Simultaneous Interferometry and Microscopy Reveal Unexpected Nickel Corrosion Dynamics in Confined Geometry

Abstract

Corrosion of nickel and its alloys in confined areas remains a serious concern for the safety of structural materials. We uniquely combined white light interferometry and microscopy with high spatial and time resolution for a real-time study of the corrosion mechanism of confined nickel surfaces in alkali metal chloride (e.g., NaCl) electrolytes (c = 1 – 100 mM) at pH = 7. A mica surface is used as crevice former in the electrochemical surface forces apparatus to establish 600-1000 µm2 confined dry areas on extended nickel surfaces. NaCl is the wetting medium of the confined zones and the electrolyte layer spreads out along all the surface. The nickel surfaces form interfacial oxides films at the beginning when no electrochemical potential is applied. Subsequent application of an electrochemical potential ramp from -400 mV to +800 mV initiates severe corrosion of the formed crevices. Surprisingly, the corrosion inside the crevices proceeds as preferential and self-catalyzed “pitting” corrosion (inhomogeneous localized dissolution with the formation of pits) within in the confined zone. Moreover, different behavior of the Nickel corrosion is observed depending on the concentration: inside the confined zone at low concentration (<10mM) and outside the crevice at higher chloride concentrations (>10mM). Our data reveal a surprising and detailed view into the initial corrosion mechanism of nickel surfaces in confined zones, and may contribute to a deeper understanding and, ultimately, prevention of localized corrosion of other metallic surfaces. A double-layer theory coupled with diffusion-limited effects provides a plausible explanation for the observed phenomena.