Effect of inhibitor-loaded halloysites and mesoporous silica nanocontainers on corrosion protection of powder coatings
Introduction
Self-healing activity of the materials is based on their feedback action. The trigger for feedback action can be an external signal applied to the material (light, mechanical force) or changes of the internal properties (like local pH-changes during corrosion process) and overall material integrity [1]. The output of the trigger is the restored functionality of the initial material, in our case corrosion protection ability of the coatings. First simple approach for providing self-healing to the organic coatings is to directly introduce corrosion inhibitors in the pretreatment, primer or topcoat layer of the coatings [2]. The idea behind is the response to the coating damage by diffusive or stimuli-induced release of the inhibitor from the coating matrix. Direct introduction of the inhibitor into the coating matrix showed successful results for some classes of the inhibitors (like phosphate pigments); however, there are difficulties with application of inhibitors with very high or low solubility in paints [3], [4]. Very low solubility of inhibitor leads to its deficit in the damaged area. In case of too high solubility, metal substrate can be protected for only a relatively short time due to rapid spontaneous leaching of inhibitor from the coating [5]. Another drawback, which can appear due to high solubility, is the osmotic pressure initiating blistering and, finally, delamination of the coating [6], [7].
The new and promising solution to avoid interaction between inhibitor and coating matrix is to encapsulate the inhibitor into stimuli-responsive capsules [6], [8]. Capsules or nanocontainers can isolate encapsulated corrosion inhibitor from coating matrix, prevent spontaneous leakage of inhibitor and, at the same time, provide controlled release of the inhibitor directly into the corroded area. There are several approaches demonstrated so far for the design of nanocontainer systems: (i) polymer containers [9], (ii) halloysites [10], (iii) nanocontainers with polyelectrolyte shell [11], (iv) layered double hydroxides [12], (v) ion-exchange organic resins [13], (vi) conductive polymer matrixes [14] and (vii) mesoporous inorganic materials [15]. Depending on the selected approach, the size of the containers can be varied from 20 nm to 50 μm and, besides permeability control, the shell can have other functionalities (magnetic, catalytic, conductive, targeting, etc.).
The current level of the development of nanocontainer-based self-healing coatings shows large number of the highly-efficient examples on the laboratory scale [16], [17]. However, there are two main difficulties for the transfer of the research to commercial applications: (i) the costs of the nanocontainers and (ii) lack of valid industrial test results (mostly salt-spray tests). The first problem requires the search for cheap nanocontainer hosts which can be available in large-scale quantities. Halloysites and mesoporous silica particles can be perfect candidates here. They are much cheaper comparing to the other types of nanocontainers and commercially available in large quantities [18]. Additional interest of using inorganic nanocontainers is mechanical and thermal stability, which allow their utilization in different coating layers (pre-treatment, primer, topcoat) subjected to high mechanical loads or significant thermal stresses.
Halloysites are defined as two-layered natural aluminosilicates chemically similar to kaolin, which have a predominantly hollow tubular structure in the submicrometer range [19]. Efficient self-healing properties of the benzotriazole and 8-hydroxyquinoline loaded halloysite nanotubes were demonstrated in zirconia-silica sol-gel coatings deposited on the surface of aluminium alloy A2024 [10]. To prevent undesirable leakage of the loaded inhibitor from the halloysite interior at neutral pH, the outer surface of the halloysite nanotubes can be modified by deposition of alternating polyelectrolyte multilayers (poly(allylamine hydrochloride)/poly(styrene sulfonate)) [20].
Another type of the nanocontainers with inorganic scaffold is mesoporous silica. It is inert to the most of the corrosion inhibitors and has high specific surface area (>1000 m2/g) [21]. The incorporation of inhibitor-loaded mesoporous nanocontainers into inorganic sol-gel coatings improved significantly the coating corrosion resistance [22]. On one hand, the coating barrier properties were enhanced by reinforcement of the coating matrix caused by introduction of mechanically stable, robust silica nanoparticles. On the other hand, the large amount of encapsulated inhibitor (up to 80 wt.%) and its controlled release upon corrosion attack provided superior corrosion inhibition. Additional advantage of the silica nanocontainers is the possibility to tailor hydrophobic surface functionality for solvent-born coatings. Mesoporous SiO2 functionalized with octyl groups and loaded with benzotriazole showed tenfold greater corrosion protection performance in polyester-based commercial coatings than for the coating without nanocontainers [23].
Despite large number of the papers devoted to the nanocontainer-based self-healing coatings, most of them use lab-scale analytical methods for characterisation of their self-healing performance: EIS, polarisation, SVET and various adapted electrochemical techniques. Only a few papers [24] analysed the efficiency of the nanocontainer-based coatings using industrial methods. Here, we attempt to decrease this “transfer gap” and present comparative analysis, done by industrial neutral salt-spray test (ISO 9227), of the corrosion protection performance of 8-hydroxyquinoline (8-HQ) loaded halloysites and mesoporous silica particles impregnated into polyester powder coating. Coatings with and without nanocontainers were tested on bare low carbon steel substrates.
Section snippets
Materials
Corrosion inhibitor 8-hydroxyquinoline, ethanol, acetone, HCl, NaOH and NaCl were purchased from Sigma-Aldrich and used without further purification. Halloysites were provided by Atlas Mining Company (Dragon mine deposit, Utah, USA) and mesoporous silica particles were purchased from Grace, USA (SYLOID® C803 silica). Halloysites are naturally occurring layered kaolin aluminosilicates with hollow tubular structure. The aluminium hydroxide and the silicon oxide layers are bond covalently with
Results and discussion
The highest loading efficiency was observed for both nanocontainer types after third loading cycle (see Experimental part). First loading cycle provided 12 wt.% inhibitor loading for halloysites and 34 wt.% of inhibitor for mesoporous SiO2, as was calculated from TGA. Next two cycles lead to the loading limit of 20 wt.% of 8-HQ for halloysites and 77 wt.% for silica. Further repetition of the loading did not result the increase of the quantity of 8-HQ in nanocontainers clearly indicating achieved
Conclusion
Summarising the data presented above, we can conclude that the effect of inhibitor-loaded nanocontainers is based on the pH-controlled release of the 8-hydroxyquinolile inside the damaged (or corroded) area of polyester powder coatings. Local increase of the pH due to the corrosion process immediately accelerates the solubility of the encapsulated 8-hydroxyquinoline provoking its diffusion from nanocontainers to the defected area where it then chemisorbs on the anodic corrosion sites [29] and
Acknowledgements
DS acknowledges ERC Consolidator grant ENERCAPSULE for financial support.
DG is grateful for the financial support of the Federal Ministry of Economics and Energy (BMWi), Germany in the framework of “EXIST Transfer of Research” program (Project “SigmA” Nr. 03EFCBB028) and G8 project “SMARTCOAT”, G8MUREFU2-FP-377-008. We thank Dr. M. Schenderlein and Mr. E. Scherke for the help in experimental work.
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