THE RELATION OF FORMULATION PARAMETERS ON THE LEACHING BEHAVIOUR AND PERFORMANCE OF ACTIVE PROTECTIVE COATINGS

Abstract

Fast, effective and irreversible corrosion inhibition via the leaching of corrosion inhibitors is essential for active corrosion protection by organic coatings. In the presence of a coating defect, water moisture permeates the coating and the corrosion inhibitor dissolve and migrate into the defect area, preventing/stopping the corrosion process. This leaching mechanism is the primary active protective mechanism in today’s aerospace corrosion inhibiting primers. The transition towards chromate-free systems for active corrosion inhibition remains a challenge. The lower effectiveness of the viable chromate alternatives and the leaching mechanism's lack of understanding are the key obstacles. The lower effectiveness of the inhibitor alternatives requires total control over the leaching rate of corrosion inhibitors. Systems must provide the minimum critical concentration to provide optimal corrosion protection. However, to ensure the metallic substrate's long-term corrosion protection, an excessive release must be avoided to prevent the inhibiting system's complete depletion. Comprehension of the leaching mechanism is a must to control the release rate of corrosion inhibiting species. This PhD's scientific objective was to understand the impact of various coating formulation parameters on the corrosion inhibitor's leaching. Aiming to unravel the relationship between the leaching mechanism, coating formulation parameters, and the resulting active corrosion protection performance. The active protective properties of the active coatings containing lithium as the corrosion inhibitor were investigated using a combined surface analytical and electrochemical techniques approach. The active protective properties were then related to the leaching rate of corrosion inhibitors, monitored using ICP-AES. The first part of the investigation involved the understanding of Li-leaching technology’s leaching properties. Through changes in the pigment volume concentration (PVC), it was corroborated that lithium follows the leaching mechanism described in the literature. First, soluble material's direct dissolution occurs, followed by Fickian diffusion and transport via the interconnected pathways. Cumulative leaching measurements elucidated the direct relation between the PVC and the total ion released. High PVCs developed a broader network of interconnect-ed soluble material which resulted in higher leaching. Leaching of barium and magnesium ions was also observed, which highlighted a direct relation between the solubility of soluble material and the leaching rate. Highly soluble material experiences faster direct dissolution to the environment, thus instigating the quicker development of the network of interconnected cavities. Cross-sectional scanning electron microscopy (SEM) of the coating revealed a decrease in particle size distributions when increasing the PVC. The effect of the particle size distribution on the leaching had never been studied. A decrease in particles size distribution led to an increment in magnesium and barium’s leaching but a decline in lithium release. Initially, it was suggested that lithium interacted more strongly with the electron-rich groups in the polymer chain, hindering transport via the interconnected pathways. With this hypothesis, the effect of the binder chemistry on the leaching of corrosion inhibitor was studied. Leaching measurements demonstrated that highly functional epoxy systems led to higher leaching rates. Therefore, it was suggested that lithium’s capability to solvate with up to a second water layer and thus increasing its ionic radius could be the reason behind lithium’s leaching decrease when lowering the particles size distribution. In this thesis, it was demonstrated for the first time the effect of the particle size distribution and the epoxy binder chemistry on the corrosion inhibitor's leaching. Furthermore, the importance of the corrosion inhibitor’s solubility to provide long-ter

Date of Award	31 Dec 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Xiaorong Zhou (Supervisor) & Carlos Avendano (Supervisor)