Scanning Kelvin Probe Studies of Filiform Corrosion on Automotive Aluminum Alloy AA6016

Scanning Kelvin probe (SKP) potentiometry was used to systematically investigate the effect of surface abrasion and subsequent heat-treatment on the open-circuit potential (OCP) of AA6016 in humid air. SKP is also used to follow the kinetics of filiform corrosion (FFC) and to determine characteristic potentials associated with the electrolyte-filled filiform head and dry filiform tail. It is shown that simply abrading with 180 grit SiC paper produces a surface potential ca. 0.2 V lower than the bulk. When the abraded sample was overcoated with a 30 μm layer of PVB (polyvinyl butyral) and exposed to HCl a fast, superficial filiform corrosion was observed in which metal loss was limited to the thickness of the surface layer (i.e., ca. 1 μm). Filiform head OCP values were similar to those of the surface layer, whereas filiform tail OCP values were similar to the bulk. A mechanism is proposed in which the ultrafine grain structure of the surface layer produces an anodic activation, and the potential difference between the surface layer and the bulk provides an increased thermodynamic driving force for corrosion. For postabrasion heat-treatment temperatures between 180 and 350°C the fast filiform process was followed by a slower, deeper form of FFC.