An environmentally friendly, non-chemical surface treatment has been developed as an alternative to phosphoric acid anodizing. This treatment is based on ion beam enhanced deposition of aluminum oxide (a-Al2O3) onto an aluminum alloy surface. Results of experiments are presented, demonstrating that the IBED surface treatment yields a surface which has improved resistance to corrosion in aqueous chloride solutions.
Aluminum and its alloys are widely used because of specific desirable properties. For example, the high strength-to- weight ratio of the Al-Cu (2000-series) and AI-Zn-Mg (7000-series) alloys, makes these alloys particularly suited as structural materials in aircraft. However, they are susceptible to various forms of corrosion such as pitting, intergranular, and exfoliation corrosion. In particular, the wrought commercial alloy 2024-T3 is highly susceptible to pitting corrosion. Particles of intermetallic phases are reported to be common sites for pit nucleation in the 2000 and 7000 series aluminum alloys.1,2,3 In a recent study by Chen at al.4 it was demonstrated that pitting in alloy 2024-T3 was associated with AI-Cu-Mg and AI-Cu-Fe-Mn intermetallic (or constituent) particles. The former act as anodic sites, whereas the latter act as cathodic sites, where pits can nucleate. Pit growth is attributed primarily to the cathodic nucleation sites (AI-Cu-Fe-Mn particles), which promote matrix dissolution, and particularly to the coalescence of individual pits at these cathodic sites.
In order to prevent rapid localized corrosion, various methods of protecting aluminum alloys have been developed. These may range from the formation of an artificially thick aluminum oxide, grown either chemically or electrochemically, to the application of various paints and coatings. Aluminum oxides occur in various structures and morphologies depending on how they are formed and on the degree of oxide hydration.5A common means to form protective oxide films is the anodizing process. Anodizing can be accomplished in various acidic solutions, such as sulfuric, phosphoric, and chromic acid. Although these treatments generally result in amorphous V-ALP., the oxide morphologies can be quite different, see Figure 1.6 It can be seen that the anodic oxides are relatively porous, and although there are processes known to seal the pores in the oxides, fully dense oxides can not be achieved. Moreover, alloying elements such as Cu, Fe, and Mn (in the case of 2024-T3) are incorporated in the oxide, and act as nucleation sites for pitting corrosion.
Ion beam enhanced deposition (IBED) of anhydrous a-AI2O3 can create a dense oxide, free of constituent particles, which may provide higher resistance to corrosion than the natural or (electro)-chemically grown aluminum oxides. A research program was conducted to demonstrate the protective nature of the ion beam enhanced deposited oxide film.
EXPERIMENTAL APPROACH
Processing
Oxide films were deposited onto the alloy 2024-T3 both electrochemically by phosphoric acid anodizing (PAA),7 and by IBED.8 Prior to anodizing, the test coupons were degreased in an industrial general purpose cleaner, and pickled in a 500 g/l HNO3 - 2.5 g/l HF solution at room temperature for 10 minutes.
