The application of sodium hydroxide in metal surface treatment process

Sodium hydroxide, commonly known as caustic soda, caustic soda, or caustic soda, is a strong corrosive alkali that is in the form of flakes, granules, or blocks. It is easily soluble in water (releasing heat when dissolved in water) and forms an alkaline solution. It also has deliquescence and is easy to absorb water vapor (deliquescence) and carbon dioxide (transformation) from the air. Hydrochloric acid can be added to test for deterioration. Easy to dissolve in water, ethanol, and glycerol, insoluble in acetone and ether. Pure product is a colorless and transparent crystal. Density 2.13g/cm3. Melting point 318 ℃. Boiling point 1388 ℃. Industrial products contain small amounts of sodium chloride and sodium carbonate, which are white opaque crystals. What are the practical applications of sodium hydroxide in metal surface treatment processes?

Oil removal application
Sodium hydroxide reacts with stearic acid esters in animal and vegetable oils to produce water-soluble sodium stearate (soap) and glycerol (glycerol). When the concentration of sodium hydroxide decreases and the pH is less than 10.5, sodium stearate undergoes hydrolysis and the oil removal effect decreases; If the concentration is too high, it will reduce the solubility of sodium stearate and surfactants, resulting in poor water washability. The dosage of sodium hydroxide is generally not more than 100g/L.
Sodium hydroxide is widely used in metal parts, such as various steels, titanium alloys, nickel, copper, and non-metallic parts such as various plastic parts for pre plating degreasing treatment. However, alkali soluble metal parts such as aluminum and zinc should not be degreased using sodium hydroxide.
Alkaline degreasing of plastic parts is suitable for ABS, polysulfone, modified polystyrene, etc. Parts such as fiberglass reinforced plastics and phenolic plastics that are not resistant to alkaline solutions are not suitable for alkaline degreasing.
2. Metal etching applications
① In the pre-treatment of aluminum alloy oxidation, a large amount of sodium hydroxide is used for alkaline etching. This method is the standard pre-treatment method for aluminum alloy oxidation, and a large amount of sodium hydroxide is also used for aluminum alloy texture etching.
② Sodium hydroxide is an important etching material and a commonly used etching method in the chemical etching process of aluminum and alloys. In the etching process of aluminum and alloys, the content of sodium hydroxide is generally controlled at 100-200g/L. As the concentration of sodium hydroxide increases, the etching speed accelerates. However, excessive concentration not only increases the cost, but also requires higher requirements for the anti-corrosion layer. Thirdly, excessive concentration of sodium hydroxide can also deteriorate the etching quality of certain aluminum materials. Its reaction is as follows: AI+NaOH+H2O=NaAIO2+H2 ↑
3. 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 where sufficient sodium hydroxide is a basic condition for maintaining solution stability; Used for pH regulation in electroless copper plating; Preparation of zinc immersion solution for aluminum alloy chemical plating/electroplating.
① Application in cyanide zinc plating
Sodium hydroxide is another complexing agent in the plating solution, which forms zinc salt ions by complexing with zinc ions, making the plating solution more stable and improving its conductivity, thereby improving the cathode current efficiency and dispersion ability of the plating solution. When the sodium hydroxide content is high, the anodic dissolution accelerates, causing an increase in zinc content in the plating solution and rough coating. 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 without sodium hydroxide, the cathodic efficiency is very low. As the concentration of sodium hydroxide increases, the cathodic efficiency gradually increases. When the concentration of sodium hydroxide reaches a certain amount (such as 80g/L), the cathodic efficiency reaches its highest value and remains basically constant thereafter.
② Application in Zinc Salt Electroplating
Sodium hydroxide is a complexing agent and a conductive salt. A slightly excessive amount of sodium hydroxide can make the complex ions more stable, have better conductivity, improve the dispersion ability of the plating solution, and enable the anode to dissolve normally. The mass ratio of zinc oxide to sodium hydroxide in zinc salt plating solution should be maintained at around 1: (10-14), with the lower limit set for hanging plating and the upper limit set for rolling plating. When the content of sodium hydroxide is too high, the anodic dissolution is too fast, the concentration of zinc ions in the plating solution is too high, and the crystallization of the coating is rough. If the content is too low, the conductivity of the plating solution is reduced, and it is easy to generate zinc hydroxide precipitation, which affects the quality of the coating.
③ Application in alkaline tin plating
The main function of sodium hydroxide in alkaline tin plating is to form a stable complex with tin salts, improve conductivity, and facilitate the normal dissolution of the anode. As the concentration of sodium hydroxide increases, the polarization effect strengthens and the dispersion ability improves, but the current efficiency decreases. Excessive sodium hydroxide makes it difficult for the anode to maintain a semi passive state and dissolves in divalent tin, resulting in a deterioration of the coating quality. So controlling the concentration of sodium hydroxide is much 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 rate of chemical copper deposition. However, excessive sodium hydroxide concentration cannot further increase the rate of copper deposition, but instead can reduce the stability of the chemical plating solution. Sodium hydroxide is also widely used in the oxidation of steel, and its concentration directly affects the oxidation rate of steel. High carbon steel has a fast oxidation rate, and lower concentrations (550-650g/L) can be used. Low carbon steel has a slow oxidation rate, and higher concentrations (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, which is prone to red ash. If the concentration of sodium hydroxide exceeds 1100g/L, magnetic iron oxide is dissolved and cannot form a film. If the concentration of sodium hydroxide is too low, the oxide film is thin and the surface is flower like, with poor protective performance.
4. Application of sewage treatment
Sodium hydroxide is a commonly used neutralizing agent and metal ion precipitant for wastewater discharged from electroplating, anodizing, and other processes.
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