Elucidation of corrosion phenomena with high lateral resolution using scanning probe techniques
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Final Report Abstract
The main topic of this project was to develop an approach based on Scanning Kelvin Probe, Scanning Kelvin Probe Force Microscopy (SKPFM) and Scanning Electrochemical Microscopy (SECM) for the fundamental investigation of microscopic mechanisms of corrosion processes. As an example, delamination was chosen. Delamination at polymer/metal interfaces is a problem of widespread technological importance: The reasons for degradation of the interface can be manifold: mechanical stress, chemical degradation at the interface or electrochemically driven delamination starting at defects. The latter case is the most vicious one, as it can be extremely fast and often precedes other degradation reactions. A reliable prediction of long-term corrosion performance is not available yet, since the processes involved in the overall corrosion process - especially for degradation processes at buried interfaces - are not known in detail. To overcome this deficiency it is important to gain especially an in depth understanding of the electrochemical processes during delamination at high lateral resolution. Kelvin probe techniques, the most powerful tools so far for the investigation of delamination, provide only indirect information on electrochemical processes at the buried interface. Since even the high resolution achievable with SKPFM can only then provide high enough resolution, when model coatings of just a few tens of nanometre thickness or even thinner are prepared, the project focused on ultra thin model coatings. In this project it was already decided at an early stage that the best approach was to apply the plasma-polymerisation technique for the preparation well defined ultra-thin films, instead of preparing latex-films or homogenous polymer films by spin-coating from solvent phase. The main advantage is the excellent control of thickness and also composition. As substrate aluminium alloy, respectively model alloy samples have been used, because on these substrates the whole delamination process, known as so called filiform corrosion, is totally controlled by processes occurring on the micro-scale. The weak sites on these alloys are intermetallic inclusions (intermetallic particles (IMPs)) that provide reactive sites on the otherwise inactive aluminium matrix, covered by insulating alumina. These reactive, mainly cathodic sites can be well resolved by SKPFM and were also expected to be detectable by Scanning Electrochemical Microscopy (SECM). As Kelvin Probe techniques do not provide kinetic information and in most cases are only applied ex-situ, they are usually not sufficient for providing in-depth information about the underlying fundamental mechanisms during delamination processes such as filiform corrosion. Two main approaches to solve this problem are followed here: trying to apply SKP and SKPFM in-situ and combining it with SECM, which directly provides kinetic information. Both approaches were a success. For the first time an in-situ study on the nanoscopic steps of filiform corrosion was successfully performed with SKPFM, including the in-situ monitoring of galvanic coupling between intermetallic particles and active head ofa filament. The experiments show that not all particles have the same activity. To a certain degree SKPFM can be used to identify the most reactive sites, which is already a very important achievement. This is achieved by monitoring the differences in potential changes upon change of the atmosphere between air and nitrogen. However, the actual local reaction rates cannot be measured with this technique. As the standard SECM technique is not that highly resolving, model aluminium alloy samples wilh larger intermetallic articles as well as pure phase intermetallics were prepared. Another problem is to find exactly the same particles that were characterised with SKP or SKPFM with SECM. However, in order to obtain more detailed information about the electrochemical processes and the correlated interface degradation, a synergetic application of different scanning probe techniques such as scanning electrochemical microscopy (SECM) and scanning Kelvin probe force microscopy (SKPFM) is of utmost importance for making progress. Hence, in parallel to the SKPFM based investigation of filiform corrosion a new experimental set-up was developed in this project where SECM and Kelvin probe are integrated in one instrument. That enables to perform Kelvin probe and SECM measurements routinely on exactly the same sample location. The excellent performance of this new instrument was demonstrated. This work is a big step forward for establishing a correlation between electrochemical activity at the buried interface and the corresponding delamination behaviour. Moreover, this work lays the basis for a novel approach for the in depth investigation of electrochemical reactions al heterogeneous electrodes in general. It was shown on the one hand that in-situ SKPFM studies of coating delamination is possible, even for slow processes such as filiform corrosion (slow processes require long term studies which is far from easy with AFM-based techniques al the high humidity required for corrosion studies). This opens up a great range of possible further applications in corrosion science. For additional insight on the correlation of potentials and their evolution as measured by Kelvin Probe methods with the actual local reaction rates the combined SKF/SECM will be an indispensable tool. However, as the spatial resolution is still limited to several microns, this method will require the preparation of suitable model samples. Hence, in-situ SKPFM studies on samples of interest are suggested to be combined with SKP/SECM studies on suitable model samples. This combination of techniques should provide a unique approach enabling a detailed investigation of the localised processes of localised corrosion problems and delamination. This approach should also be generally applicable for investigation of fundamental aspects of electrocatalysis on heterogeneous electrodes.
