Abstract
Two kinds of metal oxide coatings were fabricated on 7050 Al alloy by hard anodization (HA) and micro-arc oxidation (MAO) techniques. The microstructure, phase composition, and corrosion behavior of the two coatings were studied by scanning electron microscopy, X-ray diffraction, and corrosion tests, respectively. When compared with the HA coating, the MAO one was more effective to isolate the substrate from the corrosive environment. In addition, as confirmed by electrochemical tests, the MAO coating was of better corrosion resistance than the HA coating. Furthermore, it was revealed by neutral salt spray test that the MAO coating could protect substrate alloy over 1140 h, while the HA coating can only protect substrate alloy for 46 h due to the amorphous composition and through thickness defects (micro-pores and micro-cracks).
Highlights
Due to their superior properties of good formability, lightweight, and high strength/weight ratio, aluminum alloys have a wide usage in the aerospace industry, especially in commercial transport aircraft and military fighter aircraft [1]
The corrosion resistance of micro-arc oxidation (MAO) coating has been studied by a large number of studies and the results reveal that the MAO technique can improve the anti-corrosion properties of the substrate [15,16,17,18,19,20]
7050 Al alloy was used as an anode material, whose composition is Zn 6.2 wt %, Mg 2.10 wt %, Cu 2.40 wt %, Zr 0.12 wt %, Fe 0.15 wt %, Si 0.12 wt %, and Al balance
Summary
Due to their superior properties of good formability, lightweight, and high strength/weight ratio, aluminum alloys have a wide usage in the aerospace industry, especially in commercial transport aircraft and military fighter aircraft [1]. Different kinds of local corrosions, such as pitting corrosion [2], intergranular corrosion [3], exfoliation corrosion [4], and stress corrosion [5], occur on aluminum alloys All of these decrease the material strength and the plasticity substantially, and, greatly limit the application of these materials. In the past half century, hard anodization (HA) as a cost effective solution has been widely used to enhance the corrosion resistance of aluminum alloys by forming an amorphous and porous coating with a thickness over 25 μm [6,7,8] It is not an environmentally friendly and economical technique, since it adopts an electrolyte with high concentrations of acid, high current density, and low temperature [9,10]
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