How pigment volume concentration (PVC) and particle connectivity affect leaching of corrosion inhibitive species from coatings

Abstract Active anti-corrosion pigments, such as strontium chromate, are essential components of many corrosion protective organic coatings, since these leach out to provide active inhibition to the metallic substrate at the defective areas of the coatings arising during service from mechanical and/or environmental damages. Currently, formulators use empirical tests to determine effective inhibitor concentration, because the factors that determine leaching behaviour are poorly understood. In this study, we present insights into leaching mechanisms by correlating the microstructure of model coatings pigmented with strontium aluminium polyphosphate hydrate (SAPH) to the transport of different species. It is found that diffusion and transport of active species through the polymeric matrix does not significantly contribute to the leaching kinetics. Thus, leaching starts when inhibitor pigments are in direct contact with the environment via surface-breaking defects in the coating, and continues as long as the cluster of connected inhibitor pigments is in direct contact with the environment, until 3-dimensional connectivity is lost. Therefore, the extent, shape and size of the clusters of connected inhibitor pigments, as well as the solubility and dissolution rates of individual pigments, play important roles in the leaching process. Additionally, the 3-dimensional percolation threshold (P c ) for pigment connectivity is proposed as a critical parameter that has significant influence on the leaching rate as well as the barrier properties of corrosion protective coatings.