Effect of ethylene glycol dimethacrylate on VOC reduction, rheological, mechanical and anticorrosion properties of a hybrid sol-gel coating on AA2024-T3 and sulfuric acid anodized AA2024-T3
Graphical abstract
Introduction
High strength aluminum alloys such as AA2024 are commonly used for fuselage and structural components on aircraft due to their favorable strength to weight ratio. Cu and Mg containing precipitates dispersed in the aluminum matrix allow the strengthening of the material, however, render this alloy highly vulnerable to corrosion, specially to localized corrosion such as pitting and intergranular corrosion. Hexavalent chromium (Cr(VI))-based surface treatments have been used for >50 years to provide surface protection to aluminum critical components within the aerospace sector, where the products to which they are applied must operate to the highest safety standards, in extreme environmental conditions and for extended time periods. They have unique technical functions that confer substantial advantage over potential alternatives, despite Cr(VI) compounds are strictly regulated substances due to their toxic and hazardous nature. The corrosion protective scheme employs chromates in many steps, such as in the surface preparation (deoxidizing), in the chemical conversion coating, in the anodic coating, as sealant of the anodic coating and as leachable inhibitor in the organic coatings. They provide superior corrosion resistance and inhibition, improved paint adhesion, low electrical contact resistance, enhanced wear-resistance (in the case of anodic layers) and can be used for local repair applications. The development of alternative solutions to chromate conversion coating (CCC) and chromic acid anodizing (CAA) to eliminate the health and environmental hazards of soluble hexavalent chromium is being pursued for many years [1].
In particular, conversion coatings based on trivalent chromium (Cr(III)) salts (such as Surtec® 650, Socosurf® TCS/PACS, Bonderite® M-NT 65000 or Lanthane® 613.3) have been the first candidates for industrial replacement of Cr(VI)-based ones (Alodine®1200S/Bonderite® M-CR 1200 AERO). However, further efforts must be addressed to totally industrialize Cr-free technologies. New Cr-free conversion coatings are being investigated based on inorganic species exhibiting different chemistry such as reducible hypervalent transition metals (based on Mo, Mn, V, Tc) [[2], [3], [4], [5], [6]] difficult-to-reduce transition metal oxides (based on Zr, Ti, Hf, Ta) [2], and/or precipitated compounds from rare-earth elements [[7], [8], [9], [10], [11], [12]] or hydrotalcite [13].
As substitute to CAA, alternative anodizing processes using electrolytes such as sulfuric, tartaric, oxalic, phosphoric, boric or mixed acids [14,15] have been approached. Among these electrolytes, sulfuric acid is commonly used for anticorrosion applications [16,17]. The structure of the anodized aluminum oxide layer is nanoporous, formed by a hexagonal array of cells with cylindrical pores of diameter 25 nm to 0.3 μm and depth up to 100 μm [18], which extends from the surface of the film down to a thin, denser oxide barrier layer at the metal oxide interface [17]. In the case of AA2024, with high content of Cu as alloying element, the protection properties provided by the sulfuric acid anodizing (SAA) are reduced by the inclusion of Cu rich intermetallic particles in the metal as well as Cu ions within the oxide network [19]. In any case, in order to fully protect the underlying aluminum metal, the porous oxide layer requires a sealing treatment to prevent penetration of aggressive corrosion inducing ions or chemicals to the base metal [18]. In the past, the most effective sealing was also achieved using Cr(VI)-based compounds. Recently, Cr-free sealing processes containing nickel acetate, cobalt acetate, nickel fluoride or hydrothermal treatments are being applied as replacement for sodium dichromate sealing [19]. However, these processes provide limited corrosion protection on AA2024 [20].
