Orrico, Marianna (2025) Sustainable Materials for Corrosion Protection on Aircraft Structures. [Tesi di dottorato]

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Abstract

In the aviation industry, where aircraft are subjected to extreme climatic and operational conditions, metal component corrosion poses a critical challenge, both in terms of safety concerns and associated costs. Preventing and protecting metals from corrosion has a significant economic impact. In Europe, the annual cost of preventing the degradation of metal structures is estimated at around 200 billion euros, while in the United States, this figure reaches $276 billion[1]. Moreover, corrosion compromises the structural integrity of aircraft components, directly affecting their safety and potentially leading to severe consequences. Thanks to their light weight, low cost, durability, and excellent workability, aluminum alloys are widely used in the production of most structural aircraft components. AA2024-T3, an alloy primarily composed of aluminum and copper with small amounts of magnesium, manganese, and other elements, is one of the most commonly used alloys in the aviation industry[2]. However, the alloying elements often have electrochemical potentials that differ significantly from the aluminum matrix, making the alloy more vulnerable to corrosion[3,4]. Therefore, an effective corrosion protection system is crucial for maximizing the aircraft’s service life. Corrosion prevention systems typically consist of a multilayer structure, including a pretreatment layer such as anodizing or conversion coating, followed by an organic corrosion-inhibiting primer and a topcoat. Anodizing is an electrochemical process where critical factors like electrolyte composition, electrode and electrolyte temperature, pH, and current density influence the morphology of the anodic oxide film. Alternatively, anodization can be replaced with chemical conversion, a process that involves immersing the metal surface in solutions containing phosphate, carbonate, fluoride, and chromate. While anodizing generally enhances corrosion and wear resistance, chemical conversion offers a simpler, faster, and more cost-effective method for pretreating metal surfaces[5]. This method is particularly advantageous for protecting large aircraft components that may be difficult to anodize[6]. Additionally, pretreatment of aluminum alloys is essential for ensuring the correct application and functionality of the primer. The porous structure of the oxide layer promotes strong adhesion between the paint and the metal substrate, improving its barrier properties. Chromium and its compounds have been widely used in the aircraft industry for many years, playing a crucial role in the corrosion protection of structural components. The corrosion inhibition mechanism of hexavalent chromium compounds has been extensively studied and is now well understood. Chromates act as both anodic and cathodic inhibitors, effectively reducing the rate of metal dissolution and providing robust protection against corrosion. However, the health risks and environmental concerns associated with the use of Cr(VI) have prompted a reassessment of existing corrosion protection strategies and intensified efforts to find environmentally friendly alternatives. Under new environmental regulations, such as REACH[7], all substances containing hexavalent chromium are classified as carcinogenic, leading to their restricted use in several industries. Finding a substitute for chromates is challenging, as they are known to offer reliable corrosion protection even under extreme environmental conditions. Significant efforts are currently underway to replace Cr(VI) with environmentally friendly alternatives across all components of aeronautical corrosion protection systems[8-10]. Hexavalent chromium is commonly used both as an additive in anticorrosive primers and in the pretreatment of aluminum substrates. Hexavalent chromium-based pretreatments, such as Chromic Acid Anodizing (CAA)[11] and Chromate Conversion Coating (CCC)[12], are widely applied to form protective films. In CCC treatments, chromic acid deposits both Cr(III) and Cr(VI) oxides on the metal substrate, while in CAA, it acts as a catalyst to promote the growth of the anodic oxide layer. Both pretreatment methods offer several benefits, including self-healing properties due to chromate ions, strong adhesion to metal alloys, compatibility with paints and primers, and cost-effectiveness[13]. Research has shown that modifying the anodizing process by using sulfuric, citric, or tartaric acids as alternatives to chromic acid results in significant morphological changes to the treated surface, particularly in pore size[14]. These changes could enhance both corrosion resistance and primer adhesion. The main strategies for replacing CAA focus on the use of alternative electrolytes or their combinations[15]. Examples include Phosphoric Acid Anodizing (PAA)[16], Sulfuric Acid Anodizing (SAA)[17], and mixed sulfuric-organic acid electrolytes, such as Phosphoric-Sulfuric Acid Anodizing (PSAA)[18], Tartaric-Sulfuric Acid Anodizing (TSAA)[19,20], and Boric-Sulfuric Acid Anodizing (BSAA)[21]. Additionally, a Green Chemical Conversion Coating (GCCC)[22], based on Cr(III), considered less harmful than Cr(VI), is currently being researched as a potential replacement for CCC. Following pretreatment, aluminum alloys are further protected by an anticorrosive primer, which typically contains chromates. The application of organic coatings represents the final step in the corrosion protection process. These anticorrosive primers serve as the first barrier against environmental exposure and must actively protect the substrate in cases of damage, cracking, or delamination. Chromate ions leach from the primer, migrate to the damaged area, and undergo cathodic reduction. This reaction, combined with the anodic dissolution of the metal, forms a protective layer of chromium oxide (Cr₂O₃) that prevents further corrosion[23]. A common hexavalent chromium-based primer is a polyamide epoxy primer containing strontium chromate (SrCrO₄)[24], which functions as a corrosion inhibitor. Among Cr(VI)-free coatings, metal-based epoxy primers (MBP) are particularly promising[25]. These primers contain a specific volume of metal particles, ensuring electrical contact between the metal and the aluminum substrate, which is crucial for effective cathodic protection[26]. The substrate’s oxidative degradation is mitigated by the sacrificial oxidation of the metal pigment, thus providing cathodic protection to the aluminum alloy. However, the use of MBP primers is currently limited to aircraft maintenance procedures, which involve removing the existing coating and cleaning the damaged surface with specialized chemicals that not only clean but also activate the surface to enhance adhesion of the new primer[27,28]. In recent years, incorporating environmentally friendly components into coating formulations has become a crucial aspect of coating design[29]. Interesting advancements have been made in developing partially or entirely chromate-free coatings that utilize rare-earth compounds as corrosion inhibitors[30]. Additionally, studies have proposed the use of lithium salts[31,32], hybrid organic-inorganic pigments[33], and nanocontainers filled with corrosion inhibitors[34] for application in organic coatings. Furthermore, several micro and nanocapsules have been investigated to incorporate green inhibitors for corrosion protection in coatings, including bentonite[35], montmorillonite[36], and hydroxyapatites[37]. Recently, an increasingly prominent and promising approach for designing reliable coatings involves using zeolite as a filler and microcontainer. This method facilitates the encapsulation of inhibitory species[38] and enhances self-healing mechanisms in innovative smart coatings[39,40]. Additionally, several studies have increasingly focused on investigating antioxidant systems, such as lignin, tannic acid, and their derivatives[41,42], which have demonstrated significant potential as corrosion inhibitors[43]. Moreover, one of the most interesting methodologies for developing coatings that serve as effective barrier layers against corrosion is the Sol-Gel technique[44]. In view of these insights, the research project focused on developing innovative corrosion protection systems by studying and further exploring currently used techniques. In collaboration with Leonardo S.p.A. – Aircraft Division, a leader in aircraft manufacturing, which has initiated a phase of process revision to align with the EU’s industrial transition towards a circular economy, the Ph.D. project addressed the issue of corrosion from a sustainability perspective. This approach aimed to converge on non-toxic, low environmental impact anti-corrosion coating products by modifying existing formulations and applications in aviation, or by creating new, high-performance coatings with excellent adhesion properties on metal substrates. Various surface treatment techniques for aluminum alloys were compared, and the anticorrosive properties of a commercial Cr(VI)-free metal primer, capable of controlling the oxidative degradation of the substrate through sacrificial oxidation of the metal pigment, were evaluated. The goal was to identify the optimal synergy between the primer and the surface treatment to achieve the best corrosion resistance. Specifically, 2024-T3 aluminum alloys underwent surface treatments involving anodization and chemical conversion, including both innovative techniques and traditional methods utilizing hexavalent chromium. Some of the pre-treated aluminum samples were subjected to an aging process by exposure to air at room temperature to observe any surface morphological transformations induced by the treatment. The MBP primer was applied to the aluminum plates both before and after aging to evaluate the corrosion resistance of the different surface treatments in conjunction with the same primer. This approach provided a comprehensive overview of the corrosion behavior of pre-treated surfaces, along with analyses of surface roughness, microstructure, chemical composition, and resistance to wear and corrosion. Moreover, to enhance both the effectiveness and durability of the MBP primer, various optimization strategies were explored by modifying its composition with different percentages of aluminum powder, cerium nitrate, and tannic acid. Each component was selected for its unique properties: aluminum powder was added to improve the coating’s cathodic protection, cerium nitrate was included for its eco-friendly corrosion-inhibiting qualities, and tannic acid was chosen as a natural corrosion inhibitor due to its antioxidant properties. The corrosion resistance of the modified primers was evaluated through accelerated corrosion tests and salt spray tests conducted at Leonardo’s plant in Pomigliano d’Arco. The research and formulation of innovative primers led to the exploration of alternative solutions to traditional corrosion inhibitors. In this context, interest grew in tannic acid, recognized for its strong antioxidant properties, and zeolites, microporous materials with remarkable capabilities to adsorb and release substances. Zeolites serve as effective carriers for corrosion inhibitors due to their crystalline structure, which enables them to trap inhibitory molecules and release them gradually in the presence of corrosive agents. The objective was to leverage the properties of these two substances in the formulation and laboratory synthesis of new anticorrosive pigments. These pigments were integrated into an epoxy matrix without conventional inhibitors to evaluate their adhesive and anticorrosive properties. The analysis of the resulting data provides a comprehensive overview of the potential of the formulated coatings and the pretreatments employed, offering valuable insights for their industrialization to meet increasingly stringent sustainability requirements. In advanced industries like aeronautics, the use of sustainable coatings can significantly reduce the overall environmental impact of production and operational processes by minimizing the use of toxic substances. Additionally, it can help extend the service life of aircraft while simultaneously lowering maintenance and replacement costs, thereby promoting greater environmental responsibility.

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