ã€China Aluminum Industry Network】 Aluminum alloy has the characteristics of light weight, easy molding, high specific strength, and good corrosion resistance. It is widely used in aerospace, transportation, light industry, building materials, packaging corrosion, electrical appliances, furniture and other fields. 1]. There are more than 700,000 kinds of aluminum products, and it is called second steel. Aluminum-based steel, copper and wood are the development trends in today's world. The original aluminum alloy is of a single color and cannot meet the needs of diversified colors in the application. With the improvement of people’s living standards, the aluminum coloring products with various colors have been updated. , higher requirements, given its excellent surface functional properties [2]. Nowadays, aluminum anodized electrolytic coloring technology has been in the core technical position [3]. The level of electrolytic coloring of aluminum profiles represents the status of an aluminum profile enterprise's surface treatment technology and determines the competitiveness of aluminum profile enterprise products. In this paper, the tin-nickel double salt electrolysis coloring technology, which is widely used and widely used in aluminum industry, is studied in detail.
Second, tin-nickel double salt electrolytic coloring mechanism
At present, electrolytic coloring technology for industrial production at home and abroad is basically tin-nickel double salt and single nickel salt[4], especially tin-nickel double salt electrolytic coloring technology is widely used in industrialization, and its coloring color is generally It is a pale to deep bronze color system [5, 6]. This is a color system obtained by the scattering effect in the visible light range. Researchers at home and abroad tend to ripen the tin-nickel double salt electrolytic coloring process in the 1980s. The mechanism of electrolytic coloring was studied in depth [7]. The oxide film and the coloring mechanism were studied microscopically. However, the electrolytic coloring process was relatively complicated. Some research theories were not uniformly approved, such as the presence of metal in the electrolytic coloring film. The coloring principle of coloration, how the current in the electrolytic coloring process makes the metal ions reduce to the bottom of the oxide film through the barrier layer have different views and opinions. Studies at home and abroad have shown [8] and [9] that, regardless of the metal electrolyte's AC electrolytic coloring film, the deposits in the anodic film pores are not only crystalline metal ions but also amorphous metal oxides or hydroxides. Different metal ions are deposited in different colors [10], and the anodizing and electrolytic coloring conditions differ depending on the metal salt used.
The basic process of tin-nickel double salt electrolysis coloring is divided into three steps [11,12]: (1) Sn2+, Ni2+, and H+ reactant ions pass near the surface of the oxide film barrier layer; (2) Sn2+ and Ni2+ on the oxide film The electrons are obtained between the barrier layer and the liquid interface, H+ penetrates the barrier layer, and electrons are obtained between the interface between the substrate and the barrier layer; (3) metal is precipitated and hydrogen is produced. The reduction deposition reaction of Sn2+ at the cathode: Sn2++2e→Sn; at the same time, the hydrogen ion discharge reaction at the cathode produces hydrogen: 2H++2e→H2; due to the tin-nickel double salt electrolytic coloring process PH is about 1, reaching Without the Ni2+ reduction electrode potential, nickel ions cannot be reduced at this time and only stannous ions are reduced [3, 13].
Third, large-scale production line tin-nickel alloy double salt electrolysis coloring key technology
3.1 Effect of Process Parameters on Electrolytic Coloring of Tin-nickel Double Salts in Aluminum Profiles
3.1.1 Effect of main salt concentration on electrolytic coloring of tin-nickel double salts in aluminum profiles
In the tin-nickel double salt electrolytic coloring solution, if the concentration of stannous sulfate and nickel sulfate is lower than the process range, it is not easy to color the aluminum oxide film in the air. If the concentration of stannous sulfate and nickel sulfate is too high, it is easy. A floating color appears and is easily eluted after washing. Therefore, the control of the main salt concentration must be within the scope of the process to ensure that the color requirements are from light to dark. In general, the large-scale production line produces the champagne color system, the concentration of stannous sulfate is controlled to 4-5 g/L, and the concentration of nickel sulfate is controlled to 18 -20g/L; if bronze or black is produced, the concentration of stannous sulfate is controlled to 8-10g/L; the concentration of nickel sulfate is controlled to 28-30g/L; nickel is used in tin-nickel double salt electrolytic coloring process Ions cannot be reduced and deposited in the pores of the aluminum oxide film. Nickel ions are added to compete with the stannous ions to reduce and promote the deposition of stannous ions in the pores of the oxide film, thereby accelerating the electrolytic coloring process and shortening the electrolytic coloring time.
3.1.2 Effect of the pH value of the tank at night on the electrolytic coloring of tin-nickel double salts in aluminium profiles
In the tin-nickel double salt electrolytic coloring solution, the pH value at the tank is generally constant at about 1. When the pH value exceeds 1.5, the hydrolysis of divalent tin ions is intensified, the oxide film is eroded, and it is easily blocked by the hydroxide. The membrane pores are not colored. In this case, the reagent sulfuric acid can be used to adjust the bath. Adding sulfuric acid is an economical and effective method to increase the bath acidity. In addition, organic acids can be added to increase the bath acidity. Higher than sulfuric acid, but added to help improve the bath complexation. The pH of the bath must not be too low. When the pH of the bath is less than 0.5, the oxide film is susceptible to corrosion and difficult to tint. The colored part may also be uneven or the tint may be faded. Sometimes even completely without coloring, at the same time the pH of the bath is too low to cause the hydrogen ions to be reduced to generate hydrogen in preference to the stannous ions, which reduces the deposition rate of stannous ions and affects the electrolytic coloring effect.
3.1.3 Effect of bath temperature on electrolytic coloring of tin-nickel double salt
The increase of bath temperature will accelerate the oxidation of divalent tin ions to tetravalent tin ions, and the hydrolysis reaction speeds up. For this reason, controlling the bath temperature has important significance for maintaining the stability of the bath fluid. Another disadvantage of the bath temperature is that the bath temperature is too high. The conductivity of the coloring liquid is increased, and the reduction reaction of the stannous ions is accelerated. As the coloring speed is accelerated, the surface of the oxide film tends to float with a rough color, and the process control becomes more difficult. If the bath temperature is too low, the coloring speed is slow and only light colors can be used. The temperature control of tin-nickel double-salt electrolytic coloring bath is generally 18-22°C for large-scale production lines. If the bath temperature is controlled within the process range, the above two points can be avoided.
3.1.4 Effect of Alternating AC Voltage on Electrolytic Coloring of Tin-nickel Double Salts
Under the condition that the concentration, pH value, temperature, and coloring time of the electrolytic coloring solution are not changed, if a low voltage coloring is used, the coloring speed is slow and the color is light. If the coloring voltage is increased, the coloring speed is accelerated, and The dark color can be put on, the production of light-colored products on large-scale production lines, the AC voltage is generally controlled to 15-17V, the production of dark-colored products, the AC voltage is generally controlled to 17-19V; In addition, the AC voltage can not rise too fast, usually after approximately 40s increases the AC voltage from 0V to 17V. If the voltage rises too fast, the oxide film will be peeled off, resulting in failure to color.
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