Abstract
The modifications of the passive film formed on 316 L stainless steel surface during stepwise heating in ultra-high vacuum up to 300 °C have been studied in situ by ToF-SIMS. The pre-formed passive film (in 0.05 M H2SO4) has a bilayer structure, comprising Fe-rich, Mo-rich (outer) and Cr-rich (inner) layers. Below 100 °C, the passive film is stable. At 100 °C–250 °C, dehydroxylation and dehydration is observed. Above 250 °C, the main modification in the film is formation of chromium oxide at the expense of oxidized iron. At higher temperature, thicker Cr-rich inner layer with sharper inner/outer oxide interface is formed.
Highlights
Stainless steels are employed in a wide range of technical applications due to the formation of a protective surface oxide layer, that can provide high corrosion resistance of the material in aggressive environments [1, 2]
The passive films formed on stainless steel surfaces, typically a few nanometres thick at room temperature, are known to be duplex with an outer layer rich in iron oxide while the inner layer is enriched in chromium oxide [3, 4]
Under reduced oxygen partial pressures, at oxidation temperatures above 800°C, chromium oxide is the predominant layer on the stainless steel surface [9, 10], while for the oxide films formed at temperatures lower than 400°C, both the iron and chromium oxide were reported to grow on the SS surfaces [11,12,13,14]
Summary
Stainless steels are employed in a wide range of technical applications due to the formation of a protective surface oxide layer, that can provide high corrosion resistance of the material in aggressive environments [1, 2]. The passive films formed on stainless steel surfaces, typically a few nanometres thick at room temperature, are known to be duplex with an outer layer rich in iron oxide while the inner layer is enriched in chromium oxide [3, 4]. Oxidation of SS surface in air at most temperatures (temperatures lower than 800°C), leads to the formation of a duplex oxide layer, consisting of an outer iron oxide and an inner chromium rich layer [7, 8]. Under reduced oxygen partial pressures (lower than 10-3 mbar), at oxidation temperatures above 800°C, chromium oxide is the predominant layer on the stainless steel surface [9, 10], while for the oxide films formed at temperatures lower than 400°C, both the iron and chromium oxide were reported to grow on the SS surfaces [11,12,13,14]
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