alter the passivity and corrosion behaviour of stainless steels. These include alloying, deformation, welding and heat treatments.In this work a combination of local techniques, such as Scanning Vibrating Electrode Technique (SVET) and Scanning Kelvin Probe Force Microscopy (SKPFM), and surface and microstructural analytical techniques, such as Electron Backscatter Diffraction (EBSD), Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) is used to elucidate the influence of process variables on the corrosion resistance of stainless steels. Different manufacturing processes, such as casting, rolling or additive manufacturing (AM) (Paper V), fabrication processes, such as welding (Paper III) and deformation (Paper I and IV), and post-processing, such as heat and high pressure treatments (Paper II and V) and post-weld cleaning (Paper II) are studied.The results showed that deformation can have a major impact on microstructure but a smaller impact on corrosion. Plastic deformation decreased irreversibly the Volta potential, whereas elastic deformation did not have any permanent effect. The potential was dependent on composition, passive film thickness and deformation but not on the crystallographic orientation. Thermal oxides formed after welding were detrimental to corrosion resistance. SVET showed anodic activity on the weld areas with oxides where pitting initiated, explained by the interplay between the composition and the thickness of the oxides. Brushing combined with pickling was recommended for restoring the corrosion resistance. Long-term ageing of a Cu-alloyed 2507 caused precipitation of Cu-rich ε particles which showed the lowest potential and were most severely pitted, plus nitrides which were the most noble. Intermetallic sigma phase observed in the AM 2507 after HIP heat treatment with slow cooling resulted in a decreased corrosion resistance. However, it was concluded that AM 2507 exhibits similar or even higher corrosion resistance than conventional 2507 if appropriate post-processing parameters are applied.This thesis provides scientific insights to facilitate the correct manufacturing, fabrication and use of stainless steels.
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