This research evaluated the in situ physicochemical changes and alterations occurring in an electrolytic chromium coated steel (ECCS), surface protected by polyethylene teraphthalate (PET) copolymer, after inducing a fracture on the coating in an acid acetic-acetate medium. The delamination was characterized in the front of the failure by means of anodic and cathodic electrochemical mechanisms, and the resistance and degradation of the metal-polymer composite's substrates were analyzed by means of Raman vibrational spectroscopy. The application of an electrochemical cell to generate in situ delamination, simulating the formation of pores or artificial defects, provided information on the activity inside the substrates of the PET-coated ECCS composite as a result of the effect of the acetic acid. The anodic delamination mechanism is based on the diffusion of the electrolyte through the metal-polymer interface and the pre-existence of pores on the polymer layer. The cathodic delamination mechanism is based on the mechanical action of the gaseous hydrogen as a result of the reduction of H+. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 1658-1666, 2012