High Resolution in Situ Studies of Localized and Crevice Corrosion with Multiple Beam Interferometry and Atomic Force Microscopy

Crevice corrosion (CC) and localized corrosion of metals remains a serious concern for structural materials. Yet both, a real-time and in-situ visualization of (1) initiation of localized corrosion, and (2) corrosion within confined geometries, remains very challenging. In this contribution, we will discuss how multiple-beam interferometry (MBI) can be utilized to directly visualize corrosion processes in real-time and with Ångstrom resolution within well-defined confinement geometries^[1]. Also, we will discuss how electrochemical AFM can be utilized to study localized phenomena such as dealloying on patterned surfaces^[2]. We will particularly aim to discuss and describe techniques and electrochemical cells, that we recently developed and will discuss how they may become useful in the field of corrosion and localized corrosion.

With MBI, we can detect and track active sites of aluminum corrosion in NaCl solution within confined geometries^[3]. We find that CC of aluminum randomly initiates in the confined crevice mouth, where the distance between apposing surfaces is between 20-300 nm. We can directly track oxide dissolution/formation, and corrosion-rates as well as their retardation due to sodium vanadate inhibitors present in solution. Formation of a compacted oxide layer effectively inhibits CC in 5 mM NaCl solutions with 2.5 mM of added NaVO₃, while inhibition rapidly breaks down at chloride concentrations above 50 mM. Breakdown of the inhibition-layers is initiated by rapid dissolution of the protective oxide within the confined zone.

With our newly developed electrochemical AFM cell we can accurately track localized initiation sites of corrosion in model de-alloying experiments on patterned surfaces with nm resolution in x/y and atomic resolution in vertical directions^[4]. The time resolution of our EC-AFM is currently limited to about 10 seconds per frame. In both parts of this contribution we will discuss experiments with model systems and we aim to specifically focus on technical hurdles and pros/cons of our techniques, including an outlook into what may be achieved in the future.

References:

[1]   M. Valtiner, X. Banquy, K. Kristiansen, G. W. Greene, J. N. Israelachvili, Langmuir 2012, 28, 13080.

[2]   M. Valtiner, G. N. Ankah, A. Bashir, F. U. Renner, Review of Scientific Instruments 2011, 82, 023703.

[3]   B. R. Shrestha, Q. Hu, T. Baimpos, K. Kristiansen, J. N. Israelachvili, M. Valtiner, Journal of the Electrochemical Society 162(7), 2015, C327.

[4]   B. R. Shrestha, A. Bashir, M. Valtiner, F. U. Renner, Faraday Discussion - Corrosion 2015, in press.