Crevice corrosion (CC) of engineering metals remains a serious concern for structural materials. Yet combining both, a real-time and high-resolution in-situ visualization of corrosion within confined geometries remains very challenging. In this contribution, we will discuss how multiple-beam interferometry (MBI) and simultaneous microscopy can be utilized to directly visualize corrosion processes in real-time and with Ångstrom resolution within well-defined confinement geometries. Here we will directly compare results of accelerated corrosion tests of confined Aluminum and Nickel surfaces in a modified surface forces apparatus¹.

With MBI, we can detect and track active sites of aluminum corrosion in NaCl solution within confined geometries in real time. 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.

Interestingly, Nickel CC shows distinctly different and unexpected corrosion mechanism depending on the NaCl concentration. We find that low Chloride concentration lead to “preferential pitting corrosion” inside of confined zones, while high Chloride concentrations lead to “crevice corrosion in the crevice mouth”. We will discuss these surprising results and we will show real time high-resolution videos of CC in confined geometries that detail this behavior.

Literature

1.      Shrestha, B. R.; Hu, Q.; Baimpos, T.; Kristiansen, K.; Israelachvili, J. N.; Valtiner, M., Real-Time Monitoring of Aluminum Crevice Corrosion and Its Inhibition by Vanadates with Multiple Beam Interferometry in a Surface Forces Apparatus. Journal of the Electrochemical Society 162 2015, C327-C332