Focused ion beam and transmission electron microscopy as a powerful tool to understand localized corrosion phenomena

"The present study deals with focused ion beam (FIB) coupled with transmission electron microscopy (TEM) analysis of corrosion mechanisms in cathodic arc evaporation (CAE)-PVD CrN\/NbN multilayer coatings on aluminum alloy substrate. Samples were exposed to a 500-h ASTM B-117 salt spray test. Surface morphology of tested specimens was preliminary analyzed by SEM-EDS techniques. The corrosion rate in CAE-PVD coated components (ASTM B-117 standard) was significantly lower than uncoated substrates. Crevice corrosion in relation to surface defects (microdroplets) was observed after FIB and TEM cross-section analysis. Nevertheless, results of TEM analysis also showed a more complex crevice\/galvanic corrosion mechanism in relation to the bottom droplet\/coating voids, close to the coating\/substrate interface, which could be concurrent with usually proposed corrosion mechanisms. Moreover, modification of coating microstructure and composition is observed close to the peripheral area of droplets. These results give further and deeper insight into the corrosion mechanisms for PVD thin coatings, and could only be obtained by FIB sectioning and TEM analysis. Obtained results also allow the correct selection of the most suitable coating material and process parameters as a function of the nature of the substrate and the specific environmental conditions."