Sodium hydroxide, commonly known as caustic soda, fire soda, and caustic soda, is a highly corrosive alkali in the form of flakes, granules, or blocks. It is easily soluble in water (it releases heat when dissolved in water) and forms an alkaline solution. It is deliquescent and can easily absorb water vapor (deliquescence) and carbon dioxide (deterioration) in the air. Hydrochloric acid can be added to check whether it has deteriorated. Easily soluble in water, ethanol, and glycerol, but insoluble in acetone and ether. The pure product is a colorless and transparent crystal. Density 2.13g/cm3. Melting point 318℃. Boiling point 1388℃. Industrial products contain a small amount of sodium chloride and sodium carbonate, which are white opaque crystals. Let’s talk about the practical applications of sodium hydroxide in metal surface treatment processes.
1. For oil removal, use sodium hydroxide to react with stearic acid esters in animal and vegetable oils to produce water-soluble sodium stearate (soap) and glycerin (glycerin). When the concentration of sodium hydroxide decreases and the pH is less than 10.5, sodium stearate will be hydrolyzed and the oil removal effect will be reduced; if the concentration is too high, the solubility of sodium stearate and surfactant will be reduced, resulting in poor water washability and hydrogen oxidation. The sodium dosage generally does not exceed 100g/L. Sodium hydroxide is widely used in metal parts, such as various steels, titanium alloys, nickel, copper, etc., and non-metal parts, such as various plastic parts, for degreasing before plating. However, sodium hydroxide should not be used to degrease alkali-soluble metal parts such as aluminum and zinc. Alkaline degreasing of plastic parts is suitable for ABS, polysulfone, modified polystyrene, etc. Parts such as glass fiber reinforced plastics and phenolic plastics that are not resistant to alkaline solutions are not suitable for alkaline degreasing.
2. Metal etching application ①. In the treatment of aluminum alloy before oxidation, a large amount of sodium hydroxide is used for alkali etching. This method is the standard treatment method before aluminum alloy oxidation. A large amount of sodium hydroxide is also used for aluminum alloy texture etching. ②. Sodium hydroxide is an important etching material in the chemical etching process of aluminum and alloys. It is also a common etching method today. In the etching process of aluminum and alloys, the content of sodium hydroxide is generally controlled at 100~200g/L. , and as the concentration of sodium hydroxide increases, the etching speed accelerates. However, if the concentration is too high, it will increase the cost. The etching quality of some aluminum materials deteriorates. The reaction is as follows AI+NaOH+H2O=NaAIO2+H2↑
3. In electroplating and chemical plating applications, a large amount of sodium hydroxide is used in alkaline tin plating and alkaline zinc plating. Especially in alkaline zinc plating, a sufficient amount of sodium hydroxide is the basic condition to maintain solution stability; in electroless plating It is used for pH adjustment of electroless copper plating; used for the preparation of zinc immersion solution before aluminum alloy electroless plating/electroplating, etc. ①. Application in cyanide zinc plating. Sodium hydroxide is another complexing agent in the plating bath. It complexes with zinc ions to form zincate ions, which makes the plating bath more stable and improves the conductivity of the plating bath. Therefore, the cathode current efficiency and dispersion ability of the plating solution are improved. When the sodium hydroxide content is high, the anode dissolves faster, causing the zinc content in the plating solution to increase and the coating to become rough. If the sodium hydroxide is too low, the conductivity of the plating solution is poor, the current efficiency decreases, and the coating will also be rough. In a plating solution that does not contain sodium hydroxide, the cathode efficiency is very low. As the concentration of sodium hydroxide increases, the cathode efficiency gradually increases. When the concentration of sodium hydroxide reaches a certain amount (such as 80g/L), the cathode efficiency reaches The highest value and remains essentially constant thereafter. ②. Application in zincate electroplating: Sodium hydroxide is a complexing agent and a conductive salt. A slight excess of sodium hydroxide can make the complex ions more stable and have better conductivity, which is beneficial to improving the dispersion ability of the plating solution. , and allow the anode to dissolve normally. The mass ratio of zinc oxide to sodium hydroxide in the zincate plating solution is preferably about 1: (10~14), with the lower limit for hanging plating and the upper limit for barrel plating. When the sodium hydroxide content is too high, the anode dissolves too quickly, the concentration of zinc ions in the plating bath is too high, and the crystallization of the coating is rough. If the content is too low, the conductivity of the plating bath is reduced, and zinc hydroxide precipitation is easily generated, which affects the quality of the coating. ③. Application in alkaline tin plating. In alkaline tin plating, the main function of sodium hydroxide is to form a stable complex with tin salt, improve the conductivity, and facilitate the normal dissolution of the anode. As the concentration of sodium hydroxide increases, the polarization becomes stronger and the dispersion ability increases, but the current efficiency decreases. If the sodium hydroxide is too high, it is difficult for the anode to maintain a semi-passivated state and dissolves divalent tin, resulting in poor coating quality. Therefore, controlling the concentration of sodium hydroxide is far more important than controlling the tin salt content. Usually sodium hydroxide is controlled at 7~15g/L, and if potassium hydroxide is used, it is controlled at 10~20g/L. In the alkaline electroless copper plating process, sodium hydroxide is mainly used to adjust the pH value of the plating solution, maintain the stability of the solution and provide an alkaline environment for formaldehyde reduction. Under certain conditions, increasing the concentration of sodium hydroxide can appropriately increase the speed of electroless copper deposition, but too high a concentration of sodium hydroxide cannot increase the speed of copper deposition, but will instead reduce the stability of the electroless plating solution. Sodium hydroxide is also widely used in the oxidation of steel. The concentration of sodium hydroxide directly affects the oxidation speed of steel. High-carbon steel has a fast oxidation speed and a lower concentration (550~650g/L) can be used. Low-carbon steel oxidation The speed is slow and higher concentration (600~00g/L) can be used. When the concentration of sodium hydroxide is high, the oxide film is thicker, but the film layer is loose and porous, and red dust is prone to appear. If the concentration of sodium hydroxide exceeds 1100g/L, the magnetic iron oxide is dissolved and cannot form a film. The concentration of sodium hydroxide If it is too low, the oxide film will be thin and the surface will be shiny, and the protective performance will be poor.
4. Application in sewage treatment: Sodium hydroxide is a commonly used neutralizing agent and metal ion precipitating agent for wastewater discharged from electroplating, anodizing, etc.
Post time: Sep-04-2024
