Leaching from coatings pigmented with strontium aluminium polyphosphate inhibitor pigment- evidence for a cluster-percolation model

•Microstructure of an organic coating affects the leaching mechanism.•A cluster model describes leaching of strontium aluminium polyphosphate inhibitor pigment.•Diffusion through the polymeric binder does not significantly contribute to the total leachate concentrations.•Leaching starts when the inhibitor pigments are in direct contact with the environment via defects.•Dominant leaching mechanisms are through defects and interconnected voids/cavities directly connected to the environment. Model organic coatings were formulated with different volume concentrations of strontium aluminium polyphosphate (SAPH) inhibitor pigment. Leaching measurements were performed from the top surfaces of the coatings. The microstructure of the coatings and the distribution of phosphorus and strontium within the coatings prior to and after exposure to an aggressive environment were characterized by SEM, EDS and EDXRF. The increase in inhibitor pigment volume concentration (PVC) results in the formation of larger clusters of connected inhibitor pigments within the coatings, which coincides with higher leaching rates from the coatings with higher inhibitor PVC. It is revealed that in the presence of the large clusters, the concentrations of inhibitor pigments decreases through the entire thickness of the exposed coating. In contrast, when the inhibitor PVC is low and elongated clusters of inhibitor pigments do not span the thickness of the coating, the reduction of the inhibitor pigment concentration within the exposed coating is limited to regions closer to the coating/environment interface. On the other hand, EDS analysis shows that after exposure, the concentrations of species released from the inhibitor pigments are below the detection limit within the polymeric binder. It is also shown that the diffusion through the polymeric binder would appear to be the limiting factor that retards the leaching from the coating, suggesting that diffusion through the polymeric binder is not the dominant transport mechanism for leaching of species released from the inhibitor pigments. A cluster model appears to describe well the leaching of inhibitive species released from the SAPH inhibitor pigments incorporated into the organic coatings.