This work investigates the influence of inhibitor pigment volume concentration (PVC) on the microstructure, the barrier properties and the leaching behaviour of primer coating. The polymeric matrix of the primer coating is polyester based cross linked with blocked aliphatic polyisocyanate, whereas the active inhibitor is strontium aluminium polyphosphate hydrate (SRPP). The objectives are to correlate the PVC, primer microstructure to the leaching rate of phosphorus containing species and strontium, also, to discover the leaching mechanism and the dominant transport paths that serve as the passage for the release of phosphorus containing species and strontium. Quantitative energy dispersive X-ray spectroscopy (EDS) manifest that the concentrations of phosphorus containing species and strontium decrease throughout the entire thickness of the primer coating with relatively high SRPP PVC ( 25%). However, in a system with 10% SRPP PVC, the concentrations of phosphorus and strontium within the exposed primer coating decrease only within the regions, which are closer to the environment, whereas the concentrations of phosphorus and strontium within the regions further away from the environment are in a comparable range to that in the similar regions of the unexposed primer coating. On the other hand, microstructure characterization with scanning electron microscopy (SEM) shows that the addition and the increase of the SRPP PVC lead to formation of connection networks and clusters of connected SRPP pigment particles within the primer coatings. Also, asperities and discontinuities form on the surface of the primer coatings with the addition of the SRPP PVC, whereas the size and frequency of clusters of connected inhibitor particles and the asperities on the surface increase with the increase in the SRPP PVC. Further, the increase in the SRPP PVC leads to proportionately higher leaching rate, which could be beneficial for effective protection of the substrate, although it also leads to the depletion of the âreservoirâ from the inhibitive species during short period of exposure to environment, which is undesirable. This behaviour indicates strong microstructural influence on the leaching process and is attributed to the spatial distribution of the SRPP pigments and the formation of connection networks and clusters of connected inhibitor particles. The EDS analysis illustrates the absence of detectable concentrations of phosphorus and strontium within the polymeric regions of exposed primer coatings. Therefore, transport of phosphorus containing species and strontium (if any) is highly restricted through the polymeric binder. On the other hand, the variations of the thickness of the primer coatings with similar SRPP PVC show lower leaching rate for thicker primer layers although the weight of SRPP pigment per cm2 of the primed substrate is higher within the thicker primer. This is attributed to thicker âskin layerâ and/or lower number of discontinuities on the surface of thicker primer coatings, which indicates that the transport of the pigment species through the polymeric binder in the absence of easy transport paths is highly restricted and significantly influence the total leaching rate. Clusters of connected inhibitor pigments form with a critical size at percolation threshold (Pc). Therefore, percolation threshold (Pc) is proposed as a critical pigment volume concentration for inhibitor pigments, which together with inhibitor pigment dissolution and release properties should be considered during the primer design and formulation. If the inhibitor PVC is larger than the Pc, it could lead to significantly high release rates of inhibitive species (i.e. fast depletion) and significantly low barrier properties and 30 impedance of the primer coating. On the other hand, if the inhibitor PVC is just below the Pc, higher concentrations of inhibitive species can be released into the environment compared with the system that are further below the Pc, which is beneficial for effective corrosion protection, whereas the barrier properties and impedance of the primer coatings with active inhibitor PVC below the Pc are comparable to each other. Based on this study, a model is proposed for the leaching of inhibitive species released from the SRPP pigment. The release and leaching start at the loci of defects/discontinuities, where the inhibitor particle is in direct contact with the environment. The dissolution and release of inhibitive species lead to the formation of voids and cavities, which are filled with electrolyte. If the void forms at the locus of a particle, which belongs to a cluster of connected SRPP particles, the void is connected to another particle that can release inhibitive species into the medium of the void, which in the absence of the polymeric binder acts as an easy and direct passage for the transport of inhibitive species into the environment. Therefore, the leaching will continue until the cluster becomes completely depleted from the inhibitor pigments. On the other hand, if the void forms at the locus of an isolated and single particle, which is surrounded and encapsulated with the polymeric matrix, the leaching will stop after the dissolution and removal of the single particle. Based on this study and the proposed model, it is concluded that the diffusion of phosphorus containing species and strontium through the polymeric binder cannot contribute significantly to the total leaching rate. Therefore, the dominant transport paths for the release of inhibitive species are through the direct contact with the environment and the voids/cavities that are left behind after the dissolution and release of inhibitive pigments. The ratios of strontium to phosphorus concentrations within the leachate solutions are significantly higher than the ratio of strontium to phosphorus concentrations within the powder SRPP pigment. This is attributed to: 1-cationic exchange processes that replace strontium ions with cations from the environment such as sodium and potassium ions and 2- higher dissolution rate of strontium-rich particles compared with the phosphorus-rich and aluminium-rich particles that comprise the SRPP pigment.
|Date of Award||31 Dec 2018|
- The University of Manchester
|Supervisor||Stuart Lyon (Supervisor) & Xiaorong Zhou (Supervisor)|
Elucidation of the Influence of Inhibitor Pigment Volume Concentration on Leaching and Barrier Properties of Organic Coating
Emad, S. (Author). 31 Dec 2018
Student thesis: Phd