Unrevealing corrosion mechanism in nanometer confined gaps using white light interferometry in reflection and transmission mode

Abstract

Corrosion is a natural process, a gradual destruction of materials (usually metals) by a chemical and/or electrochemical reaction with their environment. A study of the National Academy of Corrosion Engineering estimated the annual cost of global corrosion at $2.5 trillion. So, it is no wonder that a better understanding of corrosion phenomena is a crucial task in electrochemistry. Among all types of corrosion, its occurrence in confined spaces remains one of the most difficult types to detect and prevent. Most often crevice corrosion (CC) occurs in narrow fissures where oxygen access is poor and a stagnant electrolyte solution is present. Crevice corrosion is of particular concern for stainless steel and nickel alloys in contact with chloride solutions, commonly found in marine environments. Experimentally it is a challenge to obtain in-situ information about processes in confined geometries and to establish well defined confined situations in the first place. Multiple beam interferometry (MBI) was used here to study and monitor the initial stage of the CC process of thin semi transparent layers of nickel in situ. Using mica as a crevice former in an electrochemical surface forces apparatus (SFA) allowed us to provide a deeper understanding of the initiation of the corrosion process. This novel technique provides not only an accurate real-time monitoring of corrosion in confined geometries, but is also capable to track and quantify localized nanoscopic features of the corrosion mechanism, such as pit formation with nano-to-microscale resolution. For instance, the shape of single localized corrosion sites (e.g. pits) and their depth profile can be directly tracked in real-time, yielding localized corrosion current densities in a well-defined confined geometry and currents from inside and outside the confinement can be detected. Further, we successfully extend MBI from transparent thin film materials to mirror polished bulk materials. We demonstrate the capabilities of MBI in reflection mode and compare the initial CC mechanism on confined nickel and a Ni model alloy in real time. Here, the study of bulk material was coupled with other techniques to study the morphology of the corroded samples, such as atomic force microscopy (AFM) and nano-Laue diffraction to correlate the local crystal structure, surface defects, domain orientation and dissolution process during the initiation of a corrosive process in confinement, where differences between the pure nickel and the nickel alloy mechanism were identified. Finally, to better understand the mechanism of dissolution and formation of the oxide film in different nickel alloys, similar experimental conditions were reproduced in a newly constructed Avesta type flow-cell, using time-resolved inductive coupled plasma mass spectroscopy (ICP-MS) analysis. Anodic polarization well into the transpassive region allowed us to isolate the role of different elements in protecting the alloys and follow their dissolution in real-time.