Ander Cervellera Dominguez - The effect of particle size distribution and cross-linking agent on the leaching behaviour of anti-corrosion species for long-term active corrosion protection on AA2024-T3 alloy
Presentation - pdf
A. Cervellera-Dominguezb, X. Zhoua, Peter Visserb, S.R. Gibbonc
a Corrosion and Protection Centre, School of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
b AkzoNobel, Specialty Coatings, 2171 AJ Sassenheim, The Netherlands
c AkzoNobel, Supply Chain, Research & Development, Stoneygate Lane, Felling, Gateshead, Tyne and Wear, NE10 0JY, UK
Leaching of corrosion inhibitors from organic coatings, and fast effective and irreversible corrosion inhibition are key properties for active protective coatings to provide optimal long-term corrosion protection. Previous studies suggest that leaching of the active species occurs via interconnected pathways in the coating and showed that the leaching behaviour is strongly influenced by the pigment volume concentration in coatings. However, the influence of parameters such as cross-linking agent and particles size distribution on the leach-rate is not fully understood yet.
The thorough understanding of the effect of the cross-linking agent and the particles size distribution will provide essential knowledge for the development of coatings with long-term corrosion protection. The work presented aims to correlate the release rate of active inhibitor from coatings matrix with the particles size distribution, cross-linking agent properties, moisture ingress, coating morphology, and corrosion protection.
For this purpose, model epoxy-based primers have been formulated with lithium carbonate as leachable corrosion inhibitor. Moisture ingress and corrosion properties were measured by electrochemical impedance spectroscopy (EIS). The release of lithium carbonate was monitored by ICP.
Q&A
From Jordan Petkov : Have you replenished your solution upon taking 2 ml aliquots in the Lithium leaching experiment?
Answer: Yes, basically what I was doing was that I was removing 2 ml aliquots and putting 2 ml more. I actually did the calculation as I was using 250 ml beakers, adding 2 ml or removing it doesn’t change significantly the total concentration.
From Emma Michailidou : If I understood correctly you investigate lithium salts within a primer. Have you looked into possible diffusion of lithium salts towards a topcoat and how would that affect the corrosion performance of the overall coating?
Answer: So I didn’t look at systems with topcoat. There is some literature in it and what is observed and what I would expect is that adding a topcoat will create a barrier between the primer and the environment and that could lead to blistering because of the difference in concentration and that would decrease the corrosion protection because we would not allow the lithium ions diffuse into the defect.
From Simon Gibbon : Interesting decrease in leaching with particle size with Li makes sense if connectivity has changed. How do you measure particle size of multiple different particles in coating?
From Reza Emad : Thank you very much for nice presentation. How did you reduce the particle size distribution? What was the range? Do you know the hydrated size of Li, Ba and Mg?
Answer: I don’t know the hydrated sizes. I know that lithium has two hydration layers, so the size can be quite large. I just control the PSD by grinding times. The size is up to 10 microns. See slide 11 to see the information. That is the total particles, not just the lithium, because we grind the base paint. Now we are looking into grinding them individually. If you decrease the connectivity you have less leaching. All the particles seem to be decreasing the size based on the images (slide 12)
From Bob Luigjes : Concerning the cross-linking agent – did you also look at the performance with the variation in cross-linker in resin systems only (without Lithium inhibitor) and the effect on the barrier properties?
Answer: No, I always looked at inhibitor because I was looking at the leaching.
From A Legrix : Have you tried other minerals (eg platey) that may improve corrosion resistance, eg. wollastonite or talc?
Answer: No, the project focused on understanding leaching and not the corrosion protection, I tried lithium carbonate, but also lithium phosphate and lithium oxalate to see how the solubility affects the leaching.
From Jordan Petkov : Do you go above the glass transition temperature of PVC? during grinding.
Answer: the system does get hotter during grinding. The more we grind the lower the PVC of the coating. Larger particles induces defects like cracks and pits, and changes the properties of the system.
From Peter Collins : How did the different anions you tried effect the solubility and hence corrosion protection?
Answer: for lithium we see that lithium phosphate has a very low solubility and leaches too slowly to provide corrosion, lithium oxalate leaches too fast, so it produces blistering and depletion. So what we have is a sweet spot in the middle that may not be lithium carbonate, between the solubility and corrosion protection properties that we need to find.
From Dr Tolutope Siyanbola : Adhesion is an important coating property, did you use any other polyol system aside from bisphenol A.
Answer: I looked at different bisphenol A and Bisphenol F based systems, and saw that adhesion was very similar. However, leaching of Li increased with the functionality of the binder system.