Depth Profiling of the Passive Layer on Stainless Steel using Photoelectron Spectroscopy

Abstract: The physical properties of the protective passive films formed on the surface of stainless steels under electrochemical polarization in different electrolytes were studied. The structure of these films was analyzed as a function of depth using photoelectron spectroscopy (PES).Depth profiling (using PES) of the surface layer was achieved by either changing the angle of incidence to achieve different analysis depths (ARXPS), by argon ion etching, or by varying the energy of the incoming x-rays by the use of synchrotron radiation. The use of hard x-rays with high resolution (HAXPES) provided novel quantified information about the nickel content underneath the passive films.A complex environment was found in these surface layers composed of an outermost monolayer of iron on top of a layer of chromium hydroxides covering an underlayer of chromium oxides. Molybdenum was enriched in the interface between the metal and oxide. Nickel is enriched underneath the passive film and therefore nickeloxides are only present in the surface layer in low concentrations.A comparison was performed on austenitic and duplex stainless prepared by hot isostatically pressed (HIP) or cast and forged processes. HIP stainless steel was produced using the burgeoning technique of pressing gas atomized powders together. The structure of these steels is far more homogenous with a lower porosity than that of the conventionally prepared equivalents. It was shown that hot HIP austenitic steel had better pitting corrosion resistance than its conventional counterpart.Finally, the duplex steel was cycled in a Li-ion battery to explore its potential application as a current collector. It was shown that the passive film formed in the organic solvents is similar in composition and thickness to the films formed in aqueous solutions. However, it is doubtful if steel could be used as current collector in batteries due to its high reactivity with lithium.