Causes of color difference in anodized aluminum profiles

The electrolytic coloring of aluminum profiles has good decorative properties, and therefore is widely used both domestically and internationally, especially in the surface treatment production of architectural aluminum profiles. Currently, the main process involves electrolytic coloring using tin–nickel mixed salts. The products produced are mainly champagne-colored. Compared with single nickel salt coloring, products colored by tin–nickel mixed salts have a bright color and a full tone. The main problem is the presence of color differences. Unreasonable extrusion processes and anodic coloring processes during aluminum profile production can lead to color variations in the products.

The extrusion process affects anodic coloring mainly through factors such as mold design, extrusion temperature, extrusion speed, and cooling method, all of which influence the surface condition and uniformity of the extruded profile. Mold design should ensure thorough mixing of the input material; otherwise, bright (or dark) streak defects may occur, and color separation may appear along the same profile. At the same time, the mold condition and extrusion marks on the profile surface also affect anodic coloring. Variations in extrusion temperature, speed, cooling method, and cooling time can lead to non-uniform profile structure, which also produces color differences.

Anodizing has a significant impact on the color difference in electrolytic coloring, especially in the vertical oxidation line production process, where color variations at both ends often occur. The vertical oxidation tanks are 7.5 meters deep, and temperature differences can easily form between the upper and lower tank solutions. Temperature plays a crucial role in anodizing: when the temperature is high, the oxidation tank solution accelerates the dissolution of the oxide film, enlarging the pore size on the surface of the porous anodic oxide film. Conversely, when the temperature is low, the pore size on the surface of the porous anodic oxide film is smaller. Additionally, higher temperatures result in higher porosity in the anodic oxide film, whereas lower temperatures lead to lower porosity. Electrolytic coloring primarily involves electrochemical reduction of the metal ions in the coloring solution on the barrier layer surface of the oxide film's micro-pores, causing the metal ions in the solution to deposit at the bottom of the anodic oxide film pores. These deposits scatter incident light, producing different colors— the more material deposited in the micro-pores, the darker the color. Under the same amount of electrical charge, if the same amount of metal or metal compound is deposited in areas with different temperatures, locations with higher porosity and larger surface pores will have less deposition per pore, resulting in a relatively lighter color, while areas with lower porosity and smaller pores will appear darker, causing the color variation at both ends. During anodizing, conductivity also affects the oxide film and can lead to color differences. This issue mainly occurs in horizontal production lines, due to looser clamping during the upper row process before oxidation, leading to poor conductivity in certain pieces. As a result, their oxide films differ, producing color differences after coloring.

The electrolytic coloring process can directly reveal the color difference problem. The ability of the electrolytic coloring solution to distribute current determines the uniformity of color deposition on the material. Uneven current distribution causes noticeable color differences. The current distribution ability of the tank solution mainly depends on the solution's conductivity and polarization. A certain amount of conductive salts in the coloring solution is intended to enhance conductivity. If the conductive salts are not replenished timely, conductivity decreases, and current distribution worsens, causing color differences. Additionally, additives in the coloring solution can cause characteristic adsorption, increasing polarization. Excessive consumption of these substances reduces the electrolyte's polarization, decreasing current distribution ability, and also leading to color differences. In actual production, it is necessary not only to improve the tank solution's conductivity but also to ensure that the conductive rods and copper holders maintain good conductivity. Poor conductivity results in uneven current distribution across the power line, causing color differences.