Abstract
The delamination of two commercially important, but markedly different polymer coatings from cathodically protected mild steel has been investigated. The conditions of the steel surface prior to coating, the nature of the metal-to-polymer bonding, and the failure of the coating as a result of lap shear tests has also been studied. The major analytical technique employed throughout the work was X-ray photoelectron spectroscopy (XPS). The nature of polybutadiene-to-steel adhesion has been investigated by XPS in conjunction with a novel oxide stripping/argon ion bombardment technique for interface analysis. It is concluded that the interface is more correctly described as an. interphase zone, characterised by the presence of divalent iron. The locus of failure of polybutadiene coated steel has been determined for mechanical and cathodic delamination. Cohesive failure of the polymer occurred in the former case but when polarised cathodically failure occurs in two stages; the interphase segregating with either the coating or the substrate. Reduction of the substrate oxide was observed and its relevance to the cathodic disbondment of organic coatings is discussed. The kinetics of delamination of a powder sprayed epoxy coating have been investigated for a series of substrate pretreatments. In the early stages of delamination the disbondment rate varies logarithmically with time but soon approximates to a linear dependence which decreases as interfacial path length increases. The concept of a critical disbondment velocity for the epoxy/iron oxide system is proposed and model calculations indicate that a pH of about 11 is required within the disbondment crevice to sustain this failure rate. Surface analyses have shown that although failure mechanism is essentially invariant with surface treatment; as path length increases so does the proportion of cohesive failure of the oxide. A comprehensive mechanism for epoxy delamination, in the light of both kinetic studies and interface analyses is proposed.