Corrosion studies that couple electrochemical methods with ion beam or neutron beam techniques, in situ. ​

ABSTRACT 

J.J. Noël, H. Feltham, M. Brocklebank, A. Situm, V. Kabanova, A. Vorobiev, E. Bergendal, G.K. Palsson, L.V. Goncharova, and Z. Tun.

Neutron scattering methods, particularly neutron reflectometry, and ion scattering techniques, such as Rutherford backscattering spectrometry (RBS), medium-energy ion scattering (MEIS), and other accelerator-based methods such as nuclear reaction profiling (NRP), can provide high-resolution depth profiles of solid surfaces and near-surface buried regions that are difficult or impossible to obtain by other means. Applying these analytical methods to a material in situ, while it is undergoing corrosion processes, can yield valuable information about the composition and structure of the interfacial region, including corrosion product layers and the subsurface of the specimen. However, neither neutron beams nor ion beams are compatible with aqueous electrochemical environments without special innovations.

Neutrons pass easily through substantial amounts of many types of materials, but the hydrogen present in the water of an aqueous electrolyte solution is a strong incoherent scatterer of neutrons, so the neutron beam used for in situ neutron reflectometry studies of corrosion processes must not pass through water on its way to probing the electrode/electrolyte interface. To overcome this inconvenience, we deposit a thin film of the material of interest in our corrosion experiments on a Si substrate, which we mount on an electrochemical cell such that the specimen is in contact with the electrolyte solution while the neutrons access the specimen and proceed to the detector by passing through the back of the substrate, without ever entering the water.

Likewise, ions are easily deflected by collisions with gaseous atoms and molecules in the atmosphere, so ion beam techniques are used within a chamber under ultra-high vacuum, which is incompatible with an aqueous corrosion environment. Nevertheless, we have developed an approach to enable in situ corrosion studies using ion beams by constructing an hermetically sealed electrochemical cell that can be filled with aqueous electrolyte at atmospheric pressure, then installed in the UHV chamber of an ion accelerator to enable ion beam analyses. We deposit a thin film of the material of interest on a thin SiN substrate that can act as a “window” for the ions and make this one wall of our cell. The ions then access the specimen from the back side, and those that eventually contribute to the analysis also return through the SiN window, exiting the specimen toward the detector after interacting with the material of interest.

This talk will describe the apparatus and approaches, as well as some of the experimental complications encountered, such as water radiolysis and high background signals, illustrated with examples from the ion beam measurements we have made on titanium in aqueous NaCl solutions under both open-circuit and polarized conditions and the neutron reflectivity measurements we have made in conjunction with electrochemical impedance spectroscopy to detect and quantify hydrogen absorption on corroding Ti, Zr, and Cu.