Pitting corrosion of AA7075-T6 was investigated in situ by Scanning Kelvin Probe (SKP) under droplets containing MgCl₂and NaCl. The corroded samples were analyzed by optical microscope and optical profilometer. The SKP results indicated that single regions of sustained pitting attack formed with preference at the edge of the droplets due to shortened distance of oxygen diffusion. The pits at the sustained attack sites formed anodes with supporting cathodic reactions at the surrounding sample surface. As the pits grew, the cathodic region was observed to expand, leading to elevated potentials measured at the center of the droplets. Droplets with greater initial volume formed larger areas of contact with the sample surface and accelerated corrosion attack. Overall, sustained pitting attack was observed along groups of surface particles oriented along the rolling direction. In many cases, pitting led to the formation of a secondary droplet at the periphery of the primary droplet. A mechanism is proposed for the pitting behavior of AA7075-T6 under the studied electrolyte droplets, as well as a new mechanism for the formation of secondary droplets.

This work was supported by Office of the Secretary of Defense Technical Corrosion Collaboration through the United States Air Force Academy.