The kinetics and mechanisms of filiform corrosion on aluminium alloy AA6111

Scanning Kelvin Probe (SKP) potentiometry is used to investigate the effect of surface abrasion and heat treatment on the open-circuit potential (OCP) of AA6111 in humid air. SKP is also used to follow the kinetics of filiform corrosion (FFC) and to determine characteristic potentials associated with FFC filaments. Simply abrading with 180 grit SiC produces a near surface deformed layer (NSDL) with OCP >0.2 V lower than the bulk alloy. When the abraded sample is overcoated with a 30 μm layer of PVB (polyvinyl butyral) and exposed to HCl a fast, superficial, surface-active FFC is observed in which metal loss is limited to the thickness of the NSDL (approximately 2 μm). Filiform head OCP values are similar to that of the NSDL and filiform tail OCP values are similar to the bulk. A mechanism is proposed in which the ultra-fine grain structure of the NSDL produces an anodic activation, and the potential difference between the NSDL and the bulk provides an increased driving force for corrosion. When the NSDL has been removed through surface-active FFC or by caustic etching, a slower, deeper, successive-pitting form of FFC affects the bulk alloy. The rates of both surface-active and successive-pitting FFC reach a maximum after post-abrasion heat treatment at 180 °C.