Study On New Anodising Process Of Aluminum And Its Alloys

Aluminum and its alloys are increasingly being utilized in fields such as automotive, aerospace, electronics and home appliances on account of their excellent physicochemical properties. In order to further improve of hardness, wear resistance and corrosion resistance of aluminum alloys, anodic oxidation treatment was usually carried out, and a layer of thick, dense oxide film formed on the surface of aluminum alloys. Commonly used hard anodizing process is carried out in sulfuric acid electrolyte at low-temperature and high current density. This oxidation process has harsh conditions, high energy consumption and production cost. In AC oxidation, the two electrodes can be oxidized at the same time, so the cathode material does not need and high-speed oxidation can be achieved by using high current density. Compared with DC oxidation, AC oxidation has high efficiency, low energy consumption and low cost, and has been of concern for people. AC oxide film obtained in sulfuric acid is thin, yellow, transparent, porous and less than 10μm, and has low hardness, poor wear resistance. All these limit its application. In this paper, room temperature rapid hard anodizing and AC oxidation techniques of 3005 aluminum alloy were developed by using special composite electrolyte. Effects of film morphology, structure and electrolytic parameters on film properties were studied in detail. Moreover, preparation of multi-layer structure aluminum oxide film was explored. The main research results are as follows:1. A new room temperature rapid hard anodization technique of 3005 aluminum alloy was developed. In sulfosalicylic acid mixed electrolyte, a black hard anodizing film can be formed on 3005 aluminum alloy surface at 10-25℃with a constant current density. The anodizing film has high hard and good corrosion resistance. Effects of electrolyte components and anodizing parameters on the film properties were investigated. In addition, morphologies and elements composition of the films were investigated by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the film forming process and film properties were influenced by electrolyte component, current density and agitation intensity. The optimized electrolyte solution contains sulfosolicylic acid 40-50g/L, additive A 25-40 g/L, additive B 10-25 g/L, 98%(mass fraction) of concentrated sulfuric acid 2-4ml/L. SEM result shows that the film is porous, with a pore size of 10-15nm and interpore distances of 60-70nm. EDS result shows that the main elements of the film are Al and O、C、S and Mn are also existed in the film.2. An AC oxidation technique of aluminum and its alloys was developed. In the composite electrolyte containing aromatic carboxylic acid, oxalic acid and inorganic film-forming promotor agent, a layer of gray or gray-black oxide film can be obtained by using commercial AC. The film has a certain thickness and fine decorative performance. The electrolyte compositions and electrolytic parameters are as follows: aromatic carboxylic acid 50-60 g/L, oxalic acid 20-30 g/L, inorganic film-forming promotor agent 20-30 g/L, peak current density 8-16 A/dm2, oxidation time 10-15min, temperature 10-20℃. SEM analysis shows that the oxide film is porous with 20 nm pore size and 10nm interpore distances, the oxide film hole is vertical to the basement and the hole wall is continuous and complete. EDS result shows that the main elements of the film are O and Al, a small amount of C and S elements are also contained in the film.3. Preparation of multi-layer aluminum oxide film was explored preliminarily. Different multi-layer aluminum oxide films were prepared by using different powers and electrolytes with multi-step anodizing. The experimental results show that the growth of oxide film is relatively independent and the oxidation parameters are influenced by the pre-formed oxide film. Anodizing of aluminum can proceed on the basis of the pre-formed oxide film. The pre-existed oxide film does not affect the structure of the oxide film. The method presents a new route for preparing aluminum oxide film with special structure and properties, which opens up a new field of exploring aluminum anodizing process.