Influence of volume concentration of active inhibitor on microstructure and leaching behaviour of a model primer

Model primers with different volume concentrations of active inhibitor were prepared. Strontium aluminium polyphosphate hydrate was incorporated as the active inhibitor in the polymeric matrix. Leaching measurements were performed in 0.86 M NaCl solution while the concentrations of strontium (Sr) and phosphorus (P) were monitored over 21 days. Analytical electron microscopy was employed to characterise the primers with different volume concentrations of active inhibitor. Different release behaviours were established for Sr and P in the aforementioned environment. P had an approximately linear release trend over the 21 days of immersion, whereas Sr exhibited an initially high release rate which reduced with immersion time. The different release behaviours are attributed to the higher dissolution/release rates of strontium-rich particles and a cationic exchange mechanism influencing the release of Sr. Furthermore, whilst higher volume concentrations of the active inhibitor incorporated in the primer layer resulted in the release of higher concentrations of Sr and P, it was not the only factor that led to high leaching rates of Sr and P from the primer. Variations in the coating microstructure occur as a result of the increased pigment volume concentration of the active inhibitor, which leads to the formation of heterogeneities through the thickness of the primer layers, and appears to play an important role that influences the transport paths for the inhibitive species within the primer layers.