Sol-gel approach offers a vast range of possibilities for coating design. Its ability to synergistically combine inorganic and organic moieties leads to the formation of hybrid organic-inorganic materials intermingled at molecular level through inorganic polycondensation and organic polymerization reactions taking place simultaneously. This configuration permits to balance the properties provided by the inorganic moieties, such as high adhesion to metal, hardness and scratch resistance with those provided by the organic moieties such as flexibility, reduction of the densifying temperature and enhancement of the compatibility with subsequent organic coatings. The most frequently used sol–gel formulations for the preparation of hybrid coatings for corrosion protection are based on the hydrolysis and condensation of mixtures of metal and semimetal alkoxides (Si, Zr, Ti alkoxides) in combination with organo-alkoxides. The latter are commonly organylalkoxysilanes containing functionalization such as epoxy, methacrylic and others, linked to the central atom by a non-hydrolizable and stable SiC bond. The most frequently used sol–gel formulations for the preparation of hybrid coatings for protection of AA2024 are based on mixtures of organylalkoxysilanes such as 3-glycidoxy propyl trimethoxy silane (GPTMS), methacryloxy propyl trimethoxy silane (MAPTMS), 3-phenyl-aminopropyl triethoxy silane (PAPTS) or mercapto-propyl trimethoxy silane (MPTMS), with tetraethoxy silane (TEOS) or tetramethoxy silane (TMOS) as the inorganic precursors [[21], [22], [23], [24], [25], [26]]. The combination of organylalkoxysilanes with alkoxides of transition metals as the central atom, such as Zr [[27], [28], [29], [30], [31], [32], [33], [34], [35], [36]] Ti [37] or Nb [38] has demonstrated to effectively contribute to the improvement of corrosion protection. Transition metal alkoxides catalyze the reaction/polymerization of the organic moiety [[39], [40], [41]] in particular of the epoxide group in GPTMS [35,[42], [43], [44], [45]] and of the methacrylic group in MAPTMS [34,46,47]. In the first case, the cleavage of epoxide rings by nucleophilic attack; in the second case, the radical polymerization of methacrylic groups, promote the attainment of a highly dense and crosslinked network.
Sol-gel coatings as sealant of anodized AA2024 have received less attention up to now. Anodic layers on AA2024 or AA2524 in sulfuric, tartaric‑sulfuric, and phosphoric acid baths have been sealed with sol-gel coatings prepared from precursors with Si as central atom, TEOS-GPTMS [17,[48], [49], [50]], phenyltriethoxysilane [17] and also from the combination of precursors with Si and Zr as central atom, MAPTMS-zirconium propoxide (TPOZ) [17].
The chemistry of the sol-gel process is based on hydrolysis and polycondensation of monomeric alkoxide and/or organo-alkoxide precursors. Hydrolysis reaction of Metal-OR and Si-OR bonds takes place in the presence of nucleophilic reagents such as water (basic or acidic) and is facilitated in the presence of homogenizing agents such as organic solvents. Alcohol is the byproduct of successive hydrolysis and condensation reactions that form the material network. Alcohols are considered volatile organic compounds (VOC) as they have high vapor pressure at room temperature. The use of certain types of VOC gives rise to pollutant emissions that contribute to the photochemical formation of ground-level ozone by reacting with nitrogen oxides (NOx, x = 1,2). Although alcohols (methanol, ethanol, propanol) have shown low ozone formation potential (⁓20% of ethene's potential) [51] and have mild impact on environment and health, reducing the concentration of VOC or hazardous air pollutants in the coating formulations is an important environmental goal. VOC controls in Europe affecting the coatings industry are governed by two main sets of legislation i) the paint product directive (Directive 2004/42/CE) and ii) the industrial emissions directive (IED – Directive 2010/75/EU), which limits the emissions of VOC in certain industrial activities. According to Directive 2004/42/CE, for solvent-based thin coatings (<5 μm thick) and one-component anticorrosion coatings, the limits are 400 and 500 g/L, respectively.
A highly optimized anticorrosion sol-gel formulation with low VOC content has been developed by the group of Oubaha [30] based on MAPTMS and TPOZ, which forms a methacrylate-silica-zirconia-based coating. This formulation contains an initial quantity of propanol of only ⁓10 vol%. However, in general, formulations with high equivalent solid content, result in sols with high viscosity. The viscosity of the sol is an important parameter that influences thickness and coating homogeneity depending on the deposition technique. Spray coating usually requires lower sol viscosity than dip coating. On the other hand, in order to ensure that the hardness and abrasion resistance properties of an anodic layer are maintained, the sol-gel coating as sealant should be imbedded into the porous oxide matrix [17]. In this sense, a low sol viscosity should improve the coating absorption into the porous oxide by surface capillarity.
