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Surface chemistry and diffusion of trace and alloying elements during in vacuum thermal deoxidation of stainless steel
Authors:Lin Zhu  Ali Al-Sakeeri  Filip Lenrick  Oskar Darselius Berg  Per Sjödin  Alexei A Zakharov  Axel Knutsson  Anders Mikkelsen
Institution:1. MAX IV Laboratory, Lund University, Lund, Sweden;2. Department of Physics, Lund University, Lund, Sweden;3. Department of Mechanical Engineering, Lund University, Lund, Sweden

Materials Technology & Chemistry, Alfa Laval, Lund, Sweden;4. Materials Technology & Chemistry, Alfa Laval, Lund, Sweden

Abstract:Removal of the native surface oxide from steel is an important initial step during vacuum brazing. Trace and alloying elements in steel, such as Mn, Si, and Ni, can diffuse to the surface and influence the deoxidation process. The detailed surface chemical composition and grain morphology of the common stainless-steel grade 316L is imaged and spectroscopically analyzed at several stages of in-vacuum annealing from room temperature up to 850°C. Measurements are performed using synchrotron-based X-ray photoemission and low-energy electron microscopy (XPEEM/LEEM). The initial native Cr surface oxide is amorphous and unaffected by the underlying Fe grain morphology. After annealing to ~700°C, the grain morphology is seen at the surface, persisting also after the complete oxygen removal at 850°C. The surface concentration of first Mn and then Si increases significantly when annealing to 500°C and 700°C, respectively, while Ni and Cr concentrations do not change. Mn and Si are not located only in grain boundaries or clusters but are distributed across over the surface. Both Mn and Si appear as oxides, while Cr oxide becomes metallic Cr. Annealing from 500°C up to 850°C leads to the removal of first the Mn and then Si oxides from the surface, while Cr and Fe are completely reduced to metals. Deoxidation of Cr occurs faster at the grain boundaries, and the final Cr metal surface content varies between the grains. The findings are summarized in a general qualitative model, relevant for austenite steels.
Keywords:annealing  deoxidation  diffusion  LEEM  microscopy  PEEM  steel alloys  surface  vacuum  XAS  XPS
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