Keynote: In-Situ Visualization of Non-Linear Phenomena during Metastable Pitting Corrosion on Stainless Steel

Stainless steels and many other alloys are designed to be corrosion resistant. Nevertheless they can undergo localized pitting corrosion, which may rapidly lead to their failure, with the possibility of catastrophic events as a result.  To study the initial steps of metastable pit formation in situ and in real time we adapted the ellipso-microscope for surface imaging (EMSI) [1] and a high resolution, contrast enhanced optical microscope to the electrolyte - stainless steel interface.  Utilizing these methods we have been able to explain the sudden corrosion onset by an explosive autocatalytic growth in the number of metastable pits [2]. 

By applying those different but complementary imaging methods simultaneously the correlation between oxide film weakening and the nucleation of individual pits could be confirmed.  The existence of front propagation as a component of the transition to pitting corrosion shows that characteristics of this process are consistent with the behavior of stochastic reaction-diffusion systems [3].

Recently we implemented digital in-line holography as a third imaging tool simultaneously to gather three-dimensional information within the electrolyte, in close proximity to the exposed surface. This enhances the sensitivity for small pitting events and opens new avenues for investigating and understanding the underlying processes. For instance we are now able to follow the trajectories of particles ejected from a pit [4].

Using this arsenal of in situ and real time imaging tools allows us to efficiently study methods to enhance the pitting corrosion resistance of stainless steels; initial results of simple treatments such as emerging the sample just in high purity deionized water at 90 °C for an hour, but nevertheless improving the pitting corrosion resistance in NaCl solutions dramatically, will be discussed [5].

[1]  H.H. Rotermund et al. Science 270, 608-610 (1995)

[2]  C. Punckt et al., Science 305, 1133-1136 (2004)

[3]  M. Dornhege et al., J. Electrochem. Soc. 154, C24-C27 (2007)

[4]  P.E. Klages et al., Corrosion Science 65, 128–135 (2012)

[5]  P.E. Klages et al., Electrochemistry Communications 15, 54–58 (2012)