The present work focuses on the improvement of the abovementioned corrosion protective hybrid sol-gel coating, by reducing the viscosity of the sol without any further VOC addition, with the aim to obtain thinner and more compact low-VOC coatings. The approach has involved the incorporation of ethylene glycol dimethacrylate (EGDMA), an organic precursor with low viscosity, in the methacrylate-silica-zirconia-based hybrid coating for the corrosion protection of AA2024-T3 and SAA AA2024-T3 aluminum alloys. In the first case, as replacement of CCC, in the second, as sealant of SAA surfaces. The EGDMA is a bifunctional monomer with two methacrylate moieties. The methacrylate polymerizable moiety of MAPTMS is linked to the inorganic network by the SiC bond, which is stable to nucleophilic attack of water. Oxozirconium clusters capped by polymerizable methacrylate ligands are obtained by reaction of zirconium alkoxide with methacrylic [41,52]. This combination permits the creation of a hybrid material with interpenetrated organic-inorganic network by the copolymerization of the methacrylate groups (EGDMA, MAPTMS and methacrylate-capped oxozirconium) and the polycondensation of hydroxyl and alkoxyl groups taking place in parallel. A thermal-induced initiator of polymerization has been introduced to initiate the polymerization by attacking the CC bond of the methacrylate groups. The formation of a highly crosslinked and interpenetrated network can offer a positive impact in the barrier and mechanical properties of the coating. The influence of the EGDMA concentration in the rheology and viscosity of the sols has been studied, as well as its effect on thickness, compactness and roughness of the derived coatings. The effect on corrosion protection has been studied by electrochemical techniques and exposure to neutral salt spray tests. Mechanical properties have been studied by dynamic microindentation and abrasion resistance tests.
Section snippets
Materials
1-propanol (extra pure, >99%) as solvent was purchased from Scharlab S.L. (Sentmenat, Spain). MAPTMS as hybrid precursor (≤100%) was purchased from Evonik Industries AG (Essen, Germany). EGDMA as organic precursor (purity 98%, with 90–110 ppm monomethyl ether hydroquinone as inhibitor) and 2,2′-azobis(2-methylpropionitrile) (AIBN, purity 98%) as thermal initiator were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used as received. Zirconium (IV) n-propoxide (TPOZ, 70 wt% solution in
Effect of EGDMA content in the initial viscosity, VOC content and thermal stability
The viscosity of a sol is an important parameter to control for coating deposition and depends on many parameters such as concentration, type of solvent, the chain length and branching of inorganic and organic clusters and on the degree of molecular association [55]. The viscosity of the Baseline sol was 16.4 mPa·s. This formulation presented an initial quantity of propanol of ⁓10 vol%, as displayed in Table 2. However, the maximum quantity of short-chain alcohols (VOC) in the sol, supposing
Conclusions
Thin and smooth methacrylate-silica-zirconia hybrid coatings were obtained from sols prepared by acid-catalyzed hydrolytic polycondensation of MAPTMS and TPOZ-MAAH mixtures at Si/Zr molar ratio of 4 with low VOC addition, followed by radical polymerization of EGDMA at different MAPTMS:EGDMA molar ratio of 1:0.14, 1:0.25 and 1:0.5 in the presence of thermal initiator AIBN. The addition of crescent quantity of EGDMA resulted in a progressive reduction of the sol viscosity, which at similar
CRediT authorship contribution statement
Cecilia Agustín-Sáenz: Conceptualization, Investigation, Visualization, Writing - Original draft preparation.
Eider Martín-Ugarte: Investigation.
Beatriz Pérez-Allende: Investigation.
Usoa Izagirre-Etxeberria: Supervision, Writing - Reviewing and Editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The basis of this work received funding from the European Union 7th Framework Programme for Research and Technological Development, within the project AVCOP under Grant Agreement ID 315041.
The authors thank the support of the Basque Government for the Elkartek project Frontiers-V (ref. KK2019/00077).
The authors thank Miguel Pérez-Aradros for the help with graphical abstract illustration.
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