In-situ scanning Kelvin probe (SKP) potentiometry is used to study the underfilm corrosion of organic-coated, commercial purity (CP) magnesium under conditions of high relative humidity. Following initiation by the application of aqueous hydrochloric acid (HCl) to a penetrative coating defect, underfilm corrosion is characterised by the onset of coalesced filiform-like features, which lengthen with time and propagate at a rate which is largely independent of the quantity of initiating HCl. The potentials of the advancing underfilm corrosion-front range from -1.35 to ca. -1.45 V vs. SHE, with corrosion potentials in the tail region largely similar to those of the intact coated Mg surface. The rate of filiform corrosion (FFC) advance is shown to be insensitive to the presence of oxygen, but highly dependent upon the relative humidity of the holding environment. It is proposed that the mechanism of propagation is not governed by differential aeration, but rather a differential "electrocatalytic activation" phenomenon involving anodic Mg dissolution at the leading edge of the corrosion front, galvanically coupling with hydrogen evolution on a cathodically activated corroded region behind. This proposed mechanism also accounts for observations of FFC upon immersion of CP-Mg in dilute chloride-containing electrolyte, where differential aeration would not be expected to exert an influence. (C) 2010 Elsevier Ltd. All rights reserved.