Initial cathodic reactivity of intermetallic particles in 7175 aluminum alloy buried under 6 μm thick anodized oxide layer revealed by in-situ reflective microscopy

An in-situ approach to buried metal/oxide interface is proposed, allowing to correlate an initial cathodic reactivity detected on the top surface of a several mu m thick rough oxide with the distribution of intermetallic particles (IMPs) in the substrate alloy close to metal/oxide interface. A 7175 aluminum alloy with a 6 mu m thick oxide layer (tartaric-sulfuric acid anodized and silicate sealed) was exposed to applied cathodic potential of - 1.4 V vs Ag/ AgCl in 10 mM NaCl aqueous solution for 900 s. Sites of initial cathodic reactivity generated gas bubbles and were localized on the surface thanks to high resolution in-situ reflective optical microscopy and image treatment of the recorded movie. Buried metal/oxide interface was accessed and studied ex situ by scanning electron microscopy and electron dispersive spectroscopy. Areas with high density of bubble initiation sites on the top surface correlated with the high density of IMPs in the substrate alloy close to the buried metal/oxide interface. For more than 80% of the 145 analyzed bubble initiation sites the distance between the (X,Y) position of the generated bubble center and the (X,Y) position of the nearest buried IMP was less than the average diameter of the bubble (6 +/- 2 mu m).