[Alcoa.com] The surface of Al and its alloy reacts with oxygen to form an oxide film. This passivation film can resist atmospheric corrosion, but the alumina on the film is amorphous, and the film surface is uneven and discontinuous. . Since the appearance of the aluminum anodizing process, the preparation process of the aluminum anodized film has many kinds of classifications, such as a sulfuric acid method, an oxalic acid method, an organic acid method, and a direct current method. In this paper, the samples were electropolished and then treated with two different mixed acids. The aluminum alloys were subjected to constant voltage DC anodic oxidation. The effect of oxidation time on the growth thickness of the aluminum anodic oxide films was studied, and the microstructure of the oxide films was investigated. The characterization, at the same time, analyzed the causes of some phenomena.
The experimental material is a massive as-cast 7050 aluminum alloy whose main chemical composition is (mass fraction, %): 0.12Si, 0.15Fe, 2.0~2.6Cu, 0.1Mn, 1.9~2.6Mg, 0.04Cr, 5.7~6.7Zn, 0.06Ti, 0.08~0.15Zr. A 10 mm×30 mm×10 mm metallographic sample was cut from a block of aluminum alloy, and then ground on different types of water-repellent sandpaper in order, then roughly polished, and roughened with a suspension of a large, coarse-grained Al2O3 powder mixed with water. Polishing agent, polishing machine rotation number of 400 ~ 600r/min, after the appropriate fine polishing, until the naked eye can not see obvious scratches. Rinse with water, dehydrate alcohol, and dry it for later use. Erosion with Keller's reagent (1.0% HF, 1.5% HCl, 2.5% HNO3, and 95% water in a mixture of reagents) prior to electrolytic polishing reveals the deformed layer.
Electrolytic polishing solution of 70% (volume fraction) perchloric acid 10mL and ethanol 90mL mixed solution. Temperature 80-90°C; voltage 14-18V; current density 0.1A/cm2; polishing time 20s. In the experiment, a stainless steel plate was used as the cathode, and the 7050 aluminum alloy sample was coated with an anode in an aqueous HBF4 solution. After electrolysis, the sample was rinsed with water, and the electrolysis product on the surface was washed in a 1:1 (volume fraction) nitric acid solution. After ultrasonic cleaning, alcohol dry, ready for coating. The device covered with the film is the same as that of the electrolytic polishing, with a voltage of 15 to 22 V, a current density of 0.1 A/cm2, room temperature, and a time of 5 seconds. Two kinds of film-making solution were selected, (1) the composition of HBF4 aqueous solution [5.5gH3BO3 added 15mL (20% ~ 30%) HF, make it fully dissolved into a clear solution] and then against the 500mL distilled water; (2 ) 95% to 98% (volume fraction) of 38 mL of sulfuric acid, 85% (volume fraction) of 43 mL of phosphoric acid, and 19 mL of distilled water.
According to the polarization curve of Al and its alloys to adjust the time and current density, it was found that the low voltage and small current density have a better effect on electrolytic polishing. In a certain time range, the thickness of the anodized film increases with time. The XRD composition analysis of the film surface showed that the surface of the film contains amorphous and crystalline Al2O3.
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