Study On New Coloring Anodization Technologies 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 properties. On account of bad corrosion resistance and decorative properties, aluminum alloys should be treated before using. Anodizing is one of the means of enhancing the corrosion resistance, wear resistance and decorative properties. But the conventional anodizing processes have some disadvantages, such as complex technique, high cost, and toxic electrolyte of anodizing. A new coloring anodization technique was studied and the medium-sized test of the technique was investigated in this paper. The main results obtained are presented as follows:1. A new constant voltage coloring anodization technique was developed. In oxalic acid, boric acid or 5-sulfosalicylic acid mixed electrolyte respectively, smooth lustrous and yellow, grey or champagne color anodic films with high hard, good corrosion resistance and wear resistance on aluminum surface were prepared by this process. The effects of anodizing parameters such as electrolyte component, peak current density and agitation intensity on the film properties were investigated. In addition, the morphologies of the films were investigated by scanning electron microscopy (SEM). The results show that the film forming process and film properties were influenced by electrolyte component, peak current density and agitation intensity. The conditions of every anodizing process are as follows: yellow series, oxalic acid 6~8 g/L, film-formation accelerating agent 5~10 g/L; grey series, boric acid 30~40 g/L, film-formation accelerating agent 5~10 g/L, film-formation additive A 0.5~2.0 g/L; champagne series, 5-sulfosalicylic acid 60~70 g/L, film-formation additive B 5~8 g/L, Na2WO4 0.5~1.0 g/L.2. Anodizing of hard alloys (2024) in a boric acid mixed electrolyte was developed. Smooth film which is grey and ceramic luster, showing good micro-hardness and corrosion resistance can be deposited on hard alloys (2024) by constant voltage. The research results show that chemical pre-treatment is better than physical pre-treatment for film formation; the higher of the peak current density, the thicker of the film obtained; anodizing temperature should be controlled below 25℃. The EDS results show that the content elements of the hard alloys films are O, Al, P, S and C, and no Cu element were detected. XRD investigation shows that the film of hard aluminum was non-crystal film. The surface and cross-section SEM characteristics of the hard aluminum show that the film surface is porous and the diameter of the pore is about 20 nm. The hole wall is not vertical to the substrate and continuous, which seems to be formed by small grains.3. Both electrochemical means and SEM were employed to analyze the film formation process. Furthermore, the action of each electrolyte component was investigated. The results show that film properties were obvious influenced by electrolyte components, and excellent film could be formed only when three electrolyte components were mixed together. The film-formation accelerating agent can increase the anodizing current density and thus accelerate the forming rate; the film-formation additive can make the film uniform and nice. The mechanism of film formation greatly attaches to the electrolyte components and anodizing technique. The model based on the field-assistant and volume expansion can explain the pore formation. Under the high current density, an anion polymer layer formed at the interface of aluminum and electrolytes, so the vertical oriented ability of the pore become weak and a typical anodic film was formed.4. The equipment and process parameters such as pretreatment, electrolyte components, concentration, peak current density, anodizing time, et al. in the medium-size test of coloring anodizing technique were tentatively investigated. The technological process of industry production was determined, and the problems arising during the test and the resolving methods were also discussed.