Scanning Kelvin Probe Studies of Cosmetic (Filiform) Corrosion on AA6016
Scanning Kelvin Probe (SKP) potentiometry is used to systematically investigate the effect of surface abrasion and subsequent heat-treatment on the open-circuit potential in humid air of the AA6016 surface. SKP is also used to follow the kinetics of filiform corrosion 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 produces a surface potential up to 0.5V lower than the bulk. When the abraded sample is overcoated with a 30 micron layer of PVB (polyvinyl butyral) and exposed to HCl a fast, superficial filiform corrosion (FFC) is observed in which metal loss is limited to the thickness of the surface layer. Filiform head OCP values are similar to that of the surface layer, whereas filiform tail OCP values are similar to the bulk. A mechanism is proposed in which the ultra-fine grain structure of the surface layer produces an anodic activation and the potential difference between the surface layer and the bulk provides and increased thermodynamic driving force for corrosion. For post-abrasion heat treatment temperatures up to 350°C the fast filiform process is followed by a slower, deeper form of FFC.
W.J. Poole, M.A. Wells and D.J. Lloyd
H.N. McMurray, A.J. Coleman, G. Williams, A. Afseth, G. M. Scamans, "Scanning Kelvin Probe Studies of Cosmetic (Filiform) Corrosion on AA6016", Materials Science Forum, Vols. 519-521, pp. 679-686, 2006