Real-Time Corrosion Monitoring of Aluminum Alloy Using Scanning Kelvin Probe Force Microscopy

The evolution of the Volta potential of grade AA6063-T5 aluminum alloy microstructure with various intermetallic phases (IMP) during corrosion exposure to chloride-laden thin-film electrolytes at ambient temperature has been investigated, in situ and in real-time, using scanning Kelvin probe force microscopy (SKPFM), to gain insight into local corrosion processes. Trenching around micrometer-sized IMPs, de-alloying, oxidation, and the deposition of corrosion products were observed, which led to an inversion of the electrochemical nobility of the IMPs relative to the alloy matrix. Most of the IMPs showed cathodic Volta potentials (high nobility) relative to the matrix at 40%-80% relative humidity (RH), which became anodic (low nobility) when the surface was moistened with aqueous sodium chloride at elevated moisture (76%-87% RH) producing concentrations up to 5-6 M of chloride. The change of the Volta potential correlated with the onset and progress of corrosion and a nobility inversion of IMPs relative to the alloy matrix was observed. The corrosion behavior and the reasons for galvanic activities among microstructural constituents were elucidated and brought into a broader context to understanding localized corrosion.