An early specific effect of chloride ions on the heterogeneous swelling at the metal-polymer interface in non-defective coated metals is elucidated using frequency-dependent alternating-current scanning electrochemical microscopy (AC-SECM). Spatially-resolved differences in the topography of the coated sample are visualized upon prolonged incubation with chloride-containing electrolyte, thus allowing to assume a direct evidence of chloride ion permeation through the polymer matrix simultaneous to water uptake. The implications of this finding are particularly relevant towards the development of a new mechanistic model for blister initiation in coated metals. (C) 2012 Elsevier Ltd. All rights reserved.