The delamination of polymeric coatings from steel Part 3: Effect of the oxygen partial pressure on the delamination reaction and current distribution at the metal/polymer interface

Abstract

In the third part of this paper, it is shown that oxygen reduction plays the central role in the delamination process. The oxygen partial pressure controls the potential of the local cathode at the metal/polymer interface and potential changes, which are measured after change of the oxygen activity, allow us to determine the local diffusion coefficient of oxygen in the polymer. The delamination is caused by reaction products, like radicals, which are formed during the oxygen reduction at the metal/polymer interface. However, the reaction rate of the oxygen reduction for the simple non-pigmented polymer under investigation is determined by the ohmic potential drop between the defect and the delamination frontier. This potential drop is given by the difference of the open circuit potential at the defect and the intact interface (which results from the surface treatment, such as phosphating) and the ionic conductivity between both sites. Both are confirmed by a direct measurement of the distribution of the galvanic current along the metal/polymer interface.