Aerospace alloys protective coatings

Twite R.L., Bierwagen G.P. (1998), Review of alternatives to chromate for corrosion protection of aluminum aerospace alloys. Prog. Org. Coat., 33,91. 

Heat resistant and chemically resistant coating materials are attractive for applications in the automotive, aircraft and other industries. Guerriero et al. (2011) reported that coatings made of liquid crystal thermosetting polymers (LCT) showed markedly increased interface adhesion between LCT and an aluminum alloy substrate. Compared to a commercial LCP the LCT possessed higher hardness and stiffness, and this was favorable for their wear resistance. They concluded that LCTs have potential to serve as protective coatings for aerospace applications. 

Twite R. L. and Bierwagen G. P. (1998), Review of Alternatives to Chromate for Corrosion Protection of Aluminum Aerospace Alloys , Progress in Organic Coatings, 33,91-100. 

Hybrid sol-gel/conducting polymer coatings self-healing coatings for the corrosion protection of aerospace alloys... 

The technique may be said to combine the advantages of vacuum evaporation and sputtering, so that excellent qualities of adhesion are obtained without a limitation of maximum thickness of the coating—while at the same time the rate of deposition can be comparatively high. Many metals, alloys, and compounds may be deposited, on both metallic and non-metallic articles. However, its use at present is mainly for functional and protective applications, particularly where high resistance to corrosion is required. Thus, as examples, aluminium may be deposited on various types of steel and on titanium for uses in the aerospace and defence industries—and can be regarded as a less hazardous replacement for cadmium electroplating. 

Zinc-nickel Zn-Ni alloys with 5 to 15 wt% Ni offer excellent corrosion resistance and are mainly used in the automotive, aerospace, and electronics industries. Above 15% Ni, the alloy coating becomes more noble than steel, and the corrosion-protection mechanism changes from a sacrificial to a pure physical one (comparable to pure Ni coatings, see Sect. 5.5.4.2.2). They can be electrode-posited from acid or alkaline baths. The acid baths are usually based on sulfate, chloride, sulfate-chloride, pyrophosphate, or acetate (Table 15). The system shows anomalous codeposition (see Sect. 5.5.1.2), which has been explained by a hydroxide suppression mechanism [47]. As in the case of Ni-Fe, the alkaline baths must contain complexing agents (see Sect. 5.5.4.6.2). The alloys electroplated from add haths contain approximately 10 to 14% Ni, whereas the alkaline Zn-Ni... 

The review of rare earth conversion coatings presented in this chapter delineates the wide, and ongoing, interest in the rare earths for corrosion protection. The aqueous inhibition qualities of the simple rare earths salts for a range of metals have been known for nearly 30 years. The focus of the rare earths has been primarily on cerium, with a significant amount of research also performed on lanthanum systems. The evolution of the development of the immersion process to viable commercial processes has, however, been slow. Much work has focused on aluminium alloys and a commercial process was developed for non-aerospace applications for aluminium alloys. The development of conversion coatings for other metals has been slower for a range of reasons. 

Chandrasekhar et al. [934] carried out a very detailed study of corrosion protection of the aerospace Al alloys 2024 and 7075, the former of which has a substantial Cu content. They used coatings of poly (aromatic amines), primarily derivatives of poly (diphenyl amine) P(DPA)). In this work, these CPs were compared with P(ANi) and with the commercially available chromate conversion coatings " Accelagold" and Alumigold" (Elf Atochem America). Chandrasekhar et al. chose P(DPA) derivatives because of their higher bandgaps and much better match to Fermi levels at interfaces with Al-based alloys than P(ANi), as also their proven greater oxidative and environmental stability (> 400 C in air as shown by TGA data). 

---