In Situ Study of Stainless Steel’s Passive Layer Exposed to HCL Using a Scanning Tunneling Microscope
The nature of 304 stainless steel’s passive film and how it is altered as a function of exposure time in hydrochloric acid (HCl) acid has been studied using a scanning tunneling microscope (STM). This experiment offers the distinct advantage of in situ studies, on a microscopic scale, of the electrical character of the surface in the presence of a corrosive environment. In the present paper we present evidence which shows directly how the surface structure of 304 stainless steel is altered by HCl; simultaneously the surface is probed with a STM. In particular, the in situ current, I, versus bias voltage, V, curves were measured as a function of etchant exposure time. The important region, concerning passivation, is the extent and the slope of the I-V curve near where the current is zero; this region corresponds to the material showing corrosion resistance and exhibits a low or zero slope near where the current is equal to zero. The research establishes a quantitative measure of the extent of corrosion protection, stability, and the kinetics of the passive film. The findings of this research are correlated with nanoscale STM images of the 304 stainless steel’s surface.
KeywordsScanning Tunneling Microscope Passive Film Corrosion Protection Scanning Tunneling Microscope Image Passive Region
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