Phenylphosphonic acid (H2PP) dissolved in a polyvinylbutyral coating is shown to profoundly inhibit filiform corrosion (FFC) on coated AA6111-T4. Following FFC initiation by application of aqueous HCl to a penetrative coating defect, the time-dependent extent of coating delamination was determined both optically and by repeated in situ measurements using a scanning Kelvin probe. FFC inhibition is shown to become almost complete when H2PP is >= 0.5% w/w. In-coating H2PP is also shown to depress Volta potential values associated with the intact (uncorroded) coated surface by up to 0.35 V. (c) 2007 The Electrochemical Society.