Abstract
The delamination of two commercially important, but markedly different polymer coatings from cathodically protected mild steel have been investigated by a small area ac impedance technique. Corrosion induced delamination has also been studied by large area in situ ac impedance analysis. It is concluded that the single property, which determines the kinetics of delamination and the type of failure mode is the coatings permeability to oxygen and the cation. This single property will determine whether or not conditions of high alkalinity can be achieved at the interface constituted by the polymer and the oxide. This in turn will determine whether dissolution of the oxide can occur. Evidence of ionic migration through the thin (20-50 mum) polybutadiene coating has been found. No such evidence has been found, however, in the case of thick (200-800 mum) epoxy coatings. In the light of surface science findings, (2,111,112,146) it was concluded that in the first case the predominant mechanism of failure is that of oxide dissolution, although coating degradation cannot be ruled out, whereas in the latter, it is that of the interfacial (adhesive) failure, which it is believed is bond strength dependent. The high pH generated by the cathodic reaction at the polymer/metal oxide interface will produce a permanent increase in the dielectric constant of the polymer in the immediate vicinity, so long as the polymer is sensitive to the hydroxyl ion and the external solution remains neutral or slightly alkaline. The progress of delamination could therefore be followed using the small area impedance measurements in the polybutadiene/steel system, which was found to be sensitive to the hydroxyl ion. However, this was not possible in the epoxy system as this polymer displayed no such sensitivity. Thus the two properties, which determine whether the progress of coatings delamination from metallic substrate can be followed are, firstly the coatings sensitivity to the hydroxyl ion and secondly whether the polymer system in question allows the build up of a high pH to take place. The characteristic steady linear increase in the capacitance and/or dielectric constant-time curves of polybutadiene/steel system, after its penetration by a conducting phase, has been attributed to the build up of alkalinity in the corrosion induced delaminated areas. It was concluded that such behaviour is a caution signal that the polymer/metal bond is suffering deterioration. The dielectric constant-time measurements in the initial stages of exposure to the electrolyte have been found useful in assessing the homogeneity of the polymer coating. A comprehensive appraisal as to the suitability of the ac impedance technique for studies of this nature has also been carried out.