Chapter 2

Deposition of Environmentally Compliant Cerium-Containing Coatings and Primers on Copper-Containing Aluminium Aircraft Alloys

Stephan V. Kozhukharov

University of Chemical Technology and Metallurgy, Sofia, Bulgaria

*Corresponding author, stephko1980@abv.bg; stefko1980@abv.bg

Abstract

This chapter describes the basic concepts related to the application of cerium compounds as a main alternative to the already restricted approaches of the use of toxic and environmentally unacceptable compounds. The chapter begins with a brief description of the importance of aluminium alloys for the aircraft industry and the basic corrosion forms and damages typical for these alloys. Besides the indispensability of the coating procedures for providing long-term corrosion protection, the basic multilayered coatings systems are also discussed. Following this, the basic stages and factors of deposition of cerium conversion coatings (CeCCs) as primer coating layers are described. Subsequently, various methods that involve cerium compounds as active components in upper and finishing coating layers are proposed based on the literature analysis. Furthermore, some alternatives of the cerium compounds as environmentally friendly active coating ingredients, such as organic corrosion inhibitors, are also proposed. The chapter finishes with the recent and the most actual directions for further improvement of coatings. Finally, the development of dense, self-healing, sun-light-protected, hydrophobic coatings are described.

Keywords: Aircraft alloys, corrosion protection, cerium conversion coatings, technological aspects, hybrid and nanocomposite materials, corrosion inhibitors, multifunctionality

2.1 Importance and Indispensability of the Corrosion-Protective Coating Layers

2.1.1 Employment of Reliable Materials for the Aircraft Industry

The aircraft industry provides high-speed and long-distance transport options. However, the reliability of this kind of transport services depends primarily on the ability for regular flights, regardless of the climatic conditions. During the flight, the external surface of airplanes is exposed to severe environmental conditions due to the sharp changes in temperature, caused by both the temperature difference between different altitudes and various local climatic zones the airplane passes through. Even in summer, the airplanes are exposed to low-temperature conditions at high altitude (e.g., the temperature at 10 000 m of altitude is about -70 °C, even at mid-summer). The high speeds of the modern flights lead to additional decrease in temperature, because of the resulting air flows that abrade (spoil) the airplane surface at about 850-900 km/h. Furthermore, since 1970s, following the inventions in the branch of the military aviation, promoted by the cold war, the commercial aircraft started to introduce models that enable flights even at ultrasonic speeds. The famous European commercial airplane "Concord" and its Russian analogue "Tu 144" are direct examples for ultrasonic commercial liners (Figure 2.1). As a result of these flight velocities, each flight is subject to sharp temperature changes in a wide range, usually from +40 °C to -70 °C. The endeavour for achievement of higher speeds of the flight will bring even sharper and more severe thermal shocks onto the fuselage during the flight.

Figure 2.1 Schematic images and photographs of commercial ultrasonic aircraft [1, 2].

These low temperature levels cause ice-heaping of the wings and propeller blades of the planes, leading to remarkable changes in their aerodynamics. These changes cause remarkable decrease in the efficiency, resulting in increase in the fuel spends, and might even endanger the safety of flight. Furthermore, the airplane travels through various local climate zones, with different humidity levels that form aerosols both in liquid (water drops) and solid (ice fogs) forms. Given the high speed of the flight, the abrading impact of these aerosols promotes scratching of the coating surface. Any rain water entrapment and/or water condense inside the scratches, combined with posterior volume expansion, causes disruption of the coating followed by crack proliferation. The already formed coating cracks favour localized forms of corrosion, such as filiform, pitting and crevice corrosion of the metallic body of the airplane. Each of these corrosion effects leads to stress corrosion cracking that seriously decreases the exploitation lifetime of the airplanes and can even be the reason for explosion-like splitting of the entire fuselage during flight.

Another important trend in the aviation sector is the elaboration of aircraft alternative to the airplanes. Indeed, the helicopters are among the most widely used aircraft. These have proved to enable transport service to destinations that appear to be impossible for any other kind of transport (Figure 2.2).

Figure 2.2 Image of a commercial helicopter [3].

The most important feature of the helicopter transport branch is the indispensability of these devices in rescue missions in high mountains and other inaccessible regions. Nevertheless, among the most important problems related to the aircraft industry are the flight safety, reliability and the fuel spends, which besides the economic impact, provoke environmental contamination. Undoubtedly, this is one of the main roads for overcoming connected to improvement of the constructive materials and technologies.

Regardless of the employment of entire generations of new materials as carbon fibre composites in the transport and especially in the aircraft industry, the aluminium alloys still remain the basic constructive materials [4]. Especially, AA2024 and AA7075 alloys are objects of special attention, due to their remarkable mechanical strength [5], being the basic constructional material for commercial [6] and military [7, 8] aircraft. In the former case, the aluminium fuselages render "visibility" for the aircraft and airport radars, whereas in the military air transport, the Al frames shield the on-board navigation and communication equipment against exterior electromagnetic influence. Recently, the importance of these alloys increased, due to their capabilities to be employed in the automobile industry [9-12]. In addition, the aluminium alloys encountered application in the marine industry for production of sport boats and even ships [13-15], and the automotive industry [16, 17]. The main advantage of the aluminium alloys compared to the steels is that the former are much lighter (about 2.723 tonnes/m3) than the latter (about 7.840 tonnes/m3) [18]. Finally, the aluminium alloys are valuable for military naval building, rendering to the ships lower radar detection visibility [18], compared to the steel constructions. Nevertheless, the corrosion protection by coatings is indispensable for all kinds of aluminium details and equipment. The conventional coatings are always composed of multilayer systems, where each layer has its own function [19-21]. On the other hand, the environmental restrictions regarding the employment of chromium and other heavy metals in the European Community [22, 23] and the Unites States [24, 25] impose demands for elaboration of environmentally compliant coatings.

The elaboration of quick and efficient technological approaches for deposition of cerium conversion coatings (CeCCs) is an efficient response to all these challenges. These coatings should serve as primers for upper organic, hybrid or composite layers. The behaviour of these exterior coatings, especially their aptitude for cracking, blistering or/and partial detachment, is strongly dependent on the primer coating layers. In their review article, Balgude and Sabnis [21] have remarked the state-of-the-art and the ongoing future trends in the field of development of new multilayer systems for corrosion protection. These authors even schematically illustrated their vision about the ongoing trends regarding the elaboration of advanced multilayered anticorrosion coatings, as shown in Figure 2.3.

Figure 2.3 Schematic representation of (a) conventional chromate-based coating system, (b) newly developed sol-gel-based films and (c) environmentally friendly chrome-free super primer technology [21].

2.1.2 Corrosion Phenomena, Basic Definitions and Concepts

In the literature, there are various definitions about the corrosion processes. It could be considered as undesirable loss of properties of solid materials, as consequence of their interaction with their surrounding environment. These interactions, as nature and intensity, strongly depend on the nature (composition structure and properties) of the material, the environmental conditions (composition, pH, temperature, etc.) and also on the exposed surface area of the material. The importance of the last factor is originated from the heterogeneous character of the corrosion reactions, occurring between the solid metallic surface and the liquid or gaseous environment. According to Davis [26], the corrosion processes can be divided into several basic groups, as shown in Figure 2.4.

Figure 2.4 Basic classification of corrosion processes [26].

All these forms of corrosion attack proceed on the metallic surface, resulting in decrease of the efficient cross section of the respective metallic details. Combined with the presence of mechanical tensions, they lead to stress corrosion cracking, followed by complete constructional...